PLEASE NOTE:
*
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
(1) SATELLITES SPY ON METEORITE EXPLOSIONS
Nature Science Update, 21 November 2002
(2) SMALL BUT DEADLY ASTEROID THREAT DOWNGRADED
New Scientist, 20 November 2002
(3) ASTEROID THREAT REASSESSED
BBC News Online, 20 November 2002
(4) BOLIDES, ROHRABACHER & MITIGATION
David Morrison <david.morrison@arc.nasa.gov>
(5) THE FLUX OF SMALL NEAR-EARTH OBJECTS COLLIDING WITH THE EARTH
P. Brown, R.E. Spaulding, D.O. ReVelle,
E.Taglioferri, and S.P. Worden.
(6) SPACE NEWS OP-ED: PRIORITIZING THE NEO THREAT
Dana Rohrabacher
(7) MOUNTAINS OR MOLEHILLS: SIZING UP THE IMPACT HAZARD
Mercury (Astronomical Society of the Pacific),
Nov/Dec 2002
(8) RISK FROM SMALL IMPACTS
Michael Paine <mpaine@tpg.com.au>
(9) AND FINALLY: BLAMING ASTEROIDS AS "US AEROSPACE IS
SEEKING NEW FRONTIERS
FOR PORK"
Financial Times, 20 November 2002
==========
(1) SATELLITES SPY ON METEORITE EXPLOSIONS
>From Nature Science Update, 21 November 2002
http://www.nature.com/nsu/021118/021118-7.html
Defence data lower forecast for asteroids exploding in Earth's
atmosphere.
21 November 2002
PHILIP BALL
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.
References
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
==============
(2) SMALL BUT DEADLY ASTEROID THREAT DOWNGRADED
>From New Scientist, 20 November 2002
http://www.newscientist.com/news/news.jsp?id=ns99993093
A new analysis of data from US military satellites shows that
locally
devastating impacts by small asteroids are likely only about once
in a
millennium.
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
Scientist.
However, a key uncertainty remains, says Brown: whether large and
dangerous
meteors are concentrated in streams, like this week's
unthreatening Leonid
showers.
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
=============
(3) ASTEROID THREAT REASSESSED
>From BBC News Online, 20 November 2002
http://news.bbc.co.uk/1/hi/sci/tech/2496757.stm
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
Earth.
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
================
(4) BOLIDES, ROHRABACHER & MITIGATION
>From David Morrison <david.morrison@arc.nasa.gov>
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
========================================
(5) THE FLUX OF SMALL NEAR-EARTH OBJECTS COLLIDING WITH THE EARTH
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.
ADDITIONAL MORRISON COMMENTS FROM THE PAPER: The data are based
on
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).
==============================================
(6) SPACE NEWS OP-ED: PRIORITIZING THE NEO THREAT
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.
================================
ARMAGEDDON CAN WAIT: STOPPING KILLER ASTEROIDS
>From The New York Times, 19 November 2002
--snip
this article was on CCNet 19 November 2002
-- snip
===========================
(7) MOUNTAINS OR MOLEHILLS: SIZING UP THE IMPACT HAZARD
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
future."
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
planets.
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
thing."
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.
Dodgeball
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
fronts.
+++++++++++++++++++++++++++++++++++++++++++
NEO News is an informal compilation of news and opinion dealing
with Near
Earth Objects (NEOs) and their impacts. These opinions are
the
responsibility of the individual authors and do not represent the
positions
of NASA, the International Astronomical Union, or any other
organization.
To subscribe (or unsubscribe) contact dmorrison@arc.nasa.gov.
For
additional information, please see the website: http://impact.arc.nasa.gov.
If anyone wishes to copy or redistribute original material from
these notes,
fully or in part, please include this disclaimer.
============================
* LETTERS TO THE MODERATOR *
============================
(8) RISK FROM SMALL IMPACTS
>From Michael Paine <mpaine@tpg.com.au>
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 http://www4.tpg.com.au/users/horsts/sta1047.htm
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
exploding.
regards
Michael Paine
================
(9) AND FINALLY: BLAMING ASTEROIDS AS "US AEROSPACE IS
SEEKING NEW FRONTIERS
FOR PORK"
>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
industry.
The commission, a highbrow group of experts and executives, calls
on the
federal government to rediscover, among other things, the yen for
space
exploration.
"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
earth.
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
-----------
COMMISSION ON AEROSPACE DELIVERS FINAL REPORT AND FINDINGS TO
PRESIDENT BUSH
AND CONGRESSIONAL LEADERS CONCERNING AIR TRANSPORTATION, HOMELAND
DEFENSE,
AND SPACE SECTOR HEALTH
>From Commission on the Future of the US Aerospace Industry,
18 November 2002
http://www.aerospacecommission.gov/releases/111802release.doc
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