# (307261) 2002 MS4

(307261) 2002 MS$_4$ (provisional designation 2002 MS$_4$) 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 MS$_4$, 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 MS$_4$ is large enough that some astronomers conclude that it might be a dwarf planet.
The surface of 2002 MS$_4$ is dark gray and is composed of water and carbon dioxide ices. 2002 MS$_4$ 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 (16 mi) tall and a crater-like depression that is 320 km (200 mi) wide and 45 km (28 mi) deep. 2002 MS$_4$'s topographic features are among the tallest and deepest known for Solar System bodies.

## History
### Discovery

2002 MS$_4$ 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-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.$^{:100}$ This survey was responsible for the discovery of several other large objects beyond Neptune, which includes the dwarf planets Eris, Sedna, and Quaoar.$^{:214}$
2002 MS$_4$ was found through manual vetting of potential moving objects identified by the team's automatic image-searching software.$^{:101}$ It was among the fainter objects detected, just below the survey's limiting magnitude with an observed brightness of magnitude 20.9.$^{:99,103}$ Follow-up observations were conducted two months later with Palomar Observatory's 1.52-meter (60 in) 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 2002 MS$_4$.

### Further observations

Since receiving follow-up in August 2002, 2002 MS$_4$ 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 2002 MS$_4$'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.$^{:42}$ As of 2023, 2002 MS$_4$ has been observed for over 68 years, or about 25% of its orbital period.

### Numbering and naming

2002 MS$_4$ received its permanent minor planet catalog number of 307261 from the Minor Planet Center on 10 December 2011.$^{:292}$ As of yet, it remains unnamed and the discoverers' privilege for naming this object expired ten years after its numbering.$^{:6}$ Per naming guidelines by the International Astronomical Union's Working Group for Small Bodies Nomenclature, 2002 MS$_4$ is open for name suggestions that pertain to creation myths, as required for Kuiper belt objects in general.$^{:8}$

## Orbit and classification

2002 MS$_4$ 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$^{:45}$ orbital eccentricity of 0.15. In its eccentric orbit, 2002 MS$_4$ 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. 2002 MS$_4$ last passed perihelion in April 1853, passed aphelion in February 1987, and will make its next perihelion passage in June 2123.
2002 MS$_4$ is located in the classical region of the Kuiper belt 37–48 AU from the Sun,$^{:227}$ and is thus classified as a classical Kuiper belt object or cubewano.$^{:53}$ 2002 MS$_4$'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.$^{:227,229}$ 2002 MS$_4$'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.$^{:214}$
A dynamical study in 2007 simulated 2002 MS$_4$'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,$^{:218}$ which seems to cause irregular fluctations in 2002 MS$_4$'s orbital inclination and eccentricity.$^{:225}$ Despite this, researchers do not consider 2002 MS$_4$ to be in resonance with Neptune.$^{:56}$$^{:2}$

## Observability

In the night sky, 2002 MS$_4$ 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.$^{:9}$ Combined with 2002 MS$_4$'s faint apparent magnitude of 20.5 as seen from Earth, its crowded location can make observations difficult.$^{:92}$$^{:9}$ On the other hand, 2002 MS$_4$'s location makes it viable for observing stellar occultations as there are numerous stars for it to pass in front of.$^{:9}$

