A Close Call

An asteroid makes its closest ever approach to Earth

Nov 09, 2011 | By Jeffrey Kluger for TIME

Astronomers have known for several years that November 8, 2011, would end one of two ways: either uneventfully or with a massive impact that would leave a 4-mile crater somewhere on the planet and unleash 70-foot tsunami waves on the world.

They even knew the exact time it would happen. They call it the cocktail-hour moment. It is the precise time at which asteroid 2005 YU55 will make its closest approach to Earth. Asteroid 2005 YU55 is a 1,312-foot space rock that was discovered six years ago.

The moment has come and gone. All along, the astronomers knew we had nothing to fear. They calculated that the asteroid should miss us by 201,700 miles. It sounds like a lot. But on a cosmic scale is still a close call, a full 38,000 miles closer than the moon is to us. That's a little like a car speeding down your street and touching your property with its tires. It is not enough to hurt you, but it is enough to make you uneasy.

An Extraordinary Opportunity

Scientists are excited about the close call. Don Yeomans, manager of NASA's Near Earth Object Observations program, calls it "an extraordinary opportunity" to learn more about asteroids. It also gives scientists a chance to sharpen tracking and early-warning skills. This will help when another asteroid takes better aim — which, like it or not, will happen sooner or later.

Asteroids are dangerous because there are so many of them. Scientists at the Near Earth Observations program are currently keeping their eyes on 19,500 rocks in the 330 to 3,300 foot range. Those are hardly the only asteroids of that size out there, but they're the ones near Earth.

Professional astronomers studied YU55 in detail as it passed by. They looked for the chemical signatures of carbon and water. Since asteroids are artifacts of the earliest days of the solar system, they might tell us something about how life got started on Earth, and perhaps elsewhere. "A small army of astronomers are pretty excited about it," Yeomans says.

What Happens If…?

With its massive size, Earth shouldn't be bothered by a small rock. But moving objects pack a bigger punch than their size suggests. Incoming space debris moves fast, anywhere from 22,000 to 157,000 miles per hour.

We now have technology that gives us the lead time to send up intercept missions that would destroy or deflect dangerous asteroids before they reach us. But the technology is not yet perfect. And blowing up a big asteroid could simply create many smaller boulders that would still hit us.

We should have plenty of time to sharpen these skills. The last time YU55 troubled us was 200 years ago. It will not be back around again for another 200 years. Nothing so big should come our way again until 2028, and that asteroid is expected to miss us as well. In general, we can anticipate a strike by a YU55-size object only once every 100,000 years.

NASA has not yet chosen its next goal for human space travel. Options include the moon, Mars and various asteroids. YU55 has been short-listed as one place worth visiting. This means that we may be visiting it long before it comes back to call on us.