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The Earth orbits the Sun in a sort of cosmic shooting gallery, subject to impacts from comets and asteroids. It is only fairly recently that we have come to appreciate that these impacts by asteroids and comets (often called Near Earth Objects, or NEOs) pose a significant hazard to life and property. Although the annual probability of the Earth being struck by a large asteroid or comet is extremely small, the consequences of such a collision are so catastrophic that it is prudent to assess the nature of the threat and prepare to deal with it.
Studies have shown that the risk from cosmic impact increases with the size of the projectile. The greatest risk is associated with objects large enough to perturb the Earth's climate on a global scale by injecting large quantities of dust into the stratosphere. Such an event could depress temperatures around the globe, leading to massive loss of food crops and possible breakdown of society. Such global catastrophes are qualitatively different from other more common hazards that we face (excepting nuclear war), because of their potential effect on the entire planet and its population. Various studies have suggested that the minimum mass impacting body to produce such global consequences is several tens of billions of tons, resulting in a groundburst explosion with energy in the vicinity of a million megatons of TNT. The corresponding threshold diameter for Earth-crossing asteroids or comets is between 1 and 2 km. Smaller objects (down to tens of meters diameter) can cause severe local damage but pose no global threat.
The first step in any program for the prevention or mitigation of impact catastrophes must involve a comprehensive search for Earth-crossing asteroids and comets and a detailed analysis of their orbits. Although current technology permits us to discover and track nearly all asteroids or short-period comets larger than 1 km diameter that are potential Earth-impactors, no general survey has been carried out. Of approximately 2000 Earth-crossing asteroid, fewer than 200 have actually been discovered. Most of what we now know about the population of Earth-crossing asteroids has been derived over the past two decades from studies carried out by a few dedicated observing teams using small ground-based telescopes. Currently several new asteriods are discovered each month. At this rate, however, it will require more than a century to approach a complete survey, even for the larger objects. What is required to assess the population and identify any large objects that could impact the Earth is a systematic survey that
effectively monitors a large volume of space around our planet and detects these objects as their orbits repeatedly carry them through this volume of space. In addition, the survey should deal with the long-period comets, which are thought to constitute about 10 percent of the flux of Earth impacts. Long-period comets do not regularly enter near-Earth space; however, nearly all Earth-impacting long-period comets could be detected with advance warning on the order of a year before impact with the same telescopes used for the asteroid survey.
At present no asteroid or comet is known to be on a collision course with the Earth. The chances of a collision within the next century with an object 1 km or more in diameter are very small (less than 1 in a thousand). But such a collision is possible and could happen at any time. If we did have sufficient warning, however, the incoming object could be deflected or destroyed. Cosmic impacts are the only known natural disaster that could be avoided entirely by the appropriate application of space technology. That is one reason this subject generates so much interest.
