(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.[13] It was discovered in 2002 by Chad Trujillo and Michael Brown, and has precovery images back to 8 April 1954.[2]

(307261) 2002 MS4
2002 MS4 imaged by the Hubble Space Telescope on 9 April 2006
Discovery[1]
Discovered byChadwick A. Trujillo
Michael E. Brown
Discovery sitePalomar Obs.
Discovery date18 June 2002
Designations
2002 MS4
TNO[2] · cubewano (hot)[3]: 56 
distant[1] · Scat-Ext[4]
Orbital characteristics (barycentric)[5][2]
Epoch 25 February 2023 (JD 2460000.5)
Uncertainty parameter 3
Observation arc66.22 yr (24,188 d)
Earliest precovery date8 April 1954
Aphelion47.801 AU
Perihelion35.677 AU
41.739 AU
Eccentricity0.14524
269.48 yr (98,429 d)
226.844°
0° 0m 13.167s / day
Inclination17.693°
216.075°
≈ 12 June 2123[6]
±5 days[2]
214.575°
Physical characteristics
Dimensions(823±20) × (770±34) km (projected)[7]
Mean diameter
796±24 km[7]
Flattening≥0.066±0.034[7]: 6 
14.251 h[8]: 5, 54 
7.33 h or 10.44 h (single-peaked)[9]: 158 
0.100±0.025[7]: 9  or 0.098±0.004[10]: 2  (geometric)
0.039±0.005 (Bond)[10]: 23 
B−V=0.69±0.02[11]: 6 
V−R=0.38±0.02
B−R=1.07±0.02
20.5[12]
3.56±0.03[8]: 62, 74 
3.63±0.05[7]: 9 
3.62[2][1]

    As of 2021, 2002 MS4 is 46.4 AU from the Sun.[12] It will reach perihelion, its closest point to the Sun, in 2123.[6] At 800 km (500 mi) in diameter, it is approximately tied with 2002 AW197 and 2013 FY27 (to within measurement uncertainties) as the largest unnamed object in the Solar System.

    History

    Discovery

    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 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, distant objects, including the dwarf planets Eris, Sedna, and Quaoar.[15]: 214 

    2002 MS4 was found through manual vetting of potential moving objects identified by the team's automatic image-searching software.[14]: 101  It was among the fainter objects detected, just below the survey's limiting magnitude with an observed brightness of magnitude 20.9.[14]: 99, 103  Follow-up observations were conducted two months later with Palomar Observatory's 1.52-meter (60 in) telescope on 8 August 2002.[16] 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 MS4.[16]

    The 1.2-meter Samuel Oschin telescope that was used to discover 2002 MS4 at Palomar Observatory
    Discovery images of 2002 MS4 from 18 June 2002

    Further observations

    Since receiving follow-up in August 2002, 2002 MS4 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.[17] These recovery observations significantly improved 2002 MS4's orbit, allowing for further extrapolation of its position backwards in time for identification in precovery observations.[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.[18][19]: 42  As of 2023, 2002 MS4 has been observed for over 68 years, which is about 25% of its orbital period.[2][1]

    Numbering and naming

    2002 MS4 received its permanent minor planet catalog number of 307261 from the Minor Planet Center on 10 December 2011.[18][20]: 292  As of yet, it remains unnamed and the discoverers' privilege for naming this object has expired ten years since its numbering.[1][21]: 6  Per naming guidelines by the International Astronomical Union's Working Group for Small Bodies Nomenclature, 2002 MS4 is open for name suggestions that pertain to creation myths, as required for Kuiper belt objects in general.[21]: 8 

    Orbit and classification
    The 18:11-resonant libration of 2002 MS4's nominal orbit, in a frame co-rotating with Neptune
    Top and side views of 2002 MS4's orbit (white) with Pluto and other classical Kuiper belt objects for comparison

