Centaurs are primitive icy objects that are tens of kilometers in diameter that reside for a few million years in chaotic orbits between Jupiter and Neptune. Centaurs may originate in the Kuiper Belt, which is a zone of icy objects on the edge of the solar system beyond Neptune. They can provide clues to the composition of the original solar nebula, but observing these small distant objects is difficult, and there has been controversy about their size, shape, and composition.
Kern et al. now present two long-exposure spectra from two regions of the Centaur 8405 Asbolus from the near-infrared camera and multiobject spectrograph (NICMOS) on the Hubble Space Telescope. They conclude that Asbolus is a spherical object that rotates with a period of about 4 hours (instead of the previously estimated 9 hours) and that has an impact crater on one side. The spectrum for this side shows absorption features that they attribute to an exposed deeper water-ice layer against a darkened weathered surface. The spectrum for the other side is featureless. Asbolus may have been ejected from the Kuiper Belt by a collision that produced the observed crater, and this collision may have increased its rotation rate. The chemical and structural heterogeneity of Asbolus may help explain discrepant observations of other centaurs, some of which were thought to be homogeneous objects. — LR
Astrophys. J. 542, L155 (2000).