We have a visitor -- a large asteroid called 2005 YU55 that is expected to come within approximately 201,700 miles of Earth on Tuesday, according to NASA. That's slightly less than the distance from Earth to the moon.
Asteroids often pass this close, but most are tiny. Countless thousands of pieces come plunging into the atmosphere, but they burn up without doing any harm. If they're as large as grains of sand, we may, if we're lucky, see them in the night sky as shooting stars.
But 2005 YU55 is at least 1,300 feet wide -- larger than an aircraft carrier, according to radar measurements. The last time an asteroid this big passed by was in 1976, and the next one scientists know of won't be until 2028, NASA says. (There have been some rude surprises in between, but not involving anything remotely as large.)
Don Yeomans, manager of NASA's Near-Earth Object Program Office at Jet Propulsion Laboratory, in Pasadena, Calif., said this fly-by is an opportunity to learn more about c-type -- that is, carbon-based -- asteroids, to find "clues as to what it was like when our solar system was forming."
Yeomans and his colleagues have spent years looking for and tracking so-called NEOs, to figure out whether there's a danger that any of them might hit us in the foreseeable future.
Asteroids like this one likely crashed into the young Earth almost routinely billions of years ago, carrying organic, carbon-based materials, and making life possible.
"Without objects of this type, we probably wouldn't be here," Yeomans said.
But an impact in modern times could be catastrophic. There have been a few times -- most notably 65 million years ago, at the end of the age of dinosaurs -- when impacts from space wiped out much of the life on Earth.
Ray Williamson, head of an organization called the Secure World Foundation, said he hopes 2005 YU55 will serve as a healthy alert that the nations of the world need to get together and decide what to do if some future asteroid appears to be coming too close for comfort.
"It's a really good reminder that bad things could happen," he said. "We don't know when."
All sorts of schemes have been proposed for deflecting asteroids -- everything from blowing them apart with nuclear weapons to deflecting them gently with advanced rocket engines. But since the chances of a major hit anytime soon are so low, no plan has funding or international backing. Williamson says it's worth at least making plans.
Asteroid 2005 YU55 is roughly spherical, spinning slowly. It is darker than charcoal, according to past NASA radar observations.
Its approach is particularly exciting to researchers because now, from the comfort of our planet, they can study an object they previously could only look at closely with unmanned spacecraft. Technology has improved since the last large asteroid fly-by in 1976.
The asteroid will not be visible to the naked eye. Amateur stargazers can take a look at the asteroid if they have a reflecting telescope with a light-gathering mirror six inches or more in diameter.