# (523794) 2015 RR245

(523794) 2015 RR$_{245}$, provisional designation 2015 RR$_{245}$, is a large trans-Neptunian object of the Kuiper belt in the outermost regions of the Solar System. It was discovered on 9 September 2015, by the Outer Solar System Origins Survey at Mauna Kea Observatories on the Big island of Hawaii, in the United States. The object is in a rare 2:9 resonance with Neptune and measures approximately 600 kilometers in diameter. 2015 RR$_{245}$ was suspected to have a satellite according to a study announced by Noyelles et al. in a European Planetary Science Congress meeting in 2019.

## Discovery

A first precovery of 2015 RR$_{245}$ was taken at the Cerro Tololo Observatory in Chile on 15 October 2004. It was first observed by a research team led by Michele Bannister while poring over images that the Canada–France–Hawaii Telescope in Hawaii took in September 2015 as part of the Outer Solar System Origins Survey (OSSOS), and later identified in images taken at Sloan Digital Sky Survey and Pan-STARRS between 2008 and 2016. The discovery was formally announced in a Minor Planet Electronic Circular on 10 July 2016.

## Numbering and naming

This minor planet was numbered by the Minor Planet Center on 25 September 2018 (M.P.C. 111779). As of 2021, it has not been named.

## Orbit and classification

As of 2018, 2015 RR$_{245}$ has a reasonably well defined orbit with an uncertainty of 3. It orbits the Sun at a distance of 33.8–128.6 AU once every 731 years and 6 months (for reference, Neptune's orbit is at 30 AU). Its orbit has an eccentricity of 0.58 and an inclination of 8° with respect to the ecliptic.
2015 RR$_{245}$ is among the most distant known Solar System objects. As of 2018, it is 63 AU from the Sun. It will make its closest approach to the Sun in 2093, when it will reach an apparent magnitude of 21.2.

### 2:9 resonance

Additional precovery astrometry from the Sloan Digital Sky Survey and the Pan-STARRS1 survey shows that 2015 RR$_{245}$ is a resonant trans-Neptunian object, securely trapped in a 2:9 mean motion resonance with Neptune, meaning that this minor planet orbits the Sun twice in the same amount of time it takes Neptune to complete 9 orbits. The object is unlikely to have been trapped in the 2:9 resonance for the age of Solar System. It is much more likely that it has been hopping between various resonances and got trapped in the 2:9 resonance in the last 100 million years.

## Physical characteristics
### Diameter and albedo

Its exact size is uncertain, but the best estimate is around 670 km (420 mi) in diameter, assuming an albedo of 0.12 (within a wider range of 500 to 870 km, based on albedos of 0.21 to 0.07). For comparison, Pluto, the largest object in the Kuiper belt, is about 2,374 km (1,475 mi) in diameter. Astronomer Michael Brown assumes an albedo of 0.11 and calculates a diameter of 626 km, while the Johnston's Archive gives a diameter of 500 kilometers for the primary and 275 km for the satellite, based on an assumed equal albedo of 0.135.

## Possible satellite

2015 RR$_{245}$ is suspected to be binary. If this moon exists and significantly contributes to the observed brightness of the primary, the size of 2015 RR$_{245}$ may therefore be substantially smaller than estimates that assumed the system's total brightness was from a single object; the system may be similar to that of 174567 Varda. Once the orbit of the satellite is determined, the mass and density of the 2015 RR$_{245}$ can be determined. 2015 RR$_{245}$ was observed by the Hubble Space Telescope in 2020, but did not detect the putative satellite.