20000 Varuna - Size

Size

The size of the large Kuiper belt objects can be determined by simultaneous observations of thermal emission and reflected sunlight. Unfortunately, thermal measures, intrinsically weak for distant objects, are further hampered by the absorption of the Earth's atmosphere as only the weak 'tail' of the emissions is accessible to Earth-based observations. In addition, the estimates are model-dependent with the unknown parameters (e.g. pole orientation and thermal inertia) to be assumed. Consequently, the estimates of the albedo vary resulting in sometimes substantial differences in the inferred size. Estimates for the size of Varuna have varied from 500 to 1060 km. The two most recent estimates from Spitzer are closer to the 500 kilometres (310 mi) range and inconsistent with the 2005 estimate of a size of 936 +238
−324 km, based on earlier results (900 +129
−145) and (1060 +180
−220). This inconsistency of the Spitzer results with the earlier (sub-millimetre) observations was recently addressed by the original authors (Stansberry et al.); given a number of difficulties in Varuna case, the authors are inclined to favor the sub-millimetre results (Jewitt, Lellouch) for this object over those from Spitzer.