1998 SM₁₆₅: A large Kuiper belt object with an irregular shape

The recent discovery of an ancient reservoir of icy bodies at and beyond the orbit of Neptune - the Kuiper belt - has opened a new frontier in astronomy. Measurements of the physical and chemical nature of Kuiper belt objects (KBOs) can constrain our ideas of the processes of planet formation and evolution. Our 1.8-m Vatican Advanced Technology Telescope and charge-coupled device camera observations of the KBO 1998 SM₁₆₅ indicate its brightness periodically varies by 0.56 magnitudes over a 4-h interval. If we assume a uniform albedo of 0.04, which is typical of values found in the literature for a handful of KBOs, and an "equator-on" aspect, we find 1998 SM₁₆₅ has axes of length 600 × 360 km. If our assumptions are correct, such dimensions put 1998 SM₁₆₅ among the largest elongated objects known in our solar system. Perhaps long ago, two nearly spherical KBOs of comparable size coalesced to form a compound object, or perhaps 1998 SM₁₆₅ is the residual core of a catastrophic fragmentation of a larger precursor.