From: LARRY KLAES (ljk4_at_msn.com)
Date: Mon Mar 03 2003 - 05:52:18 PST
----- Original Message -----
From: Benny J. Peiser 21111400037537
Sent: Monday, March 03, 2003 5:04 AM
To: cambridge-conference
Subject: THE GREAT IMPACT DEBATE, PART 4: ON A COLLISION COURSE FOR EARTH
THE GREAT IMPACT DEBATE, PART 4: ON A COLLISION COURSE FOR EARTH
>From AstroBiology Magazine, 3 March 2003
http://www.astrobio.net/news/modules.php?op=modload&name=News&file=article&s
id=389&mode=thread&order=0&thold=0
Summary: Our "Great Debate" series brings together a group of scientists who
are experts on asteroids and comets. This final debate concerns how we could
respond to the threat of an asteroid heading for Earth, and what sort of
projects would best serve future NEO goals.
Participants:
Clark Chapman - scientist at the Southwest Research Institute's Department
of Space Studies, in Boulder, Colorado. Member of the MSI/NIS
(imaging/spectrometer) team of the Near Earth Asteroid Rendezvous (NEAR)
mission to Eros.
Alan Harris - senior research scientist at the Space Science Institute, an
affiliate of the University of Colorado at Boulder.
Benny Peiser - social anthropologist at Liverpool John Moores University in
the UK. He has written extensively about the influence of NEO impacts on
human and societal evolution.
Joe Veverka - professor of astronomy at Cornell University in Ithaca, New
York. Principal Investigator for NASA's Comet Nucleus Tour (Contour)
mission.
Don Yeomans - (debate moderator) - Senior Research Scientist at NASA's Jet
Propulsion Laboratory in Pasadena, California, and manager of NASA's
Near-Earth Object Program Office.
Don Yeomans: As mentioned in last week's debate, an asteroid or comet larger
than a kilometer colliding with the Earth would be a very rare event. One
would only expect a collision of this type to occur every several hundred
thousand years. Nevertheless, it has happened before and it could happen
again in the near future. In the unlikely event that a sizable near-Earth
object (NEO) is found to be on an Earth-threatening trajectory, would we
have the technology to deflect the object in time so that it would pass
harmlessly past the Earth?
Clark Chapman: I think pieces of the technology are there. We have rockets
that can launch the deflection hardware, and there are well-tested means to
deliver and operate this hardware in the vicinity of a low-gravity body. In
fact, one spacecraft already has landed on an asteroid - the NEAR-Shoemaker
spacecraft landed on the asteroid "Eros" on Valentine's Day, 2001.
What has not been done is to put all the technological tools in our toolbox
together and make them work in the strange, unintuitive physical world of an
asteroid. Also, much more thinking is necessary about the diversity of
asteroid properties. We have sufficiently energetic tools to push on an
asteroid and move it, but we need to consider how we might attach any
deflection mechanism to an NEA and push it in the direction we want.
Not every one-kilometer NEA will be easy to divert. Such a body is very
massive, and a long lead-time of perhaps decades would be necessary to
succeed, even without employing new or potentially dangerous technology.
But, fundamentally, we probably could do it, provided there was sufficient
motivation: namely, an asteroid headed our way, destined to collide with
Earth some years or decades hence.
Joe Veverka: I believe that we currently are not in a position to protect
Earth from impacts by one kilometer-sized objects. The technology required
to carry out such a task exists, or it can be developed, but the effort
would be colossal by any standards.
I would argue that the question, while of academic interest, is not very
relevant from a practical point of view. In such a discussion, it is
essential to define a "horizon of concern." In other words, how far into the
future does it make sense to worry about something and take precautions?
The answer might depend on where and when we live, but right now any
planning that society does hardly extends more than a few decades into the
future, and at most perhaps to a few centuries. Planning for events that
occur on time scales of hundreds of thousands to a few million years just
doesn't make practical sense. Nor is it necessary to expend resources to
protect ourselves against events that occur on time scales of a million
years. For instance, few of us lose sleep over the fact that the sun will
turn into a red giant some 5 billion years from now.
It is only when we get down to impacts that occurred early in the 20th
century that it makes sense to discuss mitigation - for example, the
Tunguska explosion of 1908 that has been attributed to a meteoroid 60 meters
in diameter. But even for these events, which might occur every few hundred
to a thousand years, the cost of a mitigation policy must be weighed against
the likely benefit.
