CCNet 135/2002 - 21 November 2002

"However, a key uncertainty remains, says Brown: whether large and
dangerous meteors are concentrated in streams, like this week's
unthreatening Leonid showers."
--Jeff Hecht, New Scientist, 20 November 2002

"It is perhaps worth noting that the recent data lowering the
average frequency of Tunguska-class imacts does not mean we are safer
from impacts, since the hazard is dominated by larger (million megaton
or more) impactors, not Tunguska (10 magaton) impacts. Besides, the more
important public issue is not average impact rates, but whether anything
is actually on a collsion course with the Earth at this time."
--David Morrison, 20 November 2002

"So, should the estimated 50,000 NEOs in the 200-meter category be
ignored and left to fall where they may? Such an impactor could
devastate a region as effectively as two or three hydrogen bombs, or
it could trigger a nuclear war if it explodes over a nation like India or
Pakistan. But from an actuarial perspective, these relatively small
asteroids pose no greater risk than major earthquakes, hurricanes, and
volcanic eruptions. In fact, they are probably much less of a hazard to
humanity than we are to ourselves."
--Ivan Semeniuk, Mercury (Astronomical Society of the
Pacific), Nov/Dec 2002

    Nature Science Update, 21 November 2002

    New Scientist, 20 November 2002

    BBC News Online, 20 November 2002

    David Morrison <>

    P. Brown, R.E. Spaulding, D.O. ReVelle, E.Taglioferri, and S.P. Worden.

    Dana Rohrabacher

    Mercury (Astronomical Society of the Pacific), Nov/Dec 2002

    Michael Paine <>

    Financial Times, 20 November 2002


>From Nature Science Update, 21 November 2002

Defence data lower forecast for asteroids exploding in Earth's atmosphere.
21 November 2002
Every ten years an explosion equivalent to three Hiroshima atom bombs rips
through the Earth's upper atmosphere, scientists in Canada and the United
States report.

These explosions are not clandestine nuclear tests. They are natural events,
caused by the collision of asteroids with the Earth. Peter Brown of the
University of Western Ontario in Canada and co-workers have used military
satellite data to figure out how heavily our planet is being bombarded by
cosmic missiles1.

Their findings make sobering reading, but things aren't as bad as scientists
had feared. They had thought that one ten-megaton explosion, equivalent to
the biggest hydrogen-bomb detonations at the height of the Cold War nuclear
tests in the 1950s, happened every two or three centuries. Brown and
colleagues say that these occur only once a millennium.

If that's so, perhaps we can relax a little. The last 10-megaton explosion
took place only about a century ago.

In 1908, a meteorite exploded about 6 km up in the atmosphere above the
uninhabited region of Tunguska in Siberia. It levelled forests over an area
of hundreds of square kilometres. People 60 kilometres away were thrown to
the ground; reindeer herders 30 km away were blown into the air - one was
allegedly killed when he hit a tree.

Statistically speaking, it should be a very long time before we see the like
of the Tunguska explosion again.

Impact factor

Impacts of large asteroids can be catastrophic, and leave obvious footprints
such as the awesome Meteor Crater in Arizona. But they are very rare, and
invariably happened long ago. Colliding bodies called bolides, less than 100
metres or so across, tend to break up in the atmosphere and so often leave
no traces.

Brown and colleagues have gained access to a unique window on this rain of
space rock falling onto the Earth. They have scanned observations made by
classified US satellites, which the Departments of Defense and Energy use to
look for explosions that signal nuclear-weapons tests.

Between the start of 1994 and September 2002, these satellites spotted 300
events that Brown's team attribute to high-altitude bolide impacts. The
satellites see a flash lasting just a few seconds. From these flashes, the
researchers estimate the amount of energy released by the explosion.

In six years they have seen events ranging from equivalent to a tenth of a
kiloton of TNT to a few tens of kilotons. These correspond to bolides
between about 1 and 10 metres across.

A few ground-based telescopes are now dedicated to looking for bodies at
least several metres across on trajectories that will cross Earth's path.
The Lincoln Near Earth Asteroid Research (LINEAR) program in Socorro, New
Mexico, and the Spacewatch telescope in Arizona are on this watch.

