Since it formed over 4.5 billion years ago, Earth has been hit many times by asteroids and comets whose orbits bring them into the inner solar system. These objects, collectively known as Near Earth Objects or NEOs, still pose a danger to Earth today. Depending on the size of the impacting object, such a collision can cause massive damage on local to global scales. It is an astronomical certainty that sometime in the future Earth will undergo another cosmic impact. There is strong scientific evidence that cosmic collisions have played a major role in the mass extinctions documented in Earth's fossil record. That such cosmic collisions can still occur today was demonstrated graphically in 1994 when Comet Shoemaker-Levy 9 broke apart and 21 fragments, some as large as 2 km in diameter, crashed into the atmosphere of Jupiter. If these fragments had impacted on Earth instead, we would have suffered a global catastrophy!
Once the stuff of science fiction, the danger posed by these intruders in the inner solar system are now the subject of serious scientific investigation. Excellent introductions to the NEOs and the threat they pose to our planet can be found at NASA's NEO website at JPL and the Asteroid Impact web site at NASA's Ames Research Center. Another good source of information about NEO developments can be found on the news page at www.hohmanntransfer.com.
Most of the asteroids and comets in our solar system do not pose any danger to our planet. But, for every thousand or so of those objects there is one with an orbit crosses that of Earth, posing the possibility of a collision at some future time. In 1991 the U.S. Congress directed NASA to conduct workshops on how potentially threatening asteroids could be detected, and how they could be deflected or destroyed. In 1994 the House Committee on Science and Technology directed NASA, in coordination with the DOD, to work with the space agencies of other countries to identify & catalogue within 10 years the orbital characteristics of all comets and asteroids larger than 1 km in and in orbits that cross the orbit of the Earth.
In response to these mandates from Congress, over the decade of the 1990s, several programs have been undertaken to map the orbits of large NEOs that might pose a danger to Earth. These include the Lincoln Near Earth Asteroid Research (LINEAR) project, the Lowell Near Earth Object Search (LONEOS), the Near-Earth Asteroid Tracking (NEAT) project, and the Spacewatch program. These search programs have discovered many thousands of main-belt asteroids and have identified almost 2,000 NEOs.