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WORKSHOP ON SCIENTIFIC REQUIREMENTS FOR MITIGATION OF HAZARDOUS COMETS AND
ASTEROIDS
http://www.noao.edu/meetings/mitigation/index2.html

(Workshop Sponsored by NASA; co-sponsored by Ball Aerospace and Science
Applications International Corporation)

Dates: September 3 through 6, 2002
Venue: Hyatt in Arlington, Virginia

Background Rationale and Goals for the Workshop

One hundred years is approximately the time scale for a 10% probability of
an Earth impact by a 100-meter sized near-Earth asteroid, one capable of
causing substantial regional disruption or destruction of societal
infrastructure.

This is also the estimated time (~ 70 years) necessary to assure the
development of an appropriate mitigation technology and learn how to apply
it to an Earth threatening object (Belton et al, 2001).

These timescales are similar to the typical lifetime of a family from birth
through the death of grandchildren, and can be expected to be of particular
interest to contemporary society.

This confluence of timescales gives present urgency and special interest to
consideration of the scientific foundations on which near-Earth object (NEO)
collision avoidance and impact mitigation technologies must be based.

Programs for the detection of possible impactors are well in hand, and ideas
abound on how to apply the energy required to either disrupt or deflect an
incoming impactor (Hazards due to Comets & Asteroids, T. Gehrels, Ed.,
1994). Yet little published work exists to address the detailed scientific
and technical requirements for avoidance and mitigation technologies, and
whether an adequate knowledge base exists.

The need for space exploration of NEOs is widely recognized (e.g. in the
Spaceguard Survey report, Morrison, 1992; Space Surveillance, Asteroids and
Comets, and Space Debris, USAF Science Advisory Board report, 1997). More
recently, a UK Task Force on NEOs (Atkinson, 2001) recommends that an
international approach be considered that employs a coordinated set of
rendezvous missions based on inexpensive micro-satellite technology.

Even with the publication of such recommendations it is not clear, from what
has been published, that they are offered on a secure scientific and
technical basis. For example, micro-satellite spacecraft do have an
important role to play in the future scientific exploration of NEOs. Yet for
impact mitigation or collision avoidance technologies to succeed, a high
priority must be placed on scientific investigations intimately associated
with the deep interior structure and special material properties of these
objects.

Beyond revealing fundamental clues to the origins of planets, knowledge of
the deep interior structure of asteroids and comets is a requirement if one
means to apply whole-body forces to them and achieve predictable results.

To measure and characterize the needed properties encompassing mass, mass
distribution, material strengths, internal structure, shape, and spin state
(Huebner and Greenberg, 2002), novel kinds of spacecraft investigations will
be required. Locally, drilling and digging from the surface can provide some
of these data, but will probably be restricted to a limited depth. Globally,
radio and seismic wave experiments with active sources analogous to those
used in terrestrial exploration may be necessary. This will require the
development of whole new encounter technologies, and may lead to new
mitigation strategies as well.

This workshop will review what is known about the physics and chemistry of
the interiors of small cometary nuclei and asteroids with the purpose of
attaining a geophysical understanding of asteroids and comets in near-Earth
space. In addition, the workshop will work towards the following specific
goals:

Determination of the scientific requirements for those collision avoidance
and impact mitigation technologies that are considered viable. This includes
identification of measurements that are needed and the accuracy that should
be attained.

Determination of what mission models and instrumentation developments are
needed to make these measurements.

Construction of a mission and research roadmap for achieving an adequate
level of knowledge on which to base the future development of practical and
reliable collision avoidance and impact mitigation systems.

References:
Atkinson, H. 2001. UK Task Force on Near-Earth Objects. This report is best
acquired through its web page:
http://www.nearearthobjects.co.uk/neo_report.cfm

Belton, M.J.S., E. Asphaug, W. Huebner, and D. Yeomans 2001. Scientific
requirements for NEO Impact Mitigation. Presented at Asteroids 2001 meeting,
Palermo, Sicily.

Hazards due to Comets and Asteroids 1994. Edited by Tom Gehrels, University
of Arizona Press.

Huebner, W.F., and J.M. Greenberg 2002. Erice Workshop Summary on Physical
and Chemical Properties of Potential Earth Impactors, Meteoritics and
Planetary Science, In Press.

The Spaceguard Survey: Report of the NASA International Near-Earth-Object
Detection Workshop 1992. Edited by David Morrison. Pasadena, CA: Jet
Propulsion Laboratory.

USAF Scientific Advisory Board 1997. Space Surveillance, Asteroids, and
Comets, and Space Debris, Vol 1, Space Surveillance, SAB-TR-9604.

========
ABSTACTS

http://www.noao.edu/meetings/mitigation/media/invited_abs.pdf

SIZES AND STRUCTURES OF COMETS AND ASTEROIDS: WHAT IS WORTH MITIGATING, AND
HOW?

Erik Asphaug, Earth Sciences Dept. University of California, Santa Cruz
asphaug@es.ucsc.edu

Once every 20,000 years, a huge rock mass slams into an ocean basin with
little or no warning, generating a tsunami with wave energy equivalent to ~3 gigatons of TNT.
Hundred meter high waves propagate across the impacted ocean basin, obliterating coastal
cities in their wake. Hundreds of millions of lives are lost, and the cost in purely
economic damage is in the trillions of dollars. Yet we do almost nothing
about it. I am talking about volcanic island landslides. Waves spawned by
once-per-20,000 year collapses of volcanic mountain flanks (Cumbre Vieja,
Kilauea, etc.) are about the same wave energy as would be spawned by a 600 m
diameter asteroid (S. Day, pers. comm. 2002). Interestingly, 20,000 years is
also about the mean recurrence interval for 600 m NEO impacts. Smaller
island collapses (e.g. Ritter Island, 1888) are certainly more frequent than
Tunguska-type airbursts, and probably cause at least as much potential harm.
And the largest volcanic events, such as the Siberian flood basalts which
may have conspired to end the Permian, are about as rare and evidently as
deadly as the largest impact events in the present solar system. These
numbers are all quite rough, and the parallels not entirely satisfactory
(for instance, asteroids can hit suddenly and anywhere). But it helps
objectively constrain our concern with NEOs. They do represent the one
potentially catastrophic natural disaster that we think we can mitigate, yet
mitigation has its own costs and risks, and if those costs overwhelm the
costs of the underlying fundamental research, and if those risks outweigh
the hazard they are aiming to subdue, there is little point. At some small
diameter, we all agree, mitigation is not worth the trouble. What size is
that? In my talk I hope to address this with some precision, or at least
with some geophysical motivation. Any proposed mitigation scenario will be
enormously expensive to develop; $10G (~15% the cost of Space Station) is
probably a fair estimate of the cost to deflect or disrupt a 300 m diameter
NEO with appropriate lead time. In comparison, ~3% of this amount would
support a Discovery-class telescope interior to Earth (orbiting at Venus L2,
say) capable of telling us with near certainty in two decades that nothing
out there larger than 300 m is going to hit us before the next century. Of
course, we face a ~1/500 chance of learning bad news instead of good from
such a survey - i.e. that we need to prepare for a 300 m impact before 2100
- but then we'll know. From a purely fiscal perspective, it makes 500/3% =
2.10^4 times more sense to pursue advanced reconnaissance of NEAs, than to pursue
any engineered mitigation solution before its time. Reconnaissance is such an enormous
bargain that any money spent elsewhere, if taken from the same pool of
funds, is folly. This argues strongly for putting the NEO search in a
protected budget, so that it does not compete with vastly more expensive,
and in the end probably unnecessary, initiatives related to hazardous NEOs.
Yet we do speculate "what if 2002 NT7 was headed our way in 2019".
Thermonuclear asteroid mitigation - perhaps our best hope in that
one-in-a-million dire circumstance with such little lead time - can easily
be developed alongside existing weapons testing and development programs.
Indeed, research in this area can be continued, and even promoted, in a
manner that affirms Article IV of the Outer Space Treaty (prohibiting
weapons in space) and which affirms the present Comprehensive Test Ban
Treaty. Thermonuclear weapons design is done in the modern era by computer
modeling, coupled with field- and lab-testing of individual deployable
components in a manner that does not yield an explosion. Of particular
relevance is the United States Department of Energy Accelerated Strategic
Computing Initiative which oversees modeling efforts using the world's
fastest supercomputers to perform high-fidelity simulations running advanced
3D thermophysical and nuclear reaction codes. DoE-ASCI is a well-established
and well-funded research program that is already perfectly suited to oversee
model development and testing of any thermonuclear asteroid mitigation
scenario, alongside the DoE's banner goal to "shift promptly from nuclear
test-based methods to compute-based methods" (see
http://www.lanl.gov/asci/asci.html).
One need not be branded a blind optimist to presume that advanced and
benign, perhaps even profitable technologies for NEO mitigation shall be
developed in the coming centuries, so that thermonuclear asteroid mitigation
never happens. In the year 25,000 - the average time between now and the
next 300 m asteroid strike - we will presumably have better tools. But in
the interim we can learn the detailed effects of high energy explosions on
asteroids by combining existing models for asteroid impact disruption with
national security computations related to weapons performance. But a model
is only as good as its boundary conditions, and any mitigation modeling
program would have to be complemented by extensive field reconnaissance of
asteroids and comets. Which brings us back to the scientific requirements
that are the subject of this conference: how do we adequately characterize
an asteroid's geology. Rational NEO mitigation priorities are therefore
approximately as follows: (1) Link NEO impact predictions to existing
warning centers, as this can be done at almost zero cost immediately (e.g.
(http://www.prh.noaa.gov/pr/ptwc/aboutptwc.htm). (2) Complete the NEO
catalog down to about 300 m, for about $300M, within about 30 years. (3)
Determine detailed geological characteristics, for a wide range of comets
and asteroids, down to sizes of a few 100 m. The latter folds in superbly
with the goals of solar system exploration, especially since we now know
that NEOs are objects from the main belt and beyond, delivered to our
doorstep for free. These priorities alone are going to represent an uphill
but worthy battle for tax dollars. Going another step - trying to deploy
intervention mitigation at this time, beyond the conceptual stage - will be
a dramatically unsound investment until these first three steps are complete, and may in
fact hinder their timely completion by competing for funds. Moreover, and
perhaps most seriously, it may elicit a suspicion regarding the honest goals
of planetary science, if comparable plans are not also laid out for
volcanologists to mitigate the impending collapse of Cumbre Vieja.

==============
LANDER AND PENETRATOR SCIENCE AT NEOS

Andrew J Ball, Planetary and Space Sciences Research Institute,
The Open University, Walton Hall, Milton Keynes MK7 6AA, UK
a.j.ball@open.ac.uk

Some of the surface or sub-surface investigations needed to support
Near-Earth Object risk assessment and mitigation demand contact with the
surface. This talk will look at some of the conceivable experiments for
which this is the case and will highlight existing technologies and concepts
applicable to missions to the surfaces of comets and asteroids. Current
capabilities will be described and recommendations made concerning
technology development. Possibilities for surface missions include
destructive impacts, passive projectiles, payload-delivery penetrators, soft
landers, touch-and-go measurements, end-of-mission landings and various
concepts for surface or sub-surface mobility. The low gravity environment
means that a 'surface mission' may in some cases be achievable with a
spacecraft hovering at very low altitude, rather than actually landing.

=============
ADVANCES IN GROUNDBASED CHARACTERIZATION OF THE NEO POPULATION

Richard P. Binzel (MIT)

Over the past decade the growth in groundbased measurements of NEO physical
properties has struggled to keep pace with the increase in their interest and their
discovery rate. Physical parameters (such as their spectroscopic, shape, and
rotation properties) were known for only a few dozen NEOs in 1990. By 1998
measurements were in hand for about 100 objects. Today the current sample is
nearly 300 objects. These studies are revealing the population to be diverse
and in some cases seemingly bizarre, as material strength and gravity
compete to form and hold NEOs in stable shape and rotational configurations.
Beyond the opportunity to study the structural nature of the smallest
observable solar system bodies, the scientific rationale for studying
near-Earth objects also focuses on understanding the relationships between
asteroids, comets, and meteorites. Through the analysis of a large sample
groundbased spectroscopic and albedo measurements, we are beginning to
achieve good constraints on the actual compositional and size distribution
of the NEO population. These are giving insights to the main-belt and
extinct comet source regions for NEOs. We are also making substantial
progress in directly relating NEOs in space to their hand samples studied as
meteorites in the laboratory. It is the combined knowledge of size, shape,
internal structure, and composition that are most critical to addressing how
to effectively mitigate the possible impact threat posed by any particular
object.
As our basic understanding of the NEO population and its origins has
advanced, so to has the level of scientific questions we can ask. Is there
evidence for groupings (or "families") of NEOs that pinpoint common
collisional or dynamical origins? Are there "streams" of NEOs that may favor
delivery of particular types of meteorites relative to others? Is the subset
of "potentially hazardous objects" (PHAs) representative of the total NEO
population? Which NEOs are the "best" for spacecraft exploration in terms of
both accessibility and intrinsic scientific interest (taking into account
such factors as unusual structure or composition)? While the first level of
questions about the nature of NEOs can be (and is being) addressed by
"random" statistical surveys of the population, the more advanced questions
require directed studies of particular NEOs. Directed studies are inherently more
difficult because almost any given NEO makes infrequent passages near the Earth that
provide favorable opportunities for observation. In most cases objects are discovered
BECAUSE they are making a particularly favorable apparition and the best opportunity
for performing physical studies is immediate to the time of discovery. The
groundbased telescope time and aperture requirements for such directed
studies of specific NEOs is quite different from the statistical studies
that have been carried out to date. Nearly dedicated access to a modest
(4-m) aperture telescope is required for thorough characterization of
discoveries and select opportunities with large (6-10m) telescopes are
required for characterizing specific objects of high interest.