We have to keep in mind all of the other ways resources could be used to
benefit society in preserving and improving life. Even in the case of
Tunguska-type events, there are far more urgent and potentially more
beneficial uses of our resources than developing a system to protect us from
impacts by bodies a hundred or so meters across. Almost certainly more
people will die from wars, cancer, and even traffic accidents during the
next few hundred years than are likely to die from the next Tunguska.
Clark Chapman: Joe Veverka makes a major error when he compares the time
scale for a large asteroid collision with the time scale for the sun turning
into a red giant. There is ZERO chance that the sun will turn into a red
giant during the next century, or even the next billion years, according to
our robust understanding of the physics of stellar evolution. But asteroids
strike AT RANDOM. If asteroids struck like clockwork, a kilometer-sized body
every few hundred thousand years for example, then the analogy might work.
But there is roughly a one-in-several-thousand chance that a kilometer-sized
asteroid will strike during the 21st century. One could even strike
tomorrow.
One might well question what level of risk we are willing to accept by doing
nothing about one-kilometer asteroids. Joe should argue that he's willing to
accept the risk, given other higher priority concerns. But he's wrong, and
he hurts his case, to make the classic error people make about lightning
strikes and hundred-year floods: "the next one can't happen again soon." It
has nothing to do with a "waiting time" or being "over the event horizon."
Given that civilization might hang in the balance, we really should think
about this issue, despite the low probability that we will have to meet this
challenge during our lifetimes. Of course, until such an asteroid is
discovered, there certainly are weightier threats facing society, as Joe
Veverka argues.
Benny Peiser: In contrast to other, more frequent natural disasters such as
earthquakes, volcanic eruptions, tropical storms, tsunami, etc., we have
very little understanding of or experience with NEO impacts. Thus, we can
only speculate about the effectiveness of planetary protection. The question
as to whether or not we have the technology necessary for effective NEO
protection ultimately depends on the warning time we are granted by an
asteroid or comet on a collision course with Earth.
At present, we do not have any protection against a NEO about to collide
with Earth in, say, one or two years time. Estimates for the time it may
take to assemble an operative deflection system currently range from 30 to
70 years. With ongoing advancements in space and defense technologies, I am
confident that this estimate will gradually come down further.
But the real problem, as I see it, is not so much whether we have the
theoretical know-how for NEO deflection. Instead, the key challenge we will
face at some time in the future is when a NEO is found to have a significant
chance of hitting Earth. In the absence of any experience, we will be
confronted with an unprecedented crisis situation. Such an impact crisis
could happen tomorrow or it could occur in 300 years time. It could be a
small asteroid, a medium-sized comet, or an even larger object. Happily,
chances are extremely small that this will happen soon. Nonetheless, such an
event will transpire one day. And when it happens, it will be unprecedented.
By contemplating what may happen in the event of a small impact, we need to
recognize the psychological and social implications of traumatic events and
the emotional and irrational reactions they can activate. The social effects
of an impact are all too often ignored or underestimated, but they could be
extremely grave. Such effects perhaps could be even more disruptive than the
physical damage and economic costs. Some people may experience problems
dealing with even a small impact due to its totally random and "terrorizing"
nature. It will certainly stir up anxieties - not least because the impact
is likely to be blown out of proportion by the mass media. Some people will
blame their governments, space agencies, and astronomers for failing to
protect them from cosmic disaster. Then it will not be sufficient to issue
the mantra of 'statistical risk estimates.'
Don Yeomans: If you were given the means, what scientific or engineering
project (or any other endeavor) would be highest on your list to better
understand these near-Earth objects, or to possibly reduce the threat that
these objects pose to Earth?
Clark Chapman: The theme of NEO impacts with Earth and other planets has a
strong scientific legitimacy, even if dangerous impacts in our lifetimes are
unlikely. I believe that asteroids and comets are of exceptional importance
in the scientific understanding of the solar system. Yet it took 25 years
from the first asteroid mission study before the first dedicated asteroid
mission (the NEAR Shoemaker mission to Eros) was accomplished.
I believe future studies of NEOs should combine the purely scientific
interest in these bodies with the public interest of impact hazard
mitigation, as well as the potential utilization of asteroid materials. This
includes theoretical studies, Earth-based telescopic observations, and
space-based missions of increasing sophistication.
