University of Hawaii astronomers have discovered a new type of asteroid, whose orbits lie completely within the orbit of the Earth.
Previously, all known asteroids traveled in an orbit farther from the Sun than the Earth, over at least a portion of their journey.
"All other efforts to discover asteroids on a collision course with the Earth are being directed at a region of the sky almost opposite the Sun," said David Tholen, planetary astronomer at the Institute for Astronomy.
"The significance of this discovery is that we would have otherwise never found this new asteroid because it apparently doesn't travel to that region of the sky being scanned by other search efforts."
If such an asteroid's orbit around the Sun intersects with the Earth's orbit, it could hit the Earth and we would have never seen it coming, said Tholen. We would have been caught unaware by an asteroid approaching us from the daytime side of the sky, he said.
Tholen and graduate student Robert Whiteley made the observation using a specialized camera fitted on the University of Hawaii's 2.24-meter telescope atop Mauna Kea last February.
While scanning the dusk and dawn skies to assess the size and number of asteroids within the Earth's orbit, Whiteley spotted the object, since designated 1998 DK36, on his computer screen, shortly after Tholen had recorded the images at Mauna Kea Observatory and sent them to Whiteley's computer via the Internet.
Additional observations made the following night made it possible to compute a preliminary orbit of the object around the Sun. Tholen said the exact size and shape of the asteroid orbit remain uncertain. However, the orbit's farthest point from the Sun could be determined relatively accurately, and it appears to be very close to, but slightly inside the orbit of the Earth.
The asteroid is thought to be about 40 meters in diameter, similar in size to the one that flattened the Tunguska region of Siberia on June 30, 1908, as well as the iron object that produced Meteor Crater in Arizona 50,000 years ago.
Could it collide with the Earth?
"We were unable to obtain enough observations to perform a formal probability calculation, though the best-fitting orbit has the object passing an apparently safe 750,000 miles from the Earth's orbit," said Tholen. "To do a better job with such discoveries, we really need to have a telescope that we can dedicate to such difficult observations."
"1998 DK36 is nothing to lose sleep over," said Tholen. "It's the ones we haven't found yet that are of concern."