==============
UNDERSTANDING THE DISTRIBUTION OF NEAR-EARTH OBJECTS

William Bottke (SwRI), Alessandro Morbidelli (Obs. Nice) and Robert Jedicke
(U. Arizona)

The orbital and absolute magnitude distribution of the Near-Earth Objects
(NEOs) is difficult to compute, partly because known NEOs are biased by
complicated observational selection effects but also because only a modest
fraction of the entire NEO population has been discovered so far. To
circumvent these problems, we have used numerical integration results and
observational biases calculations to create a model of the NEO population
that could be fit to known NEOs discovered or accidentally rediscovered by
Spacewatch. This method not only yields the debiased orbital and absolute
magnitude distributions for the NEO population with semimajor axis a < 7.4
AU but also the relative importance of each NEO replenishment source. We
list a few of our key findings here, with a full accounting given in Bottke
et al. (2002a, Icarus 156, 399). Our best-fit model is consistent with 960
+/- 120 NEOs having absolute magnitude H < 18 and a <7.4 AU, with
approximately 55% found so far. Our computed NEO orbital distribution, which
is valid for bodies as faint as H < 22, indicates that the Amor, Apollo, and
Aten populations contain 32%, 62%, and 6% of the NEO population,
respectively. We estimate that the population of objects completely inside
Earth's orbit (IEOs) arising from our NEO source regions is 2% the size of
the NEO population. Overall, our model predicts that 37 +/- 8%, 25 +/- 3%,
23 +/-9%, 8 +/- 1%, and 6 +/- 4% comes from the nu_6 resonance, the
intermediate-source Mars Crossing (IMC) region (i.e., a population of
Mars-crossing asteroids with perihelion q > 1.3 AU located adjacent to the
main belt), the 3:1 resonance, the outer main belt, and the Jupiter-family
comet region, respectively. The influx rates needed to replenish the NEO
population and the identification of extinct comets in the Jupiter-family
comet region will also be discussed. Applying the results of this model, our
team has also developed a method for determining the debiased albedo/orbital
distribution of the NEOs (Morbidelli et al., 2002, Icarus, in press). Our
work shows that an observationally complete NEO population with diameter D >
0.5 km should contain 53% bright objects (e.g., S-type asteroids like 433
Eros) and 47% dark objects (e.g., C-type asteroids like 253 Mathilde). By
combining our orbital distribution model with our albedo distribution model,
and assuming that the density of bright and dark NEOs is 2.7 and 1.3 g
cm^-3, respectively, we estimate that the Earth should undergo a 1000
megaton (MT) collision every 64,000 years. On average, the bodies capable of
producing 1000 MT blasts are those with H < 20.5; only 18% of them have been
found so far. We have also combined our debiased NEO population results with
a survey simulator in order to investigate the time needed by existing NEO
surveys to find 90% of the NEOs larger than 1 km diameter. In our most
realistic survey simulations, we have modeled the performance of the LINEAR
survey over the 1999 -2000 (inclusive) period (Jedicke et al. 2002, Icarus,
in press.). Tests indicate that our survey simulator does a reasonable job
at reproducing LINEAR's NEO detections over this time frame. For this
reason, we have some confidence that extending our simulator results into
the future will also produce realistic results. Our results indicate that
existing surveys (as of January 2001) will take another 33 +/- 5 years to
reach 90% completeness for D > 1 km asteroids. Our predicted timescale to
reach the Spaceguard goal is longer than other recent estimates because our
undiscovered NEOs have a very different orbital distribution than our
discovered NEOs. Conversely, advances in survey technology over the last
6-12 months have allowed LINEAR to improve their limiting magnitude (J.
Evans, personal communication), such that they can now find fainter objects
than they could as of January 2001. We are still investigating
the implications of their changes (and improvements made to other NEO
surveys), but our test results suggest that the Spaceguard goal could be
achieved as soon as 2014, better than the 2035 estimate given above. We
believe this issue will need to be continually revisited over the next
several years as surveys get better at finding NEOs. We have not yet
attempted a cost-benefit analysis, but our results suggest that a local-area
network of telescopes capable of covering much of the sky in a month to
limiting magnitude V ~ 21.5 may be administratively, financially, and
scientifically the best compromise for reaching 90% completion of NEOs
larger than 1 km diameter by 2008. We find that distributing survey
telescopes in longitude/latitude may produce a 25% savings in the time
needed to reach the Spaceguard goal. This value can be used to assess the
relative merits of a southern hemisphere NEO survey against factors like
cost, time needed to reach operational status, etc. Our results also
indicate that a space-based satellite survey on an orbit inside Earth orbit
(e.g., perihelion near Mercury) would offer significant advantages over
terrestrial surveys, such that a Discovery-class mission to discover NEOs
might be warranted. For more information on these topics, please go to:
http://www.obs-nice.fr/morby/ESA/esa.html

================
WHAT WE KNOW AND DON'T KNOW ABOUT ASTEROID SURFACES

Clark R. Chapman, Southwest Research Inst., Boulder CO

One of the most fundamental aspects of mitigating an impact threat by moving
an asteroid involves physical interaction with the asteroid. Whether one is bathing
the asteroid surface with neutrons, bolting an ion thruster or mass driver onto the
surface, or trying to penetrate the surface in order to implant a device
below the surface, we need to understand the physical attributes of the
surface. Of course, we must understand the surface of the particular body
that, most unluckily, is eventually found to be headed for Earth. But, in
the meantime, it will advance our ability to design experiments and
understand data concerning the particular body if we have thought, in
advance, about the range of surface properties we might encounter. We
already know, from meteorite falls, that asteroidal materials can range from
strong nickel-iron alloy (of which most smaller crater-forming meteorites,
like Canyon Diablo, are made) to mud-like materials
(like the remnants of the Tagish Lake fireball event). But the diversity
could be even greater, especially on the softer/weaker end of the spectrum,
because the Earth's atmosphere filters out such materials. That is why many
meteoriticists doubt that we have any macroscopic meteorites from a comet.
We could readily expect some icy, snowy, frothy, and dusty materials on the
surfaces of asteroids and comets, and perhaps still stranger materials (e.g.
with the structure of styrofoam). A common framework for thinking about
asteroid surfaces is to extrapolate from our very extensive knowledge of the
lunar regolith. Indeed, there is a considerable literature concerning
asteroid regoliths (mostly published in the 1970s and 1980s) based on
theoretical extrapolation from lunar regolith models and on inferences from
what are termed "regolith breccia" meteorites. These studies suggested that
we should expect both similarities and differences from our lunar
experience, for asteroids several km in diameter and larger. Less thought
was given to smaller asteroids, except that at small sizes there must
eventually be a transition to a "bare rock in space." The Earth-approaching
asteroid Eros is large enough that it was expected to have a roughly
lunar-like regolith, although perhaps somewhat coarser and less well mixed.
A major surprise from the NEAR Shoemaker mission to Eros is that its surface
is totally unlike the Moon's, particularly at spatial scales of centimeters
to tens of meters - just the scales relevant for human interaction with an
asteroid. The Moon is covered with a well-churned regolith (basically a
sandy soil, with occasional larger rocks and boulders, especially near
recent craters large enough to have penetrated the several-meter-deep
regolith down to bedrock), and its surface is characterized by innumerable small craters.
Eros, on the other hand and despite its lunar-like appearance at spatial
scales larger than ~100 meters, has been found to have relatively few
craters tens of meters in size, and almost no craters cm to meters in size.
Instead, the surface of Eros is dominated by countless rocks and boulders,
except in localized flat areas (nearly devoid of both craters and rocks)
that have been called "ponds".
The lesson is that extrapolations from meteoritical and lunar studies proved
wrong. Evidently, our generalized understanding of the processes that shape
asteroid surfaces is wrong in one or more fundamental ways. The way that we
can really tell what an asteroid surface is like is to measure it directly
rather than to theorize about it. It is tempting to draw inferences from the
NEAR Shoemaker data about what the surfaces of asteroids, or at least of
S-type asteroids, are like. Indeed, it is the best evidence that we have.
But, as indicated above, the NEAR Shoemaker surprises are not yet
understood, although some hypotheses have been offered. And there is much
that we don't know. The ponds are thought by many to be deposits of fine
particulates (e.g. electrostatically levitated dust), but our best
resolution is only a couple of cm and we do not even know for sure that
these surfaces aren't solid and hard. Although the NEAR Shoemaker spacecraft
landed on Eros' surface, we never got to see the gouges it may have made.
Even once the NEAR Shoemaker data are thoroughly analyzed, it is not clear
how relevant the interpretations will be for asteroid mitigation, which will certainly
involve much smaller bodies. In terms of self-gravity, the
multi-hundred-meter body we might want to deflect from Earth impact is as
different from Eros as Eros is from the Moon. These bodies will likely have
essentially no modern regolith on them at all. But there may be legacy
regoliths, evolved on the larger bodies from which the small bodies were
formed...or almost any kind of unexpected structure.

===========
IMPACT PROBABILITIES AND LEAD TIMES

Steve Chesley (JPL/Caltech) and Tim Spahr (CfA, Harvard Univ.)

The most important requirement, scientific or otherwise, for any impact
mitigation is the recognition of the hazard, since, in the absence of a
perceived impact risk, there is neither the incentive nor the capability to
address the threat. Therefore, the success of any potential mitigation
effort will rely heavily upon our ability to discover, track and analyze
threatening objects. In this presentation we will consider the effectiveness
of the present surveying and monitoring capabilities by bombarding the Earth
with a large set of simulated asteroids that is statistically similar to the
impacting population. Our aim is to see how many of these impactors might be
recognized as threatening, and what is the reliability and expected lead
time for such recognition. To begin, we form a large set of "typical"
impactors. For this purpose we use the debiased NEA population model
developed by Bottke et al. (2000, Science 288, 2190). Starting with a very
large population of NEAs we derive a set of 1000 impactors by first reducing
the population to those for which the minimum orbital separation, or MOID,
is low enough to permit an impact. Impactors are sampled from this low MOID
set according to the fraction of their orbital period that they spend within
the Earth-capture cross-section of the Earth's orbit, a value that can range
from as much as a few percent for Earth-like orbits down to 10^-9 for
low-MOID cometary orbits. This sampling approach allows for the more
hazardous orbital classes, such as low inclination, Earth-like or tangential
orbits, to have appropriately increased prominence among the simulated
impactors. The orbital characteristics of the impacting population are
important from a mitigation perspective in terms of both discovery and
deflection efforts and these issues will be addressed. Given a set of
impactors one can ask whether and when they would be discovered by various
NEO surveys with differing sky coverages and brightness limits. To approach
these questions we run survey simulations, recording detections for various
object sizes. This allows us to infer the distribution of warning times as a
function of size. If there is a warning before an impact, the warning time
will generally be measured either in years or else in weeks. In the former
case mitigation by disruption or deflection of the object may be feasible,
while in the latter case mitigation will be limited to evacuation of the
impact region, etc. Detectability at the final apparition is often very
challenging since the objects will tend to have rather slow sky-plane motion
and will generally be located far from the heavily-searched opposition
region. This means that in many cases, especially for the smaller objects,
if a last-minute detection does occur it is not likely to be until the
object is close enough for the parallactic motion to be detectable,
generally a few weeks before impact. The detection lead time is important in
determining the time available for mitigation, but it is not the only
factor. There is some delay between the discovery of the asteroid and the
recognition that it poses a threat worthy of mitigation. The idea of
continually monitoring the ever-evolving asteroid orbit catalog for
possibilities of impact is fairly new, and the first automatic collision
monitoring system was fielded less than three years ago. Today there are two
independent and parallel systems, at JPL and the Univ. of Pisa, that are
operating continuously to scan for potential impacts. These efforts have
been very successful at detecting potentially hazardous future encounters
for newly discovered asteroids and reporting the results to the NEO
community. Follow-up observers have responded enthusiastically with
observations that permit the hazard assessment to be refined and usually
eliminated. We will consider a few impact case studies to understand how
rapidly after discovery the probability of an impending impact can be
expected to increase as time passes, and in particular to understand how
this affects the lead time for mitigation.

============
THERMOPHYSICAL PROPERTIES OF COMETS AND ASTEROIDS INFERRED FROM FIREBALL
OBSERVATIONS

Mario Di Martino, INAF - Osservatorio Astronomico di Torino

Fireballs are very important events to derive basic physical information on
near-Earth objects in a size range for which detection using conventional
astronomical techniques is particularly difficult. The observable features
of these events give relevant information about the physical properties of
their parent bodies, and their likely origin. This may be important, for
instance, to better evaluate the relative abundance of bodies having a
likely cometary origin. At the same time, a better estimate of the frequency
of fireball events can put essential constraints on the general trend of the
NEO size distribution, by providing data referring to an interval of the
mass spectrum that is very poorly known at present. The major problem in
fireball observations, however, is that currently only a minor fraction of
the events are actually detected and recorded, and detections occur mostly
in the form of serendipitous discoveries made by satellites devotedto other
purposes. The situation can drastically improve if dedicated observing
facilities will be developed. Due to the large areas of sky to be monitored
for efficient fireball detection, the development of dedicated space-based
facilities is strongly needed.

============
MISSION CONCEPTS FOR NEO CHARACTERIZATION

Richard Dissly and Rich Reinert, Ball Aerospace & Technologies Corp.

The scientific characterization of potentially hazardous Near Earth Objects
will require a series of spacecraft missions to fully address the
measurements required for the optimized implementation of any mitigation
strategy. In addition, current surveys of the NEO population can benefit
tremendously from space-based observational missions. This talk will cover
both reconnaissance and survey mission concepts. Mission concepts will be
discussed in reference to the scientific questions they are designed to
address. Measurement implementation strategies drive mission design some
examples:
* What measurements can be made remotely?
* What measurements require a part or all of the spacecraft to contact the body?
* Is the contact on the surface or sub-surface?
* Is the contact long-term or an ephemeral event?

Future mission architectures so categorized will be compared to previous and
current mission designs. The discussion will assess the technical maturity
of future concepts and make preliminary estimates of the associated costs.
This talk will also address technology developments that can facilitate
suggested measurements.

