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

The recent discovery of an ancient reservoir of icy bodies at and beyond th e orbit of Neptune-the Kuiper belt-has opened a new frontier in astronomy. Measurements of the physical and chemical nature of Kuiper belt objects (KB Os) can constrain our ideas of the processes of planet formation and evolut ion. Our 1.8-m Vatican Advanced Technology Telescope and charge-coupled dev ice camera observations of the KBO 1998 SM165 indicate its brightness perio dically varies by 0.56 magnitudes over a 4-h interval. If we assume a unifo rm 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 o f length 600 x 360 km. If our assumptions are correct, such dimensions put 1998 SM165 among the largest elongated objects known in our solar system. P erhaps 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 cat astrophic fragmentation of a larger precursor.