The small size and high geometric albedo of the Haumea cluster member 1995 SM55 from a stellar occultation and ground based photometry

The Trans-Neptunian Object with provisional designation 1995SM55 is one of the few members of the Haumea orbital cluster and therefore it is a very relevant body to study through the occultation technique in order to accurately determine size, shape and geometric albedo.  A very wide campaign involving 50 telescopes was arranged to observe a stellar occultation by this body on 25 February 2024, given that an astrometric update soon before the occultation indicated a good chance of detecting the event. Taking into account that the involved star was relatively bright, at G=12.4 mag, accessible to numerous instruments, the probability of success was high. In the end 7 positive detections from 5 different observatories were obtained and 33 sites obtained negative results. From a preliminary analysis of the occultation results together with ground based photometry data on 1995SM55 that we have gathered over more than a decade with various telescopes, we have been able to determine an accurate projected size and geometric albedo for this object. It turns out that the body is remarkably small for its brightness and therefore it has a high geometric albedo. Given the orbital and spectral similarities of 1995SM55 with Haumea, a high albedo, comparable to that of Haumea and a small size was expected. The final geometric albedo turned out to be even higher than that of Haumea.