==============
SCIENTIFIC REQUIREMENTS FOR ENABLING FUTURE TECHNOLOGIES

Alan W. Harris, JPL

I am, at present, an observational astronomer, specializing in physical
observations of asteroids, especially of the Near-Earth variety. It
therefore seems likely that I should advocate intensive physical
observations of NEAs in order to characterize the one that may get you.
Instead, however, I will argue that we already know the range of physical
properties of NEAs well enough that the problem with respect to mitigation
is not a lack of knowledge of the range possible NEA properties. Instead it
is our lack of knowledge of the specific properties of the one with our name
on it: its size, mass, density, composition, material strength, whether it
is one body or two, and so forth. We know that NEAs range from meteoroids to
dinosaur-killers ten or twenty kilometers across, from near dust balls to
solid iron, from spheres to long skinny pencils-in-the-sky and even binary
objects. The only way we can know the specific properties of
the one with our name on it is to find it. Additional physical studies will
not do much to narrow down the range of possibilities, so if one insists on
being prepared, one must simply deal with the entire range of possibilities
of sizes, orbits, and physical states of the entire population, which we
actually know quite well enough. Thus surveys must remain the most important
astronomical endeavor relating to the impact hazard. That being said, I will
advocate continued, and hopefully increased, physical studies for two
reasons. First, the survey discoveries currently being made represent a
superb opportunity for scientific investigations apart from the hazard
issue. It borders on criminal neglect to not take advantage of these
opportunities for physical studies for their scientific return alone.
Consider that NASA has spent, and continues to spend, hundreds of millions of dollars
on missions to obtain high-resolution images of small bodies. Ground-based radars are
capable of yielding comparable quality results (perhaps somewhat inferior in resolution
but superior to flybys in time resolution), e.g. from the recent (future as
I write this) close passage of the newly discovered 2002 NY40 in mid-August.
Most of what we know about NEA binaries has been gleaned by rapid-response
observations of recent discoveries. This is 100% true of the many tiny
super-fast rotators found. We would not even know this population exists if
it weren't for rapid follow-up observations (and some raving speculations of
theorists). The second justification of physical observations does have
indirect application to the hazard issue. In order to characterize the NEA
population as it is discovered one must obtain at least a statistical sample
of properties. The most fundamental physical parameter is size,
characterized most simply by absolute magnitude H. Even this is not well
determined by the surveys and should be refined by well-calibrated
photometric observations. Since absolute magnitude is the fundamental metric
for tracking progress of the survey, this much should be done for every
discovered object. In addition, at least a statistically significant
sampling of other properties, spectra and radiometric albedo, should be
undertaken so as to "calibrate" the transformation from sky brightness to a reasonable
estimate of physical size of objects. Returning to the matter of enabling mitigation
technology, I will not speculate on how to kill an asteroid, other than to
posit that it will require rendezvous. In the distant past, I naively
speculated that one might deflect an asteroid by a standoff nuclear blast,
causing spallation of a surface layer leading to recoil of the main body. It
now appears very likely that most NEAs larger than a couple hundred meters
in diameter are not monolithic and indeed are almost certainly disjoint
"rubble piles" of some sort or another. Spallation is not likely to work.
Any other method of deflection I can imagine will require rendezvous. Thus
we should consider the requirements of rendezvous missions, not simply flyby
ones. In another life, I was (maybe still am) a celestial mechanic, so I
will next speculate a bit on "getting there."
It has often been said that NEAs are the easiest targets in the solar system
for space missions. This is especially true for flyby missions to an
asteroid on a collision trajectory with the Earth. Given a few orbits in
advance, all you have to do is barely escape the Earth and park in an orbit
with a slightly different period to move ahead or behind the Earth's
position in orbit as needed to effect the close flyby (or impact). This has
led some folks to speculate that a cheap mitigation system could be put
together out of a few spare ICBMs and standard nukes already on hand. I
maintain this is so unlikely to be effective that it should not be
contemplated or advocated. For rendezvous missions, NEAs are only easy
targets if you get to choose the target; e.g. 4660 Nereus is unquestionably
an easy rendezvous target. Unfortunately, if nature chooses the target for
you (the one with your name on it), it is not likely to be easy.
Simplistically, the velocity needed to match orbits with such an object is
approximately equal to the impact velocity it will have when it hits
(hopefully achieved at least a few orbits sooner). The mean (RMS) impact
velocity of NEAs for actual discovered orbits is around 20 km/sec. I once
heard no less an authority than Werner Von Braun himself declare that a
Saturn-V could send a Volkswagen to Pluto, which is a similar delta-v task
to a rendezvous with an "average" NEA. It is not a task suitable for a
surplus ICBM to achieve such a launch velocity (15 km/sec plus Earth
escape). Indeed, with chemical propulsion current launch vehicles couldn't
get much more than a shoebox to such a velocity. There are tricks to reduce
the launch energy requirements, such as gravity assist trajectories, but
these are time consuming and have limitations. Thus it seems to me that
the most important "enabling technology" for impact mitigation is the
development of advanced high-energy propulsion systems. We must first enable
simply getting there before worrying over much about what to do when we
arrive. I must conclude, however, that even "getting there" is not cheap and
simple, and combined with the extraordinarily low probability of needing to
"get anywhere," it seems to me unjustified to do more than paper studies in
advance of the actual discovery of a threatening NEA. High-energy propulsion
systems are probably worth developing for other reasons (like going to Pluto
without rebuilding a Saturn-V), but the impact hazard by itself hardly
justifies doing so. Ceterum censeo machinas ad sidera errantia deflectenda
struendas non esse.

=============
SCIENTIFIC REQUIREMENTS FUR UNDERSTANDING THE NEAR-EARTH ASTEROID POPULATION

Alan W. Harris, DLR Institute of Space Sensor Technology and Planetary
Exploration, Berlin

A vital prerequisite for the development of an effective mitigation strategy
for hazardous near-Earth asteroids (NEAs) is a thorough understanding of
their physical nature and mineralogical composition. The deflection of an
object on collision course with the Earth would require the use of
considerable force, the successful application of which would depend on
prior knowledge of parameters such as mass, shape, strength, and structure.
Recent experience has shown that much can be learned about individual
objects from fly-by and rendezvous missions and such missions would play the
dominant role in gathering mitigation-relevant information once a dangerous
potential impactor had been identified, provided sufficient time were
available before the impact. In the meantime, it is important to study the
NEA population in general to enable the most likely physical characteristics
of a potential future impactor to be anticipated as accurately as possible.
Groundbased, airborne, and satellite observatories offer a wide range of
techniques with which large numbers of near-Earth asteroids can be remotely
sensed, including lightcurve measurements, visible to thermal-infrared
photometry, visible to near-infrared reflectance spectroscopy, and radar.
The merits of techniques most useful from the point of view of NEA hazard
assessment and mitigation, and the type of information each can provide, are
discussed. The interdependency of the interpretation of data from the
various observing
techniques is emphasized.

==============
GEOLOGY OF ASTEROIDS: IMPLICATION OF SPIN STATES REGARDING INTERNAL
STRUCTURE AND SOME IMPLICATIONS OF THAT STRUCTURE ON MITIGATION METHODS

K. A. Holsapple, University of Washington

The design of asteroid and comet collision mitigation strategies depends
crucially on knowledge of the body's internal structure and mechanical
properties; but those are poorly known. While we have clues, definitive
information eludes us. Planning for mitigation requires focused efforts; not
only for discovery, but also methods for the determination of internal
structure and properties, and the study of the science of proposed
deflection or disruption methods. A natural approach to looking for such
clues about the makeup of asteroids is to study the implications of
their observed size, shape and spin. Those properties are available from the
analysis of the lightcurves of well over 1000 asteroids, including about 100
NEA's. An asteroid's size, shape and spin produces internal stresses from
the gravitation and rotational forces. In turn, the asteroid must be
sufficiently strong to resist those stresses. Thus, a minimum strength can
be deduced by an analysis of those internal stresses. Knowledge of that
required strength gives constraints on the internal structure. I have obtained closed-form
algebraic expressions that give equilibrium stress states as a function of size, ellipsoidal shape
and spin (Holsapple, 2001). Further, those equilibrium states must also
satisfy constraints of stability, which further narrows the possibilities
(Holsapple, 2002). The stable states of equilibrium are then compared to
strength models to determine the required strength. Geological materials are
mostly modeled as granular materials with a Mohr-Coloumb strength, in which
the allowable shear strength is related to the confining pressure, that
relation depending on the cohesion (strength at zero confining pressure) and
the so-called angle of friction. The results are surprising: almost all
known asteroids are within the limits allowed by a cohesionless material
with some reasonable angle of friction. Further, most are well within the
limits for relatively low angles of friction, on the order of 20?. As a
result, while we cannot rule out the possibility of additional strength, all
of those asteroids need only have the strength of a porous rubble-pile
granular structure.
If indeed many bodies do have such a rubble-pile structure, we must study
the implications of such a porous structure on proposed mitigation schemes.
Even if the gross properties of an asteroid are not those of a rubble-pile,
the presence of a porous regolith may also have a dramatic effect on
mitigation. I also present some calculations emphasizing the importance of
porosity on proposed (Ahrens and Harris, 1994; Melosh et al., 1994)
mitigation methods. Methods utilizing surface and buried nuclear or chemical
explosions may be reduced in effectiveness by a factor of five or so by
porosity. Methods using the kinetic energy of an impactor may also reduced
in effectiveness by a factor of five. Even more dramatically, methods using
energy deposition and blow-off may be reduced by a factor of 103 in
effectiveness. For example, the use of a standoff nuclear weapon in the
megaton range would not have any appreciable effect on diverting a 10km
porous-surface asteroid or comet.

References:
Holsapple, K.A., Equilibrium Configurations of Solid Ellipsoidal
Cohesionless Bodies, Icarus, Volume 154, Issue 2, pp. 432-448 (2001).
Holsapple, K.A., Rubble pile asteroids: Stability of equilibrium shapes,
Proc. Lunar Planet. Sci. Conf. XXXII, (2002).
Ahrens, T. J. and Harris, A. W., "Deflection and fragmentation of near-earth
asteroids", in Hazards Due to Comets and Asteroids, ed. by T. Gehrels
(1994).
Melosh, H. J., Nemchinov, I. V. and Zetzer, Y. I., "Non-nuclear strategies
for deflecting comets and asteroids", in Hazards Due to Comets and
Asteroids, ed. By T. Gehrels (1994).

=============
THE SCIENTIFIC REQUIREMENTS OF FUTURE MITIGATION TECHNOLOGY

R. Kahle 1,2 and Ch. Gritzner 1

1 Dresden University of Technology, Institute for Aerospace Engineering,
Mommsenstrasse 13, 01069 Dresden, Germany
2 DLR, Institute of Space Sensor Technology and Planetary Exploration,
Rutherfordstrasse 2, 12489 Berlin, Germany

Introduction: Currently, various ideas for the diversion and/or disruption
of potentially hazardous objects (PHO) exist. Among them are systems that
are technologically feasible at present, e.g. kinetic energy impactors and
nuclear explosives. Others are currently not available within the designated
size but might be possible with some effort, e.g. propulsion systems
(chemical, nuclear), and solar concentrators. Some systems seem to be too
far off to be realized within the next decades such as mass drivers, solar
sails, and surface layers (Yarkovsky effect). Besides, there are also
futuristic technologies such as laser systems, eater, "cookie cutter", and
the use of antimatter. The use of mass drivers as well as solar sails would
probably demand for large and heavy mechanical structures and might thus
never become realistic mitigation options. The same can be expected from the
utilization of the Yarkovsky effect. Further, the
application of kinetic energy impactors or nuclear explosives might even
worsen the situation in case of an unintended disruption of the NEO, which
could cause multiple impacts on Earth (firestorms). Here, two mitigation
concepts will be discussed that could become attractive alternatives in the
mitigation of hazardous objects: the solar concentrator system and the
magnetospheric propulsion.
Solar Concentrator: The application of solar concentrators for NEO
mitigation was discussed first by Melosh et al. [1]. The basic idea of this
technology is to concentrate solar radiation onto the NEO surface with a
lightweight (parabolic) reflector. Depending on duration and intensity of
illumination, the material within the spot will be heated up and vaporizes.
The evaporated material accelerates to a speed of about 1 km/s and delivers
an impulse to the NEO. Although the generated thrust is small (order of
magnitude: 10^1 to 10^2 N) it will suffice to deflect the NEO from its
collision course with Earth if sufficient lead-time is given (years). Such a
system could be operated for the duration of several months, which would
lead to a slight increase in semi-major axis of the hazardous NEO. For
technology demonstration a small satellite could be built within short time.
When equipped with instruments, e.g. mass spectrometer, material
properties of the target NEO could be studied at same time.
Magnetospheric Propulsion: The idea of magnetospheric propulsion is related
to the solar sail concept concerning that both tap the ambient solar energy
to provide thrust to a spacecraft. But, solar sails suffer from their
mechanical structure - if large spacecraft or even small asteroids have to
be propelled, physical limits will be reached, e.g. the system mass and
problems accompanied by deploying that large structures. Thus, Winglee et
al. [2] invented a revolutionary propulsion concept for interplanetary space
missions (named Mini-Magnetospheric Plasma Propulsion - M2P2). This system
creates a magnetic bubble that will intercept the solar wind. At a distance
of 1 AU the solar wind particle density is about 6 cm^-3 moving at a speed
of about 300 to 800 km s^-1. This results in a constant dynamic pressure of
2 nPa. If the magnetic field cross section is large enough a continuous
force (order of magnitude: 10^1 N) could be provided. We propose to use such
a system for NEO diversion. Although the generated thrust is low, this
system could be operated for a long duration (several months) to divert a
PHO.
Summary: Both technologies, solar concentrator and magnetospheric
propulsion, could be developed
within short time. For demonstration the systems could be scaled to small
satellites (about 200 kg). An overview of both systems, relevant physical
parameters for the interaction, a brief conceptual analysis, and examples
for orbit diversion will be presented.

References:
[1] Melosh, H.J. et al., Non-nuclear strategies for deflecting comets and
asteroids, in: T. Gehrels (ed.), Hazards due to comets and asteroids, pp.
1111-1132, 1994.
[2] Winglee, R.M. et al., Mini-Magnetospheric Plasma Propulsion: Tapping the
energy of the solar wind for spacecraft propulsion, Journal of Geophysical
Research, Vol. 105, No. A9, pp. 21,067-21,077, 2000.

=============
PEERING INSIDE NEOS WITH RADIOWAVE TOMOGRAPHY

Wlodek Kofman

The radar technique has been successfully applied for many years for Earth
and planetary observations, while the tomography has been used for many
years in medical studies and in non-destructive analysis of materials. The
Synthetic Aperture Radar is a good example of the use of the radiowave
imaging inverse scattering technique in radar application. The idea to use
radiowave sounding to study the interior of comets is applied in the CONSERT
experiment for the ROSETTA mission. In this presentation, we start with a
discussion on the ability of the radar tomography to observe the interior of
asteroids. Then, after a description of the relevant radar parameters
necessary to define in a very general way the radar designed for this
purpose, we discuss the principle of Radar Transmission and Radar reflection
tomography and compare these two methods, discussing their differences. With
the example of the CONSERT experiment which was developed for a cometary
mission, and which will be launched in January 2003 on the ROSETTA mission,
we show how the transmission tomography is used. The CONSERT system is
briefly described; simulation results of the inversion methods, and how to
infer the interior of the comets from measurements, are shown. The
propagation of the waves in the material medium is addressed with special
attention concerning the attenuation coefficient in the cometary and
asteroid materials covering their likely composition. This parameter is
essential for the determination of the frequency and bandwidth of the radar.
It is thus clear that for asteroid interior peering, we should use low
frequency radars, surely below 50 MHz, and even this
will not guarantee a total penetration. The monostatic reflection radar
tomography is probably the only solution. The accuracy of the satellite
positioning relatively to the surface of the object, which has to be very
high, of the order of a fraction of the wavelength, is an additional
argument for the use of low frequency radars. We discuss the expected radar
performances and show that for small kilometric bodies, the radar reflection
tomography is a good approach to study the interior of asteroids. Finally,
radar specifications are proposed.

