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The mission of the Spaceguard program is to identify asteroids or comets that have the potential of impacting the Earth. In particular the system is to find and warn of impactors large enough to cause global property damage and loss of life. Impacts by large Earth approaching asteroids and comets pose a great threat to life and property. The yearly probability of the Earth being struck by a large object 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. 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. At the request of the U.S. Congress, NASA has carried out a preliminary study to define a program for dramatically increasing the detection rate of Earth-crossing objects.
The scientists working with Spaceguard believe that the greatest risk from cosmic impacts 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 (except 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 potential threat of death from the skies is the only reason the Spaceguard was created and will continue to operate. The cooperation between scientists and astronomers is amazing. Most new discoveries cause a little stir until they are able to be images more and a correct trajectory is obtained. Most new objects are given a Torino scale score of 1 to begin with, but nearly every object is found to be of no threat to the Earth whatsoever.
There is a good article on Wikipedia about the Spaceguard program. Here on Universe Today there are good articles on Spaceguard and Spaceguard Two. Astronomy Cast offers a great episode on the topic of finding an equation to explain all of the forces in the universe.