A giant asteroid expected to zip within 5 million miles of Earth early today poses no danger but offers a scientific treat.

The asteroid, which is a half-mile to one mile in diameter, will be 20 times farther from Earth than the moon when it zooms past at some 49,000 mph, flying nearest just before 6 a.m. Sunday, said astronomer Eleanor Helin.But "astronomically speaking, that's a close pass," said Helin of the Jet Propulsion Laboratory in Pasadena, Calif., who discovered the asteroid dubbed 1991 AQ on Jan. 14.

The pass will give astronomers a chance to study the object to learn more about its origin and perhaps gain information that could help predict and prevent more dangerous future encounters, she said.

About 1,500 asteroids larger than a half-mile in diameter are known to cross Earth's path, and astronomers have identified about a dozen of the objects, she said.

Asteroids are rocky extraterrestial bodies, most of which orbit the sun. But about 1 percent of known asteroids have orbits that cross those of planets, including Earth.

The asteroid, which returns to Earth about every three years, also crosses the orbits of Venus and Mars, she said. "It gets kicked about" during each encounter, which could alter the orbit into a collision course with Earth, she said.

"This object may, in tens of thousands of years, very well be menacing," she said, adding that a collision between Earth and an object that size "would cause a lot of devastation."