Based on these two discoveries, the Centaurs and scattered disc objects are now considered a single class of asteroids. Spacewatch astronomers search for asteroids by taking repeated scans of certain patches of the sky. It takes a CCD detector about 30 minutes to acquire a single image. The astronomers then compare images taken at different times. If there is an asteroid in any of the images, it will appear to move across the picture.
Asteroids travel at different rates, depending on where they are in the solar system.
Near Earth Asteroids move between three-tenths of a degree to 30 degrees a day. UA astronomers Tom Gehrels and Robert McMillan, who currently leads Spacewatch, founded the Spacewatch Project in 1980 to survey for asteroids and comets that might pose a threat to Earth. Originally, astronomers assumed that it was just a single object drifting on the outskirts of the solar system. But in 1991 Spacewatch observers found the second Centaur, Pholus. Today a whole population of 16 Centaur asteroids is known, and Spacewatch discovered about half of them.