LINEAR and Spacewatch have seen far fewer bolides because big ones are
rarer. But Brown's satellite data for small objects matches up well with the
telescope data for bigger objects. This, the first direct measure of the
smaller impact events, indicates that there is, on average, a 5-kiloton
explosion every year - that's one-third the size of the Hiroshima bomb.
Brown, P., Spalding, R. E., ReVelle, D. O., Tagliaferri, E. & Worden, S. P.
The flux of small near-Earth objects colliding with the Earth. Nature, 420,
294 - 296, (2002). |Article|
Nature News Service / Macmillan Magazines Ltd 2002


>From New Scientist, 20 November 2002
A new analysis of data from US military satellites shows that locally
devastating impacts by small asteroids are likely only about once in a

The benchmark for such impacts is a 1908 blast that levelled 2000 square
kilometres of forest in the Tunguska area of Siberia. Scientists calculate
that a 50- or 60-metre object exploded in the atmosphere with the force of
10 megatons of TNT.

But no other well-documented case is known and this size of object is too
small to spot reliably in space, so estimates of their frequency are
sketchy. The previous best guess suggested such blasts were likely every 200
to 300 years.

Now Peter Brown of the University of Western Ontario says the odds are more
in our favour. The evidence comes from military satellites that were looking
out for nuclear weapons tests. During eight years of observations, the
satellites were able to record 300 meteors exploding in the atmosphere.

Taking advantage of acoustic data on 19 events, Doug ReVelle of the Los
Alamos National Laboratory for the first time was able to calibrate how much
energy each blast released, so Brown could plot how many objects of
different sizes hit over the period.

Ground damage

They found that the frequency of objects in the one- to 10-metre range
decreases with size. The mathematical function that describes this decrease
is same as that determined for near-Earth asteroids larger than 50 metres,
from astronomical observations.

Brown says this means the same function should hold for the difficult 10- to
50-metre range. On this basis, a one-megaton blast should occur on average
every 130 years, while a 50-kiloton blast will occur about every decade. A
26-kiloton explosion - like the one recorded over the Mediterranean in June
- will hit about once every three to four years.

The ground damage caused depends on both the size of the explosion and its
altitude. "Somewhere in the hundreds of kilotons range you start getting
effects on the ground, and certainly in the megaton range," Brown told New

However, a key uncertainty remains, says Brown: whether large and dangerous
meteors are concentrated in streams, like this week's unthreatening Leonid

US military officials only started recording all meteor data after a
50-kiloton blast in February 1994, and this interval is short enough to miss
streams like the Leonids that peak about once a century.

Journal reference: Nature (vol 420, p 294)
Jeff Hecht

Copyright 2002, New Scientist


>From BBC News Online, 20 November 2002
By Dr David Whitehouse
BBC News Online science editor 
Every year a small asteroid explodes in the Earth's atmosphere with an
energy equivalent to 5,000 tonnes of TNT, according to new information.
The assessment comes from researchers who have studied about 300 impacts
from space observed by US military surveillance satellites.

The scientists now estimate an object of the size that exploded over central
Siberia in 1908 causing widespread devastation only strikes the Earth every
1,000 years or so. This is far less frequent than had been thought.

The asteroid impact assessment has been published in the journal Nature.

Military data

Asteroids with diameters smaller than 50-100 metres that collide with the
Earth usually do not hit the ground as a single body. Rather, they detonate
in the upper atmosphere.

A new analysis of the flashes of light from these exploding asteroids is
possible because of data provided by the US Department of Defense from
military satellites.

Positioned in geostationary orbit, the satellites have a view of an entire
hemisphere of the Earth and, because they are designed to detect light from
rockets being launched, they are able to see the light flashes from space
impacts as well.

Between February 1994 and September 2002, about 300 impact events were seen.
>From the intensity and duration of the light flashes, and some basic
physics, it was possible to calculate the size of the incoming asteroids.

The researchers led by Dr Peter Brown, of the University of Western Ontario,
Canada, estimate that every month an object explodes in the upper atmosphere
with an energy equivalent to 300 tonnes of TNT.

'Harmful intruders'

Every 10 years, an object with the energy of 50 kilotonnes impacts the

An object like the one that struck Tunguska in central Siberia in 1908 hits
us, on average, every 1,000 years or so. That object had an energy
equivalent to 10 megatonnes of TNT. If such an object were ever to strike an
inhabited area, millions of people could be killed.

Recently scientists have expressed concern that upper atmosphere explosions
caused by small asteroids could be mistaken for nuclear detonations,
especially during times of international tension.

Surveying the latest data, Dr Benny Peiser, of Liverpool John Moores
University, UK, told BBC News Online: "This new research reinforces our view
that we are constantly bombarded by cosmic debris large enough to be
misinterpreted as a nuclear attack.

"The findings are a compelling warning that we need to start scanning the
skies for small, but potentially harmful intruders."

And commenting in the journal Nature, Dr Robert Jedicke, of the University
of Arizona, US, says the study has "linked the fields of meteor and
comet/asteroid planetary astronomy in a manner which shows that they are not
merely distant cousins."

