Those tumbling asteroids

Once a small planet has been discovered (see 1. inst. Messenger No. 6, Sept. 1976) and its orbit determined, we can keep track of it and find it again in the sky at any time as a faint speck of light, moving along between the fixed stars. Then we can study it further by spectroscopy and photometry (measurement of its magnitude and colours). Whereas its spectrum is normally very similar to that of the Sun (reflected sunlight from the asteroid's surface), its light-curve may tell us its rotation period, and possibly, after a long series of preeise measurements, the shape and direction of the rotation axis. These quantities are not trivial; f. inst. the behaviour of minor planets of the same family (similar orbits) is of importance for our understanding of their origin.