===============
GEOPHYSICAL CONSTRAINTS ON NEO MITIGATION STRATEGIES

H. J. Melosh, Lunar and Planetary Lab, University of Arizona, Tucson, AZ
85721

The success of any proposed mitigation strategies depends on two major
factors: How massive is the NEO and how much lead time do we have? A
secondary issue is what is the NEO made of and how are its various parts
arranged. The essential object of deflecting an asteroid or comet away from
an impending impact with the Earth is to change its velocity. Given the
astronomical distances likely to separate the NEO from the Earth at the time
the threat is discovered, only a small velocity change (perhaps a few
cm/sec) is necessary, but even such small changes are difficult to achieve
for objects 1 km or more in diameter, which may have masses in the range of
10^12 kg. The direction in which the velocity impulse is applied is
important for orbiting objects. An impulse in the direction of the orbital
motion is much more effective in changing the position of
an object than an impulse in any perpendicular direction. Deflection
scenarios range from a single impulse delivered long before the impact, such
as the jolt delivered by a nuclear explosion or impact of another asteroid,
to long-duration low accelerations delivered by solar evaporation or a mass
driver. In any case, the deflection process is likely to be limited by the
energy available, not the reaction mass available, so the optimum use of the
available energy is to move as much mass as possible, not to eject it at
high speeds. The success of a given deflection strategy may depend strongly
on the physical and chemical nature of the NEO. The methods envisioned for
deflecting a solid silicate rock may differ considerably from those
effective against a porous aggregate. Recent spacecraft studies indicate
that the density of asteroids runs the gamut from nearly solid silicates
(Eros) to highly porous aggregates (Mathilde). Theoretical cratering studies
and limits on rotational period suggest that most asteroids larger than a
few km in diameter are thoroughly fractured by smaller impacts. The
effective strength of any given NEO may thus vary over a wide range,
especially if it is composed of mechanically independent blocks, or even
possesses a satellite, as do about 15% of NEOs. Even stony asteroids may
contain volatiles that could affect the success of some deflection
scenarios. These considerations make it important to implement a program in
which determination of the physical and chemical properties of NEOs are a
major component of any large-scale deflection strategy.

================
SCIENCE AND PUBLIC PERCEPTION (PANEL)

David Morrison
NASA Astrobiology Institute

Impacts are different from other more familiar hazards. The impact risk is
primarily associated with extremely rare events - literally unprecedented in
human history. They are the extreme example of a hazard of low probability
but immense consequences. Although there is a chance of order one in a
million that each individual will die in any one year from an impact, it is
not the case that one out of each million people dies each year from an
impact. Further, impacts threaten not just individuals but civilization
itself. For many people, impacts are therefore a greater concern than is
implied by simple numerical risk estimates, since a large impact could
destroy much that is uniquely human. Others, of course, prefer (perhaps
unconsciously) to play the odds, based on the very low probability that any
major impact will occur within our lifetimes. Scientists (aided by
Hollywood) have succeeded in alerting the world to the existence of an
impact hazard, and astronomers have successfully undertaken the Spaceguard
Survey, focused (so far) on the threat of global disaster from collision
with a NEA of diameter greater than 1 km. We have not yet established any
goals beyond 2008. Should we continue the present survey to push the
completeness limits for large asteroids to 95% or 99%? Should we raise the
bar and build the larger telescopes that will be required to achieve
completeness at smaller sizes, say 300 m? Or should we begin to develop
technology to change the orbits of asteroids? To answer such questions, the
NEO science community needs to engage in active dialog with other
professionals with greater experience in disaster mitigation and national
security. We need to consider the societal context of NEO searches and of
approaches to mitigation. These social and political considerations will
play an important role in determining what priority will be placed on
protecting our planet from cosmic impacts. We also have a responsibility to
the public. Every few months this issue is thrust into the public spotlight,
usually by a report that a newly-discovered asteroid poses (temporarily)
some low-probability hazard of colliding with the Earth. There is a
temptation to play up such stories, even though most scientists realize that
the issue will likely evaporate when a few more observations are made. Some
members of our community like to appear on TV, and others feel this is a
good way to garner public support for our work.
We need to ask ourselves if it is really to our advantage to use these
opportunities to gain media and public attention, especially when we know
the risk is actually extremely small. There is a serious potential down-side
if we cry "wolf" too often. Our credibility is at stake, and hence our
ability to inform the public and perhaps to influence the decision makers.
We also need to be concerned about confusions between large impactors and
small ones. Understanding kiloton-energy bollides that explode in the
atmosphere is important, but this is entirely different from the search for
dangerous asteroids. Similarly, there is an orders-of-magnitude difference
in the hazard from large asteroids (larger than a couple of kilometers) and
that from smaller, Tunguska-class impacts that have no global consequences.
When we blur these distinctions, we confuse the public and sometimes even
ourselves. An example is the recent interest in establishing a government
coordinating and warning center. The implication of this suggestion is that
we will have many warnings to issue. I don't think so. The frequency of even
the smallest impacts that do surface damage is no more than one per century.
Even with a perfect survey, the warning center might therefore issue fewer
than one warning per human lifetime. Does this make sense? These are all
issues of public communication, but they ultimately depend on our own
ethical commitment to deal with the impact hazard in a responsible, honest
way.

================
RADAR RECONNAISSANCE OF POTENTIALLY HAZARDOUS ASTEROIDS AND COMETS

Steven J. Ostro, JPL/Caltech 300-233 Jet Propulsion Laboratory, Pasadena, CA
91109-8099 ostro@reason.jpl.nasa.gov

Groundbased radar is an intelligence-gathering tool that is uniquely able to
reduce uncertainty in NEO trajectories and physical properties. A single
radar detection secures the orbit well enough to prevent loss of newly
discovered asteroids, shrinking the instantaneous positional uncertainty at
the object's next close approach by orders of magnitude with respect to an
optical-only orbit. This conclusion, reached initially by Yeomans et al.
(1987) through Monte Carlo simulations, has been substantiated
quantitatively by comparison of residuals for radar+optical and optical-only
positional predictions for recoveries of NEAs during the past decade (Ostro
et al. 2002). Integration of an asteroid's orbit is afflicted by
uncertainties that generally increase with the length of time from epochs
spanned by astrometry. Eventually the uncertainties get so large that the
integration becomes meaningless. The duration of accurate orbit integration
defines our window of knowledge about the object's whereabouts. Presumably
we want to find out if any given NEO might threaten collision, and if so, we
would like as much warning as possible. Radar extends NEO trajectory
predictability intervals far beyond what is possible with optical data
alone, often approaching the end of this millennium (e.g., 1999
JM8; Benner et al. 2002). For 2002 FC, an eight-week arc of
discovery-apparition optical astrometry could not reliably identify any
close Earth approaches before or after 2002, but with Arecibo astrometry
from May 24 and Goldstone astrometry from June 6 (the object's last radar
opportunity until 2040), close approaches could be identified reliably
during the 1723 years from 488 to 2211. At this writing, with a much longer,
3.3-month optical arc, the corresponding intervals are 1951 years with radar
(464 to 2415) and 137 years without it (2002 to 2139).
For asteroid (29075) 1950 DA, analysis of the radar-refined orbit (Giorgini
et al. 2002) revealed that there will be a possibly hazardous approach to
Earth in 2880 that would not have been detected using the original
half-century arc of pre-radar optical data alone. This event could represent
a risk as large as 50% greater than that of the average background hazard
due to all other asteroids from now through 2880, as defined by the Palermo
Technical Scale (PTS value = +0.17). 1950 DA is the only known asteroid
whose danger could be above the background level. The uncertainty in the
probability of a collision in 2880 is due mostly to uncertainty in the
Yarkovsky acceleration, which depends on the object's shape, spin state, and
global distribution of optical and thermal properties. This example
establishes the fundamental inseparability of asteroid physical properties
and long-term prediction of their trajectories: if we take the
hazard seriously, physical characterization must be given high priority. For
most NEAs, radar is the only Earth-based technique that can make images with
useful spatial resolution (currently as fine as ~10 m). With adequate
orientational coverage, delay-Doppler images can be used to construct
geologically detailed three-dimensional models (e.g., Hudson et al. 2000),
to define the rotation state, and to constrain the internal density distribution. The
wavelengths used for NEAs at Arecibo (13 cm) and Goldstone (3.5 cm), in
combination with the observer's control of the transmitted and received
polarizations, make radar experiments sensitive to the surface's bulk
density and to its roughness at scales larger than a centimeter (e.g., Magri
et al. 2001). The fact that NEAs' circular polarization ratios (SC/OC) range
from near zero to near unity means that the surfaces of these objects are
extremely variegated. In many cases, NEA surfaces have more severe
small-scale roughness than that seen by spacecraft that have landed on the
Moon, Venus, Mars, or Eros (whose SC/OC is near the NEA average of ~0.3).
Radar-derived shape models of small NEAs open the door to a wide variety of
theoretical investigations that are central to a geophysical understanding
of these objects. With realistic models, it is possible to
explore the evolution and stability of close orbits (e.g., Scheeres et al.
1998) with direct application to the design of spacecraft rendezvous and
landing missions. Given information about the internal density distribution,
one can use a shape model to estimate the distribution of gravitational
slopes, which can constrain regolith depth and interior configuration. A
shape model also allows realistic exploration (Asphaug et al. 1998) of the
potential effectiveness of nuclear explosions in deflecting or destroying
hazardous asteroids. The most basic physical properties of an asteroid are
its mass, its size and shape, its spin state, and whether it is one object
or two. Radar is uniquely able to identify binary NEAs, and at this writing,
has revealed six (Margot et al. 2002 and references therein, Nolan et al.
2002), all of which are designated Potentially Hazardous Asteroids (PHAs).
Analysis of the echoes from these objects is yielding our first information
about the densities of PHAs. Current detection statistics suggest that
between 10% and 20% of PHAs are binary systems. The risk of a
civilization-ending impact during this century is about the same as the risk
of a civilization-ending impact by a long-period comet (LPC) during this
millennium. At present, the maximum possible warning time for an LPC impact
is probably between a few months and a few years. Comet trajectory
prediction is hampered by optical obscuration of the nucleus and by
uncertainties about nongravitational forces. Radar reconnaissance of an
incoming comet would be the most reliable way to estimate the size of the
nucleus (Harmon et al. 1999) and would be valuable for determining the
likelihood of a collision.

References
Asphaug E. et al. (1998). Disruption of kilometre-sized asteroids by
energetic collisions. Nature 393, 437-440.
Benner L. A. M. et al. (2002). Radar observations of asteroid 1999 JM8.
Meteoritics Planet. Sci. 37, 779-792.
Giorgini J. D. et al. (2002). Asteroid 1950 DA's encounter with Earth in
2880: Physical limits of collision probability prediction. Science 296,
132-136.
Harmon J. K. et al. (1999). Radar observations of comets. Planet. Space Sci.
47, 1409-1422.
Hudson R. S. et al. (2000). Radar observations and physical modeling of
asteroid 6489 Golevka. Icarus 148, 37-51.
Magri C. et al. (2001). Radar constraints on asteroid regolith compositions
using 433 Eros as ground truth. Meteoritics Planet. Sci. 36, 1697-1709.
Margot J. L. et al. (2002). Binary asteroids in the near-Earth object
population. Science 296, 1445-1448.
Nolan M. C. et al. (2002). 2002 KK_8. IAU Circ. No. 7921.
Ostro S. J. et al. (2002). Asteroid radar astronomy. In Asteroids III (W.
Bottke, A. Cellino, P. Paolicchi, and R. P. Binzel, Eds.), Univ. of Arizona
Press, Tucson.
Scheeres D. J. et al. (1998). Dynamics of orbits close to asteroid 4179
Toutatis. Icarus 132, 53-79.
Yeomans D. K. et al. (1987). Radar astrometry of near-Earth asteroids.
Astron. J. 94, 189-200.

=============
CLOSE PROXIMITY OPERATIONS AT SMALL BODIES: ORBITING, HOVERING, AND HOPPING

D.J. Scheeres, Department of Aerospace Engineering The University of
Michigan Ann Arbor, MI 48109-2140
scheeres@umich.edu

Central to any characterization or mitigation mission to a small solar
system body, such as an asteroid or comet, is a phase of close proximity
operations on or about that body for some length of time. This is an
extremely challenging environment in which to operate a spacecraft or
surface vehicle. Reasons for this include the a priori uncertainty of the
physical characteristics of a small body prior to rendezvous, the large
range that can be expected in these characteristics, and the strongly
unstable and chaotic dynamics of vehicle motion in these force environments.
To successfully carry out close proximity operations about these bodies
requires an understanding of the orbital dynamics close to them, a knowledge
of the physical properties of the body and the spacecraft, and an
appropriate level of technological sensing and control capability on-board
the spacecraft. In this talk we will discuss the range of possible dynamical
environments that can occur at small bodies, their implications for
spacecraft control and design, and technological solutions and challenges to
the problem of operating in close proximity to these small bodies.

=============
MISSION OPERATIONS IN LOW GRAVITY, REGOLITH AND DUST

Derek Sears, Shauntae Moore, Shawn Nichols, Mikhail Kareev and Paul Benoit.
Arkansas-Oklahoma Center for Space and Planetary Sciences and Department of
Chemistry and Biochemistry, University of Arkansas, Fayetteville, Arkansas
72701

Introduction
Scientific investigations should be a component of impact mitigation studies
because knowledge of the nature of the asteroid is necessary for the
development of deflection techniques and predicting the effects of
atmospheric and terrestrial impact. We are developing a proposal for the
Hera mission, as mission to reconnoiter three asteroids and take samples
from three locations on each. We are interested in the asteroid-meteorite
connection and all this has to imply for the origin and evolution of the
solar system and the relationship between our Sun and other stars. In these
connections, we have performed experiments with simulated regolith and dust
on NASA microgravity facility (the KC-135), which should also provide
insights into mission operations in low gravity, regolith, and dust.
The Microgravity experiments
The aircraft lies about forty parabolas in a 2.5-hour flight in groups of 10
separated by 10 minutes of flat flight. Each parabola is about 2 minutes
duration and can be considered as having four phases, positive gravity
(during climb), negative gravity (when objects in the plane continue to
climb when the plane reaches the top of the parabola), microgravity (as the
plane descends at almost free-fall) and recovery (as the plane comes out of
descent). The duration of microgravity is about 25 seconds. We conducted
experiments during three campaigns, flying twice in each campaign, for a
total of 240 parabolas. During the first campaign, 317 one-inch Plexiglas
tubes filled with various sand and iron mixtures were flown. Separation of
iron and sand was determined from image analysis of photographs of the tubes
after flight and from the measurements of removed samples. For the second
campaign, two six-inch diameter Plexiglas cylinders containing sand iron
mixtures in approximately chondrite grain sizes and proportions were
observed with digital cameras The separation of iron and sand was noted and
any structures resembling the ponds on Eros were looked for. The third
campaign was essentially a test of the Honeybee Robotics touch-and-go
surface sampler. This device consists of two counter-rotating cutters that
eject material into a cylindrical container with front doors, to allow
collection, and a trap door below to allow ejection into the spacecraft
container. The collector was mounted on a vertical rail inside a double
walled enclosure and attempts were made to sample four surface stimulants,
sand, sand and iron mixtures, sand and gravel mixtures and concrete. It is
particularly helpful to compare the test results in microgravity with the
results in the laboratory.

