Wired Science

Will an Asteroid Hit Us?

Having the good fortune to uncover a meteorite buried in the ground in Kansas is pretty awesome, but it implies the obvious: At some point, a huge asteroid crashed into Earth. And if it has happened before, occasionally with devastating consequences—it wiped out the dinosaurs, after all—then we can count on it happening again.

The big questions are when and how bad it will be. Sure, a meteorite may have wiped out much of life 65 million years ago, but that doesn't mean it's going to do the same to us anytime soon. But according to a number of astronomers, there is a real risk that at least a regionally disastrous impact could happen sometime in the next few hundred years. And while the good news is that we would most likely have the ability to deflect an imminent hit, the bad news is that we just don't know what's coming—because we're not looking very vigorously.

"In the next 50 years, there's a 1 percent chance that an object [of substantial] size will hit the earth and hurt people. But in the next several thousand years, there's only a 1 percent chance that it won't happen," says Robert Gold, a space engineer at the Johns Hopkins University Applied Physics Laboratory . "This is the greatest natural threat to the survival of mankind."

To be fair, NASA has undertaken a few efforts in recent years to address our limited knowledge of what's out there. There are an estimated 1,100 near-Earth asteroids greater than 1 kilometer in diameter lurking out in space, and the Spaceguard Survey, a NASA-sponsored program initiated in 1998, will have identified about 80 percent of them by the end of next year. Of those they have already found, none, thankfully, are on a collision course with Earth.

But it's not just large asteroids that we need to worry about. Those larger than just 25 meters in diameter can penetrate Earth's atmosphere, and the dense little rocks can do a lot of damage even if they don't make it all the way to the ground. A meteor that exploded about five to 10 miles above ground in Siberia in 1908 felled 80 million trees over 2,150 square kilometers—but as it was over an unpopulated area, no one was known to have been killed. And asteroids that hit the ground, even if they don't hit anyone, can stir up so much dust as to affect light levels and temperature and even destroy ecosystems. "You wouldn't necessarily have a great deal of death, but you'd have a great deal of destruction," says Noam Izenberg, a planetary scientist at the Johns Hopkins Applied Physics Laboratory—what might be "the equivalent of a major war."

Despite the fact that there are about 3 million asteroids greater than 25 meters in diameter closely surrounding us—and about 100,000 over 140 meters—there is currently no effort to identify them. In 2005, Congress gave NASA one year to come up with plans for a survey that would identify 90 percent of all 140-meter-wide or larger near-Earth asteroids by 2020. NASA came back to Congress in 2006, however, to say that while it could complete such a project using a combination of ground- and space-based telescopes, it would cost on the order of $1 billion and is, therefore, too expensive to implement. Since then, Congress has made no recommendation or decision on the matter, though it will hold a congressional hearing on the subject on Nov. 8.

Clark Chapman, a planetary scientist at the Southwest Research Institute in Boulder, Colo., argues that the $1 billion price tag is steeper than it needs to be. If Congress were to extend the 2020 deadline slightly, then the price would go down to $150 million, or $10 million every year over the course of 15 years, he says. This is just a little over double what NASA is currently spending each year to track 1-kilometer-wide asteroids.

And the sooner scientists identify all these potentially dangerous asteroids, the better, because the further away they are, the easier they are to divert. "Give me a lever long enough and I can move the world," says Izenberg. In other words, he says, "The more time you have to act, the smaller the action has to be." If scientists want to move an Earth-bound asteroid seven years before impact, they have to change its speed by 1 centimeter per second; if they instead do it 70 years in advance, they only have to change it by 1 millimeter per second, says Gold. And luckily, asteroid trajectories are stable enough to predict 100 years into the future, meaning that scientists could determine which asteroids, if any, are expected to crash into Earth in the next century.

And then they could do something about them. For distant asteroids, this might be as simple as sending out a medium-sized spacecraft to fly side-by-side with the asteroid for a while. Their mutual gravity would change the course of the asteroid slightly, and a little goes a long way when the asteroid is far away. With closer asteroids, direct impact with a spacecraft or rocket would be the best course of action, says Gold, but this is riskier, as it's not in anyone's best interest to break up the rock. "Then it might just spray all over the atmosphere instead of just coming down in one place," he notes.

With the closest and biggest asteroids, standoff nuclear explosion might be the only way, because nuclear weapons have far more energy per mass than other explosives, Gold says. But again, breaking the asteroid up into smaller pieces is a bad idea—you just want to change its speed enough for it to miss Earth. Getting that exactly right, he says, requires careful knowledge of the asteroid's shape, mass and composition.

While this may sound difficult, scientists are, on the whole, confident that a devastating asteroid impact could be prevented. The only things that are necessary, they say, are knowledge and time—knowledge of the asteroid and where it is headed, and time to devise a diversion plan and carry it out before the asteroid hits us. For the most part, with those tools in hand, disaster need not strike. But without them, who knows?

"You don't want to cry wolf, you don't want to say the sky is falling, because it's not, and you don't want to make more of it than there is, but you also don't want to make less of it than there is," Izenberg says. Identifying these asteroids is certainly worth doing, he adds, "on the merits of protecting the earth and civilization as we know it."