It is not a question of if, but rather a question of when the next large asteroid
will strike Earth. The readily apparent cratering record of the Moon and the
increasingly revealed cratering record of Earth attest to a long history of
cosmic collisions. That history continues today, as dramatically shown by the
splash of comet Shoemaker-Levy 9 into Jupiter in 1994. The resulting Earth-sized
blemishes on the tops of Jupiters clouds attest to the wallop packed even
by relatively small impacting bodies. Small impacts on our planet, such as the
one that occurred over Tunguska, Siberia, in 1908, may happen on average once
per thousand years. A larger impact, one capable of severely affecting the climate
with potentially devastating effects for civilization, may occur on average
every million years or so. Much more rare, perhaps an average of every 100 million
years, are impacts posing the threat of mass extinctions.
There is both good news and bad news in the fact that geologic time scales describe the frequency of major impacts. The good news is that no contemporary Geotimes readers are likely to witness or suffer from a devastating cosmic impact during their lifetimes. Thus we scientists and our current political leaders can be happily complacent about asteroid impacts and very likely get away with it! The bad news is that there are no guarantees. Just because an event is unlikely doesnt mean it wont happen today, next week, next year, or any time in this century. If we choose to ignore the asteroid issue, we are living dangerously and passing the problem down to future generations.
Thus from a standpoint of prudence or inevitability, there are asteroids in our future. Current technology now has the capability to transform the uncertainties and probabilities of the asteroid problem into deterministic answers. We can discover and track the largest asteroids near the Earth and determine whether any are on a collision course in the coming centuries.
Thanks largely to the pioneering push by geologist Gene Shoemaker, NASA adopted the current Spaceguard Goal to discover 90 percent of the near-Earth asteroids larger than 1 kilometer by the year 2008. (One kilometer is the estimated size capable of causing major climatic effects.) So far, about one-half of the estimated 1,100 of these objects have been discovered, occasionally creating sensational headlines when preliminary orbits show small (but still existing) probabilities for a collision during a future pass by Earth.
Fortunately, further measurements allowing refined orbits have, to date, removed all substantial threats from all known objects. With each discovery and orbit determination ruling out any foreseeable hazard, we are gaining assurance that the odds remain in our favor for not suffering a global impact catastrophe in our lifetimes.
While the risk of a global- or civilization-threatening impact is now being retired, the risk remains for the more frequent smaller-scale impacts capable of causing local or regional damage. A cost/benefit analysis recently completed by a NASA-appointed team recommends pushing the survey limits well below 1 kilometer, down to a size of 140 meters. This study team finds that dropping the discovery goals down to this size reduces the total asteroid risk (over all sizes) by 90 percent. Achieving this goal is cost effective when considering the potential worldwide casualties from both land impacts and tsunami-generating ocean impacts. One or more dedicated large telescopes (with apertures of 4 meters or more) operating for several decades could complete the job. Completing the census substantially faster could require performing the survey from space.
Fortunately, the most likely outcome of current and expanded surveys is that no sizeable object will be found to be on a collision course with Earth over the next century. In essence, we are buying cosmic insurance in the form of knowledge that our near-term future is safe. Unfortunately we can be certain that surveys will yield false positives, with asteroids impacting the media but not the actual Earth. Two types of false positives will occur: Type I is already a familiar scenario, in which a preliminary orbit shows a future close encounter with Earth, where the normal measurement uncertainties initially do not rule out a collision; Type II is where an object is discovered to be on a definitive collision course, but the size of the object is too small to penetrate the atmosphere intact and cause any substantive harm.
A key step astronomers must take to minimize media frenzy over false positives is to organize a central information source, similar to the National Hurricane Center. The role of this center will be to convey with a single authoritative voice the consensus findings of international experts regarding whether any asteroid close encounter merits public or governmental concern. Continuing with the present situation of multiple information sources, where even just a subtle difference becomes polarized by the media, serves only to erode the credibility of professional astronomers.
If we allow ourselves to think the unthinkable, where a sizable object is discovered to be on a definitive collision course, for example 30 years from now, what would we do? In spite of its lack of Hollywood appeal, deflection rather than destruction is the best approach. But pushing on an asteroid may be extraordinarily tricky, depending on whether the body is intact or severely fractured. Some models for asteroid interiors even suggest they may be rubble piles, reassembled blocks having no coherency between them, held together only by their mutual gravity. Slow and gentle propulsion, such as that provided by an ion drive or harnessed from solar radiation pressure, holds the best promise. Yet for these slow and gentle schemes to work, time must be on our side. The only way to put time on our side is for the surveys to move forward now.
Our next step, in concert with surveys, is to understand what these objects are: Know thy enemy. Here is where scientists, especially geologists and geophysicists, have their role. The questions we as scientists ask seeking to understand the compositions and internal structures of these space mountains are the same questions that must be answered in order to achieve effective mitigation.
While we might relish the idea of geology field trips to these worlds, neither the science alone nor the combination with the impact hazard concern justifies the cost of sending humans (rather than robotic spacecraft) to do the job. My view only changes if asteroids become a human destination as part of a stepping stone strategy for operationally testing people and equipment with the ultimate goal of missions to Mars. Many near-Earth asteroids provide destinations for six-month to one-year missions in interplanetary space, shorter than the risk of multi-year missions to Mars. Resource utilization may be another driver for going to the asteroids, where low launch costs and economic viability are essential for this future path. Completed surveys will reveal accessible and promising destinations, making our asteroid future more likely filled with friends than foes.