### Occultations

2002 MS$_4$ occultations observed in 2019–2022$^{:5,18B.4}$ :
| Date | Star apparent magnitude (V-band) | Positive detections | Negative detections | Number of telescope locations | Continents observed |
| 09 Jul 2019 | 15.00 | 2 | 4 | 10 | South America |
| 26 Jul 2019 | 17.78 | 3 | 0 | 3 | South America |
| 26 Jul 2019 | 15.45 | 1 | 0 | 1 | North America |
| 19 Aug 2019 | 16.51 | 2 | 0 | 2 | North America |
| 26 Jul 2020 | 14.76 | 2 | 0 | 5 | Africa |
| 8 Aug 2020 | 14.62 | 61 | 40 | 116 | Europe, Africa, Asia |
| 24 Feb 2021 | 16.51 | 1 | 1 | 2 | South America |
| 14 Oct 2021 | 15.83 | 2 | 0 | 14 | North America |
| 10 Jun 2022 | 15.1 | 3 | 0 | 3 | North America, Africa |
Stellar occultations by 2002 MS$_4$ occur when it passes in front of a star and blocks out its light, causing the star to dim for several seconds until 2002 MS$_4$ emerges.$^{:2}$ Observing stellar occultations by 2002 MS$_4$ can provide precise measurements for its position, shape, and size.$^{:1}$$^{:35}$ Due to parallax between Earth, 2002 MS$_4$, and the occulted star, occultations by 2002 MS$_4$ may only be observable to certain locations on Earth. For this reason, 2002 MS$_4$'s orbital trajectory and ephemeris must be precisely known before occultation predictions can be reliably made.$^{:2}$$^{:35}$
To facilitate occultation predictions for 2002 MS$_4$, astronomers of the European Research Council's Lucky Star project gathered astrometric observations of 2002 MS$_4$ from 2009–2019 to reduce its orbital uncertainty and utilized the Gaia catalogues for high-precision positions of stars.$^{:2}$ From 2019 to 2022, the Lucky Star project organized campaigns for astronomers worldwide to observe the predicted occultations by 2002 MS$_4$, yielding nine successfully-observed occultations by the end of the period.$^{:1,3}$ The first successfully-observed occultation by 2002 MS$_4$ 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.$^{:2,18B.4}$ More successful observations of 2002 MS$_4$'s occultations took place on 26 July and 19 August 2019, which provided highly precise astrometry that helped refine later occultation predictions.$^{:2}$
On 8 August 2020, the Lucky Star project organized a large observing campaign for 2002 MS$_4$, which would occult a relatively bright star of apparent magnitude 14.6 and be observable over densely-populated regions in multiple continents.$^{:4}$ 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.$^{:4,18B.1–3}$ The observers of the occultation found no evidence of rings, cometary jets, or natural satellites around 2002 MS$_4$.$^{:9}$ This is the most extensive participation in a TNO occultation campaign as of 2023.$^{:1347}$$^{:9}$ Thanks to the large amount of positive detections across various locations, the global shape outline and topography of 2002 MS$_4$ could be seen clearly for the first time.
* Map showing the location of telescopes that participated in the 8 August 2020 occultation campaign. Telescopes within the path of 2002 MS$_4$'s shadow (region between the two solid blue curves) made positive detections (blue and red points), whereas telescopes outside the path made negative detections (green points).
* 2002 MS$_4$'s projected shape revealed by the many positive detection chords from the 8 August 2020 occultation (blue with red error bars). A massive topographic peak and depression is visible along 2002 MS$_4$'s limb in the northeast direction.

