It seems like something out of a science fiction novel. A giant asteroid hurtling towards the Earth, threatening to destroy all in its path. Well in this case, it wasn't a pulp novel, but a real asteroid heading for a direct collision with our planet. In this instance the asteroid in question "TC3" didn't actually strike the Earth's surface, instead breaking up in the atmosphere over The Sudan on October 7th of this year. The asteroid hit the atmosphere releasing force equivalent to a kiloton of explosive, quite an impressive sight if you happened to be lucky enough to see it in the area. The rest of us can see video clips of the impact online, taken from earth orbiting satellites. This was closely followed on the 17th of October by what was believed to be an actual Impact near the remote community of Wallace Rock Hole west of Alice Springs in the NT Australia. In this case residents reported bright lights, the sound of an explosion and even feeling an earth tremor. The Australian Government earth sciences department Geoscience Australia, has confirmed it was almost certainly a meteorite.
While no one was harmed in these incidents, what would happen if a much larger asteroid was to strike the Earth's surface? The damage could be catastrophic, especially if the asteroid was to land in a populated region. As we can see, asteroid/Earth collisions are something not solely confined to the realm of science fiction, but can and do happen. While it is relatively rare for an asteroid large enough to make it through the Earth's atmosphere without burning up, it is a certainty that it will happen eventually.
The prospect of a cataclysmic event involving an asteroid colliding with the Earth may not keep all of us up at night, but the danger is real and there is research being done into how to protect the planet from such a disaster. The odds of a collision with a sufficiently large asteroid or comet in the near future are slim but it does bear mentioning that according to the U.S. National Aeronautics and Space Association (NASA), there are currently nearly a thousand different objects ( both asteroids and comets) which are classified as PHAs (potentially hazardous asteroids).
The PHA classification includes objects which have a very low probability of ever striking the Earth, but there are enough possible risks to the planet that TC3's close call with Africa has spurred greater public interest in the topic of planetary defense. The first official conference on planetary defense was held last winter in Washington D.C. and served as a forum for astronomers, astrophysicists and other experts in the field to discuss the approaches which should be explored in the interests of protecting the Earth from a possible future asteroid impact. Another conference (the first held by the International Academy of Astronauts) will convene in spring of 2009 and engage in a further discussion of the issues involved in planetary defense.
It is already known that an impact event involving an asteroid or comet can be utterly devastating. There have been scattered recorded impact events throughout human history, but one needs look no farther back than 1908 to what is called the Tunguska event. A large area near the Tunguska River, a remote area of Siberia, was essentially flattened by the air burst of a comet or asteroid disintegrating in the air over the area. That's right, the object never even made it to the surface yet still caused this sort of devastation. Even twenty years later when photographed by Leonid Kulik's expedition to the region, the damage was still readily apparent - entire forested areas had been flattened as if by a giant hand. And all of this damage was caused by an object estimated to be no larger than 100-200 feet across.
The need for some kind of planetary defense strategy is obvious; if a relatively small object such as caused the wholesale destruction seen in Tunguska, what would happen if a larger object was headed for a collision with a densely populated area? Even a Tunguska sized object would be sufficient to destroy most of a major metropolitan area. The result would be a tragedy perhaps unparalleled in human history.
While the development of a viable planetary defense system is still years away; likely many years away, we now possess the necessary technology to watch the skies and have ample warning of any possible impact event. We know that the probability of any kind of large scale impact event is low in the near future - hopefully this will not make mankind complacent about the need to protect ourselves and our planet from such events. The great interest taken by the public in events such as the recent collision with TC3 may help to focus attention on the steps which need to be taken to protect us from the rare but devastating risk of asteroid and comet impact events.
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