(528381) 2008 ST291

Background

2008 ST291 was discovered on 24 September 2008 by American astronomers Megan Schwamb, Michael Brown and David Rabinowitz at the Palomar Observatory in California. No earlier precovery images are known. The Minor Planet Center numbered it on 18 May 2019, but as of 2025 it has not been named.

The object is locked in a 1:6 resonance with Neptune, meaning it completes roughly one orbit for every six orbits Neptune makes. Its orbit has a semi-major axis of 99.89 AU, an eccentricity of 0.58, and an inclination of 21° with respect to the ecliptic. It ranges from 42.3 AU at perihelion to 157.5 AU at aphelion, and it last came to perihelion in 1954. Currently it is located 60.9 AU from the Sun.

Physical size estimates vary depending on assumed albedo and absolute magnitude. Johnston's Archive gives a diameter of about 536 km assuming an albedo of 0.126. Mike Brown estimates 549 km using an albedo of 0.09 and a magnitude of 4.6. The Asteroid Dynamic Site records a brighter magnitude of 4.3, yielding 612 km with the same albedo formula. Using the average of these magnitudes and a standard albedo range of 0.25–0.05, possible sizes span 345–773 km.

Did You Know?

• Its orbit takes 998 years and 4 months, with a semi-major axis of 99.89 AU.
• Estimated diameter ranges from 345 to 773 km, depending on assumed albedo and magnitude.