## Physical characteristics

History of diameter estimates for 2002 MS$_4$ :
| Year of Publication | Diameter (km) | Method | Refs |
| 2008 | 726.2+123.2 −122.9 | thermal (Spitzer) | $^{:173}$ |
| 2009 | 730+118 −120 | thermal (Spitzer, remodeled) | $^{:291}$ |
| 2012 | 934±47 | thermal (Herschel) | $^{:10}$ |
| 2020 | 770±2 | occultation (9 Jul 2019) | |
| 2022 | <810±70 | occultation (26 Jul 2019) | |
| 2023 | 796±24 | occultation (8 Aug 2020) | |
Results from the extensively observed 8 August 2020 occultation show that 2002 MS$_4$ has a shape close to that of an oblate spheroid, with an equatorial diameter of 814 km (506 mi) and a polar diameter of up to 770 km (480 mi).$^{:5}$ 2002 MS$_4$'s mean diameter from these dimensions is 796 km (495 mi), which places it between the diameters of the two largest asteroids, Ceres and Vesta.$^{:5}$ It is unknown whether 2002 MS$_4$'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.$^{:8}$ 2002 MS$_4$ 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.$^{:245}$$^{:2}$$^{:1}$$^{:iii}$ With measurement uncertainties considered, it is tied with 2002 AW197 and 2013 FY27 (diameters 729–807 km and 659–820 km, respectively) as the largest unnamed object in the Solar System.
2002 MS$_4$ 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.$^{:4,7,10}$ This thermal emission-derived diameter disagrees with the occultation-derived diameter; if both the thermal emission measurements and occultation-derived diameter are correct, then 2002 MS$_4$ would be emitting more thermal radiation than predicted if it were a non-rotating, simple airless body.$^{:68,70,73}$ It is not yet clear why 2002 MS$_4$ seems to be emitting excess thermal radiation; it could be possible that either there is an unknown satellite of 2002 MS$_4$ contributing to the excess thermal emission,$^{:9}$ or the predictions for 2002 MS$_4$'s thermal emission behavior are inaccurate.$^{:73}$
The mass and density of 2002 MS$_4$ is unknown since it has no known moons; otherwise, estimation of its mass would have been possible by Kepler's third law.$^{:35}$ Without a known mass and density, it is not possible to determine whether 2002 MS$_4$'s spheroidal shape is due to hydrostatic equilibrium, which would qualify it as a dwarf planet.$^{:10}$ Inferring from its diameter and albedo, 2002 MS$_4$ is probably not in hydrostatic equilibrium since it lies within the 400–1,000 km (250–620 mi) 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.$^{:1,8}$ Otherwise, if 2002 MS$_4$ is in hydrostatic equilibrium, then its density could be estimated from its oblateness and rotation period.$^{:8}$ However, both of these properties are poorly known for 2002 MS$_4$, so only its minimum and maximum possible densities could be estimated.$^{:8}$ Assuming a Maclaurin spheroid as the equilibrium shape for 2002 MS$_4$, the ranges of possible densities are 0.72–8.0 g/cm$^3$ and 0.36–3.9 g/cm$^3$ for possible rotation periods of 7.44 and 10.44 hours, respectively.$^{:8}$

### Surface

2002 MS$_4$ has a gray or spectrally neutral surface color, meaning it reflects similar amounts of light for wavelengths across the visible spectrum.$^{:6}$ In Barucci et al.'s classification scheme for TNO color indices, 2002 MS$_4$ 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.$^{:1294,1296}$ 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 2002 MS$_4$'s surface. The large Kuiper belt object 120347 Salacia was observed by JWST to have a similar surface composition as 2002 MS$_4$. Preliminary modeling of 2002 MS$_4$'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 2002 MS$_4$'s surface according to Cook et al.'s preliminary model, although they have not been detected in 2002 MS$_4$'s JWST spectrum. Volatile ices such as methane were also not detected in 2002 MS$_4$'s JWST spectrum. The lack of volatiles on 2002 MS$_4$'s surface agrees with its low geometric albedo of 0.1 determined from observations by the New Horizons spacecraft, which indicates 2002 MS$_4$ has a very dark and unevolved surface in contrast to the bright and volatile-rich dwarf planets like Pluto.$^{:2,18–19}$ New Horizons observations of 2002 MS$_4$'s phase curve indicate that the icy regolith grains on the object's surface are rough and irregularly shaped.$^{:19}$

