(307261) 2002 MS4

' (provisional designation ') 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,, , and have a diameter close to 800 km, which makes them the largest unnamed objects in the Solar System. is large enough that astronomers consider it a possible dwarf planet.

The surface of is dark gray and is composed of water and carbon dioxide ices. has been observed through stellar occultations, which have revealed massive topographic features along the outline of its shape. These features include a mountain-like peak that is 25 km tall and a crater-like depression that is 320 km wide and 45 km deep. 's topographic features are among the tallest and deepest known for Solar System bodies.

Discovery
was discovered on 18 June 2002 by astronomers Chad Trujillo and Michael Brown at Palomar Observatory in San Diego County, California, United States. The discovery formed part their Caltech Wide Area Sky Survey for bright, Pluto-sized Kuiper belt objects using the observatory's 1.22 m Samuel Oschin telescope with its wide-field CCD camera, which was operated jointly with the nightly Near Earth Asteroid Tracking program at Palomar. This survey was responsible for the discovery of several other large objects beyond Neptune, which includes the dwarf planets, , and.

was found through manual vetting of potential moving objects identified by the team's automatic image-searching software. It was among the fainter objects detected, just below the survey's limiting magnitude with an observed brightness of magnitude 20.9. Follow-up observations were conducted two months later with Palomar Observatory's 1.52 m telescope on 8 August 2002. The discovery was announced by the Minor Planet Center on 21 November 2002 and the object was given the minor planet provisional designation of.

Further observations
Since receiving follow-up in August 2002, remained unobserved for more than nine months until it was recovered by Trujillo at Palomar Observatory on 29 May 2003, followed by observations by Wolf Bickel at Bergisch Gladbach Observatory in Germany in June 2003. These recovery observations significantly reduced the uncertainty of 's orbit, allowing for further extrapolation of its position backwards in time for identification in precovery observations. 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. , has been observed for over 68 years, or about 25% of its orbital period.

Numbering and naming
received its permanent minor planet catalog number of 307261 from the Minor Planet Center on 10 December 2011. As of yet, it remains unnamed and the discoverers' privilege for naming this object expired ten years after its numbering. Per naming guidelines by the International Astronomical Union's Working Group for Small Bodies Nomenclature, is open for name suggestions that pertain to creation myths, as required for Kuiper belt objects in general.

Orbit and classification
is a trans-Neptunian object (TNO) orbiting the Sun beyond Neptune with an orbital period of 269 years. Its semi-major axis or average orbital distance from the Sun is 41.7 astronomical units (AU), with a moderate orbital eccentricity of 0.15. In its eccentric orbit, comes within 35.7 AU from the Sun at perihelion and 47.8 AU at aphelion. It has an orbital inclination of nearly 18° with respect to the ecliptic. last passed perihelion in April 1853, passed aphelion in February 1987, and will make its next perihelion passage in June 2123.

is located in the classical region of the Kuiper belt 37–48 AU from the Sun, and is thus classified as a classical Kuiper belt object or cubewano. '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. 's present orbit is far enough from Neptune (minimum orbit intersection distance 6.6 AU) that it no longer experiences scattering from close encounters with the planet.

A dynamical study in 2007 simulated 's orbital evolution over a 10-million-year timespan and found that it may be in an intermittent 18:11 mean-motion orbital resonance with Neptune, which seems to cause irregular fluctations in 's orbital inclination and eccentricity. Despite this, researchers do not consider to be in resonance with Neptune.

Observability


In the night sky, is located near the Milky Way's Galactic Center in the southern celestial hemisphere. It has been passing through that region's dense field of background stars since its discovery. Combined with 's faint apparent magnitude of 20.5 as seen from Earth, its crowded location can make observations difficult. On the other hand, 's location makes it viable for observing stellar occultations as there are numerous stars for it to pass in front of.

Occultations
Stellar occultations by occur when it passes in front of a star and blocks out its light, causing the star to dim for several seconds until  emerges. Observing stellar occultations by can provide precise measurements for its position, shape, and size. Due to parallax between Earth,, and the occulted star, occultations by may only be observable to certain locations on Earth. For this reason, 's orbital trajectory and ephemeris must be precisely known before occultation predictions can be reliably made.

