An asteroid 132 feet in diameter is circling the sun entirely within Earth's orbit and doesn't appear to pose any threat to the planet, an astronomer says.

Still, it's been difficult to collect enough observations to know for sure whether asteroid 1998 DK36 could collide with Earth, said David Tholen of the University of Hawaii Institute for Astronomy.Previously, asteroids have been discovered only in orbits farther from the sun than the Earth. Because 1998 DK36 is inside Earth's orbit, the asteroid is always near or behind the sun, making it difficult to see.

The asteroid appears to be passing about 750,000 miles away - nothing to lose sleep over, Tholen said this week.

"It's the ones we haven't found yet that are of concern," he said.

Another astronomer told The New York Times it's possible the object doesn't orbit the sun entirely within Earth's orbit.

"That orbital calculation was based on just two brief observations of the object, one lasting seven minutes on the night of Feb. 23 and the other on Feb. 24 for four minutes," Dr. Gareth Williams of the Central Bureau for Astronomical Telegrams in Cambridge, Mass., was quoted as saying by the Times Friday.

"Of the solutions we plotted, some showed the asteroid's orbit to be entirely within the Earth's orbit, but others showed part of the orbit outside the Earth's orbit," he said.

Tholen and Robert Whitely observed the asteroid with a specialized camera mounted on the university's 88-inch telescope that sits atop the dormant volcano Mauna Kea in Hawaii.