Some results
The major result of the three campaigns, in terms of implications for
mission operations on the surfaces of asteroids and comets were:

· Particle size sorting of the surface material occurs readily.
· Segregations that occurred early in the process are retained during
considerable amounts of subsequent activity
· It was difficult to "see-through" the periods of negative g, which are an
artifact of the KC-135 tests and would not be present during sample
collection on an asteroid. A collector that works well on the ground worked
far less well under microgravity conditions where movement of the disturbed
surface in all directions but mostly away from the collector was a big
problem. Clogging of moving parts in such a dusty environment was also a
problem.
Lessons for mission operations on asteroids
· Limitations of KC-135 tests. In our experiment in the plane, it has been
difficult to "see through" the negative-g phase, and attempts to retain the
sample as the plane transitions from positive g to microgravity were
difficult given the time and physical constraints operative. It might be
better to use drop towers (although they only provide typically 5 seconds of
microgravity) or the Shuttle (which is expensive).
· Most methods of sample collection will produce segregations in unconsolidated
surface materials that would seriously degrade the scientific value of the samples. The
surface will be easily disturbed and material distributed widely.
Segregations produced early in the collection process can be retained after
fairly large amounts of subsequent mechanical agitation. Therefore (1) the
collector should disturb the surface as little as possible, (2) attempts
should be made to collect rocks (or clods) as well as dust and fine
regolith.
· During the development phase of equipment designed for operation on an
asteroids or comets, it is probably safe to assume that the collector will
perform to a much lower efficiency than on Earth, where gravity retains
material and where we have ample experience. With this in mind, sample
collectors with the minimum of moving parts and with as much dust protection
as possible are preferred, and collectors which cover or retain the surface
materials as they are collected stand the best chance of success of
recovering the most scientifically valuable samples.

=============
SEISMIC INVESTIGATIONS OF ASTEROID AND COMET INTERIORS

James D. Walker and Walter F. Huebner, Southwest Research Institute,
SanAntonio,Texas 78228

For a Near Earth Object on a potential collision course with Earth, any
mitigation technique will require a knowledge of the composition and
structure of the NEO. In particular, the density, strength, and cohesiveness
of the NEO, either an asteroid or a comet, will be required. Quantitative
information about the internal composition and structure of an asteroid or
comet can be obtained through active seismology. Active seismology requires
a source of the seismic disturbance and detectors (geophones or
seismometers) to measure the sound waves produced in the asteroid or comet
body. There are two approaches to producing seismic waves: explosive charges
and impactors. The active seismology program conducted on the Apollo 14, 16,
and 17 flights used both. On each of the flights the astronauts carried
explosives, either to be launched in a grenade launcher or to be placed by
hand as seismic source. On two of the flights a hand-held thumper consisting
of exploding bridge wires was also used as a seismic source. These
experiments allowed a partial determination of the structure of the lunar
surface in the vicinity of the landing site. Also, information about the
Moon's structure was gleaned from the seismic traces produced by the impact
of the LMs and SIVBs. Some of these results will be reviewed. Next, given a
size of an asteroid or comet and some assumptions about composition, the
requirements for explosive charge size or impactor momentum in order to
obtain signals that can be measured by various seismometers will be discussed.
The size of the charge ties into the coupling between the
explosive and the surface material of the asteroid or comet. Experiments are
being performed to examine the coupling of small explosive charges with
relation to depth into the surface. Large increases in efficiency result.
The corresponding impulse loadings from impacts will be discussed, including
what size impactors and impact velocities lead to similar seismic signals.
Information about the required loading on the surface is then available as
input for mission design, and well as determining seismometer sensitivity
requirements.

============
POSTER ABSTRACTS
http://www.noao.edu/meetings/mitigation/media/poster_abs.pdf

THE DEEP IMPACT DISCOVERY MISSION

M. F. A'Hearn , L.A. McFadden, C.M. Lisse, D.D. Wellnitz (U.Md), M.J.S.
Belton, (Belton Space Initiatives), A. Delamere (Ball Aerospace and
Technologies Corp), K.P. Klaasen (JPL), J.Kissel (MPI), K.J. Meech
(U.Hawaii), H.J. Melosh (U. Arizona), P.H. Schultz (Brown U.), J.M. Sunshine
(SAIC), J. Veverka (Cornell U.), and D.K. Yeomans (JPL)

The Deep Impact mission, two spacecraft, a flyby and an impactor, will
explore beneath the surface of comet 9P/Tempel 1. The impactor will excavate
a crater. Imagers and a spectrometer observe the collision, ejecta curtain
and the crater, making a direct comparison of the newly excavated interior
to that previously emitted into the comet's coma. Launching together in
January, 2004, for a 1.5 year cruise, encounter and impact will be July,
2005. Twenty-four hours before, the two spacecraft will separate. The flyby
spacecraft will be slowed and diverted to miss the comet by 500 km. Closest
approach occurs ~14 minutes post impact. The impactor, a mostly copper mass
of 370 kg, continues under autonomous guidance to hit the comet in a sunlit
area. Telescopic observations complement the spacecraft data. The flyby
includes a medium resolution imager with narrow-band and medium-band filters
and 10 mrad fov monitoring the comet nucleus at high time resolution during
and following impact determining fundamental nucleus properties. The high
resolution imager with medium -band filters, follows crater formation
(spatial resolution 17 m/pixel at impact and 1.4 m/pixel final image). The
infrared spectral imaging module will collect spectra between 1-4.8 microns
continuously before, during and after impact comparing compositions and
looking for spatial variations. The impactor targeting sensor, a white light
imager collects high speed images until just before impact. Highest
resolution will be 20-30 cm/pixel. An S-band transmitter sends the images to
the flyby, then relays them back to Deep Space Network receivers on Earth.
We will determine the comet's shape, morphology, albedo and crater density.
We will time and map the crater ejecta curtain and debris to determine
surface properties (porosity and compressibility) and gravitational force at
the comet. We will analyze spectral maps for photometric and compositional
variations both before and after impact. With laboratory simulations of the
impact we have explored the range of possible crater sizes (diameter and
depth) and ejecta evolution. If gravity controls crater growth (strengthless
particulate surface), the crater may be as large as 120m and 25m deep.
Smaller diameters will occur if the surface is highly compressible or
exhibits strength. Ball Aerospace designed and is building the spacecraft
and instruments. Mission design and operations is carried out at JPL under
its project management.

===========
IMPACT : A SPACE CONTRIBUTION TO MONITORING THE THREAT OF POTENTIALLY
HAZARDOUS CELESTIAL BODIES

L. Bussolino and R. Somma, Alenia Spazio, Strada Antica di Collegno 253,
Torino, Italy

IMPACT is the acronym for "International Monitoring Program for Asteroids
and Comets Threats" coming out as proposal to the Agencies and Government
institutions from a series of studies funded by the italian region PIEMONTE
throughout the Civil Protection Bureau, the Italian Space Agency and the
European Space Agency in different period of time and performed by the
Planetology Group of the Astronomical Observatory of Torino in Italy and the
Alenia Spazio, the major Italian aerospace company, for the engineering
design part. The key point of the study is concerning the best continuation
till the completion of the activities of discovery as well as the physical
and mineralogical characterization of the potentially hazardous celestial
bodies, including a certain families not easy to be seen by the ground
telescopes: the new outcome is the utilization of satellites in orbit around
the Earth or in other position, in any case suitable for discovering objects
type Inner Earth Orbit. The present paper will ponder a synthesis of the
activities performed during these series of studies where the space
technology, if conveniently integrated with the Earth networks, appears to
offer a valuable contribution to the PHA detection and characterization,
fundamental activities basic for the risks mitigation. An international
approach is then proposed for monitoring this threat.

================
THE IMPACT IMPERATIVE

Jonathan W. Campbell, NASA/MSFC

The Asteroid and Comet impact problem has been with us for millions of
years. Only recently however has our awareness expanded to realize that
there may be a problem. Our collective awareness as a civilization is now
expanding as we learn more. The critical question that remains to be
answered is whether our awareness will expand to point that we will take
action in time.
Given sufficient priority, we now have the technological capability to begin
building a means for deflecting asteroids and comets. These include Earth,
LEO, and/or Lunar based laser facilities; transporting the laser to the
object; and transporting nuclear devices to the object.
All approaches depend on ablative processes to accomplish deflection. The
laser uses slow ablation to minimize fragmentation and gradually shape the
orbit. A laser facility has the advantage of being able to respond quickly
to a sighting. A nuclear approach requires time of transport and if the
explosion is external to the object use rapid, massive ablation to change
the orbit. For an explosion inside, the ablation creates gas pressures that
may fragment the object and if vented properly could create a jet effect for
orbit shaping. An equally challenging part of this problem is early warning,
early detection, and continuous tracking. Again, given sufficient priority,
we have the technological means (radar and ladar) in the near term to
address this part of the overall problem. It is imperative that the space
priorities in our National and World community's be realigned to place
impact mitigation first. Technological roadmaps must be redrawn orienting us
towards solving this problem first.

===============
PHYSICAL CHARACTERIZATION OF NEOS BY MEANS OF REMOTE OBSERVATIONS FROM SPACE

A. Cellino (Torino Astronomical Obs.), K. Muinonen (Helsinki Obs.), E.F.
Tedesco (TerraSystems, Inc.), M. Delbo` (Torino Astronomical Obs.), S.
Price, M. Egan (Air Force research Lab.), L. Bussolino (Alenia Aerospazio)

Physical characterization of NEOs is essential for a better understanding of
the properties and histories of these objects, and to develop credible
techniques for hazard mitigation. Many of the relevant physical parameters
describing the internal structures of NEOs can only be accurately derived
from local "in situ" investigations by space probes. However, remote sensing
is still very useful to provide valuable information on the distributions of
important physical parameters such as size, geometric albedo and spectral
reflectance. Moreover, space-based observations can more readily detect
objects having orbits that are mostly or totally interior to the Earth's
orbit. We are currently conducting a study funded by the European Space
Agency to assess the options and do preliminary design and performance
trade-off analyses for a dedicated space-based NEO observatory. Initial
results indicate that observations spanning a wavelength interval including
the peak thermal emission between 5 and 12 microns, are needed and suitable
to attain the scientific goals of the mission. Different orbital options for
the satellite are also being investigated with the leading candidates being
orbits around the L2 Lagrangian points of either the Earth or Venus. Both
options present advantages and drawbacks that must be carefully assessed.
This presentation provides the initial results of the study and a more
detailed rationale for the options considered.

==============
IMPLICATIONS OF THE NEAR MISSION FOR INTERNAL STRUCTURE

Andrew F. Cheng, The Johns Hopkins Applied Physics Laboratory, Laurel, MD
20723

On 14 February 2000, the Near Earth Asteroid Rendezvous spacecraft (NEAR
Shoemaker) began the first orbital study of an asteroid, the near-Earth object 433 Eros.
Almost a year later, on 12 February 2001, NEAR Shoemaker completed its mission by
landing on the asteroid and acquiring data from its surface. Previously, on
June 27 1997, NEAR performed the first flyby of a C-type asteroid, 253
Mathilde. These two asteroid databases provide a basis for inferences to be
made regarding physical properties and internal structure relevant to
mitigation. NEAR Shoemaker's study of Eros found an average density of 2.67
+/- 0.03, almost uniform within the asteroid. No evidence was found for
compositional heterogeneity or an intrinsic magnetic field. The surface is
covered by a regolith estimated at tens of meters thick. A small center of
mass offset from the center of figure suggests regionally nonuniform regolith thickness or
internal density variation. Blocks have a non-uniform distribution
consistent with emplacement of ejecta from the youngest large crater. Some
topographic features indicate tectonic deformations. Several regional scale
linear features have related orientations, suggesting a globally
consolidated internal structure. Structural control of crater shapes hints
that such internal structure is pervasive. Eros is interpreted to be
extensively fractured but without gross dislocations and/or rotations - it
was not disrupted and reaccumulated gravitationally. Some constraints can be
placed on its strength. The consolidated interior must support a shear
stress at least on the order of a few bars. Crater morphologies can be
interpreted as suggesting a "strength" near the surface of a few tens of
kPa.
The Eros flyby of Mathilde revealed a heavily cratered surface with at least
5 giant craters (close to geometric saturation). Mathilde's density was
unexpectedly low at 1.3 +/- 0.3, indicating a high porosity. Such a high
porosity may be consistent with a rubble pile structure. This high porosity
is key to understanding Mathilde's collisional history, but there are
structural features, such as a 20-km long scarp, and polygonal craters
indicating that Mathilde is not completely strengthless. At least one of its
structural components appears coherent over a few tens of km.

================
IMPACTS FROM POROUS FOAM TARGETS: POSSIBLE IMPLICATIONS FOR THE DISRUPTION
OF COMET NUCLEI AND LOW-DENSITY ASTEROIDS

Daniel D. Durda (Southwest Research Institute, 1050 Walnut Street Suite 426,
Boulder CO 80302), George J. Flynn and Tobyn W. VanVeghten (Department of
Physics, State University of New York Plattsburgh, Plattsburgh, NY 12901)

Recent observations by the NEAR spacecraft of asteroid 253 Mathilde [1],
determinations of the densities of other C-type main-belt asteroids
accompanied by bound natural satellites [2], laboratory measurements of the
porosities of meteorites [3,4], and the bulk densities of interplanetary
dust particles [5], indicate that many impact targets in the solar system
are quite porous, having bulk densities significantly lower than the density
of their constituent minerals. Love et al. [6] have shown that it requires
significantly more energy to produce craters of the same size in porous
targets than in non-porous targets. Chapman et al. [7] have suggested that
the four largest craters on the asteroid Mathilde, which exceed the
conventionally accepted size limit for crater production without catastrophic disruption or
"surface resetting", may be explained by shock dissipation in a porous
target. We performed a series of impact experiments at the NASA Ames
Vertical Gun Range (AVGR) in October 2001 and May 2002 to examine the
response of very porous foam targets to various impacts. We conducted a
total of four shots into two ~10-cm diameter closed-pore polystyrene
(Styrofoam) spheres and two 22.9x10.5x7.8 cm blocks of finely-textured,
open-pore foam that is usually used as a rigid mounting base for floral
arrangements. All impacts were performed with the AVGR impact chamber
evacuated to a pressure of about 0.5 Torr. For shot 011010, we suspended an
11.4-cm diameter (30.5 g) Styrofoam sphere, having a bulk density of ~0.6
gm/cm^3, from the ceiling of the AVGR chamber, and impacted it with a
1/8-inch aluminum sphere having a speed of 1.92 km/s (powder gun mode). Such
an impact could simulate the impact of a strong, nickel-iron projectile into
a very low-density/high-porosity comet or weak, porous asteroid. We expected
beforehand that the impactor might perhaps simply burrow its way through the
Styrofoam sphere and emerge out the other side, leaving the sphere more or
less intact. Instead, the result was a catastrophic disruption, leaving only
cm-scale shards of debris throughout the impact chamber. For shot 011011, we
cut a 1/4-inch (11.4 mg) spherical projectile from the same Styrofoam
material as the 8.9-cm diameter (15.5 g) target sphere. The Styrofoam
projectile was carefully loaded into a plastic sabot and fired in powder gun
mode at a speed of 1.68 km/s. Somewhat unexpectedly, the projectile survived
the launching process intact, although it did "pancake" into a somewhat
lenticular disk during flight. Once again, the resulting impact was much
more catastrophic than we anticipated, yielding the same, almost explosive
disruption of the target sphere. The mass distributions of fragments
resulting from the disruption of the two polystyrene spheres from shots
011010 and 011011 resemble the power law-like fragment distributions
commonly observed for disruptive impacts into more conventional rock or ice
targets. In contrast to the closed-pore foam spheres for shots 011010 and
011011, the targets for shots 020501 and 020502 were open-pore foam blocks
with dimensions of 22.9x10.5x7.8 cm, having a bulk density of ~0.2 gm/cm^3.
Projectiles were fired at an angle of 45 deg to the normal of the largest
face. For shot 020501, we impacted the block with a 1/8-inch aluminum sphere
at a speed of 1.12 km/s (powder gun mode). The projectile tunneled
essentially unimpeded through the body of the block, leaving no crater in
the surface and carving a cylindrical path completely through the block
somewhat larger in diameter than the projectile itself. The entry hole was
elliptical, measuring ~4x6 mm, and the exit hole was elliptical,
measuring ~7x11 mm.