Copyright 2002, BBC


>From David Morrison <>

NEO News (11/20/02) Bolides, Rohrabacher & Mitigation

Dear Friends & Students of NEOs:

Following are four recent publications of interest.

(1) Paper published today in Nature by Peter Brown (University of Western
Ontario) and colleagues reporting on the flux of impacting objects in the
1-10 m size, derived from 8 years of surveillance satellite data on bolides
entering the Earth's atmosphere. They conclude that the average largest
annual impact event is 5 kilotons, a value that falls almost exactly on the
extrapolated impact-frequency diagram recently developed by Alan Harris of
the Space Science Institute (and published in the review chapter by Morrison
et al. of the forthcoming book Asteroids III). Based on the combination of
rapidly increasing astronomical surveys of NEAs larger than 100 m together
with the new "ground truth" data from atmospheric impacts, it appears that
the average impact flux for NEAs from 1 m to 10 km in size is now reasonably
secure. (This range covers a factor of 10^12, or a billion billion, in
energy). The corresponding frequency for Tunguska-size impacts (10 megatons)
is once per millennium, as concluded independently in both the Morrison et
al. and the Brown et al. publications.

(2) Opinion (op-ed) article from Space News (21 Oct issue) by Representative
Dana Rohrabacher, the Chair of the House Space and Aeronautics Subcommittee
of the U.S. Congress. Rohrabacher expresses concern that insufficient
attention and funding are available for NEO studies, especially for work on
possible mitigation technology.

(3) Featuure article from the New York Times (19 Nov issue) discussing the
broad issues of the NEO impact hazard and focusing on discussion at the NASA
mitigation meeting held in September.

(4) Article from Mercury Magazine (Nov/Dec issue) that also reports
primarily on the recent mitigation discussions.

It is perhaps worth noting that the recent data lowering the average
frequency of Tunguska-class imacts does not mean we are safer from impacts,
since the hazard is dominated by larger (million megaton or more) impactors,
not Tunguska (10 magaton) impacts. Besides, the more important public issue
is not average impact rates, but whether anything is actually on a collsion
course with the Earth at this time.

David Morrison


P. Brown, R.E. Spaulding, D.O. ReVelle, E.Taglioferri, and S.P. Worden.

Nature 420: 294-296 (21 November 2002)

SUMMARY (press release): In the past eight years, US Department of Defense
satellites scanning the Earth for evidence of nuclear explosions have
detected nearly 300 optical flashes caused by small (1-10 m) asteroids
exploding in the upper atmosphere. This has provided a new estimate of the
flux of near-Earth objects colliding with the Earth, which P. Brown of the
University of Western Ontario, Canada, and colleagues publish in this week's
Nature. The revised estimate suggests that Earth's upper atmosphere is hit
once a year by objects that release energy equivalent to five kilotons of
TNT. The object that exploded above Tunguska in June 1908 was a 'small'
asteroid, yet big enough to flatten 2,000 square kilometres of Siberian
forest. Brown and colleagues calculate that Tunguska-like (ten-megaton)
events are likely to occur about once every 1,000 years. This is more
encouraging than the previous estimate, from ground-based observations, of
once every 200 to 300 years. This work "has linked the fields of meteor and
comet/asteroid planetary astronomy in a manner which shows that they are not
merely distant relatives," says Robert Jedicke of the University of Arizona,
Tucson, in an accompanying News and Views article.

ABSTRACT: Asteroids with diameters smaller than 50-100 m that collide with
the Earth usually do not hit the ground as a single body; rather they
detonate in the atmosphere. These small objects can still cause considerable
damage, such as occurred near Tunguska, Siberia, in 1908. The flux of small
bodies is poorly constrained, however, in part because ground-based
observational searches pursue strategies that lead them preferentially to
find larger objects. A Tunguska-class event -- the energy of which we take
to be equivalent to 10 megatons of TNT -- was previously estimated to occur
every 200-300 years, with the largest annual airburst calculated to be about
20 kilotons TNT equivalent [reference to Shoemaker 1983].  Here we report
satellite records of bolide detonations in the atmosphere over the past 8.5
years. We find that the flux of objects in the 1-10 m size range has the
same power-law distribution as bodies with diameters greater than 50 m. From
this we estimate that the Earth is hit on average annually by an object with
about 5 kton equivalent energy, and that Tunguska-like events occur about
once every 1000 years.

observations made by US Department of Defense and Department of Energy
space-based systems in geostationary orbitsS  We corrected the number
distribution based on the percentage coverage of the Earth's surface, which
varied from 60% to 80% S there are 300 bolides in our sample S energies were
calculated based on a model 6000 K temperature for the flash S 13 calibrated
examples provide best data S agrees with Spacewatch observations of very
small NEAs (Rabinowitz et al. 2000)...infrasound acoustic data (for 19
events only) give slightly higher value of 10 kton for annual event S most
impactors are asteroidal and not cometary judged from depth of penetration S
average impact interval for 10 megaton body is 1000 (+800, -200) years S in
agreement with recent work by Al Harris (e.g., Morrison et al. 2003, review
chapter in Asteroids III book).


