The Lightcurve of 4179 Toutatis: Evidence for Complex Rotation

The Apollo asteroid 4179 Toutatis passed within 0.0242 AU of Earth in December 1992, and photometry was obtained by observers from at least 25 sites around the world, at solar phase angles between 121· and 0.2·. The phase curve is well described in the H, G system with a mean H of 15.3 and a slope parameter G of 0.10 ± 0.10. However, the rotational lightcurve is very unusual. The amplitude is large (1.2 magnitudes) and the rotation period is extremely long (several days). Most remarkably, the lightcurve does not appear to be periodic: it is unlikely that a single rotation period can account for the lightcurve even when the rapidly changing viewing and illumination geometry during the close Earth approach is taken into account, though strong lightcurve minima recurred approximately every 7.3 days. The likely explanation is that Toutatis has complex, tumbling, rotation with a characteristic period between 3 and 7 days. As noted by A. W. Harris (1994 Icarus 107, 209-211), the damping time scale from complex to simple rotation for a small, slowly rotating asteroid like Toutatis is so long that complex rotation is expected, but Toutatis is the first asteroid to show such strong observational evidence for complex rotation.