The majority often reside in the orbit between Mars and Jupiter. Comets, also remnants of the early solar system, are partly comprised of ice and reside in the outer reaches of our solar system and beyond. Occasionally a comet wanders into the inner solar system. The asteroids between Mars and Jupiter can be as large as several hundred miles across. Of the 2,000 NEAs tracked to date, most are between one and ten miles in diameter.
Comets are typically a few miles in diameter.
The U.S. government has considered a project that would use existing scientific and military space surveillance systems to survey the sky to catalog and watch for objects on a collision course with Earth. These efforts are a good start, but NSS members should insist that the U.S government supplement them with many more telescopes and a much more systematic approach. The key is to be able to speed up the process of identifying potentially threatening objects to within 10 or 20 years, as compared to 200 years at our current rate. NSS should support additional scientific and exploration missions to these objects to study their composition and vulnerabilities. Information provided through such study is vital to determining the best option for mitigation, which currently include destruction or deflection through detonation of a missile or nuclear device, or vaporization.