By Dana Rohrabacher
21 October 2002
This year NASA Administrator Sean O'Keefe gave us his vision of NASA's
mission for the future. Inclusive within that mission is understanding and
protecting the Earth's environmental resources and ecosystem.

Conspicuously absent from the administrator's list, however, is the
potential threat posed by Near-Earth Objects (NEOs). As chairman of the
House Science space and aeronautics subcommittee, I heard disturbing
testimony in 1998 that the NEO threat should be taken seriously.

Further, within the last several months, the media has reported three events
involving asteroids that have Earth-crossing orbits with the potential for a
close encounter or collision with the Earth. Although these asteroids passed
the Earth within a distance several hundreds of thousands of miles, in
astronomical terms they missed our planet by a hair.

Thus, the subject of NEOs is no longer considered to be just science
fiction. Unfortunately, there is no U.S. government agency responsible for
responding to the NEO threat or even how to mitigate that threat.

Planetary defense advocates have proposed a wide range of options for
mitigating asteroids roughly one kilometer or more in size. An asteroid that
size can cause enormous damage. The options that have been discussed range
from establishing a Natural Impact Warning Clearinghouse, supported by
military space-based surveillance satellites gathering data for possible
international distribution, to using advanced propulsion technologies for
getting us off the planet and possibly setting-up shop on the moon for the
future preservation of humankind.

Despite these innovations, all agree that more information regarding the NEO
population is needed before asteroid mitigation becomes credible.

In 1998, NASA initiated the Spaceguard Program. Its goal is to catalog by
2008 all Near-Earth objects, or at least 90 percent of those that are at
least one kilometer in size. NASA has surveyed slightly more than 600 of
these large asteroids thus far and their program appears to be on track with
the use of ground-based telescopes. However, the surveillance of Near-Earth
Objects does not appear to be a high priority at NASA.

Some have proposed that military space-based surveillance satellites play a
role as part of an early warning of asteroid impacts, especially those that
move toward the Earth from the sun. Last June, an asteroid roughly less than
one kilometer in size and spotted three days after its flyby came within
75,000 miles of the Earth, where it went undetected due to the sun's glare.

Space-based assets, however, should not be viewed as a panacea but rather as
a possible complement to ground-based telescopes dedicated to the detection
of NEOs of all sizes. Surveys of smaller asteroids with the potential to
destroy cities, countries and global climate, should also be vigorously
tracked. The National Research Council recommended, in a recently published
report, that NASA partner with the National Science Foundation to design,
build and operate a survey facility, such as the Large-Aperture Synoptic
Survey Telescope, so as to accomplish the objective of assessing the
population of NEOs down to 300-meters in diameter and providing a measure of
the impact hazard. Ascertaining the relative critical nature of long-period
comets also contributes to gauging the impact hazard to Earth.

The question now before us is what can be done today? One of the critical
aspects of cataloging asteroids is keeping track of what has already been
identified. Amateur astronomers can play a crucial role in this regard. They
can help strengthen existing government capabilities for tracking natural
space objects by encouraging private citizens to observe the heavens.

My bill H.R. 5303 ("Charles 'Pete' Conrad Astronomy Awards Act") provides
the vehicle for private citizens to take an active role in the government's
efforts to conduct NEO surveys by encouraging amateur astronomers to
discover new and track previously identified asteroids, particularly those
that threaten close approach with the Earth. It also is my way to honor Pete
Conrad, an explorer of the highest caliber, for his tremendous contribution
to aerospace during the last nearly 40 years. He commanded Apollo 12, and
during that mission became the third man to walk on the moon.

It should be noted that recent analysis of an orbiting object identified by
an amateur astronomer suggests it is the remains of a Saturn 5 third stage
most likely from Pete Conrad's Apollo mission. I find no better way to honor
Pete Conrad than to establish an annual astronomer's award for future
asteroid discoveries in his name.

