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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.
|Joe Veverka maintains that it is not necessary to expand resources to prepare for events that occur on time scales of millions of years: for example, the sun turning into a red giant (like the star shown above) 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.
|The NEAR Shoemaker spacecraft.
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.'
|This color image of Eros was acquired by NEAR's multispectral imager on February 12, 2000, at a range of 1100 miles (1800 kilometers).
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.
|Click here for larger image. Asteroid belt estimated to contain over 1 million asteroids with diameter exceeding one kilometer.
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.
|Artist's concept of Muses-C spacecraft, flying down toward the asteroid.
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.
Related Web Pages
Great Impact: Part I
Great Impact: Part II
Great Impact: Part III
Impact Hazards Website
NASA/JPL Near Earth Object Program