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. There is no doubt that sometime in the future Earth wil suffer another cosmic impact; the only question is "when?". 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 hit Earth instead, we would have suffered global catastrophes of the kind that inspire science fiction movies:
The dangers 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 the following websites:
Most of the asteroids and comets in our solar system pose no danger to our planet. But, for every thousand or so of those objects, there is one with an orbit crosses that of Earth, raising the possibility of a future collision. 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. This mandate led to the Spaceguard Survey Report in 1992. 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 and catalogue within 10 years the orbital characteristics of 90% of all comets and asteroids larger than 1 km and in orbits that cross the orbit of Earth.
Following the 2003 NASA report from the Near-Earth Object Science Definition Team, Congress went even further and in 2005 assigned NASA the task of detecting 90% of near-Earth objects with a size greater than 140 meters in diameter by the year 2020.
In response to these mandates from Congress, several programs have been undertaken to map the orbits of large NEOs that might pose a danger to Earth. These include the following projects:
These search programs have discovered hundreds of thousands of main-belt asteroids, and have identified thousands of NEOs. They have made great progress toward meeting the Congressional mandate and have cataloged most, but not all, of the 1-km and larger NEOs -- the ones that are most likely to produce a global catastrophe, such as a mass extinction should they collide with Earth. Pan-STARRS will complete the survey of all 1-km diameter objects, and will detect most of the dangerous objects down to 300 meters in diameter -- objects that can cause major regional catastrophes should they hit the Earth.
What can be done if one of these surveys finds an asteroid on a collision course with the Earth? Scientists and engineers at the B612 foundation are looking at ways of using a spacecraft to gently change the orbit of an asteroid. One promising approach is the "gravity tractor" invented by NASA astronauts Ed Lu and Stan Love.
Next: Pan-STARRS and NEOs