The act contains three categories of awards to be presented annually to
amateur astronomers who: Discover the largest new asteroid having a
near-Earth orbit; discover asteroids using information derived from
professional sources and track newly discovered asteroids; and provide the
greatest service to update the Minor Planet Center's catalogue of known
asteroids. At a time when we seek greater public interest and participation
in the national space program, it is my hope that H.R. 5303 will bring
greater attention to the NEO issue by prompting a new generation of
Americans to pursue careers in engineering, science and astronomy.

Dana Rohrabacher is chairman of the Space and Aeronautics Subcommittee of
the House Science Committee.


>From The New York Times, 19 November 2002


this article was on CCNet 19 November 2002

-- snip


Mercury (Astronomical Society of the Pacific), Nov/Dec 2002

By Ivan Semeniuk

For Mike Belton the impact hazard wasn't a personal problem until he saw the
numbers. As a former Kitt Peak planetary astronomer, Belton has long been
aware of the infamous connection between the extinction of the dinosaurs 65
million years ago and a large comet or asteroid impact. But he also knows
the odds are heavily stacked against such an event occurring again for
millions of years.

Then Belton read a magazine article that changed his perspective. The
article featured a graph showing how the likelihood of an impact increases
as the size of the impactor decreases. What caught his eye were not the big
dinosaur-killers at one end of the graph, but the far smaller and more
numerous objects at the opposite end. These lesser rocks assault our planet
at the rate of one every few millennia. Just one can deliver enough energy
to destroy a region the size of New York state, killing tens of millions of
people, or generate tsunamis that could devastate coastlines. Given their
frequency, Belton realized there was a good chance -- maybe one in five --
that one would arrive within the next couple of generations of his family.
"That kind of shook me up," he recalls.

Belton has since become an active member of the Near-Earth Object (NEO)
community and advocates paying more attention to potential impactors of
intermediate size. Objects in that range have diameters between about 100
meters and 1 kilometer. The upper limit is the approximate threshold for a
worldwide catastrophe (the dinosaurs were done in by a 10-kilometer body).
The lower limit is about twice the size of the object that exploded over the
Tunguska region of Siberia in 1908,
flattening 2,000 square kilometers of forest and killing entire herds of
animals. The next time an impact rattles Earth, it's almost certain to come
in near the bottom end of that range -- quite possibly before the end of
this century. "Small impactors happen at rates which are of interest in
human terms," says Belton. "I find that a compelling reason to learn more
about them."

This past September, Belton co-chaired a NASA-sponsored workshop in
Washington where he made his case for learning more and learning more
quickly about the near-Earth objects (NEOs) that threaten us. Among the
attendees were representatives from NASA, the Pentagon, the National Science
Foundation, the aerospace industry, and most of the leading scientists
involved in the NEO community. What emerged was a remarkably wide-ranging
discussion -- one that reveals the impact hazard to be a much more
complicated and subtle issue than was apparent a decade ago.

Size Matters

If there is one question that best sums up the current state of thinking
about the impact hazard, it is this: At what size do we need to act? In the
shooting gallery that is our solar system, everyone agrees we are the target
of both cannonballs and BBs. The hard part is deciding where
to drawn the line that separates them.

For practical reasons, that line is now set at 1 kilometer. Not only are
objects of this diameter a global threat (no matter where they hit, we're
all affected to some degree), they are also the easiest to spot. Under a
mid-1990s congressional mandate, NASA currently funds search efforts to the
tune of about $3.5 million per year, including MIT's Lincoln Near-Earth
Asteroid Research (LINEAR) program, JPL's Near-Earth Asteroid Tracking
(NEAT) program, the University of Arizona's Spacewatch survey, and the
Lowell Observatory Near-Earth-Object Search (LONEOS). The explicit goal of
the Spaceguard Survey is to find by 2008 90% of the estimated 1,000 to 1,500
NEOs 1 km or larger (about 630 had been found as of October 22, 2002). "The
existing commitment to 1 km and larger is to retire the risk," says Tom
Morgan, who heads NASA's NEO group. "By the end of this decade we'll be able
to tell you if any of these objects presents a threat in the foreseeable

Within the NEO community there is little doubt this level of search is worth
the effort. "When we first adopted 1 km as a goal, it was just a
no-brainer," says Space Science Institute astronomer Alan Harris, formerly
of the Jet Propulsion Laboratory. "Such objects can wipe out a fair
percentage of Earth's population, and the cost of finding them all in a
decade or so is about $50 million."

But as Harris points out, this lopsided cost/benefit ratio begins to level
off when it comes to intermediate size objects. "If you go to smaller sizes,
the amount of disaster you prevent gets less and the cost of actually
finding them goes up," says Harris. "At some point it's going to crossover.
You basically can't afford the insurance for what you're getting."

