It will still be far away -- about 201,700 miles from our little blue planet, which is a little less than 8,000 miles in diameter. That's closer than the moon's orbit (239,000 miles on average), but NASA says emphatically that the asteroid will miss us. It has been tracking it, like thousands of other so-called Near-Earth Objects, since it was first spotted six years ago, and its path is well known.
"We're extremely confident, 100 percent confident, that this is not a threat," said Don Yeomans, the manager of NASA's Near Earth Object Program at the Jet Propulsion Laboratory in California. "But it is an opportunity."
This asteroid happens to be a C-type -- one rich in carbon-based molecules. Billons of years ago, when the solar system was new and full of debris, asteroids like it probably crashed into the young Earth almost routinely, carrying organic, carbon-based materials, and making life possible. Scientists would like to probe its chemical makeup, at least remotely.
"Without objects of this type, we probably wouldn't be here," Yeomans said.
The factor that has put 2005 YU55 on the public's radar is that it 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 they didn't anything remotely as large.) It is moving at about 29,000 mph relative to Earth's surface.
Asteroids often pass this close, but most are tiny. Countless thousands of pieces come plunging into the atmosphere every day, 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.
Jay Melosh, a professor of Earth and atmospheric sciences at Purdue University, did some calculations and said that if 2005 YU55 were to hit us, it would blast out a crater 4 miles across and 1,700 feet deep. Think a magnitude 7 earthquake and 70-foot-high tsunami waves.
As it is, the asteroid won't even be visible to the naked eye. At its brightest, it will look like a magnitude 11 star -- about 100 times dimmer than the limit of human vision if one is looking at the night sky from a dark place with clear weather. Amateur astronomers may be able to see it slowly moving against the stellar background.
"The good news," said Alan MacRobert of Sky & Telescope magazine in an email, "is that you should be able to spot the asteroid with your telescope if it has an aperture of at least 6 to 8 inches."
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 NASA radar observations. After today's encounter, it will disappear into the void, following an elliptical orbit that takes it as close to the sun as Venus and as far away as Mars.
The Associated Press contributed to this story.