Optical observations

How are asteroids discovered from photographs?

Discovering asteroids is not so easy. Unlike planets, asteroids are too small to be seen in a telescope as a disk with features. Asteroids look just like stars (in fact, the word asteroid means "star like").
To distinguish asteroids from background stars, scientists use the fact that asteroids are nearer, and therefore move across the sky much faster than stars. In this way, asteroids can be detected from long exposed photographs or from a set of photographs of the same portion of sky, taken in rapid succession.

The Photograph on the left is an image of 4179 Toutatis
(by Alain Maury and Derral Mulholland).
on the right,
a photograph of Geographos 1620 (taken by Petr Pravec)

In fact, if a long exposure photo is taken while the telescope is pointed at a fixed set of stars, any asteroid present in the field of view will leave a streak. This because the asteroid moves much faster than the stars (photo on the left). When instead of a single long exposure photo, a series of multiple exposures is taken and a film made out of them, the asteroid can actually be seen moving across the background sky (animation on the right).

CCD: The revolution of new technologies

Asteroids are small, confronting them with stars and planets, and therefore, even being nearer from us, they are not very bright. For this reason, it is not always possible to detect them from photos.
In the last years, this search has been greatly aided by new technologies such as new computer controlled telescopes and CCD cameras. The first instruments are very useful to automatically follow the motion of celestial objects allowing to control an incredible number of positions. Computers allow very fast comparison between images taken of the same part of sky at different times.

CCD (or charged couple devices) are electronic light detectors made of finely array arrays of semiconductor picture elements called pixels. The CCD is placed on the focal plane of the telescope, replacing the old photographic plate. When light hits one pixel, its energy causes positive and negative charges to separate, leaving free electrons in the pixel. These electrons are counted all over the picture and provide an image of the light pattern seen by the telescope.