While NASA has not yet decided on that crossover point, Harris suspects it
lies somewhere around 200 to 300 meters. The proposed Large Synoptic Survey
Telescope (LSST), an 8.4-meter instrument with a whopping 7 square-degree
field, would spot most of these intermediate-sized NEOs over the course of a
few decades. Like most of his colleagues, Harris supports the $120 million
LSST, which could be built by 2010, but he suggests that searching for 50-
to 100-meter objects might require resources comparable to all the optical
telescopes on Earth.

So, should the estimated 50,000 NEOs in the 200-meter category be ignored
and left to fall where they may? Such an impactor could devastate a region
as effectively as two or three hydrogen bombs, or it could trigger a nuclear
war if it explodes over a nation like India or Pakistan. But from an
actuarial perspective, these relatively small asteroids pose no greater risk
than major earthquakes, hurricanes, and volcanic eruptions. In fact, they
are probably much less of a hazard to humanity than we are to ourselves.

"In the real world we have a limited budget," says Colleen Hartman, NASA's
director of solar system exploration. "The community needs to come up with a
logical analysis for going below 1 km. Only then can we get a buy-in from
the American taxpayer."

Taking Stock

The wide variety of opinions about NEOs grows wider still when the
discussion turns to strategies for investigating them directly. While there
have been successful comet and asteroid flybys -- and even NEAR-Shoemaker's
impromptu landing on Eros -- Belton argues these encounters have not
provided what scientists need most for impact prevention: detailed internal
profiles of different types of NEOs. In Belton's view, the surest way to get
that information is to create a new
category of mission with its own dedicated budget.

Part of Belton's rationale stems from the increasingly obvious fact that
NEOs are a diverse lot. Like meteorites, some are stone, some are iron, and
others are made of a mishmash of material that predates the formation of the

Even the physical structure of NEOs varies. While some asteroids are
monoliths comprising single, solid pieces of rock, others are suspected of
being loosely assembled rubble piles. Curiously, this difference appears to
be size-dependent. An analysis of asteroid rotation rates reveals that
almost all asteroids above 300 meters are spinning slower than the speed at
which a rubble pile would fly apart. Meanwhile, asteroids of smaller size
typically spin much faster. This suggests
larger asteroids are fragile composites while smaller ones are uniform
chunks. If so, it may explain why a surprising number of NEOs -- perhaps 5
to 10% -- come in pairs. Just passing near a planet may produce enough of a
gravitational tug to split a large but weakly cemented asteroid in two.

With so much diversity, no single mission can hope to provide all the data
needed to successfully prevent a major impact. Multiple missions to multiple
targets are required. "The less you know about asteroids, the more likely it
is you'll have to do something drastic to divert an incoming body," says
Erik Asphaug of the University of California, Santa Cruz. "The scenario that
really makes me nervous isn't being bonked on the head by an asteroid. It's
preparing for it in the wrong way."

University of Michigan planetary scientist Dan Scheeres agrees: "Clearly you
would want to have a diversity of missions to go out and look at different
types of asteroid morphologies. If you have a plan for dealing with them,
you want to make sure it will work across the spectrum."

Belton's preferred model for an asteroid mission is based on the
multi-target approach. His proposed spacecraft, called Deep Interior, would
rendezvous with a number of NEOs in succession, using radar tomography and
seismic techniques to map them from the inside out. "The purpose of Deep
Interior is to understand the physical structure of these small bodies so
that you can do something about them," he says.

While developing the proposal Belton found such a mission would be difficult
to achieve for $300 million. This places it beyond the upper limit for a
low-cost Discovery-class mission such as Lunar Prospector or NEAR-Shoemaker.
Hence Belton's proposal for a separate class of mission specifically aimed
at gathering information for impact mitigation. It's an approach that flies
in the face of current NASA practice, where NEO-related projects, like all
space exploration missions, are justified on the basis of science alone.

"I think the public has this perception that the reason we're visiting NEOs
is because they're hazardous," says Asphaug. "But from a science point of
view, when you're proposing a mission, it has been the kiss of death to say
that you're doing it because you want to save humanity."

Colleen Hartman doesn't see a conflict between science and impact mitigation
as mission objectives. "If you do the intellectual experiment of asking what
it is we need to know in order to begin down the path of mitigation, I think
you'd be doing exactly what we're doing," she says.

Diversionary Tactics

The task of deflecting or destroying an incoming NEO makes a great premise
for a Hollywood action flick, but as an engineering challenge it has at
least as much potential to become a dark comedy. The comedy stems from the
fact that it could be remarkably difficult to persuade an asteroid,
particularly a rubble pile, to step aside. Like the Black Knight of Monty
Python fame, such an asteroid can lose bits and pieces during our attempts
to divert it, and still maintain a collision course with Earth.

