(307261) 2002 MS4 (provisional designation 2002 MS4) is a large trans-Neptunian object in the Kuiper belt, which is a region of icy planetesimals beyond Neptune.

It was discovered on 18 June 2002 by Chad Trujillo and Michael Brown during their search for bright, Pluto-sized Kuiper belt objects at Palomar Observatory.

To within measurement uncertainties, 2002 MS4, 2002 AW197, and 2013 FY27 have a diameter close to 800 km (500 mi), which makes them the largest unnamed objects in the Solar System.

2002 MS4 was discovered on 18 June 2002 by astronomers Chad Trujillo and Michael Brown at Palomar Observatory in San Diego County, California, United States.

[1] The discovery formed part their Caltech Wide Area Sky Survey for bright, Pluto-sized Kuiper belt objects using the observatory's 1.22-meter (48 in) Samuel Oschin telescope with its wide-field CCD camera, which was operated jointly with the nightly Near Earth Asteroid Tracking program at Palomar.

[14]: 100  This survey was responsible for the discovery of several other large objects beyond Neptune, which includes the dwarf planets Eris, Sedna, and Quaoar.

[18] Seven precovery observations from Digitized Sky Survey plates were identified by astronomer Andrew Lowe in 2007; the earliest of these was taken on 8 April 1954 by Palomar Observatory.

[3]: 53  2002 MS4's high orbital inclination qualifies it as a dynamically "hot" member of the classical Kuiper belt, which implies that it was gravitationally scattered out to its present location by Neptune's outward planetary migration in the Solar System's early history.

[7]: 5  It is unknown whether 2002 MS4's equator is being viewed obliquely or edge-on from Earth's perspective, so it is possible that the object's actual polar diameter may be smaller, or have a greater oblateness, than observed in the August 2020 occultation.

2002 MS4 was previously thought to have a larger diameter of 934 km (580 mi), according to infrared thermal emission measurements made by the Spitzer and Herschel space telescopes in 2006 and 2010.

[7]: 8 2002 MS4 has a gray or spectrally neutral surface color, meaning it reflects similar amounts of light for wavelengths across the visible spectrum.

[10]: 2, 18–19  New Horizons observations of 2002 MS4's phase curve indicate that the icy regolith grains on the object's surface are rough and irregularly shaped.

[7]: 7  Another depression feature about 10 km (6.2 mi) wide and 11 km (6.8 mi) deep was detected by a single telescope from Varages, France during the occultation; this depression feature partially occulted the star as 2002 MS4 emerged, which resulted in the star brightening gradually instead of instantly.

[41][42] The topographic peak on 2002 MS4 has a height comparable to Mars's tallest mountain, Olympus Mons, and the central mound of the Rheasilvia crater on asteroid Vesta.

[7]: 9  The depression's width takes up about 40% of 2002 MS4's diameter, which is comparable to the largest crater-to-diameter ratios seen in Saturn's moons Tethys and Iapetus.

[7]: 9  The trans-Neptunian dwarf planets Pluto and Charon do not exhibit such large craters on the other hand,[f] as their largest crater-to-diameter ratios are 10.5% and 18.9%, respectively.

It is difficult to measure 2002 MS4's rotation period photometrically with telescopes on Earth since the object is obscured in a dense field of background stars.

[9]: 118 [7]: 7  Due to 2002 MS4's spheroidal shape and possible surface albedo variations, its light curve only exhibits very small fluctuations in brightness (amplitude 0.05–0.12 mag[8]: 85 ) over time as it rotates.

[7]: 7 [8]: 73  The first attempts at measuring 2002 MS4's rotation were made with the Sierra Nevada Observatory's 1.5-meter telescope in August 2005, but it did not observe the object long enough to identify any periodicities in its light curve.

[47]: 922  Another trajectory using a single Jupiter gravity assist for a 2040 launch date could bring a spacecraft to 2002 MS4 over a minimum duration of 13 years.