Following are the abstract and introduction to a chapter written for the book Asteroids III (edited by William Bottke, Alberto Cellino, Paolo Paolicchi, and Richard P. Binzel), to be published by the University of Arizona Press, Tucson (2003). For the full text of the article including figures and references, click here.
NASA Astrobiology Institute
Alan W. Harris
NASA Jet Propulsion Laboratory
Clark R. Chapman
Southwest Research Institute, Boulder
The small fraction of the asteroids with Earth-crossing or Earth-approaching orbits is of special interest to us because many will eventually impact our planet. The time-averaged impact flux as a function of projectile energy can be derived from lunar cratering statistics, although we have little information on the possible variability of this flux over time. Alternatively, we can use current observations of Near Earth Asteroids (NEAs) to derive the size distribution and flux of impactors. The effects of impacts of various energies can be modeled, using data from historic impacts (such as the K-T impactor 65 million years ago) and the observed 1994 bombardment of Jupiter by fragments of Comet Shoemaker-Levy 9. Such models confirm that the terrestrial biosphere is highly vulnerable to severe perturbation from impacts, so that even such a small event as the K-T impact (by a projectile 10-15 km in diameter) can lead to a mass extinction. Combining the impact flux with estimates of environmental and ecological effects reveals that the greatest contemporary hazard is associated with impactors near one million megatons energy. The current impact hazard is significant relative to other natural hazards, and arguments can be developed to illuminate a variety of public policy issues. These include the relative risk of different impact scenarios and the associated costs and probabilities of success of countermeasures. It is generally agreed that the first step is to survey and catalogue the larger NEAs, and we review the status of the Spaceguard Survey, which has already discovered more than half of the NEAs larger than 1 km diameter, out of a total polulation estimated to be between 1000 and 1200. We compare the efficiency of survey approaches and consider the challenges of international coordination and the problems and opportunities associated with communicating the results with the press and the public. It is also important to reflect on how the impact hazard might be dealt with by both national governments and international decision-making bodies, and to anticipate ways of mitigating the danger if a NEA were located on an apparent Earth-impact trajectory. As the most extreme known example of a natural risk with low probability but severe global consequences, the NEA impact hazard calls for the most careful consideration and planning.
Among the asteroids, those populations that can impact the Earth have a special status. We generally refer to them as Near Earth Asteroids (NEAs), a category that includes many objects (with perihelion out to q = 1.3 AU) that are not currently on threatening orbits (Chapman et al., 1994, Rabinowitz et al., 1994, Shoemaker et al., 1994). Closer to home are the Earth-crossing asteroids (ECAs) or their subgroup the potentially hazardous asteroids (PHAs). In this chapter we will generally use the broader term, NEAs, or even NEOs (near-Earth objects), a term that embraces comets as well. Because of their unstable, planet-approaching orbits, the NEAs have impacted the surfaces of the planets in the inner solar system, including the Earth, influencing both geological and biological evolution. Since there is reason to expect further impacts in the future, the NEAs are a topic with profound political and societal overtones. The impact hazard represents the intersection of asteroid science with public welfare and governmental policy. As Carl Sagan frequently pointed out (e.g., Sagan, 1994), the long-term future of human civilization is linked to our ability to understand and ultimately to control the impact environment of our planet.
It is only during the past two decades that scientists have become aware of the scope of the asteroid impact hazard. This topic was broadly reviewed in 1993, leading to publication of a thousand-page book Hazards Due to Comets and Asteroids (Gehrels, 1994) that remains the primary reference in this field. With surprising speed, this concern has been communicated to governments and the public (e.g., Morrison et al., 1994). Due to the advocacy of NEA researchers (with timely publicity from the collision of Comet Shoemaker-Levy 9 with Jupiter and two feature movies), policy makers and their constituents are aware that impacts are possible. It is less clear, however, that decision-makers are convinced that any major action needs to be taken to deal with the impact hazard. The advocacy role of the science community is pivotal, because the abstract nature of the low-probability threat diminishes the likelihood of a response by either policy makers or their constituents. In this chapter we discuss both the "facts" of the impact hazard and the associated issues of public perception and governmental response.
Much of the material in this chapter is associated with estimating the frequency of impacts and evaluating their consequences, particularly for the Earth's biosphere. In the abstract, the hazard lends itself to such statistical analysis. However, from a policy perspective we do not need precise estimates of either the frequency of impacts or their consequences. We recognize that the actual risk is not statistical; if there is any sizable NEA on a collision course with the Earth, it can be found and the impact predicted decades (or more) in advance. If and when this happens, our attention will focus on that particular object and the circumstances of its predicted impact.
At first, there was considerable skepticism toward proposals for a comprehensive survey to identify any potential impactor decades in advance. Perhaps influenced by their experience with antimissile concepts, many members of the U.S. and Russian defense communities proposed various schemes that could be used to shoot down incoming asteroids given a few days, or even a few hours, of warning (e.g., papers from a Los Alamos workshop collected by Canavan et al., 1993). However, there is no warning system in place or likely to be built that would focus on such a short-term threat. Almost any asteroid that is on an impact trajectory will repeatedly pass close to the Earth on previous orbits, with multiple opportunities for detection. An optical survey system has negligible probability of finding it on its final plunge to Earth, relative to discovery on some previous close pass. The Spaceguard Survey, discussed in detail later in this chapter, is just such a comprehensive optical search, with nearly continuous coverage of the space around the Earth to distances of order 108 kilometers. Already, we have found and calculated accurate orbits for more than half of the thousand-odd NEAs larger than 1 km. None of these poses any impact threat on the timescale of a human lifetime. On the other hand, it is still impossible to say anything about the orbits of the undiscovered ones. This Spaceguard Survey approach also has limited use against long-period comets. Fortunately, these comets constitute a rather small fraction of the total impact threat, and we generally omit them from consideration in this chapter.
While it is highly improbable that a large (diameter > 1 km) NEA will hit the Earth within our lifetimes, such an event is entirely possible. In the absence of specific information, such a catastrophe is equally likely at any time, including next year. Society needs to be prepared to deal with this eventuality. In the meantime, however, the search for possible impactors will inevitably lead to false positives, NEAs that appear for some time to be a real threat. We need to consider the effect of such reports on society. As we discuss in the final sections of this chapter, impact hazard studies can be considered an applied science; that is, science applied to tangible needs of society. In determining an optimum or even advisable hazard mitigation strategy, the reaction of society to scientific information on the hazard should be considered. The NEO community has a social responsibility to ensure that its message is not just heard but comprehended by society at large. Since the hazard knows no national boundaries, it also behooves us to seek solutions that recognize the international constituency with a stake in impact prediction and prevention.
For full text of the paper including figures and references, click here.