The Lightcurve and Colors of Unusual Minor Planet 1998 WU 24

Abstract

Minor planet 1998 WU 24 is unusual in having the orbital characteristics of a Halley family comet but showing no sign of cometary activity. We present optical data that reveal a double-peaked light-curve with a period of 7.283±0.003 h and a peak-to-peak amplitude of ∼0.54 mag. Infrared spectroscopy and quasi-simultaneous BVRIJHK photometry reveal a featureless K-band spectrum and colors typical of D-type asteroids, and suspected “bare” comet nuclei (B-V=0.78±0.034, V-R=0.53±0.037, V-I=0.99=0.035, V-J=1.67±0.043, V-H=2.10±0.076 and V-K=2.34±0.102). Image profiles from co-added frames in the R band indicate no apparent cometary activity, with an implied upper limit to the dust production rate of 150 g s −1. Assuming a D-type albedo of 0.04 we derive a spherical equivalent diameter of 5.28±0.07 km although the lightcurve amplitude implies an irregular body with an axial ratio of 1.64 : 1. We conclude that 1998 WU 24 is probably an inactive comet nucleus.