Should an extinction-level asteroid begin plummeting toward Earth, it will take more than that Aerosmith song they play at proms to save the planet. Allow astronomer Neil deGrasse Tyson to elaborate on humanity's options for survival.
The chances that your tombstone will read "Killed by Asteroid" are about the same as they'd be for "Killed in Airplane Crash."
Solar System debris rains down on Earth in vast quantities — more than a hundred tons of it a day. Most of it vaporizes in our atmosphere, leaving stunning trails of light we call shooting stars. More hazardous are the billions, likely trillions, of leftover rocks — comets and asteroids — that wander interplanetary space in search of targets.
Most asteroids are made of rock. The rest are metal, mostly iron. Some are rubble piles — gravitationally bound collections of bits and pieces. Most live between the orbits of Mars and Jupiter and will never come near Earth.
But some do. Some will. More than a thousand known asteroids are classed as "potentially hazardous," based on size and trajectory. Currently, it looks doable to develop an early-warning and defense system that could protect the human species from impactors larger than a kilometer wide. Smaller ones, which reflect much less light and are therefore much harder to detect at great distances, carry enough energy to incinerate entire nations, but they don't put the human species at risk of extinction.
Every few decades, on average, house-sized impactors collide with Earth. Typically they explode in the atmosphere, leaving no trace of a crater. Once in about a hundred million years, though, Earth is visited by an impactor capable of annihilating all life-forms bigger than a carry-on suitcase.
One killer asteroid we've been monitoring is Apophis, which is large enough to fill the Rose Bowl. On Friday the 13th, April 2029, it will dip below the altitude of our communication satellites. If its trajectory on that day passes within a narrow range of altitudes called the "keyhole," then the influence of Earth's gravity on its orbit will guarantee that seven years later, in 2036, on its next trip around the Sun, the asteroid will hit Earth directly, likely slamming into the Pacific Ocean. The tsunami it creates will devastate all the coastlines of the Pacific Rim. If Apophis misses the keyhole in 2029, we'll have nothing to worry about in 2036.
A more recent discovery, half the size of Apophis, is expected to pass Earth at a distance of a million miles in 2023 and ten million miles in 2028, has been stirring up the scaremongers but rates only a 1 on the 1–10 scale of impact hazards. Unscarily named 2011 AG5, it will become much more visible and trackable during 2013. Earth's gravity could conceivably convince it to collide with us in 2040, but NASA deems that a remote chance.
Some people would like to blow potentially hazardous rocks out of the sky with a nuclear bomb. Others would deploy a radiation-intensive neutron bomb (the Cold War–era bomb that kills people but leaves buildings intact) to induce a recoil and alter the asteroid's orbit. A kindler, gentler approach would be to nudge it into a different orbit with slow but steady rockets that have somehow been attached to one side - or with a solar sail, which harnesses the pressure of sunlight for its propulsion.
The odds-on favorite solution, however, is the gravitational tractor. This involves parking a probe in space near the killer asteroid. As their mutual gravity draws the probe to the asteroid, an array of retro rockets fires, instead causing the asteroid to draw toward the probe and off its collision course with Earth.
Saving the planet requires commitment. First we have to catalogue every object whose orbit intersects Earth's, then task our computers with carrying out the calculations necessary to predict a catastrophic collision hundreds or thousands of orbits into the future. Meanwhile, space missions would have to determine in great detail the structure and chemical composition of killer comets and asteroids.
If humans one day become extinct from a catastrophic collision, we would be the laughing stock of aliens in the galaxy, for having a large brain and a space program, yet we met the same fate as that pea-brained, space program-less dinosaurs that came before us.