To facilitate occultation predictions for, astronomers of the European Research Council's Lucky Star project gathered astrometric observations of from 2009–2019 to reduce its orbital uncertainty and utilized the Gaia catalogues for high-precision positions of stars. From 2019–2022, the Lucky Star project organized campaigns for astronomers worldwide to observe the predicted occultations by, yielding nine successfully-observed occultations by the end of the period. The first successfully-observed occultation by took place in South America on 9 July 2019, which yielded two positive detections and four negative detections from the 10 participating telescope locations; the remaining four telescopes were affected by poor weather. More successful observations of 's occultations took place on 26 July and 19 August 2019, which provided highly precise astrometry that helped refine later occultation predictions.

On 8 August 2020, the Lucky Star project organized a large observing campaign for, which would occult a relatively bright star of apparent magnitude 14.6 and be observable over densely-populated regions in multiple continents. A total of 116 telescope locations from Europe, North Africa, and Western Asia participated in the campaign and yielded 61 positive detections and 40 negative detections, with the remaining 15 telescopes inhibited by poor weather or technical difficulties. The observers of the occultation found no evidence of rings, cometary jets, or natural satellites around. This is the most extensive participation in a TNO occultation campaign. Thanks to the large amount of positive detections across various locations, the global shape outline and topography of could be seen clearly for the first time.

Physical characteristics
Results from the extensively observed 8 August 2020 occultation show that has a shape close to that of an oblate spheroid, with an equatorial diameter of 814 km and a polar diameter of up to 770 km. 's mean diameter from these dimensions is 796 km, which places it between the diameters of the two largest asteroids, Ceres and Vesta. It is unknown whether '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. is the 10th (or 11th if counting Pluto's moon Charon) largest known TNO. Because of its large size, it is considered a dwarf planet candidate by astronomers. With measurement uncertainties considered, it is tied with and  (diameters $823$ and $770$, respectively) as the largest unnamed object in the Solar System.

was previously thought to have a larger diameter of 934 km, according to infrared thermal emission measurements made by the Spitzer and Herschel space telescopes in 2006 and 2010. This thermal emission-derived diameter disagrees with the occultation-derived diameter; if both the thermal emission measurements and occultation-derived diameter are correct, then would be emitting more thermal radiation than predicted if it were a non-rotating, simple airless body. It is not yet clear why seems to be emitting excess thermal radiation; it could be possible that either there is an unknown satellite of  contributing to the excess thermal emission,  or the predictions for 's thermal emission behavior are inaccurate.

The mass and density of is unknown since it has no known moons, otherwise estimation of its mass would have been possible by Kepler's third law. Without a known mass and density, it is not possible to determine whether 's spheroidal shape is due to hydrostatic equilibrium, which would qualify it as a dwarf planet. Inferring from its diameter and albedo, is probably not in hydrostatic equilibrium since it lies within the 400-1000 km diameter range where TNOs are typically observed with very low densities, presumably due to having highly porous interior structures that have not gravitationally compressed into solid bodies. Otherwise, if is in hydrostatic equilibrium, then its density could be estimated from its oblateness and rotation period. However, both of these properties are poorly known for, so only its minimum and maximum possible densities could be estimated. Assuming a Maclaurin spheroid as the equilibrium shape for, the ranges of possible densities are $796 km$ and $≥0.066$ for possible rotation periods of 7.44 and 10.44 hours, respectively.

Surface
has a gray or spectrally neutral surface color, meaning it reflects similar amounts of light for wavelengths across the visible spectrum. In Barucci et al.'s classification scheme for TNO color indices, falls under the BB group of TNOs with neutral colors, whose surface compositions characteristically have a high fraction of water ice and amorphous carbon but low amounts of tholins. Near-infrared spectroscopy by the James Webb Space Telescope (JWST) in 2022 revealed the presence of crystalline water ice, amorphous water ice, and carbon dioxide ice in 's surface. The large Kuiper belt object 120347 Salacia was observed by JWST to have a similar surface composition as. Preliminary modeling of 's JWST spectrum by Cook et al. suggests that the water ice on the object's surface consists of micrometer-sized grains and the carbon dioxide ice consists of a mix of coarser, micrometer-sized grains to finer, sub-micrometer-sized grains. Tholins should also exist on 's surface according to Cook et al.'s preliminary model, although they have not been detected in 's JWST spectrum. Volatile ices such as methane were also not detected in 's JWST spectrum. The lack of volatiles on 's surface agrees with its low geometric albedo of 0.1 determined from observations by the New Horizons spacecraft, which indicates has a very dark and unevolved surface in contrast to the bright and volatile-rich dwarf planets like Pluto. New Horizons observations of 's phase curve indicate that the icy regolith grains on the object's surface are rough and irregularly shaped.