Part of the problem is that the most powerful tools available are not
particularly well suited to the task. A hydrogen bomb delivers a bigger
burst of energy, pound for pound, than anything else we have at hand. But
energy alone cannot budge a NEO. What is needed is an efficient way of using
the energy of a nuclear explosion to produce momentum. Depending on its
internal structure, an asteroid may absorb the push of a nearby explosion by
deforming -- the same effect you get from punching a pillow. Even worse, an
explosion could break up a NEO into several radioactive chunks. "If we're
going to think about deflecting NEOs," says Jay Melosh of the University of
Arizona, "we have to know how they're going to respond mechanically."

Recently a number of alternate solutions for moving NEOs have been proposed.
They range from mass drivers that fling bits of rock away to create
momentum, giant airbags that can shepherd a loose rubble pile into a new
orbit, or focusing sunlight with giant mirrors to excavate jets of vaporized
rock that push the asteroid in a desired direction. Some schemes even
involve covering a threatening NEO with white chalk or metallic foil to
enhance the tiny recoil it gains from reflecting sunlight. As Harris says,
"We have matured from the idea of just building a bomb and nuking the

For small, hard asteroids, nukes might do the job, although Asphaug finds
the risk of space-borne nukes more sobering than the risk of small NEOs. For
rubble piles, other methods will likely yield more fruitful results. But
whichever solution proves to be the best, says Asphaug, "The main goal
should be to start the process moving now so that when we need to do
something we'll be prepared to do it with plenty of lead time." The lead
time for a mitigation mission, according to a 1997 U.S. Air Force study, is
about 15 years, with a cost of about $1.2 billion.

But as some presenters at the workshop pointed out, time may not be the only
commodity needed to prevent an impact. In the years leading up to an
anticipated collision with Earth, a hazardous NEO may be almost impossible
to reach without expending vast amounts of fuel. "NEOs are the easiest
things in the solar system to get to -- if you get to choose the target,"
says Alan Harris. "But nature may not be so kind when it's choosing the
target for you."

As Harris explains, looking at the average orbital characteristics of nearby
asteroids leads to a sobering conclusion. The total change in spacecraft
velocity (a proxy for fuel use) required to reach and rendezvous with a
typical NEO is in the neighborhood of 20 km per second. "That's just about
what it would take to either land on Pluto or put a spacecraft in orbit
around Pluto," says Harris.

To surmount the velocity barrier, future NEO missions may ultimately depend
on a low thrust propulsion system, such as the ion drive used on the
recently concluded Deep Space 1 mission, or more powerful nuclear-electric
rockets. Harris lists the development of such systems as just one of the key
precursor technologies that must be developed for successful impact
mitigation. In addition, new propulsion technologies can also be applied to
normal space exploration, particularly missions to the outer solar system.


The comedian Emo Philips has a joke about asteroids. If an asteroid is
coming toward you, he says, you don't have to blow it up the way they do in
the movies. You just have to slow it down long enough for your country to
rotate out of the way.

Humor lies in the unexpected, and the unexpected idea in Philips' punch line
is that anyone facing an Armageddon-like impact would choose to pass the
problem onto to someone else. Yet as the Washington workshop made clear, the
real impact hazard is an issue loaded with choices --
and with opportunities to pass the problem along both to other players and
to future generations.

Currently, many organizations and individuals have an interest in various
aspects of the impact hazard, from searching to exploration to developing
methods for deflection. But no single agency, military or civilian, assumes
responsibility for the problem as a whole.

According to Jay Melosh, that many not be a problem. Melosh contends that
some aspects of the impact hazard are overrated and that it's too early for
a centralized Office of NEO Mitigation. The need may only arise if one of
the current search programs finds a sufficiently large
asteroid with a good chance of hitting Earth in the next few decades. But
Melosh concedes, "the current situation is confusing."

>From Belton's perspective, one of the main reasons for mounting the workshop
was to get a good start on putting together a national program for dealing
with NEOs. After all, he says, "You don't just ask someone to move a rock
the size of a city that's traveling at 20 kilometers per
second -- they have to learn how to do it."

But which rocks to move and which to leave alone? That question remains open
after the workshop. It could well be the impactors we are most likely to
encounter in the foreseeable future are so small its not worth the expense
to try to stop them. They may even cost lives, but so will plenty of other
natural hazards over the next century or two. On the other hand, just one
city-buster exploding over North America could swing public and political
opinion from complacency to concern in the same way that terrorism became a
high priority in the aftermath of 9/11.

