Asteroid or stripped Comet?

The above images are of the same object, 1998 WU24. This is a small object with an elongated orbit like Halley's comet, but unlike the comet it does not seem to have any gas or dust associated with it. Perhaps it is rocky, or perhaps it is a comet nucleus that has either run out of gas or has temporarily stopped being active. When the above image was obtained with UFTI at UKIRT the object was in the inner solar system and was moving quite quickly. Consequently, over a 9 minute period it appears as a streak with respect to the stationary background stars (left). However on combining the images in the frame of the asteroid (right) it appears to be a point source while all the stars are smeared out. This is a useful technique because, once all the light from the object is in one spot, one can more easily measure its brightness. By observing it through different filters, and with follow-up observations with (for example) the CGS4 spectrometer, astronomers are able to ascertain physical parameters and the chemistry of surface materials for this and other, similar, solar-system bodies.

Our thanks to John Davies for allowing us to show his data.