    2002 MS4 is a trans-Neptunian object (TNO) orbiting the Sun beyond Neptune with an orbital period of 269 years.[5][lower-alpha 1] Its semi-major axis or average orbital distance from the Sun is 41.7 astronomical units (AU), with a moderate[3]: 45  orbital eccentricity of 0.15.[5] In its eccentric orbit, 2002 MS4 comes within 35.7 AU from the Sun at perihelion and 47.8 AU at aphelion.[5] It has an orbital inclination of nearly 18° with respect to the ecliptic.[5] 2002 MS4 last passed perihelion in April 1853 and will make its next perihelion passage in June 2123.[23][6]

    2002 MS4 is located in the classical region of the Kuiper belt 37–48 AU from the Sun,[24]: 227  and is thus classified as a classical Kuiper belt object or cubewano.[3]: 53  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.[24]: 227, 229  2002 MS4's present orbit is far enough from Neptune (minimum orbit intersection distance 6.6 AU)[1] that it no longer experiences scattering from close encounters with the planet.[4][24]: 214 

    A dynamical study in 2007 simulated 2002 MS4'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,[24]: 218  which seems to cause irregular fluctations in 2002 MS4's orbital inclination and eccentricity.[24]: 225  Despite this, researchers do not consider 2002 MS4 to be in resonance with Neptune.[4][3]: 56 [10]: 2 

    Observability
    2002 MS4's position in the constellation Scutum in 2020, moving eastward across the brightest areas of the Milky Way

    On the night sky, 2002 MS4 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.[10]: 9  Combined with its faint apparent magnitude of 20.5 as seen from Earth,[12] 2002 MS4 at its crowded location can make Earth-based observations difficult.[9]: 92 [10]: 9  On the other hand, 2002 MS4's location makes it viable for observing stellar occultations as there are numerous stars for it to pass in front of.[10]: 9 

    Occultations
    2002 MS4 occultations observed in 2019–2022[7]: 5, 25–26 
    DateStar apparent
    magnitude
    (V-band)    		Positive
    detections    		Negative
    detections[lower-alpha 2]    		Number of
    telescope locations[lower-alpha 3]    		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 MS4 occur when it passes in front of a star and blocks out its light, causing the star to dim for several seconds until 2002 MS4 emerges.[7]: 3  Observing stellar occultations by 2002 MS4 can provide precise measurements for its position, shape, and size.[7]: 1 [8]: 35  Due to parallax between Earth, 2002 MS4, and the occulted star, occultations by 2002 MS4 may only be observable to certain locations on Earth. For this reason, the 2002 MS4's orbital trajectory and ephemeris must be precisely known before occultation predictions can be reliably made.[7]: 3 [8]: 35 

    To facilitate occultation predictions for 2002 MS4, astronomers of the European Research Council's Lucky Star project gathered astrometric observations of 2002 MS4 from 2009–2019 to reduce its orbital uncertainty and utilized the Gaia catalogues for high-precision positions of stars.[25][7]: 3  From 2019–2022, the Lucky Star project organized campaigns for astronomers worldwide to observe the predicted occultations by 2002 MS4, yielding nine successfully-observed occultations by the end of the period.[7]: 1, 3  The first successfully-observed occultation by 2002 MS4 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.[25][7]: 3, 25–26  More successful observations of 2002 MS4's occultations took place on 26 July and 19 August 2019, which provided highly precise astrometry that helped refine later occultation predictions.[26][7]: 3 

    On 8 August 2020, the Lucky Star project organized an exceptionally large observing campaign for 2002 MS4, which would occult a relatively bright star of apparent magnitude 14.6 and be observable over densely-populated regions in multiple continents.[7]: 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.[7]: 4, 24  This is the most extensive participation demonstrated from a TNO occultation campaign as of 2023.[27]: 1347 [7]: 9  Thanks to the large amount of positive detections across various locations, the global shape outline and topography of 2002 MS4 could be seen clearly for the first time.[28][7]