Topographic features
The 8 August 2020 occultation revealed massive topographic features along 's northeastern outline, or limb, which notably includes a crater-like depression 322 ± 39 km wide and 45.1 ± 1.5 km deep, and a $14.251 h$ ($7.33 h$)-tall peak near the rim of the depression. Another depression feature about 10 km wide and 11 km deep was detected by a single telescope from Varages, France during the occultation; this depression feature partially occulted the star as emerged, which resulted in the star brightening gradually instead of instantly. The elevations of these observed topographic features lie beyond the maximum elevation of 6–7 km expected for an icy body of 's size, signifying that the object may have experienced a large impact in its past. It would be possible for to support its massive topographic features if its material strength increases toward its core. Topographic features on other TNOs have been previously observed through occultation, such as which has a depression feature at least 8 km deep.

The topographic peak on has a height comparable to Mars's tallest mountain, Olympus Mons, and the central mound of the Rheasilvia crater on asteroid Vesta. If 's topographic peak is a mountain, then it would qualify as one of the tallest known mountains in the Solar System. It is possible that this topographic peak may actually be an unknown 213 km-diameter satellite that was passing in front or behind during the occultation, but this scenario is unlikely according to Bruno Sicardy, one of the occultation team members. A satellite of this size would not be large enough to explain 's excess thermal emission.

If 's massive depression is a crater, then it would be the first observation of a massive crater on a TNO. The depression's width takes up about 40% of 's diameter, which is comparable to the largest crater-to-diameter ratios seen in Saturn's moons Tethys and Iapetus. For context, Tethys's largest crater Odysseus takes up about 43% of its diameter, while Iapetus's largest crater Turgis takes up about 40% of its diameter, but they are much shallower than the purported crater. The trans-Neptunian dwarf planets Pluto and Charon do not exhibit such large craters on the other hand, as their largest crater-to-diameter ratios are 10.5% and 18.9%, respectively. The depth of 's massive depression takes up 5.7% of 's diameter and exceeds those seen in the largest craters of other Solar System bodies of comparable size: the largest crater of Saturn's moon Mimas has a depth of up to 10–12 km and Vesta's Rheasilvia crater has a depth of up to 25 km.

Rotation and light curve
The rotation period of is uncertain and its rotational axial tilt is unknown. It is difficult to measure 's rotation period photometrically with telescopes on Earth since the object is obscured in a dense field of background stars. Due to 's spheroidal shape and possible surface albedo variations, its light curve only exhibits very small fluctuations in brightness (amplitude 0.05–0.12 mag ) over time as it rotates. The first attempts at measuring '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. Subsequent observations by the Galileo National Telescope in June–July 2011 took advantage of passing in front of a dark nebula, which enabled it to determine possible periods of either 7.33 hours or 10.44 hours. On the other hand, observations by the Canada–France–Hawaii Telescope in July–August 2013 measured a rotation period of 14.251 hours, with other less probable rotation period aliases of 8.932 and 5.881 hours.

New Horizons
The New Horizons spacecraft observed during 2016–2019, as part of its extended Kuiper belt mission after its successful Pluto flyby in 2015. was 15.3 AU away from the spacecraft when it began observations on 13 July 2016, and was 12.0 AU away from the spacecraft when it ended observations in 1 September 2019. New Horizons had the unique vantage point of observing and other TNOs while it was inside the Kuiper belt, which allowed the spacecraft to observe these objects at high phase angles (>2°) that are not observable from Earth. By observing how 's brightness changes as a function of phase angle, the object's phase curve could be determined, which can reveal the light scattering properties of 's surface regolith. In addition to significantly improving the knowledge of 's phase curve, the observations by New Horizons also significantly improved the precision of 's orbit.

Proposed
has been considered as a possible exploration target for future missions to the Kuiper belt and beyond, such as NASA's Interstellar Probe concept. A 2019 study by Amanda Zangari and collaborators identified several possible trajectories to for a spacecraft that would be launched in 2025–2040. For a spacecraft launched in 2027–2031, a single gravity assist from Jupiter could bring a spacecraft to over a minimum duration of 9.1–12.8 years, depending on the excess launch energy of the spacecraft. Another trajectory using a single Jupiter gravity assist for a 2040 launch date could bring a spacecraft to over a minimum duration of 13 years. A 2038–2040 launch trajectory using a single Saturn gravity assist could bring a spacecraft to over a minimum duration of 16.7 years,  while a 2038–2040 launch trajectory using two gravity assists from Jupiter and Saturn could bring a spacecraft to  over a minimum duration of 18.6–19.5 years.