A 100-foot diameter asteroid will pass within 26,500 miles of Earth on Thursday, the closest-ever brush on record by a space rock, NASA astronomers said.
The asteroid's close flyby, first spied late Monday, poses no risk, NASA astronomers stressed.
"It's a guaranteed miss," astronomer Paul Chodas, of the near-Earth object office at NASA's Jet Propulsion Laboratory, said Wednesday.
"Our calculations show that it will make its closest approach to Earth on Thursday at about 5 p.m. EST," he told CBS Radio News. "It's going so fast that it travels the distance from the Earth to the Moon in about 15 hours."
The asteroid, 2004 FH, was expected to streak over the southern Atlantic Ocean. It should be visible through binoculars to stargazers across the southern hemisphere, as well as throughout Asia and Europe, said astronomer Steve Chesley, also of JPL.
The best view, said Chodas, will be from southern Africa.
"It's a fast-moving object — not as fast as a satellite, mind you, but one of the fastest-moving astronomical objects that we would have ever seen," he said.
Professional astronomers around the globe scrambled Wednesday to prepare for the flyby, which could provide an unprecedented chance to get a close look at the asteroid, he added. The asteroid will pass within the moon's orbit.
Similarly sized asteroids are believed to come as close to Earth on average once every two years, but have always escaped detection.
"The important thing is not that it's happening, but that we detected it," Chesley said.
"This object is probably around 100 feet in diameter, so it was too faint to discover when it was farther from the earth, farther along in its orbit, but since it's quite close to the earth now, it's brightening up," he told CBS News Producer Charles White.
Astronomers found the asteroid late Monday during a routine survey carried out with a pair of telescopes in New Mexico funded by the National Aeronautics and Space Administration. Follow-up observations on Tuesday allowed them to pinpoint its orbit.
"It immediately became clear it would pass very close by the Earth," Chesley said.
Astronomers have not ruled out that the asteroid and our planet could meet again sometime in the future, but it may not be a disaster.
"If it really was on a collision course with the earth, it would most likely break up in the atmosphere into a lot of smaller pieces and not cause much damage when it hit the Earth," Chodas said.