    Physical characteristics
    History of diameter estimates for 2002 MS4
    Year of
    Publication    		Diameter
    (km)    		MethodRefs
    2008 726.2+123.2
    −122.9    		 thermal
    (Spitzer)    		 [29]: 173 
    2009 730+118
    −120    		 thermal
    (Spitzer, remodeled)    		 [30]: 291 
    2012 934±47 thermal
    (Herschel)    		 [13]: 10 
    2020 770±2 occultation
    (9 Jul 2019)    		 [25]
    2022 <810±70 occultation
    (26 Jul 2019)    		 [31]
    2023 796±24 occultation
    (8 Aug 2020)    		 [7][lower-alpha 4]

    Results from the extensively observed 8 August 2020 occultation show that 2002 MS4 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).[7]: 9  2002 MS4's mean diameter from these dimensions is 796 km (495 mi).[7]: 1  It is uncertain 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.[7]: 8 

    2002 MS4 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.[32]: 245 [7]: 1 [8]: iii  With measurement uncertainties considered, it is approximately tied with 2002 AW197 and 2013 FY27 (diameters 729–807 km[33] and 659–820 km,[34] respectively) as the largest unnamed object in the Solar System.

    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.[13]: 4, 7, 10  If these thermal emission measurements are correct alongside 2002 MS4's occultation-derived diameter, then 2002 MS4 would be emitting more thermal radiation than predicted for its size if it were a non-rotating, simple airless body.[8]: 68, 70, 73  It is not yet clear why 2002 MS4 seems to be emitting excess thermal radiation; it could be possible that either there is an unknown satellite of 2002 MS4 contributing to the excess thermal emission,[7]: 9  or the predictions for 2002 MS4's thermal emission behavior are inaccurate.[8]: 73 

    The mass and density of 2002 MS4 is unknown since it has no known moons, which would have made mass estimates possible by Kepler's third law.[8]: 35  Without a known mass and density, it is not possible to absolutely determine whether 2002 MS4 is truly large enough to have become spherical by hydrostatic equilibrium, which would qualify it as a dwarf planet.[35]: 10  Inferring from its diameter and albedo, 2002 MS4 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.[36]: 1, 8 

    If 2002 MS4 is in hydrostatic equilibrium on the other hand, then its density could be estimated from its oblateness and rotation period.[7]: 8–9  However, both of these properties are poorly known for 2002 MS4, so only its minimum and maximum possible densities could be estimated.[7]: 8  Assuming a Maclaurin spheroid as the equilibrium shape for 2002 MS4, the ranges of possible densities are 0.72–8.0 g/cm3 and 0.36–3.9 g/cm3 for possible rotation periods of 7.44 and 10.44 hours, respectively.[7]: 9 

    Surface
    Artistic comparison of Pluto, Eris, Haumea, Makemake, Gonggong, Quaoar, Sedna, Orcus, Salacia, 2002 MS4, and Earth along with the Moon

    2002 MS4 has a gray or spectrally neutral surface color, meaning it reflects similar amounts of light for wavelengths across the visible spectrum.[11]: 6  In Barucci et al.'s classification scheme for TNO color indices, 2002 MS4 falls under the BB group of TNOs with neutral colors.[37]: 1294  Near-infrared spectroscopy by the James Webb Space Telescope (JWST) in 2022 revealed the presence of water ice in 2002 MS4's surface.[38] No signs of volatile ices such as methane were detected in JWST's spectrum of 2002 MS4.[38] This is consistent with 2002 MS4'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.[10]: 2, 18–19 

    Topographic features
    2002 MS4's projected shape as seen from positive detections of the 8 August 2020 occultation (blue). A massive topographic peak and depression is visible along 2002 MS4's limb in the northeast direction.

    The 8 August 2020 occultation revealed massive topographic features along 2002 MS4's northeastern outline, or limb, which notably includes a crater-like depression 322 ± 39 km (200 ± 24 mi) wide 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]: 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 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.[7]: 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 MS4's size, signifying that the object may have experienced a large impact in its past.[7]: 7, 10  It would be possible for 2002 MS4 to support its massive topographic features if its material strength increases toward its core.[7]: 7  Topographic features have been previously observed through occultation in other TNOs, such as (208996) 2003 AZ84 which has a depression feature at least 8 km (5 mi) deep.[39][40]

