LARGEST EARTH-APPROACHING ASTEROID DISCOVERED?

Dr. Edward Bowell
Lowell Observatory
1400 W. Mars Hill Road
Flagstaff AZ 86001
928-774-3358
ebowell@lowell.edu

2001 August 14

Michael Van Ness, an observer for the Lowell Observatory Near-Earth-Object Search (LONEOS) program at Lowell Observatory, Flagstaff, Arizona, discovered a rare type of asteroid on the night of July 28. The object, called 2001 OG108, is an asteroid in a highly elongated orbit that takes it from the Earth's vicinity to beyond the planet Uranus.

2001 OG108 is one of only 14 known asteroids of a rare class called Damocloids, named after (5335) Damocles, the first asteroid found of its type. Damocloids have orbits that resemble those of objects like comet Halley, so it has been conjectured that they are the exhausted remains of comets that have lost their volatile components---gas and ices. Possibly, Damocloids originated in the Oort Cloud, a vast reservoir of comets, most of which orbit the Sun between 30,000 and 100,000 times the Earth's distance.

Until Van Ness' discovery, only one other Damocloid, also discovered by LONEOS, had an orbit that brings it closer to the Sun than the Earth. 2001 OG108 is also unusual because it is large. Edward Bowell, Director of the LONEOS program, estimates that the asteroid is between 5 and 10 miles in diameter, which likely makes it larger than any other known Earth-crossing asteroid. An Earth-crossing asteroid is one that, at its closest approach to the Sun, is closer than the Earth.

Fortunately, 2001 OG108 poses no near-term threat of collision to Earth. The closest it can come to our planet is about 30 million miles. Perhaps of significance for the long-term evolution of its orbit is that the asteroid can approach Jupiter within 100 million miles. Jupiter's far greater mass---300 times that of the Earth---may have a much larger influence on the shape of the asteroid's orbit.

On the night of discovery, Van Ness noticed that 2001 OG108 moved unusually quickly compared to other asteroids in the same part of the pre-dawn sky. He reported its position to the Minor Planet Center in Cambridge, Massachusetts, which, in turn put out a Web alert to observers worldwide. Over the following two weeks, nine other observers from seven countries followed the asteroid. The Minor Planet Center released the results of their observations on August 13.

Van Ness has been an observer for LONEOS since 1999. He has been responsible for the discovery of 14 near-Earth asteroids (out of a total of about 1,400 known), and two comets. He is a graduate student of anthropology at Northern Arizona University, Flagstaff.

LONEOS, located at Lowell Observatory's Anderson Mesa Station, 15 miles southeast of Flagstaff, has been in operation since 1998. It consists of an automated 23-inch Schmidt telescope with a powerful CCD camera. Moving objects, mainly asteroids and comets, are detected automatically, though observer confirmation is required for anything that looks unusual. LONEOS is mainly funded by NASA to discover asteroids that could one day strike the Earth. Its observers have discovered 87 near-Earth asteroids and 14 comets.

2001 OG108 is currently moving toward the Sun through the main asteroid belt between Mars and Jupiter. At its closest to Earth, in April 2002, it will be passing rapidly by the pole star, and will be bright enough for amateurs with moderate size telescopes to follow its motion visually. Because the asteroid may be a burned out comet, astronomers will be observing it closely to determine its chemical composition and to try to detect any remnant outgassing.