Joe Veverka: To assess the risk that NEOs pose to Earth, we not only need to
know how many there are and how big they are, but we need to know what they
are made of and how they are put together. Telescopic observations have done
a splendid job in finding what's out there, and a pretty decent job in
determining how big these bodies are. The next important step is direct
exploration by spacecraft of carefully selected NEOs to determine their
precise geochemistry and internal structures. Missions are needed to return
a sample from each of these bodies for detailed geochemical analyses and to
determine the average density of each object. Such samples would give us
accurate data on what these bodies are made of and how they are put
together. This information will be essential for evaluating the risk and
planning a mitigation strategy, if needed.
Alan Harris: I have always felt that, given the very low chance that
anything out there "has our name on it," we should not expend resources on
impact mitigation unless we discover something to mitigate against. However,
I would soften this position in the same way that one might buy a "whole
life" insurance policy rather than term life insurance, so that even if you
don't die in the prescribed term of the policy, you at least have something
like a savings account in return. Therefore I think we might favor research
programs that have intrinsic scientific interest and that also contribute in
some way to potential mitigation, if that should ever come to be necessary.
The landing of NEAR-Shoemaker on Eros already is in this category: a
valuable practice exercise for something almost certainly necessary if we
were to need to deflect an asteroid, and also scientifically valuable in
itself.
Another example could be a rendezvous and landing mission to an asteroid to
probe the interior structure of an asteroid - rubble pile, monolithic rock,
or what? This exercise would give us further insights into possible modes of
deflection. Or perhaps we could implant transponders on an asteroid in order
to practice precision orbit tracking, making sure we could monitor the
progress of a deflection maneuver. The scientific payoff, even if the
deflection technology were never needed, would be to look for wobbles in the
asteroid rotation that could help probe the interior of the body. We also
could look for very slight variations in the orbit, perhaps due to radiation
pressure, and that would help us understand the evolution of small bodies
into Earth-crossing paths.
I remain opposed to major defense programs to protect against an
undiscovered "enemy" asteroid that has only a one in ten thousand chance of
existing. I believe that the danger of having such a "defensive" system,
which almost certainly would involve rockets and nuclear bombs, exceeds the
security it provides. However, any part of the preparation that can be
accomplished at modest cost might be justified, so long as it will yield a
scientific return as a side benefit.
Benny Peiser: I'm glad to hear that Al has softened his position on future
efforts to boost the study and our understanding of impact mitigation. I
have always been skeptical of the customary NASA view that no funds should
be provided for impact mitigation research until we are faced with an
impending impact threat. This sounded too unreasonable to me. Traditionally,
the main argument has been that no supplementary resources should be
allocated to examine a highly implausible scenario. But nobody is asking for
additional funds.
Space agencies around the world are already spending billions of dollars
each year on space exploration and scientific research. As Al points out,
the landing of the NEAR-Shoemaker spacecraft on Eros shows that scientific
space missions easily can be designed so they include mitigation aspects
without the need for additional funding.
Future missions should progressively incorporate NEO and impact mitigation
components. This would ensure that we gradually learn to decode and handle
the multifaceted compositions of asteroids and comets. Such a policy would
be the best remedy to reassure an increasingly concerned public that the NEO
and space communities are taking adequate steps to take control of our
cosmic environment.
In the next twenty-five years, I would like to see the first space mission
aim to nudge an asteroid out of its orbit. After landing a spacecraft on an
asteroid (NEAR-Shoemaker), striking at a comet (Deep Impact) and bringing
back samples from an asteroid (MUSES C), the most captivating, and certainly
the most popular NEO mission ever would be an attempt to shift a
medium-sized space rock out of its orbit. In many ways, this would be the
first attempt in all of history to change the course of cosmic nature.
Clark Chapman: A NASA-sponsored workshop on "scientific requirements for
mitigation" last autumn went a long way towards demonstrating that there is
great similarity between the kinds of missions one would fly to study the
nature and origin of NEAs, and those that one would fly to learn how to push
on an asteroid, if it were ever necessary to do so.
A focused motivation to try to move a small NEA in a controlled manner in
the next dozen years, as advocated by the B612 Foundation, could reap an
enormous scientific pay-off as well as take a major step toward
understanding the practicalities of how to move a such a body. If the
endeavor involved "bombs in space," as Al Harris fears, then I would be
hesitant too. But last autumn's workshop made it clear that the appropriate
technology in most instances involves long-acting, low-thrust propulsion.
This is in order to move the asteroid gently, in a controlled fashion, and
not risk breaking the body up into a dangerous swarm of pieces. I don't see
such technology as being especially dangerous, although international
oversight of such endeavors will always be the prudent way to go.
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