In many ways, 9/11 offers an apt comparison for scientists working on the
impact hazard. That's because it may not be possible to prepare society for
a hazard for which there is no historic memory. And once it occurs, it is
difficult to put the event in perspective alongside other
more familiar hazards.

For many in the NEO community, perspective was the most valuable outcome of
the Washington workshop. "It's very interesting to see all aspects of the
problem brought together in one place at one time," says Scheeres. "When
that happens you learn to view all components of the risk relative to each
other, and relative to other risks."

Scheeres remarks are a reflection of a new era in the history of the impact
hazard. In less than a generation we're gone from almost no awareness of the
rocks that bombard our planet to treating the discovery of potential
impactors as headline news. In the future, what it more likely is continued
maturing of the issue. The impact risk is real and deserves our attention,
but it must also become part of the way we think about survival on all


NEO News is an informal compilation of news and opinion dealing with Near
Earth Objects (NEOs) and their impacts.  These opinions are the
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>From Michael Paine <>

Dear Benny

Peter Brown's study is very useful for helping to quantify the impact
threat. However I would have to question whether a 1 megaton event would
cause "serious regional damage". Using a formula provided in Rogue Asteroids
and Doomsday Comets, the area of devastation (trees knocked over?) would be
about 400 square km - the size of a small city. That is probably a
pessimistic estimate because the explosion would occur at a higher altitude
than the optimum for a nuclear weapon (on which the formula is based). Of
course the odds of such an explosion occurring over land are about one third
(ie average interval once every 300 years) and the odds of an explosion over
a "populated" area maybe about one tenth (once every 1000 years).

One of the simulations I carried out with John Lewis's software gives an
idea of the potential fatalities from small impactors. The frequency will
change due to the latest estimates of small impactor numbers but the
consequences can be gauged from the simulation.
(see for details)

NEOS 13 to 49m in DIAMETER
7% of events involved fatalities
Average of 10,000 fatalities per fatal event
Mean explosive yield 2.82Mt

In his book Lewis points out that, for this size range, airbursts by stony
asteroids are rarely a concern. Most of the serious fatal events arise from
impacts by iron meteoroids/asteroids that reach the ground or shallow angle
impacts that allow the object to penetrate deep into the atmosphere before

Michael Paine

>From Financial Times, 20 November 2002

If you were worried about Hyperpower America's designs on the globe, wait
till you read this week's recommendations from the joint
congressional-presidential commission on the future of the US aerospace

The commission, a highbrow group of experts and executives, calls on the
federal government to rediscover, among other things, the yen for space

"Future progress . . . will result in new opportunities on earth and open
the solar system to robotic and human exploration and eventual colonisation"
(our italics).

Cleverly, it clothes its dispassionate request for resources in the argot of
the age: national security. Ominously, some would say presciently, it calls
for investment in space-based communications, surveillance and
reconnaissance systems to provide for "planetary defence". Apparently, the
big threat to the US in the coming century comes not from rogue states or
terrorists but from asteroids. Helpfully, the report comes with a striking,
full-colour artist's impression of a huge inter-planetary object hitting the

It is tempting to scoff at all this as another example of Trekkie-Nation, a
testament to the powerful hold that inferior Hollywood television
productions from the 1960s and 1970s have on Americans' imagination. Indeed,
at one point the document seems to betray its intellectual provenance when
it says the US should "boldly pioneer new frontiers in aerospace technology,
commerce and exploration".

But the report needs to be taken seriously - carrying, as it does, the
imprimatur of such as the president of Lockheed Martin and the head of a
prestigious think-tank. It is likely to have some powerful backers - the
Republican-controlled Congress may prove sympathetic to its national
security case and Dick Cheney, the vice-president, is known to have pursued
a keen interest in the commission's deliberations

And some recommendations are worthy - upgrading the US's dangerously
antiquated air traffic management system and improving the regulation of
aircraft production and maintenance.

But it does read suspiciously like a wish-list of lifetime employment
opportunities for US companies. There is much hand-wringing about the
inequities of the uneven global playing field, where US companies are often
bested by competition from subsidised competitors in Europe and elsewhere.
The report is right that the US must press for enforcement of World Trade
Organisation rules in the market. But the cause of free trade will not be
helped by huge US contracts for questionable space projects.

Indeed, it looks as though planetary defence is just a clever way of getting
around WTO rules. After all, who can argue with giving hundreds of billions
of dollars to Boeing and Lockheed when the cause is not increasing their
dominance of global aerospace markets, but saving the very planet from
destruction? It's a subsidy, Jim, but not as we know it.

Copyright 2002: Financial Times Group


>From Commission on the Future of the US Aerospace Industry, 18 November 2002

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