(528381) 2008 ST291
, provisional designation , is a 1:6 resonant trans-Neptunian object located in the outermost region of the Solar System that takes almost a thousand years to complete an orbit around the Sun. It was discovered on 24 September 2008 by American astronomers Megan Schwamb, Michael Brown and David Rabinowitz at the Palomar Observatory in California, with no known earlier precovery images. Numbering and naming. This minor planet was numbered by the Minor Planet Center on 18 May 2019 (). As of 2019, it has not been named. Orbit and classification. is located at the 1:6 Neptune resonance of 99  AU meaning that it completes roughly 1 orbit for every 6 orbits Neptune makes. It orbits the Sun at a distance of 42.3–157.5 AU once every 998 years and 4 months (semi-major axis of 99.89 AU). Its orbit has an eccentricity of 0.58 and an inclination of 21° with respect to the ecliptic. Currently located at 60.9 AU from the Sun, the object came to perihelion in 1954. Physical characteristics. Based on an absolute magnitude of 4.4, is estimated by the "Johnston's Archive" to be about in diameter, assuming a typical albedo of 0.09 for trans-Neptunian  objects. Astronomer Mike Brown estimates a slightly smaller 549 km from the same albedo and a fainter 4.6 magnitude. The Asteroid Dynamic Site records a brighter 4.3 magnitude, which calculates to 612 km using the same albedo (and same formula as "Johnston's"); using the average of these magnitudes and a standard assumed minor planet albedo range of 0.25 ~ 0.05, possible sizes of 345 to 773 km are produced.