    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 4 Vesta.[40] If 2002 MS4's topographic peak is a mountain, then it would qualify as one of the tallest known mountains in the Solar System.[40] 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 MS4 during the occultation, but this scenario is unlikely according to Bruno Sicardy, one of the occultation team members.[7]: 10, 34 [40] A satellite of this size would not be large enough to explain 2002 MS4's excess thermal emission.[7]: 34 

    If 2002 MS4's massive depression is a crater, then it would be the first observation of a massive crater on a TNO.[7]: 10  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. 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.[7]: 10  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.[7]: 10 

    Rotation

    The rotation period of 2002 MS4 is uncertain. Light curve observations of 2002 MS4 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 0.05±0.01 mag. Observations made in June and July 2011 took advantage of 2002 MS4 moving in front of a dark nebula.[9]: 118  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 0.12±0.01 mag, in addition to less probable rotation periods of 8.932 and 5.881 hours.[8]: 43, 53, 74 

    Exploration

    New Horizons
    2002 MS4 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 MS4 labeled

    From July 2016 to September 2019, 2002 MS4 was observed by the New Horizons spacecraft, as part of its extended Kuiper belt mission.[10]: 11  The observations significantly improved the knowledge of 2002 MS4's orbit and phase curve behavior from the scattering properties of its surface.[41][42]

    See also

    Notes
    1. These orbital elements are expressed in terms of the Solar System Barycenter (SSB) as the frame of reference.[5] Due to planetary perturbations, the Sun revolves around the SSB at non-negligible distances, so heliocentric-frame orbital elements and distances can vary in short timescales as shown in JPL-Horizons.[22]
    2. Telescopes that were affected by poor weather or technical problems are not counted as negative detections.
    3. Telescopes that are located in the same place or are located very close together are considered single locations.
    4. Rommel et al. previously reported a diameter of 800±24 km in a 2021 conference talk about their preliminary 8 August 2020 occultation results.[28] This diameter estimate has been superseded by the more recent estimate of 796±24 km in their final paper published in 2023.[7]

    References
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    2. "JPL Small-Body Database Browser: 307261 (2002 MS4)" (2020-06-28 last obs.). Jet Propulsion Laboratory. Retrieved 18 June 2022.
    3. Gladman, Brett; Marsden, Brian G.; VanLaerhoven, Christa (2008). "Nomenclature in the Outer Solar System" (PDF). The Solar System Beyond Neptune. University of Arizona Press. pp. 43–57. arXiv:astro-ph/0702538. Bibcode:2008ssbn.book...43G. ISBN 9780816527557. S2CID 14469199.
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    5. "JPL Horizons On-Line Ephemeris for 307261 (2002 MS4) at epoch JD 2460000.5". JPL Horizons On-Line Ephemeris System. Jet Propulsion Laboratory. Retrieved 19 June 2022. Solution using the Solar System Barycenter. Ephemeris Type: Elements and Center: @0)
    6. "JPL Horizons On-Line Ephemeris for 307261 (2002 MS4) from 2123-Jan-01 to 2124-Jan-01". JPL Horizons On-Line Ephemeris System. Jet Propulsion Laboratory. Retrieved 28 June 2022. (Perihelion occurs when deldot changes from negative to positive. Uncertainty in time of perihelion is 1-sigma from JPL Small-Body Database.)
    7. Rommel, F. L.; Braga-Ribas, F.; Ortiz, J. L.; Sicardy, B.; Santos-Sanz, P.; Desmars, J.; et al. (August 2023). "A large topographic feature on the surface of the trans-Neptunian object (307261) 2002 MS4 measured from stellar occultations". Astronomy & Astrophysics. in press. arXiv:2308.08062. doi:10.1051/0004-6361/202346892. S2CID 260926329.
    8. Peng, Jinghan (September 2023). Phase Dependent Variation in the Reflectivity of Kuiper Belt Object 2002 MS4 (PDF) (MSc thesis). University of Victoria. hdl:1828/15363. Archived (PDF) from the original on 9 September 2023. Retrieved 9 September 2023.
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