2002 MS4 imaged by the Hubble Space Telescope on 9 April 2006
|Discovered by||Chad Trujillo,|
Michael E. Brown
|Discovery date||18 June 2002|
|Epoch 31 May 2020 (JD 2459000.5)|
|Uncertainty parameter 3|
|Observation arc||23,443 days (64.18 yr)|
|Earliest precovery date||8 April 1954|
|272.62 yr (99,575 d)|
|0° 0m 13.015s / day|
|Dimensions||≈842 × 688 km (projected)|
726±123 km (thermal)
|7.33 h or 10.44 h (single-peaked), 14.66 h or 20.88 h (double-peaked)|
(307261) 2002 MS4 is a large classical Kuiper belt object and a possible dwarf planet in the Kuiper belt, a region of icy planetesimals beyond Neptune. It was discovered in 2002 by Chad Trujillo and Michael Brown. 2002 MS4 has been observed 74 times, with precovery images back to 8 April 1954.
As of 2019[update], 2002 MS4 is 46.5 AU from the Sun. It will reach perihelion, its closest point to the Sun, in 2122. It is approximately tied (to within measurement uncertainties) as the largest unnamed object in the Solar System.
In 2007, an archive search revealed the existence of several precovery observations of 2002 MS4, the earliest of which was taken on 8 April 1954, as part of the Digitized Sky Survey at Palomar Mountain.
In July 2016, 2002 MS4 was observed by the New Horizons spacecraft, as part of its extended Kuiper belt mission. The observations significantly improved the knowledge of 2002 MS4's orbit and phase curve.
2002 MS4 has a similar orbit and current position to Quaoar, although with higher eccentricity and inclination, orbiting every 272.6 years. 2002 MS4 belongs to the class of dynamically hot Kuiper belt objects. It is in an intermittent 18:11 orbital resonance with Neptune.
The Spitzer Space Telescope estimated it to have a diameter of 726±123 km, while the Herschel Space Telescope estimated it to be 934±47 km. If the latter case is true, 2002 MS4 would be comparable in size to the dwarf planet Ceres, and would also be the largest Solar System object without a name as of 2020[update].[a] 2002 MS4 is one of the 10 largest TNOs currently known and large enough to be considered a dwarf planet under the 2006 draft proposal of the IAU. A stellar occultation by 2002 MS4 was observed on 26 July 2019 from British Columbia, with a single chord of 831 km. Another stellar occultation on 19 August 2019 suggested that 2002 MS4 may be highly oblate in shape, with a projected dimension of 842 × 688 km.
2002 MS4 does not have any known moons orbiting it, thus an accurate mass estimate cannot be made. Based on its size, Brown lists it as nearly certain to be a dwarf planet, however, its low albedo may imply the opposite: dark, mid-sized bodies such as this, less than about 1000 km in diameter and with albedos less than about 0.2, have likely never collapsed into solid bodies, much less differentiated or relaxed into hydrostatic equilibrium, and thus are unlikely to be dwarf planets.
As of 2019[update], the rotation period of 2002 MS4 is unknown. Observations in 2005 and 2011 showed possible periods of either 7.33 hours or its alias 10.44 hours (single-peaked), or twice those values for the double-peaked solution, with a small light curve amplitude of 0.05±0.01 mag. Light curve observations of 2002 MS4 are difficult because of the dense field of background stars it is crossing. Observations made in June and July 2011 took advantage of 2002 MS4 moving in front of a dark nebula.