In general, Earth might collide with one of three types of quite common space objects: meteoroids, asteroids, and comets. Of the three, meteoroids are actually of the least concern if they are found to be in a collision course with Earth. Meteoroids, which the International Astronomical Union vaguely defines as larger than an atom but smaller than an asteroid, are small enough that the heat generated by their entry into the Earth's atmosphere is enough to burn them up entirely.
From the surface, the average meteoroid is most distinctive because of the telltale trail or flash it leaves in the atmosphere as it plunges downward. Occasionally the Earth passes through a particularly dense region of meteoroids, and the result is an extensive series of such entries, known as a "meteor shower." For example, the Perseid meteor shower occurs in August every year, as the Earth passes through a debris cloud left on the orbital path of the comet Swift-Tuttle. Swift-Tuttle is a 17-mile-wide comet whose 133-year-long orbit stretches from just within Earth's orbit around the Sun, at its closest point, to well beyond Pluto's at its farthest, roughly 51 AU (51 times as from the Sun as te Earth).
If larger objects - like Swift-Tuttle itself - were to be on a collision with Earth, however, the consequences would be much more severe. Large asteroid and comet impacts are necessarily very rare, since there are far fewer of them in the solar system than there used to be. However, these space objects are both larger and faster than the small meteoroids we watch plunge into the atmosphere every August, so that if they entered the atmosphere, they would not be burned up. An asteroid several dozen metres across burst over Tunguska, Russia in 1908, unleashing a force roughly equivalent to a thermonuclear bomb. One perhaps a kilometre across would cause continental devastation.
Worse yet, one ten kilometres (six miles) across would cause global devastation, probably endangering the survival of the human race and most other large animal species. If it landed in the ocean, the force of the impact would cause tsunamis thousands of feet high. Either on land or at sea, enough dust and debris would probably be thrown up to blanket the Earth in a chilling cloud for years, wiping out plant life and the animals that depend upon it. The asteroid which slammed into the Yucatan Peninsula 65 million years ago, creating the 110-mile-wide Chicxulub crater, was about six miles across and struck the surface with a force equivalent to about 100 million megatons of TNT, orders of magnitude greater than the entire world's arsenal of nuclear weapons.
Such cataclysmic impact events happen only very rarely, perhaps on the order of once every 100 million years or so. However, because even one such collision could threaten the future of human civilization, NASA has taken a keen (if under-funded) interest in identifying near-Earth asteroids which could collide with our planet in the near future. Several surveys attempt to identify and track all near-Earth objects and rule out possibilities with Earth over the coming decades, and ideally coming centuries.
NASA currently uses the Torino Scale to identify objects which might collide with Earth in the future, and, if so, how severe the effects might be. As of June 2010, only one object is listed, at risk level 1 (meaning a very minimal chance of collision, based on preliminary estimates): 2007 VK184, which will pass close by the Earth in 2084. The European organization NeoDYS also lists asteroid 1950 DA as a "2" on the scale, because of a close approach that asteroid will make in 2880. The highest score on the Torino Scale belonged to the asteroid Apophis, which climbed as high as 4 after its discovery in 2004 before astronomers were able to confirm that a close pass expected to occur in 2029 had an acceptable margin of safety.
Despite the best efforts of near-Earth object surveys, without enough funds to do the job comprehensively, there is still a chance that an extremely large asteroid could be on a collision course with Earth without us even realizing it. In 1989, the asteroid Asclepius, large enough to collide with a force equivalent to 600 megatons of TNT, passed through the same position in space which Earth had been in just six hours before. Similarly sized objects are regularly detected by the near-Earth object surveys only after they have swung past Earth and are already heading away.
Still, if a large asteroid was detected on a collision course with Earth, there are a variety of proposals for what to do. Most physicists now agree that the most popular method, as seen in movies like Armageddon - sending nuclear weapons to blow up the asteroid in question - would not be helpful, since the fragments would still be on a collision course with Earth and would, overall, deliver a comparable amount of kinetic energy despite the breakup. Instead, most schemes now involve some method of slowly tugging the asteroid off course, years in advance, so that it misses the Earth altogether. This includes slamming a heavy spacecraft into the asteroid to nudge it off course (a so-called "kinetic impactor"), or parking such a spacecraft on a trajectory parallel to the asteroid, so that its gravity gradually pulls the asteroid's trajectory away from Earth (a so-called "gravity tractor").
In all probability, however, if an asteroid was detected on a collision course with Earth today, the first result would be panic and political chaos. There are no international agreements to determine which nation or nations would be responsible for responding to an incoming asteroid, or how they would do so. While the United States would probably ultimately take the lead in any asteroid defence mission, chaotic international negotiations could delay the efforts considerably. Perhaps the most nightmarish scenario one can imagine is that, even if the technology to deflect an asteroid could have been developed and deployed in time, political institutions spent so long bickering over what to do that, by the time these issues of responsibility were settled, it was too late to actually deflect the asteroid after all.