Big asteroid has close encounter with Earth

This radar image of asteroid 2005 YU55 was generated from data taken in April of 2010 by the Arecibo Radar Telescope in Puerto Rico.

CAPE CANAVERAL, Florida (Reuters) - A black asteroid as big as an aircraft carrier zoomed past Earth on Tuesday, delighting astronomers who trained telescopes on the ancient body in hopes of learning more about its composition and origin.

With a diameter estimated at 400 meters, or about a quarter of a mile, Asteroid 2005 YU 55 is the biggest asteroid to make a close pass by Earth since 1976.

During its closest approach, which occurred at 6:28 p.m. EST, it was inside the orbit of the moon, about 200,000 miles above the planet. It posed no threat to either.

Thousands of professional and amateur astronomers were tracking the asteroid with telescopes, seeking to learn more about what it is made of, how fast it spins and ultimately, where it came from.

"It was pretty easy to find," astronomer Ronald Dantowitz, director of the student-run Clay Center Observatory in Brookline, Massachusetts, told Reuters. "It's moving differently than the stars are moving. It looks like a giant rock floating through space."

With automated controls for tracking, the asteroid appeared to be fixed in position, while background stars were a blur, Dantowitz said. The asteroid, however, was moving at about 30,000 mph.

Astronomers believe YU 55 has been visiting Earth for thousands of years, nudged out of the main asteroid belt between Mars and Jupiter after a gravitational elbowing by Jupiter.

Computer models simulating the asteroid's path for the next 100 years show there is no chance it will hit Earth or the moon during that time, said Don Yeomans with NASA's Jet Propulsion Laboratory in Pasadena, California.

Astronomers do not expect it will be a threat beyond that time either but have not run the models beyond 100 years.

Asteroids are of interest to scientists trying to piece together how the solar system formed.

"It costs millions of dollars to send a spacecraft for a close encounter with an asteroid," Dantowitz said. "Instead, this one is coming to us. It's literally streaming through our backyard."

YU 55 is believed to be one of the more common type, carbon-rich asteroids, albeit a large one. Its ancient rocks may contain water, metals and other materials that could be useful for space explorers.

NASA's next human space venture beyond the International Space Station, a $100 billion orbital research outpost flying about 240 miles above Earth, is a mission to an asteroid, targeted for 2025.

(Editing by Tom Brown and Xavier Briand)