Planetary Defense. The close passage of asteroid 2004FH (42,000 kilometers from Earth) brings to mind the question as to whether or not anything can be done about one that is going to hit. The short answer is, not much.
Asteroid 2004FH, being only 100 feet across, was small and probably would have detonated in the atmosphere. However, even that sort of impact could ruin a citys day. The Tunguska event of 1908 was either a small icy comet or meteorite that detonated about ten kilometers up with a force of up to 30 megatons. Had that occurred over a major city, the results would have been catastrophic.
Air Force Space Command has been working on the planetary defense mission, sending the Clementine probe up, and testing lightweight technology developed via the Strategic Defense Initiative of the 1980s. The asteroid flyby portion of Clementines mission failed, and Clementine II, a mission to dart asteroids, was cancelled in 1997. However, a NASA probe, NEAR-Shoemaker, launched by a Delta II, entered into orbit around the asteroid Eros on February 14, 2000, and eventually landed on it. This was an important, if unheralded milestone in carrying out the mission of planetary defense. If you can land a space probe on an asteroid, you can land another kind of spacecraft on an asteroid.
The real challenge, however, is finding and tracking an asteroid to get a long-enough lead time to apply that technology. Asteroid 2004FH was detected on March 15th less than four days before its close pass. NEAR-Shoemaker took 28 months to develop and four years to approach Eros. There may also be an issue with booster availability. The Titan 4 is probably the booster that would be used. However, the Air Force has a total of ten left, albeit with no launches scheduled (prior to 2004 there had been as many as five launches a year present capability is eight launches a year, but can surge to twelve). The Delta III booster, which is almost as capable as the Titan 4, failed in two launches (destroyed in 1998 after going off-course, the next flight put a satellite in the wrong orbit. A third launch was with a dummy payload). The Titan II, a refurbished ICBM, and the previously mentioned Delta II could also be used to launch an interceptor.
Should an asteroid be headed for Earth, the interceptor to rendezvous with it would probably be carrying a nuclear weapon. The objective would be to either deflect the asteroid by detonating the nuclear weapon a short distance from the surface of the asteroid or to go for a hard kill. The latter would be accomplished through a nuclear detonation that would either disintegrate or fragment the asteroid by placing the nuclear weapon in contact with the asteroids surface. Harold C. Hutchison (firstname.lastname@example.org)