It begins to get on your nerves. Every now and then -- a few times a year, depending on what you count -- an asteroid goes whizzing past Earth at fairly close range, reminding us that sometimes outer space isn't quite as, well, spacious as we may like to think.
This month's visit will be by a fast-moving space rock called 2012 DA14, which will pass about 17,200 miles from Earth's surface on Feb. 15. It's only about 150 feet across, so astronomers say not to bother to look for it in the sky -- but it will be closer than the communications satellites that ring the planet, 22,000 miles away.
NASA scientists have had a lot of time to plot the orbit of 2012 DA14, and they say they are quite sure it will miss us. There are dust-sized pieces of debris plowing harmlessly into the atmosphere all the time; we see some of them as shooting stars at night. Impacts like the one that wiped out the dinosaurs, 65 million years ago, are separated by tens of millions of years.
"But it has happened and it can happen again," said Humberto Campins of the University of Central Florida. "So as a species it is important we learn all we can about asteroids in case we have to deflect one. And there are other reasons for us to investigate. Asteroids could provide precious resources both to Earth and to space travelers, and they hold secrets to how our planet and life on it formed."
So, say scientists, it's worth keeping a lookout -- and worth paying attention when something the size of 2012 DA14 passes by. The chances that any one object will hit us are very small, but almost inevitably there will be asteroids or comets headed our way, with serious consequences that we may now be able to prevent.
Edward T. Lu is a physicist and former astronaut who flew two space shuttle missions and spent six months on the International Space Station. For skeptics who think asteroid impacts are science fiction, he said, check what happened in Siberia in 1908.
A 330-foot meteor exploded in the atmosphere above the Tunguska River with an impact 1,000 times more powerful than the atomic bomb that destroyed Hiroshima. The force was enough to destroy an area the size of San Francisco.
Lu now heads the non-profit B612 Foundation, a group dedicated to hunting down asteroids before they hit Earth. B612 wants to launch the first privately funded deep-space mission: Sentinel, a space telescope to orbit the sun and map the inner Solar System in search of asteroids that could smash into Earth.
NASA already watches with ground-based telescopes, but every now and then there's a surprise -- a hunk of rock or metal that passes near Earth without much advance notice.
"For every one we know about, there are about 100 more we don't know about," Lu said. "We have to find the other 99."
"Once we find an asteroid," he said, "it is possible for us to predict its trajectory. We know the government wants to discover asteroids big enough to wipe out the planet, but we also want to find those that could wipe out a city the size of New York, or Hong Kong, or Houston."
Fifteen years ago, in the movie "Armageddon," Bruce Willis led a team of roughneck astronauts who tried to blow up a threatening asteroid with a nuclear weapon. Lu and other scientists -- even those who spend their careers watching for threats -- say the reality would be much less dramatic. If you spot an incoming asteroid well in advance -- and that's the idea behind Sentinel -- you just have to nudge it ever so slightly.
Lu and astronaut Stan Love have proposed a space tug, a rocket that would launch to the same orbit as an asteroid threatening to hit the Earth, and push it just enough to make it harmless.
"You don't have to change much. One hundred thousandth of a mile an hour is enough, 10 years ahead of time, to cause an asteroid to miss the rendezvous with Earth," Lu said.
With this month's visitor, 2012 DA14, no action will be necessary. Some scientists will use the opportunity to take a relatively close-up look at a passerby; others say to think of it as a sort of shot across our bow.
"We have been given a gift, the ability to protect our planet," Lu said. "We shouldn't squander it."