FINDING NEW ONES
A handful of programs at professional observatories are devoted to the search for asteroids. One approach is to photograph a portion of sky, wait a day or two, and re-photograph it. By comparing the two images, any object that moved between the exposures—a definite asteroid candidate—can be identified.
A second method uses CCD detectors to image a portion of sky through a telescope. As the chip records the field of view, it electronically subtracts a reference image for that area containing just the stars. Anything not in the reference image will stand out clearly.
Neither of these approaches is perfect. The film technique imposes delays, but covers a much wider field of view. The CCD method produces results at the telescope in real time (or close to it), but it covers only a tiny portion of sky in each exposure.