Size and albedo of Kuiper belt object 55636 from a stellar occultation

A second look at KBO 55636 Kuiper belt objects (KBOs), small icy bodies orbiting in the Solar System beyond Neptune, are difficult observational targets. Occasionally one is spotted causing a stellar occultation, when data relevant to its structure can be obtained and a few properties deduced. But the stellar occultation of 9 October 2009, involving KBO 55636, a member of the water-ice rich Haumea collisional family, was a landmark event taking the observational astronomy of KBOs to a new level. Following several years of tracking of bright KBOs, it was possible to predict an imminent occultation, and to train a number of telescopes in its direction. Despite the vagaries of weather and other observational hazards, more than one (in fact two) separate telescopes were able to make the observation. With two different angles of view to work with, more accurate figures can be calculated: KBO 55636's mean radius of 143 ± 5 km, and its geometric albedo of about 0.9 in the V band, reveal it to be smaller than previously thought and, like its parent body, highly reflective. The dynamical age implies either that KBO 55636 has an active resurfacing mechanism, or that fresh water ice can persist for billions of years in the outer reaches of the Solar System. KBO 55636 (2002 TX 300 ) is one of the Kuiper belt objects — specifically, a member of the water-ice-rich Haumea KBO collisional family. Here, observations are reported of a multi-chord stellar occultation by KBO 55636. Calculations show that KBO 55636 is smaller than previously thought and, like its parent body, is highly reflective. The dynamical age implies either that it has an active resurfacing mechanism, or that fresh water-ice in the outer Solar System can persist for gigayear timescales. The Kuiper belt is a collection of small bodies (Kuiper belt objects, KBOs) that lie beyond the orbit of Neptune and which are believed to have formed contemporaneously with the planets. Their small size and great distance make them difficult to study. KBO 55636 (2002 TX 300 ) is a member of the water-ice-rich Haumea KBO collisional family 1 . The Haumea family are among the most highly reflective objects in the Solar System. Dynamical calculations indicate that the collision that created KBO 55636 occurred at least 1 Gyr ago 2 , 3 . Here we report observations of a multi-chord stellar occultation by KBO 55636, which occurred on 9 October 2009 ut . We find that it has a mean radius of 143 ± 5 km (assuming a circular solut