(307261) 2002 MS4

(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, and has precovery images back to 8 April 1954.
, is 46.4 AU from the Sun. It will reach perihelion, its closest point to the Sun, in 2123. At in diameter, it is approximately tied with and (to within measurement uncertainties) as the largest unnamed object in the Solar System.
History
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 Kuiper belt objects using the observatory's 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, distant objects, including 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 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 improved '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, which is 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 has expired ten years since 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 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. Its high orbital inclination qualifies it as a dynamically "hot" member of the classical Kuiper belt, which implies it was gravitationally scattered out to its present location by Neptune early in its 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
On 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 its faint apparent magnitude of 20.5 as seen from Earth, at its crowded location can make Earth-based 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, the '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 an exceptionally 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. This is the most extensive participation demonstrated from 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 and a polar diameter of up to . 's mean diameter from these dimensions is . It is uncertain 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 approximately tied with and (diameters and , respectively) as the largest unnamed object in the Solar System.
was previously thought to have a larger diameter of , according to infrared thermal emission measurements made by the Spitzer and Herschel space telescopes in 2006 and 2010. If these thermal emission measurements are correct alongside 's occultation-derived diameter, then would be emitting more thermal radiation than predicted for its size 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, which would have made mass estimates possible by Kepler's third law. Without a known mass and density, it is not possible to absolutely determine whether is truly large enough to have become spherical by 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 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.
If is in hydrostatic equilibrium on the other hand, 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 and 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. Near-infrared spectroscopy by the James Webb Space Telescope (JWST) in 2022 revealed the presence of water ice in 's surface. No signs of volatile ices such as methane were detected in JWST's spectrum of . This is consistent with 's low geometric albedo of 0.1 determined from New Horizons observations, which indicates the object has a very dark and unevolved surface depleted in volatiles.
Topographic features
The 8 August 2020 occultation revealed massive topographic features along 's northeastern outline, or limb, which notably includes a crater-like depression wide deep, and a -tall peak near the rim of the depression. Another depression feature about wide and deep was detected by a single telescope from 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 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 have been previously observed through occultation in other TNOs, such as which has a depression feature at least 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 4 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 -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, the Odysseus crater takes up about 43% of Tethys's diameter, while the Turgis crater takes up about 40% of Iapetus's diameter. 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.
Rotation
The rotation period of is uncertain. Light curve observations of are difficult because of the dense field of background stars it is crossing. 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 . Observations made in June and July 2011 took advantage of moving in front of a dark nebula. Observations by the Canada–France–Hawaii Telescope from July–August 2013 measured a rotation period of 14.251 hours with a light curve amplitude of , in addition to less probable rotation periods of 8.932 and 5.881 hours.
Exploration
New Horizons
From July 2016 to September 2019, was observed by the New Horizons spacecraft, as part of its extended Kuiper belt mission. The observations significantly improved the knowledge of 's orbit and phase curve behavior from the scattering properties of its surface.
See also
List of Solar System objects by size
Notes
References
External links
2002 MS4 Precovery Images
(307261) 2002 MS4
307261
Discoveries by Michael E. Brown
Discoveries by Chad Trujillo
307261
Objects observed by stellar occultation
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