For shot 020502, we cut a 1/4-inch spherical projectile from the same foam
material as the target block. The foam projectile was loaded into a plastic
sabot and fired in powder gun mode. Unfortunately, but not unexpectedly, the
projectile essentially disintegrated during the firing process, resulting in
a shower of foam "dust" being launched toward the target. The surface of the
foam block target displayed minor scattered traces of the penetration of the
projectile debris, but otherwise yielded no useful cratering or disruption
data. Levison et al. [8] compared orbital distribution and survey discovery
models of Oort cloud comets to observations of populations of dormant comets
and concluded that 99% of new comets evolving inward from the Oort cloud
must physically disrupt (as did comet C/1999 S4 LINEAR; [9]), citing buildup
of internal volatile pressure as a possible mechanism. We surmise that the
closed-pore Styrofoam that we chose as a target material for the first two
shots prevented the interior of the target spheres from being fully
evacuated during the pump down of the impact chamber. Thus, an internal
pressure probably built up, leading to increased surface and internal
stresses in the target spheres that were released when their surfaces were
penetrated by the impactors. Although not the simple burrowing or
compression cratering outcomes we were anticipating (as we indeed observed
in the case of the open-pore floral foam blocks), these results may
nonetheless bear some relevance to impacts (either rare natural ones, or
artificial ones arranged by curious humans) onto comet nuclei. The Giotto
images of Comet Halley and the Deep Space 1 images of Comet Borrelly both
showed localized jets of gas and dust emission, suggesting that most of the
surface of each of these comets was protected from sublimation by a surface
crust impervious to gasses. The relatively collisionally pristine surfaces
of volatile rich, dynamically young Oort cloud comets,
or surface crusts built up on collisionally and dynamically evolved Kuiper
belt comets through the sublimation and loss of ices with retention of
rocky/dusty debris, might allow internal volatile pressure to build up
within a comet nucleus. Such internal pressures might be released in a
violent manner during even small impacts, contributing to the complete
disruption of a comet nucleus.

References
[1] Yeomans, D. K., J.-P. Barriot, D. W. Dunham, R. W. Farquhar, J. D.
Giorgini, C. E. Helfrich, A. S. Konopliv, J. V. McAdams, J. K. Miller, W. M.
Owen Jr., D. J. Scheeres, S. P. Synnott, and B. G. Williams 1997. Science
278, 2106 2109.
[2] Merline, W. J., L. M. Close, C. Dumas, C. R. Chapman, F. Roddier, F.
Menard, D. C. Slater, G. Duvert, C. Shelton, and T. Morgan 1999. Nature 401,
565 568.
[3] Consolmagno, G. J., and D. T. Britt 1998. Meteoritics Planet. Sci. 33,
1231 1240.
[4] Flynn, G. J., L. B. Moore, and W. Klock 1999. Icarus 142, 97 105.
[5] Flynn, G. J. and S. R. Sutton 1993. Lunar Planet. Sci., 21, 541-547.
[6] Love, S. G., F. Horz, and D. E. Brownlee 1993. Icarus 105, 216-224.
[7] Chapman, C., W. Merline, P. Thomas, and the NEAR MSI-NIS Team 1998.
Meteoritics & Planetary  Science, 33, A30.
[8] Levison, H. F., A. Morbidelli, L. Dones, R. Jedicke, P. A. Wiegert, and
W. F. Bottke, Jr. 2002. Science 296, 2212 2215.
[9] Boehnhardt, H. 2001. Science 292, 1307 1309.

===================
A SPACE-BASED VISIBLE/INFRARED SYSTEM FOR THE CHARACTERIZATION AND DETECTION
OF NEOS

M.P. Egan (AFRL/XP), Y.J. King, P.D. LeVan, B.J. Tomlinson, & B. Flake
(AFRL/VSSS), & S.D. Price (AFRL/VSB, VSS)

We present the technical capability for a modest sized (third to half meter)
space-based visible/infrared instrument to accurately determine the
diameters of NEOs and to augment their discovery by extending the survey
beyond the limitations of ground-based instruments. Previous analysis
demonstrated the measurement capabilities for accurate size determinations
(Price and Egan, 2001) and the detection/discovery efficiencies of such a
system for objects 200 meter in diameter and larger (Tedesco et al., 2000).
The Air Force Research Laboratory's research program in developing
spacecraft/sensor technology in the critical areas of focal plane arrays,
cryocoolers, on-board signal processing and integrated spacecraft structures
is key to being able to field a light-weight, cost effective satellite.
Mid-Infrared focal plane arrays are being developed for space observation
applications. The mature Si:As FPA technology will be described, as will be
other innovative technologies for both the infrared and visible wavelength
regions. Current candidates for low background, Mid-Infrared applications
require cooling to almost 10 Kelvin. Active low temperature cryogenic
cooling for Mid-Infrared sensing applications is being
addressed within the Space Vehicles Directorate of the Air Force Research
Laboratory (Davis et al. 2001, Tomlinson et al. 2001) to address mid to long
term DoD mission requirements. Ten Kelvin cooling technology will soon reach
protoflight capability, provides tremendous savings in payload mass versus
stored cryogen systems, and greatly increases the payload performance (with
increased cooling load capability) and lifetime (10 years and longer). Trade
studies will be shown that evaluate the performance versus maturity levels
of the subsystem technologies.

S.D. Price and M.P. Egan, Infrared Characterization of Near Earth Objects,
Adv. Space Sci., 28 1117 -1127, 2001. E.F. Tedesco, K. Muinonen and S.D.
Price, Space-Based Infrared Near-Earth Asteroid Survey Simulation, Planetary
and Space Science, 48, 801-816, 2000.
T. Davis, B. J. Tomlinson, and J. Ledbetter, Military Space Cryogenic
Cooling Requirements for the 21st Century, International Cryocooler
Conference 11, 1-9, 2001.
B. J. Tomlinson, T. Davis, and J. Ledbetter, Advanced Cryogenic Integration
and-Cooling Technology for Space-Based Long Term Cryogen Storage,
International Cryocooler Conference 11, 749-758, 2001

===============
ASTEROID 1950 DA'S ENCOUNTER WITH EARTH IN A.D. 2880

J.D.Giorgini 1 ,S.J.Ostro 1 , L.A.M.Benner 1 , P.W.Chodas 1 , S.R.Chesley 1
, R.S.Hudson 2 , M.C.Nolan 3 , A.R.Klemola 4 , E.M.Standish 1 , R.F.Jurgens
1 , R.Rose 1 , D.K.Yeomans 1 and J.-L.Margot 5

1 Jet Propulsion Laboratory
2 Washington State University
3 Arecibo Observatory
4 UCO/Lick Observatory
5 California Institute of Technology

Initial analysis of the numerically integrated, radar- based orbit of
asteroid (29075)1950 DA indicated a 20- minute interval in March 2880 during
which the 1.1-km object might have an Earth impact probability of 0.33%.
This preliminary value was supported by both linearized covariance mapping
and Monte Carlo methods. The dynamical models, however, were limited to
gravitational and relativistic point-mass effects on the asteroid by the
Sun, planets, Moon, Ceres, Pallas, and Vesta. Subsequent extended modeling
that included perturbations likely to affect the trajectory over several
centuries generally implies a lower impact probability, but does not exclude
the encounter. Covariance based uncertainties remain small until 2880
because of extensive astrometric data (optical measurements spanning 51
years and radar measurements in 2001), an inclined orbit geometry that
reduces in-plane perturbations, and an orbit uncertainty space modulated by
gravitational resonance. This resonance causes the orbit uncertainty region
to expand and contract along the direction of motion several times over the
next six centuries rather than increasing secularly on average, as is
normally the case. As a result, the 1950 DA uncertainty region remains less
than 20,000 km in total extent until an Earth close-approach in 2641
disrupts the resonance. Thereafter, the same uncertainty region extends to
18 million km along the direction of motion at the Earth encounter of 2880.
We examined 11 factors normally neglected in asteroid trajectory prediction
to more accurately characterize trajectory knowledge. These factors include
computational noise, Galilean satellite gravity, galactic tides,
Poynting-Robertson drag, major perturbations due to the gravitational
encounters of the asteroid with thousands of other asteroids, an oblate Sun
whose mass is decreasing, planetary mass uncertainties, acceleration due to
solar wind, radiation pressure and the acceleration due to thermal emission
of absorbed solar energy. Each perturbation principally alters the
along-track position of 1950 DA, either advancing or delaying arrival of the
object at the intersection with the orbit of the Earth in 2880. Thermal
radiation (the Yarkovsky effect) and solar pressure were found to be the
largest accelerations (and potentially canceling in their effects, depending
on which of two possible radar-based pole solutions is true), followed by
planetary mass uncertainty and perturbations from the 64 principle
perturbing asteroids identified from an analysis of several thousand. The
Earth approach distance uncertainty in 2880 is determined primarily by
accelerations dependent on currently unknown physical factors such as the
spin axis, composition, and surface properties of the asteroid, not
astrometric measurements. This is the first case where risk assessment is
dependent on the determination of an object's global physical properties. As
a result of this dependency, no specific impact probability is quoted here
since the results would vary with our assumptions of the numerous
uncertainties and dynamic models. Within decades, thousands of asteroids
will have astrometric datasets of quality comparable to 1950 DA's and
similarly have their long-term collision assessments limited by physical
knowledge.
1950 DA's trajectory dependence on physical properties also illustrates the
potential for hazard mitigation through alteration of asteroid surface
properties in cases where an impact risk is identified centuries in advance.
Trajectory modification could be performed by collapsing a solar sail
spacecraft around the target body, or otherwise altering the way the
asteroid reflects light and radiates heat, thereby allowing sunlight to
redirect it over hundreds of years.

The next radar opportunity for 1950 DA will be in 2032. The cumulative
effect of any actual Yarkovsky acceleration since 2001 might be detected
with radar measure ments obtained then, but this would be more likely during
radar opportunities in 2074 or 2105. Ground-based photometric observations
might better determine the pole direction of 1950 DA much sooner.

Reference :
Giorgini,J., et al, Science 296, 132-136 (2002).
http://neo.jpl.nasa.gov/1950da
E-Mail:Jon.Giorgini@jpl.nasa.gov

=============
HOW WELL DO WE UNDERSTAND THE COMETARY HAZARD?

Matthew Knight and Michael A'Hearn, University of Maryland

A preliminary study of comets discovered by amateur astronomers finds that a
significant fraction should have been found by surveys prior to their
discovery by amateurs. A sample of 34 comets discovered by amateurs between
1990 and 1999 contained at least 7 comets which should have been in the
field of view of at least one of the following surveys prior to discovery:
the Palomar Digital Sky Survey (DPOSS), the Second Palomar Observatory Sky
Survey(POSS ii), or the Second Epoch Southern Red Survey (AAOR). Extension
of this analysis to other available catalogs is expected to increase the
number of pre-discovery observations. While the preliminary sample displays
no apparent trends in orbital elements or ecliptic latitude-longitude, it is
hoped that a larger sample will reveal trends in the distributions of the
amateur-discovered comets. A better understanding of the selection effects
which allow amateurs to detect these comets and/or prevent surveys from
detecting them is critical for the success of future surveys as well as the
search for potentially hazardous comets and asteroids.

======================
DEFLECTING IMPACTORS AT 90°

Claudio Maccone, Member of the International Academy of Astronautics
Via Martorelli, 43 - 10155 Torino (TO) - Italy
E-mail: clmaccon@libero.it

In a recent paper (Acta Astronautica, Vol. 50, No. 3, pp. 185-199, 2002)
this author gave a mathematical proof that any impactor could be hit at an
angle of 90° if hit by a missile shooted not from the Earth, but rather from
Lagrangian Points L3 or L1 of the Earth-Moon system. Based on that
mathematical theorem, in this paper the author shows that:
1) This defense system would be ideal to deflect small impactors, less than
one kilometer in diameter. And small impactors are just the most difficult
ones to be detected enough in advance and to a sufficient orbital accuracy
to prove that they are impactors indeed.
2) The deflection is achieved by pure momentum transfer. No nuclear weapons
in space would be needed. This is because the missiles are hitting the impactor at the optimum
angle of 90°. A big steel-basket on the missile head would help.
3) In case one missile was not enough to deflect the impactor off its
Earth-collision hyperbolic trajectory, it is a wonderful mathematical
property of confocal conics that the new slightly-deflected impactors
hyperbola can certainly be hit at 90° by another and slightly more eccentric
ellipse! So, a sufficient number of missiles could be launched in a sequence
from the Earth-Moon Lagrangian points L3 and L1 with the absolute certainty
that the SUM of all these small and repeated deflections will finally throw
the impactor off its collision hyperbola with the Earth.

