The lightcurve and colors of unusual minor planet 1998 WU24

Minor planet 1998 WU24 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 lightcurve with a period of 7.283 +/- 0.003 h and a peak-to-peak amplitude of similar to0.54 mag. Infrared sp ectroscopy 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 come tary activity, with an implied upper limit to the dust production rate of 1 50 g s(-1). Assuming a D-type albedo of 0.04 we derive a spherical equivale nt diameter of 5.28 +/- 0.07 km although the lightcurve amplitude implies a n irregular body with an axial ratio of 1.64: 1. We conclude that 1998 WU24 is probably an inactive comet nucleus. (C) 2001 Academic Press.