Rotation rates of Kuiper-belt objects from their light curves

Very little is known about the physical properties of Kuiper-belt objects', due to their relatively small size and large distance from the Earth. For example, a Kuiper-belt object with a diameter of 300 km at a typical distan ce of similar to 30 AU would subtend an angle of only 0.014 arcsec. It is t herefore possible to investigate their surface markings, shapes and rotatio nal properties only through variations in the light that they reflect (thei r light curves). Here we report a survey of optical light curves from Kuipe r-belt objects. Variations are observed only for the faintest objects in th e survey. We can rule out eclipsing binary objects and variations in the su rface markings as the origin of these light curves, suggesting that the obs erved variations are due to the rotation of irregularly shaped objects. Irr egular shapes may be limited to the smallest Kuiper-belt objects because th e material strength in their inner regions is sufficient to maintain the sh ape against the weight of the overlying material. If, however, all of the o bjects in our survey are of essentially the same size, then the intrinsical ly faintest ones may be composed of a stronger and darker material than the brighter ones.