==============
COMET/ASTEROID PROTECTION SYSTEM (CAPS): A SPACE-BASED SYSTEM CONCEPT FOR
REVOLUTIONIZING EARTH PROTECTION AND UTILIZATION OF NEOS

Daniel D. Mazanek, NASA Langley Research Center, Hampton, Virginia USA

There exists an infrequent, but significant hazard to life and property due
to impacting asteroids and comets. Earth approaching asteroids and comets
are collectively termed NEOs (near-Earth objects). These planetary bodies
also represent a significant resource for commercial exploitation, long-term
sustained space exploration, and scientific research. The goal of current
search efforts is to catalog and characterize by 2008 the orbits of 90% of
the estimated 1200 near-Earth asteroids larger than 1 km in diameter.
Impacts can also occur from short-period comets in asteroid-like orbits, and
long-period comets which do not regularly enter near-Earth space since their
orbital periods range from 200-14 million years. There is currently no
specific search for long-period comets, smaller near-Earth asteroids, or
smaller short-period comets. These objects represent a threat with
potentially little or no warning time using conventional terrestrial-based
telescopes. It is recognized, and appreciated, that the currently funded
terrestrial-based detection efforts are a vital and logical first step, and
that focusing on the detection of large asteroids capable of global
destruction is the best expenditure of limited resources. While many aspects
of the impact hazard can be addressed using terrestrial-based telescopes,
the ability to discover and provide coordinated follow-up observations of
faint and/or small comets and asteroids is tremendously enhanced, if not
enabled, from space. It is also critical to ascertain, to the greatest
extent possible, the composition and physical characteristics of these
objects. A space-based approach can also solve this aspect of the problem,
both through remote observations and rendezvous missions with the NEO. A
space-based detection system, despite being more costly and complex than
Earth-based initiatives, is the most promising way of expanding the range of
objects that could be detected, and surveying the entire celestial sky on a
regular basis. Finally, any attempt to deflect an impacting NEO with any
reasonable lead-time is only likely to be accomplished using a space-based
system. This poster presentation provides an overview of the Comet/Asteroid
Protection System (CAPS), and discusses its primary goal of identifying a
future space-based system concept that provides integrated
detection and protection through permanent, continuous NEO monitoring, and
rapid, controlled modification of the orbital trajectories of selected comets and asteroids.
The goal of CAPS is to determine whether it is possible to identify a "single" lunar based or
orbiting system concept to defend against the entire range of threatening
objects, with the ability to protect against 1 km class long-period comets
as the initial focus. CAPS would provide a high probability that these
objects are detected and their orbits accurately characterized with
significant warning time, even upon their first observed near-Earth
approach. The approach being explored for CAPS is to determine if a system
capable of protecting against long-period comets, placed properly in
heliocentric space, would also be capable of protecting against smaller
asteroids and comets capable of regional destruction. The baseline detection
concept advocates the use of advanced, high-resolution optical/infrared
telescopes with large, mosaic image plane arrays, coordinated telescope
control for NEO surveying and tracking, and interferometric techniques to
obtain precision orbit determination when required. The primary orbit
modification approach uses a spacecraft that combines a high energy power
system, high thrust and specific impulse propulsion system for rapid
rendezvous, and a pulsed laser ablation payload for changing the target's
orbit. This combination of technologies may offer a future orbit
modification system that could deflect impactors of various compositions
without landing on the object. The system could also provide an effective
method for altering the orbits of NEOs for resource utilization, as well as
the possibility of modifying the orbits of smaller asteroids for impact
defense. It is likely that any NEO defense system would allow for multiple
deflection methods. Although laser ablation is proposed as the primary orbit
modification technique, alternate methods, such as stand-off nuclear
detonation, could also be part of the same defensive scenario. Advanced
technologies and innovation in many are as critical in adequately addressing
the entire impact threat. Highly advanced detectors that have the ability to
provide the energy and time of arrival of each photon
could replace current semi-conductor detectors in much the same way as they
replaced photographic plates. It is also important to identify synergistic technologies that can
be applied across a wide range of future space missions. For example,
technologies permitting humans to traverse the solar system rapidly could be
highly compatible with the rapid rendezvous or interception of an impactor.
Likewise, laser power beaming (visible, microwave, etc.) may be applicable
for space-based energy transfer for remote power applications, as well as
NEO orbit modification.
The vision for CAPS is primarily to provide planetary defense, but also
provide productive science, resource utilization and technology development
when the system is not needed for diverting threatening comets and
asteroids. The vision is for a future where asteroids and cometary bodies
are routinely moved to processing facilities, with a permanent
infrastructure that is capable and prepared to divert those objects that are
a hazard. There is tremendous benefit in "practicing" how to move these
objects from a threat mitigation standpoint. Developing the capability to
alter the orbits of comets and asteroids routinely for non-defensive
purposes could greatly increase the probability that we can successfully
divert a future impactor, and make the system economically viable. It is
likely that the next object to impact the Earth will
be a small near-Earth asteroid or comet. Additionally, a globally
devastating impact with a 1 km class long-period comet will not be known
decades, or even years, in advance with our current detection efforts.
Searching for, and protecting ourselves against these types of impactors is
a worthwhile endeavor. Current terrestrial-based efforts should be expanded
and a coordinated space-based system should be defined and implemented. CAPS
is an attempt to begin the definition of that future space-based system, and
identify the technology development areas that are needed to enable its
implementation.

================
A REQUIREMENTS AND MISSIONS ROADMAP FOR NON-TERRESTRIAL EMPIRICAL
VERIFICATION OF NEO EFFECTS THRESHOLDS: OBJECTIVES FOR THE DETERMINATION OF
TRUE LOWER LIMITS ON ATMOSPHERIC PENTRATIONS AND GLOBAL EFFECTS

Drake A. Mitchell, MIT '87, Planetary Defense

Recent computer-based simulations have investigated the atmospheric penetrations
of Near-Earth Objects (Hills and Goda, 2001[1]), their sub-global effects (Lewis, 2000
[2]), and the extended cratering process (Kring and Durda, 2001 [3]).
Simulations of global-effects thresholds are expected (Holsapple, 1981 [4],
1993 [5]; Holsapple and Housen, 2002 [6]; Mitchell, 2002 [7]).
In a responsible, robust, and cost-justified campaign to attack the NEO
problem, such simulations would be verified, e.g. calibrated, by space-based
empirical investigations in non-terrestrial planetary environments.
Simulation verification objectives and requirements are proposed that would
also synergistically verify both the true annualized economic exposure to
the hazard, and the viability of the many technologies and methodologies
that have been proposed for NEO hazard mitigation missions but that have
never been realistically tested or even adequately simulated.
Several classes of space-based platforms are reviewed for NEO surveillance,
reconnaissance, modification, resource utilization, and deflection objectives. Mission
optimizations and synergies are identified. It is shown that the proposed
investigations can be achieved within existing or modified international
conventions for the peaceful uses of outer space, and within the economic
parameters justified by a program that would finally pass legal tests of
negligence, i.e. specific programmatic and budgetary standards, e.g. $75
billion expended by 2010. Guidelines for such a program are derived from
analyses of the Manhattan Project under the leadership of Lt. General L. R.
Groves, and the subsequent development of the United States Nuclear Navy
under the leadership of Admiral H. G. Rickover.
[1] http://abob.libs.uga.edu/bobk/ccc/cc071702.html See #7, "[5]"
[2] http://abob.libs.uga.edu/bobk/ccc/cc012602.html See #4, "[3]"
[3] http://www.lpi.usra.edu/meetings/lpsc2001/pdf/1447.pdf
[4]
http://adsbit.harvard.edu/cgi-bin/nph-iarticle_query?bibcode=1981LPICo.449..
.21H
[5]
http://adsbit.harvard.edu/cgi-bin/nph-iarticle_query?bibcode=1993AREPS..21..
333H
[6] http://www.lpi.usra.edu/meetings/lpsc2002/pdf/1857.pdf
[7] http://abob.libs.uga.edu/bobk/ccc/cc062502.html See #6, "[17]"

==============
DETERMINING HIGH-ACCURACY POSITIONS OF COMETS AND ASTEROIDS

Alice K B Monet,  US Naval Observatory Flagstaff Station

Beginning in 1991 with the Galileo spacecraft encounter with Gaspra, the
USNO Flagstaff Station has been providing highly-accurate astrometry of
comets and asteroids to NASA/JPL in support of a variety of missions and
observing programs. Over the years, no effort has been spared to attain the
greatest possible accuracy. This has led to improvements in hardware,
detectors, supporting electronics, observing strategies, astrometric
analysis, and - perhaps most significantly - in astrometric reference
catalogues. USNO is proud to have contributed to the many successful
encounters, flybys, radar ranging experiments, and improved orbits for
targets of particular interest. While each solar system body seems to
present its own peculiar observing challenges, we have developed a certain
level of confidence in our astrometric methods. If an object is detectable
with our instrumentation, we can accurately determine its position. In this
report, I will discuss what we have come to regard as the key elements in a
successful astrometric campaign. These include a wide-field of view and
target-appropriate centroiding algorithms. Perhaps the most important is an
accurate, dense, reference catalogue of faint objects. In recent years, the
Naval Observatory has produced a number of such catalogues - most notably
the USNO-A2.0 catalogue and the UCAC. The 8-inch FASTT telescope has also
been used to densify regions of the TYCHO catalogue, for particular
applications. At the time of this Workshop, new versions or expansions to
these existing catalogues are under development, and new survey programs are
being planned which will yield yet-more accurate and dense reference grids.
All of these factors contribute to improved accuracy for asteroid and comet
positions. Certainly, the accuracy of the astrometric positions is one of
the essential ingredients in the effort to identify those comets and
asteroids which pose a potential threat to our planet.

==============
WARNING THE PUBLIC ABOUT ASTEROID IMPACTS

David Morrison, NASA Astrobiology Institute

Unlike other natural hazards, the impact of an asteroid can (in principle)
be avoided entirely by deflecting the object while it is still several years
(and hundreds of millions of kilometers along its orbit) from the Earth. The
requirement is to predict the potential impact sufficiently far in advance.
The NASA Spaceguard Report of 1992 articulated the strategy of carrying out
a comprehensive survey of NEAs, taking advantage of the fact that impacts
are very rare and that NEAs will typically pass close by Earth thousands of
times before they hit. Under these circumstances, it is extremely likely
that any impact will be predicted decades or centuries in advance (if at
all). The chances of finding a NEA on its final plunge to Earth are
negligible. This is true whatever the magnitude (size) limit of the search.
The lead-time for a Tunguska-class impactor (60 m diameter) is no different
from that of a civilization-threatening impactor (2 km diameter), once we
have invested in the larger telescopes that are needed to reach such small
NEAs. The purpose of the Spaceguard Survey is to provide long-lead warning
of possible impacts. To date, there have been no such confirmed warnings,
nor was any expected. However, during the past 5 years there has been
approximately one warning issued in the press per year (e.g., 1997 XF11,
1999 AN10, 1950 DA, 2002 MN, and 2002 NT7). Of these, only 1950 DA was
legitimate, and the low-probability chance of a collision with this asteroid
does not materialize for nearly a millennium. The others were all cases
where a rather poorly defined orbit indicated a possible (but very
improbable) impact. Additional observations and orbital calculations
eliminated this low-probability threat within a few days. While such media
scares may have helped sensitize the public to the impact hazard, they have
also demeaned the credibility of astronomers in the public eye. There is the
potential for disutility in such warnings, which undermine confidence in the
asteroid surveys and distract the public from more important issues.
Astronomers are learning to play down such false alarms, but most of us have
concluded that it is undesirable to suppress information and impossible to control the media.
As survey capabilities improve and we discover more and more of the NEA
population, we can expect more such media flaps,
unfortunately. There is, however, the legitimate question of a real
confirmed warning, which could be issued when sufficient data are
accumulated to provide a secure orbit. The most likely such case will
indicate a significant probability (above 10%) of impact several decades in
the future, by an object near the lower size limit of the surveys that are
current at that time. If the object is smaller than 50 m diameter, there
will be no danger of penetration to the surface or troposphere. If it is
between 50 and 100 m diameter and is not targeted toward a densely populated
region, it may be best to begin planning for possible evacuations. If it is
larger than 100 m, undoubtedly proposals will be made to intercept and
deflect it. The issue arises of what organization, national or
international, should issue such a confirmed warning. One proposal is to
assign this responsibility to the U.S. Air Force Space Command, where a
permanent NEA warning center might be established. The primary purpose of
this paper is to examine the possible role of such a warning center. How often will it
be activated? The Earth can expect an impact from a Tunguska-size asteroid (60 m) about
once per millennium. With present survey telescopes the chances of predicting such an
impact are very small, but a survey could be constructed that would operate
even down to such sizes. Meanwhile, the frequency of impact of the 1-km NEAs
that are the focus of the current Spaceguard Survey is about once per
million years. Thus today we would anticipate that the warning center might
issue a confirmed warning of an impact at the 10% probability level about
once every 100,000 years. If we had a survey that targeted completion at the
50-m level, such a warning might be issued about once every 50-100 years.
This is the maximum frequency, since impactors smaller than 50 m dissipate
their energy in the upper atmosphere. This is not very much work to keep a
permanent center staffed and operational. On the other hand, if the proposed
center anticipates issuing warnings much more frequently (say every year,
for example), then it will quickly lose its credibility, since the vast
majority of such warnings will be false alarms. It is difficult to envision
how a warning center devoted to the NEA impact hazard can be justified given
the infrequency of expected impacts or even of credible possibilities of
impacts. Warning centers make sense for severe storms today, and they would
also make sense for earthquakes if we knew how to predict them. But for
events as infrequent as asteroid impacts, this is not a credible option.

================
IMPACT OF GAIA ON NEAR-EARTH-OBJECT COLLISION PROBABILITY COMPUTATION

K. Muinonen (Univ. Helsinki Observatory, Finland), J. Virtanen (Univ.
Helsinki Observatory, Finland), and F. Mignard (Observatoire de la Cote
d'Azur, France)

We are studying the effects of high-precision astrometric observations on
the computation of near-Earth object (NEO) orbits and collision
probabilities. In addition to standard astrometry, we are examining
differential astrometry, that is, either differences of two positions from
standard astrometry or the actual sky-plane motion. GAIA, the next
astrometric cornerstone mission of ESA, is due for launch no later than
2011. The duration of the GAIA survey will be 5 years, the limiting
magnitude equals V = 20 mag, and full sky will be covered some dozen times a
year. In particular, GAIA promises to provide an unprecedented NEO search
across the Milky Way area typically avoided by groundbased searches. The
extraordinary precision of the astrometry, varying from 10 micro-arcseconds
at V = 15 mag to a few milliarcseconds at V = 20 mag, will have a major
impact on NEO orbit computation, in particular, on the derivation of NEO
collision probabilities and the assessment of the collision hazard. In
addition to standard positional astrometry, GAIA will obtain differential
astrometric observations: it promises to detect an object's motion across
the field of view. The accuracy of the GAIA astrometry imposes a challenge
for orbit computers, as an NEO's size, shape, and surface properties will
have an effect on the astrometry. This effect will depend on the NEO
orientation with respect to the Sun-NEO-GAIA plane and, in particular, on
the solar phase angle (the angle between GAIA and the Sun as seen from the
NEO). We show tentative simulations about the improvement of NEO orbits by
the GAIA data. Finally, we show predicted NEO detection statistics for the
GAIA mission.

