Close Encounters: Is the Sky Falling ... Again?

(Sept. 8) -- This evening, a visitor from outer space will pass a hair's breadth from Earth. A space rock known as 2010 RF12 will whisk so close that even though it is too small to do any damage if it hit the planet, it will be visible for roughly an hour through a hobbyist's telescope.

Scientists warn that this innocuous rock is a wake-up call. There are plenty of boulders in the solar system that are only slightly bigger, making them large enough to kill thousands of people but hard to spot until it's almost too late.

"We spotted these things right now only because we happened to be looking in the right place at the right time," said Lindley Johnson, head of NASA's near-Earth objects program. "I have to be honest here. There's a deficit in capability."

It was only Sunday that astronomers in Arizona noticed a space rock, or asteroid, that squeaked by the planet this morning, and its slightly smaller companion, which will make its closest approach to Earth just past 5 p.m. EDT. Amateur astronomers who live in Europe and have a decent-sized telescope should be able to see the second asteroid.

The observatory that nabbed the asteroids is part of a global network dedicated to finding large space rocks that have Earth in their cross hairs. After years of patiently trawling the skies, astronomers have discovered almost 90 percent of the "killer" asteroids. These behemoths, which measure 3,000 feet across or larger, are big enough to annihilate much of humanity.

That's not an ivory-tower concern. The dinosaurs were believed to have been wiped out by an asteroid the size of a small city that walloped Earth 65 million years ago.

It would take an asteroid only half the size of a football field to kill thousands of people, if it landed on New York City or Tokyo. Such rocks impact the Earth perhaps every few centuries, perhaps once a millennium, said David Asher of the Armagh Observatory in Northern Ireland.

Our inability to spot them well before impact "is a matter of some concern," said Alan Fitzsimmons of Queen's University Belfast. "The problem is that these objects are so small and therefore so faint that we cannot find them" until they're very close to Earth. Telescopes simply aren't powerful and numerous enough to spot all the potentially dangerous objects in the sky.

"We probably cover less than 10 percent of the sky that you [would] need to have assurance that you would pick something up several days in advance," said NASA's Johnson. A warning of even a few days would be enough to allow for an evacuation.

Fortunately, the Earth is so vast and people are spread so widely that the odds are against a murderous asteroid hitting a heavily populated area, said Alan Harris, a top asteroid expert at the Space Science Institute. He cites an asteroid that was spotted as it hurtled toward Earth in 2008. It exploded harmlessly over the desert of Sudan, providing lots of data for scientists and killing no one.

"The good news, such as it is, is that the surveys are getting better and better at finding these things," Harris said via e-mail.

Still, Johnson said, "there's a ways to go before we have a leak-proof system."