Assuming that the human race does not destroy itself through nuclear war, chemical pollution of the environment, global climate change, or a collapse of the food supply, the next likely natural threat to our species will probably come from outer space, in the form of a large asteroid. An asteroid several miles wide, if it slammed into the surface of our planet, would unleash a wave of destruction threatening most large life forms - indeed, it is believed that one such impact, at Chicxulub, Mexico, destroyed the dinosaurs 65 million years ago. Quite simply, humans should worry about an asteroid hitting the earth because that sort of impact could occur again, and this time, it could be we rather than the dinosaurs who face the end of our species.
- About Impact Events -
Scientists refer to a collision between the Earth and an asteroid - or any other space object, for that matter - as an impact event. Minor and insignificant impact events occur every day as small numbers of meteors hit the Earth. However, larger asteroids and comets are a much greater threat due to their greater mass and potentially greater velocity. An object just a few dozen metres wide could release energy equivalent to a nuclear explosion, as occurred over Tunguska, Russia, in 1908. Had this occurred over Europe instead of Siberia, the consequences for Western progress would have been immense.
Larger objects get progressively more devastating. An asteroid a mile wide would be capable of devastating large regions. An asteroid several miles wide would unleash global devastation, including enormous tsunamis if it strikes the oceans (which it probably would, given that they cover most of the Earth's surface), a shockwave racing around the globe, and so much debris thrown into the atmosphere that they would be a temporary but dramatic global cooling event.
In the distant past, these large asteroids have been blamed for major extinction events, like the extinction of the dinosaurs (and much other large life) at the end of the Cretaceous period, 65 million years ago. In that case, scientists have even pinpointed the likely candidate: an asteroid which slammed into Chicxulub, on the Yucatan Peninsula. Others of the half-dozen major extinction events in Earth's history may also have been caused in part by asteroid impacts, although no commonly accepted evidence for this has yet been put forward.
We are fortunate that, thanks to the massive gravitational influence of Jupiter, the density of asteroids in the solar system is much lower than it probably would be otherwise, so that these major impacts occur millions of years apart. Nevertheless, smaller impacts occur much more frequently, and would still have devastating consequences if they occurred in highly populated regions. When the fragments of Comet Shoemaker-Levy 9 crashed into Jupiter several years ago, none was more than a mile across, but the largest single impact unleashed an explosive force of millions of megatons of TNT, equivalent to several hundred times humanity's entire nuclear arsenal.
- Monitoring and Deflecting Space Objects -
Today, locating and tracking near-Earth objects in space is a relatively low priority for space programs around the world, but the United States is developing a growing catalogue of objects large enough to potentially threaten life on Earth. As they are identified, new objects are assessed and their future orbits compared to Earth's to judge the chances of an impact event. Objects are then charted on the Torino Scale, a ten-point scale ranging from 1 (there is a very low chance that an object could hit the Earth, but further studies will probably rule this out) through to 10 (an impact capable of a global extinction event will certainly occur in the near future).
Right now no known object has a score higher than 1, although the asteroid Apophis reached as high as 4 several years ago because of the chance of a collision in 2029 or 2037, since ruled out. There is still a chance that we would not realize an impact was going to occur until shortly before the asteroid entered the atmosphere.
If an asteroid is confirmed to be on a collision course with Earth, then attention would turn to how to deflect it. (Destroying it, as is attempted in movies like Armageddon, is usually dismissed: even if we could reliably break up a large asteroid with nuclear explosives, the resulting debris would consist of several extremely large chunks, most or all of them still on a collision course with Earth.) Asteroid deflection technologies are entirely theoretical today, but most involve some way of deflecting the asteroid slightly off course, so far in advance that it ultimately misses the Earth by a comfortable margin. This could involve a so-called kinetic impact (a heavy but non-explosive spacecraft slamming into the asteroid to knock it off course), a gravity tractor (which parks in an orbit next to the asteroid and slowly pulls it off course via gravity), or other more exotic technologies.