================
COMMUNICATING ABOUT COSMIC CATASTROPHES

Brendan M. Mulligan, CIRES, Univ. Colorado (Boulder) and Clark R. Chapman,
Southwest Research Inst. (Boulder)

The history of the Earth, and all the bodies in the solar system, has been
marked by cosmic catastrophes of epic proportions: impacts due to asteroids
and comets. Large-scale impacts have occurred in the past and, despite a
decline in impact flux, the potential for future impacts constitutes a
legitimate threat to human civilization. Communicating about the risk that
near-Earth objects (NEOs) pose to the general public presents a serious
challenge to the astronomical community. Although the NEO hazard has a
unique character, comparisons with other natural hazards can readily be
drawn and lessons can certainly be learned from years of experience that
other researchers have in risk communication. Just as specialists dealing
with other hazards have done, the NEO community has addressed the challenge
of risk communication by developing tools, most notably the Torino Impact
Hazard Scale, capable of conveying useful information to a diverse audience.
Numerous researchers and commentators have critiqued the scale, some
suggesting modifications or proposing particular significant revisions.
These critiques have dominantly focused on the Scale's perceived technical
weaknesses, neglecting the central issues concerning its ability to inform
the public in a satisfactory way. For instance, an issue that has already
been dealt with in other cases (e.g. the "terrorism scale" of the U.S. Dept.
of Homeland Security) concerns the degree to which the wording in the public
scale tells people what they should specifically do in response to a particular warning
level. The American Red Cross, for example, tabulated different responses that might be
appropriate for different groups (individuals, families, neighborhoods,
schools, and businesses) as to how they should respond to a particular level
of security threat. Similar clarification of the Torino Scale might be in
order. We hardly expect the public to "carefully monitor" an NEO predicted
as having a Torino Scale "1" close encounter; those words were intended for
astronomers. But given recent hype in popular media concerning 2002 NT7,
further clarification for science journalists about appropriate levels of
response for different interest groups (astronomers, space agency or
emergency management officials, ordinary citizens) might be appropriate. The
NT7 hype was further confused by media reference to the event's numerical
value on another scale (PTS) that is only a year old and is intended for
technical purposes only. Again, the existence of multiple scales occurs for
other natural hazards. But, despite internal debates about how to announce
an earthquake Magnitude and the existence of multiple seismic scales, the
public has been shielded from such internal, technical dissension and has
become quite comfortable with Magnitudes, even though the appellation
"Richter" has officially disappeared. Clearly, the NEO community's efforts
to help the public place in context any news about possible future impacts
remain only partially effective; NEO impact predictions continue to be met
with confusion, misunderstanding, and sensationalism. The Torino Scale value
is not the only information about impacts available to the public and,
indeed, scales of any sort are not the only way to bring some convergence
into public discussion of particular predictions. Astronomers have a public
responsibility to develop simple protocols for honestly but understandably
communicating about the inherently tiny chances of potentially huge
disasters that characterize the impact hazard. Drawing from experience with
other scales, we advocate that the IAU and other players and entities
develop policies grounded in previous experience that can ensure accuracy,
consistency, and clarity in reports of impact predictions. Only if we get
our scientific house in order can we demand responsibility on
the part of the science communicators and journalists who constitute the
next link in the chain of communication.

=================
USING A SOLAR COLLECTOR TO DEFLECT A NEAR EARTH OBJECT

James F. Pawlowski, Human Exploration Science Office, Johnson Space Center,
Houston, TX.

Of all the various non-nuclear techniques for deflecting a Near Earth Object
(NEO) on a collision course with Earth, one of the most promising methods
uses a solar collector. This method was studied by H.J. Melosh et al* and
uses a solar collector to focus the Sun's rays on the NEO's surface.
Evaporation by heat creates a thrust which modifies the NEO's trajectory
over a period of time. Such a technique has an advantage because it neither
requires stabilizing nor landing on the NEO. As the NEO rotates under the
illuminated spot, fresh material is brought into the heated area so
evaporation is continuous. Furthermore it does not, for the most part,
depend on the composition of the NEO. It can evaporate stony or icy bodies
but probably not iron NEOs, but these are rare. The steady push also
minimizes the danger of disrupting the NEO in contrast to a severe impulse.
There are a number of technical hurdles to overcome in maturing this
technique, but none seem improbable or any more difficult than any other
methods.

*Melosh, H. J., Nemchinov, I. V., Zetzer, Y. I. : 1994, Hazards Due to
Comets and Asteroids, PP. 1119 -1127

==============
NEAR EARTH OBJECT EXPLORER (NEOX): A HIGH PERFORMANCE AND COST_EFFECTIVE
SPACECRAFT FOR NEO EXPLORATION

Rich Reinert and Richard Dissly, Ball Aerospace & Technologies Corp.

We present the design and describe the capabilities of a Solar Electric
Propelled (SEP) microsatellite appropriate for a cost-effective and
comprehensive program of NEO exploration.
Use of the Xenon-ion SEP approach proven on NASAs DS-1 Mission provides the
NEOX S/C with 12km/s of Delta-V. Previous mission studies show that this Delta V will
allow a single NEOX S/C to rendezvous with one to two NEOs when launched
from an Ariane-5 ASAP, and with three to four NEOs when launched by a
Delta-II. A spacecraft mass <200kg provided by advanced technology enables
launch as a secondary payload (e.g., Ariane-5 ASAP) or launch of multiple
spacecraft from a single dedicated launch vehicle (e.g., 4 from a Delta II
7925). These low-cost launch options can enhance prospects for NEO
exploration and characterization, as up to 16 NEOs could potentially be
characterized using multiple NEOX spacecraft manifested on a single Delta-II
launch vehicle. An interesting alternative would be to launch one to four
vehicles annually as secondary payloads on the Ariane-5 LV. Possibly the
modest cost of these secondary launches could be provided as a contribution
by ESA in return for carriage of ESA payloads. The NEOX spacecraft is
designed to support a 20kg science payload drawing 100W average during SEP
cruise, with >1kW available to instruments during a NEO orbital phase when
the SEP thrusters are not powered. Rendezvous and NEO orbit will provide
determination of the target object mass and density, and will allow for
multiple phase angle imaging. The spacecraft is 3-axis stabilized with
better-than 1 milliradian pointing accuracy to serve as an excellent imaging
platform, and the telecommunications system can support a downlink data rate
of 6.4 kbps at 3 AU earth range. We will present candidate instrument suites
and further discuss the advanced but proven technologies that make this
spacecraft design possible.

=================
IMAGING THE INTERIORS OF NEAR-EARTH OBJECTS WITH RADIO REFLECTION TOMOGRAPHY

Ali Safaeinili and Steven J. Ostro, Jet Propulsion Laboratory

Scenarios for mitigation of asteroid/comet collisions include the use of
explosives to deflect or destroy the projectile (Ahrens and Harris 1995).
However, as demonstrated by Asphaug et al. (1998), the outcome of explosive
energy transfer to an asteroid or comet (via a bomb or a hypervelocity
impact) is extremely sensitive to the pre-existing configuration of
fractures and voids. A porous asteroid (or one with deep regolith)
significantly damps shock wave propagation, sheltering distant regions from
impact effects while enhancing energy deposition at the impact point. Parts
of multi-component asteroids are similarly preserved, because shock waves
cannot bridge inter-lobe discontinuities. Thus our ability to predict the
effect of detonating a nuclear device at an asteroid or comet will rest on
what we know about the object's interior. Information about the interiors of
near-Earth objects is extremely limited. Results from NEAR-Shoemaker's
year-long rendezvous of Eros (Prockter et al. 2002, Veverka et al. 2000)
suggest that it is somewhat consolidated, with a pervasive internal fabric that runs
nearly its entire length and affects some mechanical responses such as fracture
orientation. However, Eros' detailed internal arrangement of solid and
porous domains is unknown, and in any case, Eros is not hazardous and is
orders of magnitude more massive than any potentially hazardous asteroid.
For much smaller asteroids whose shapes have been reconstructed from
ground-based radar imaging (e.g., Hudson and Ostro 1995, Hudson et al. 2000)
and for radar-detected comet nuclei (Harmon et al. 1999), some interesting
but non-unique constraints on density distribution have resulted. We would
like to suggest that Radio Reflection Tomographic Imaging (RRTI) (Safaeinili
et al.) is an optimal technique for direct investigation of the interior of
a small body by a spacecraft in orbit around it. The RRTI instrument's
operating frequency is low enough so that its radio signals are able to
probe the target body's interior. The data obtained by RRTI is
three-dimensional since it consists of wideband echoes collected on a
surface around the object. This three-dimensional data set can be operated
on to obtain the three-dimensional spatial spectrum of the object. The
inversion of the RRTI data can yield the three-dimensional distribution of
complex dielectric constant, which in turn can reveal the presence of void
spaces, cracks, and variations in bulk density. The mathematical basis of
the technique is similar to that of ultrasonic reflection tomography (Kak
and Slaney 1988) and seismic imaging (Mora 1987). Design of a spaceborn RRTI
instrument for a small-body rendezvous can be based on the heritage from
other planetary radar sounders like MARSIS (Picardi et al. 2001) and radar
sounding experiments used to study glaciers (Gudmandsen, 1971) or
contemplated for searching for a Europa ocean (Johnson et al. 2001).
However, unlike these planetary radar sounding instruments, RRTI of NEOs
would exploit the spacecraft's access to all sides of the body. Global views
of the object make it possible to solve for the three-dimensional dielectric
constant variations within the object down to the size of the shortest
observing wavelength. RRTI is distinctly different from radio transmission
tomography techniques (e.g. the CONSERT experiment on Rosetta; Kofman et al.
1998) whose purpose is not imaging but rather to study material properties
of radio-transparent comets. RRTI is an imaging technique that uses a
co-located transmitter and receiver, and therefore does not require that the
illuminating signal pass entirely through the target. Therefore, an RRTI
system can be used to image the interiors of both comets and asteroids
throughout the volume penetrated by the radar echoes. The volumetric
dielectric properties of the asteroid or comet can be reconstructed using
least-squares inversion (e.g., a conjugate gradient search; Safaeinili and
Roberts 1995, Lin and Chew 1996) driven by the observed difference between
model-predicted radio echoes and the measured radio signals. A 24
computationally less intensive and reasonably accurate inversion is possible
with the Born approximation, which ignores multiple reflection within the
target and linearizes the dependence of the scattered field on dielectric
variations. See our poster for examples of simulated RRT images of the
interiors of very simple models.

References
1. Ahrens, T. J., and A. W. Harris (1995). Deflection and fragmentation of
near Earth asteroids. In Hazards Due to Comets and Asteroids (T. Gehrels,
ed.), Univ. of Arizona, pp. 897-927.
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INFERRING INTERIOR STRUCTURES OF COMETS AND ASTEROIDS BY REMOTE OBSERVATIONS

Nalin Samarasinha, National Optical Astronomy Observatory

Detailed determinations of the interior structures of comets and asteroids require
space missions equipped with suitable instruments. While such missions are
essential for the furtherance of our knowledge on the interior structures of comets and
asteroids, cost considerations alone may force such studies to be focused on a selected set
of targets. Additional useful and complementary information on the interior
structures can be derived by studying the spin states of asteroids and spin
states and activity of comets, primarily via groundbased studies. Structural
information based on rotation depends on (a) fastest spin rates for an
ensemble of asteroids (or comets) and (b) the damping time scale for
non-principal axis rotators. I will discuss capabilities and limitations of
both these procedures for determining structural parameters. In the case of
comets, activity and associated effects could provide additional useful
information on the interior structure. I will also discuss how activity and
splitting events could affect the size distribution of cometary nuclei and
by extension a significant fraction of NEOs.

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EDDY CURRENT FORCE ON METALLIC ASTEROIDS

Duncan Steel, University of Salford, UK

In order to make accurate predictions of the future orbital evolution of
Earth-approaching asteroids it is necessary to take into account
non-gravitational forces. As Giorgini et al. (Science, 296, 132-136, 2002)
have recently shown, radiation forces depending on the surface properties of
a specific relatively large asteroid will affect whether it will impact the
Earth some centuries hence. Since the surface area varies as r^2 the
perturbation varies as 1/r, and so smaller asteroids may be affected on
shorter time scales. Astronomers studying meteoroids and interplanetary dust
have studied such radiative perturbations for some decades, and also
considered the Lorentz and Faraday forces due to interactions with the
interplanetary magnetic field strength. For objects of asteroidal size the
perturbations produced are much smaller than the radiation-induced effects. Another
class of force due to the magnetic field is the eddy current force that would act on a
metallic asteroid. This depends on the (square of the) gradient of the interplanetary
magnetic field, which may be substantial at sector boundaries or in a
turbulent magnetic field. It may thus act only episodically. This force is
always dissipative, slowing down the object in question. The important point
about the eddy current force is that it varies as r^3 and so the
perturbation produced will be size/mass independent. On the other hand,
voids within an asteroid will inhibit the eddy currents and so limit the
force imposed. Rough calculations of the eddy current force indicate that it
is much smaller than the radiative forces, but show that the internal
structure of an asteroid may be significant with
regard to specifying its dynamical evolution.

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NEA DEFLECTION: SOMETIMES RESONANT RETURNS ARE OF NOT MUCH HELP

G.B. Valsecchi and A. Carusi, IASF-CNR, via del Fosso del Cavaliere 100,
00133 Roma, Italy

The Delta V needed to deflect a Near-Earth Asteroid (NEA) in order to
prevent a collision with the Earth can be significantly lower if the NEA in
question has a close encounter with our planet before the one in which the
collision is bound to happen. In fact, Carusi, Valsecchi, D'Abramo and
Boattini (2002, Icarus, in press) show that, in the hypothetical case of the
2040 collision of (35396) 1997 XF11, which would be preceded by an Earth
encounter in 2028 putting the asteroid in a resonant orbit, if the
deflection takes place a short time before 2028, then Delta V
is about two orders of magnitude smaller than the one needed in case the
deflection takes place a short time after 2028. The amount of the Delta V
saving is strictly related to the different mean motion perturbations
imparted by the 2028 Earth encounter to two fictitious particles on nearby
trajectories; the difference in mean motion leads to along-track separation
and this, in turn, leads to different b-plane coordinates in 2040.
Valsecchi, Milani, Gronchi and Chesley (2001, Astron. Astrophys., submitted)
give for these quantities analytic expressions that turn out to be in good
agreement with the numerical integrations in the case of (35396) 1997 XF11.
The formulae show that the ratio between the separation of the b-plane
coordinates at the second encounter and the separation at the first
encounter increases essentially linearly with time; however, the
coefficient of the linear increase varies significantly as a function of
some of the orbital parameters of the asteroid, and can become very small in
some cases. When this happens, one can a priori expect that a significantly
reduced Delta V saving would be obtained with a pre-first-encounter
deflection of a NEA impacting at a resonant return. As a practical example,
we discuss the case of 1994 GV, a very small (H of approx 27) NEA that has,
among others, a Virtual Impactor (VI) that, after an encounter with the
Earth in 2031, hits the Earth at a resonant return in 2048. We present
numerical integrations showing that, as expected, the Delta V saving
obtained with a pre-2031 deflection of the 2048 VI associated with 1994 GV
is more than an order of magnitude smaller than the Delta V saving obtained
with a pre-2028 deflection of the 2040 VI associated with (35396) 1997 XF11.

CCCMENU CCC for 2002