### Topographic features

The 8 August 2020 occultation revealed massive topographic features along 2002 MS$_4$'s northeastern outline, or limb, which notably includes a crater-like depression 322 ± 39 km (200 ± 24 mi) wide and 45.1 ± 1.5 km (28.02 ± 0.93 mi) deep, and a 25+4 −5 km (15.5+2.5 −3.1 mi)-tall peak near the rim of the depression.$^{:7}$ 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 MS$_4$ emerged, which resulted in the star brightening gradually instead of instantly.$^{:7}$ The elevations of these observed topographic features lie beyond the maximum elevation of 6–7 km (3.7–4.3 mi) expected for an icy body of 2002 MS$_4$'s size, signifying that the object may have experienced a large impact in its past.$^{:6,9}$ It would be possible for 2002 MS$_4$ to support its massive topographic features if its material strength increases toward its core.$^{:6}$ Topographic features on other TNOs have been previously observed through occultation, such as (208996) 2003 AZ84 which has a depression feature at least 8 km (5 mi) deep.
The topographic peak on 2002 MS$_4$ has a height comparable to Mars's tallest mountain, Olympus Mons, and the central mound of the Rheasilvia crater on asteroid Vesta. If 2002 MS$_4$'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 (132 mi)-diameter satellite that was passing in front or behind 2002 MS$_4$ during the occultation, but this scenario is unlikely according to Bruno Sicardy, one of the occultation team members.$^{:9,25}$ A satellite of this size would not be large enough to explain 2002 MS$_4$'s excess thermal emission.$^{:25}$
If 2002 MS$_4$'s massive depression is a crater, then it would be the first observation of a massive crater on a TNO.$^{:9}$ The depression's width takes up about 40% of 2002 MS$_4$'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 2002 MS$_4$ crater.$^{:9}$ 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.$^{:9}$ The depth of 2002 MS$_4$'s massive depression takes up 5.7% of 2002 MS$_4$'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 (6.2–7.5 mi)$^{:424}$ and Vesta's Rheasilvia crater has a depth of up to 25 km (16 mi).

### Rotation and light curve

The rotation period of 2002 MS$_4$ is uncertain and its rotational axial tilt is unknown. It is difficult to measure 2002 MS$_4$'s rotation period photometrically with telescopes on Earth since the object is obscured in a dense field of background stars.$^{:118}$$^{:7}$ Due to 2002 MS$_4$'s spheroidal shape and possible surface albedo variations, its light curve only exhibits very small fluctuations in brightness (amplitude 0.05–0.12 mag$^{:85}$) over time as it rotates.$^{:7}$$^{:73}$ The first attempts at measuring 2002 MS$_4$'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.$^{:31,92}$ Subsequent observations by the Galileo National Telescope in June–July 2011 took advantage of 2002 MS$_4$ passing in front of a dark nebula, which enabled it to determine possible periods of either 7.33 hours or 10.44 hours.$^{:94}$ 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.$^{:43,53,74}$

## Exploration
### New Horizons

The New Horizons spacecraft observed 2002 MS$_4$ during 2016–2019, as part of its extended Kuiper belt mission after its successful Pluto flyby in 2015.$^{:8}$ 2002 MS$_4$ was 15.3 AU (2.29 billion km; 1.42 billion mi) away from the spacecraft when it began observations on 13 July 2016, and was 12.0 AU (1.80 billion km; 1.12 billion mi) away from the spacecraft when it ended observations on 1 September 2019.$^{:8}$ New Horizons had the unique vantage point of observing 2002 MS$_4$ 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.$^{:1}$ By observing how 2002 MS$_4$'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 2002 MS$_4$'s surface regolith.$^{:1}$ In addition to significantly improving the knowledge of 2002 MS$_4$'s phase curve, the observations by New Horizons also significantly improved the precision of 2002 MS$_4$'s orbit.
* 2002 MS$_4$ imaged by the New Horizons spacecraft in July 2016, from a distance of 15.3 AU (2.3 billion km; 1.4 billion mi)
* New Horizons trajectory through the Kuiper belt, with positions of nearby KBOs including 2002 MS$_4$ labeled

### Proposed

2002 MS$_4$ 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 2002 MS$_4$ 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 2002 MS$_4$ over a minimum duration of 9.1–12.8 years, depending on the excess launch energy of the spacecraft.$^{:922}$ Another trajectory using a single Jupiter gravity assist for a 2040 launch date could bring a spacecraft to 2002 MS$_4$ over a minimum duration of 13 years.$^{:922}$ A 2038–2040 launch trajectory using a single Saturn gravity assist could bring a spacecraft to 2002 MS$_4$ over a minimum duration of 16.7 years,$^{:925}$ while a 2038–2040 launch trajectory using two gravity assists from Jupiter and Saturn could bring a spacecraft to 2002 MS$_4$ over a minimum duration of 18.6–19.5 years.$^{:923}$