It has happened before; someday, odds are, it will happen again. Sixty-five million years ago a five-mile-wide object hit the YucatAn Peninsula and wiped out the dinosaurs. In 1908 a 200-foot rock blew up over Tunguska, Siberia, flattening thousands of square miles of forest. A meteorite just half a mile wide could throw enough debris into the atmosphere to block out the sun, plunging us into a years-long night. It happens every 100,000 years or so, but it could happen any time.
Is there anything Earth can do to protect itself? The planet has taken a first step, developing a rudimentary early-warning system. Of the roughly 2000 Near-Earth Objects (NEOs) more than half a mile wide, scientists have charted about 300. Two programs in the United States watch for asteroids: Spacewatch, in Arizona, and Near-Earth Asteroid Tracking (NEAT) in Maui. ““They cover less than 10 percent of the sky per month,’’ says David Morrison, director of space at NASA Ames Research Center. ““With the present system … the first you would know is when you felt the impact and saw the fireball over the horizon.’’ In 1991 Morrison chaired the Spaceguard Survey, a NASA study requested by Congress that recommended an array of telescopes, three in each hemisphere, at about $10 million a year–10 times the current budget. That program has not been funded, but telescopes in China and Italy are coming on-line, and a close call might stimulate more action.
An alarm system is necessary but hardly sufficient. With a little notice, scientists are confident they could go to a war footing and knock the intruder out of the sky. ““If you had something coming at you that was a couple of kilometers across, and you only had a year’s warning, then you have a couple of different options,’’ says Greg Canavan, an expert in NEO interception at Los Alamos National Laboratory, ““but basically they all involve nuclear explosives.’’ Planners would prefer not to wait for the alarm to go off; they’re awash in schemes they’d like to see tested (diagram). It’s Ronald Reagan’s Star Wars program, with a vengeance. In one scenario, a rocket carrying 10 tons of lead might knock an asteroid off course, or a series of projectiles might turn it to rubble. The tricky part is making sure the pieces are under 65 feet in diameter, or they could obliterate a city instead. The favorite plan calls for a ““standoff’’ blast, where the nuke explodes next to the object. Done right, the blast and the debris coming off the surface impart enough momentum to change the rock’s direction.
Given enough time, earth-bound defenders might resort to more elegant solutions. A worldwide Spaceguard system could map the skies thoroughly enough to give a century of lead time before an asteroid starts its final approach. ““If you have that much time you don’t really have to nudge an object very hard,’’ says Canavan. The exact method would depend on the object’s size, composition and distance from Earth, but basically once it started on a new heading, it would stay on it. Gentler, more exotic technologies like mass drivers, lasers or solar sails might give the necessary push.
For more than a decade, U.S. and Russian nuclear strategists have talked about their common enemies from space. The United Nations has sponsored conferences, too. ““During the cold war, I was always hoping we would find an incoming threat,’’ says Tom Gehrels of Spacewatch. ““Then we could all go after a common enemy.’’ Today, even fewer obstacles stand in the way of a united front. ““The biggest problem in this whole field,’’ says Canavan, ““has just been getting anybody to take this seriously.’’ In other words, keep watching the skies.
At the moment, Earth is defenseless against a large comet or asteroid headed for the planet. Scientists and other star warriors are exploring a variety of defensive schemes. The ideas under discussion:
