 2002 MS4 imaged by theHubble Space Telescope on 9 April 2006 | |
| Discovery[1] | |
|---|---|
| Discovered by | Chadwick A. Trujillo Michael E. Brown |
| Discovery site | Palomar Obs. |
| Discovery date | 18 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 1 | |
| Observation arc | 66.22 yr (24,188 d) |
| Earliestprecovery date | 8 April 1954 |
| Aphelion | 47.801AU |
| Perihelion | 35.677 AU |
| 41.739AU | |
| Eccentricity | 0.14524 |
| 269.48yr (98,429d) | |
| 226.844° | |
| 0° 0m 13.167s / day | |
| Inclination | 17.693° |
| 216.075° | |
| ≈ 10 June 2123[6] ±0.4 days[2] | |
| 214.575° | |
| Physical characteristics | |
| Dimensions | (823±20) × (770±34) km (projected)[7] |
| 796±24 km[7] | |
| Flattening | ≥0.066±0.034[7]: 5 |
| 14.251 h[8]: 5, 54 7.33 h or10.44 h (single-peaked)[9]: 158 [a] | |
| 0.100±0.025[7]: 8 or0.098±0.004[10]: 2 (geometric) 0.039±0.005 (Bond)[10]: 23 | |
| Temperature | 65K[11] |
| B−V=0.69±0.02[12]: 6 V−R=0.38±0.02 B−R=1.07±0.02 | |
| 20.5[13] | |
| 3.56±0.03[8]: 62, 74 3.63±0.05[7]: 8 3.62[2][1] | |
(307261) 2002 MS4 (provisional designation2002 MS4) is a largetrans-Neptunian object in theKuiper belt, which is a region of icyplanetesimals beyondNeptune. It was discovered on 18 June 2002 byChad Trujillo andMichael Brown during their search for bright,Pluto-sized Kuiper belt objects atPalomar Observatory. To within measurement uncertainties,2002 MS4,2002 AW197, and2013 FY27 have a diameter close to 800 km (500 mi), which makes them the largest unnamed objects in the Solar System.2002 MS4 is large enough that some astronomers conclude that itmight be a dwarf planet.
The surface of2002 MS4 is dark gray and is composed of water andcarbon dioxide ices.2002 MS4 has been observed throughstellar 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 acrater-like depression that is 320 km (200 mi) wide and 45 km (28 mi) deep.2002 MS4's topographic features are among thetallest and deepest known for Solar System bodies.
2002 MS4 was discovered on 18 June 2002 by astronomersChad Trujillo andMichael Brown atPalomar Observatory inSan Diego County, California, United States.[1] The discovery formed part their Caltech Wide Area Sky Survey for bright,Pluto-sizedKuiper belt objects using the observatory's 1.22-meter (48 in)Samuel Oschin telescope with its wide-fieldCCD camera, which was operated jointly with the nightlyNear Earth Asteroid Tracking program at Palomar.[14]: 100 This survey was responsible for the discovery of several other large objects beyondNeptune, which includes thedwarf planetsEris,Sedna, andQuaoar.[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'slimiting magnitude with anobserved brightness ofmagnitude 20.9.[14]: 99, 103 [b] 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 theMinor Planet Center on 21 November 2002 and the object was given theminor planetprovisional designation of2002 MS4.[16]
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 byWolf Bickel atBergisch Gladbach Observatory in Germany in June 2003.[17] These recovery observations significantly reduced the uncertainty of2002 MS4's orbit, allowing for further extrapolation of its position backwards in time for identification inprecovery observations.[18] Seven precovery observations fromDigitized Sky Surveyplates were identified by astronomer Andrew Lowe in 2007; the earliest of these was taken on 8 April 1954 byPalomar Observatory.[18][19]: 42 As of 2023[update],2002 MS4has been observed for over 68 years, or about 25% of itsorbital period.[2][1]
2002 MS4 received its permanentminor 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 expired ten years after its numbering.[1][21]: 6 Per naming guidelines by theInternational Astronomical Union'sWorking 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
2002 MS4 is atrans-Neptunian object (TNO) orbiting theSun beyond Neptune with an orbital period of 269 years.[5][c] Itssemi-major axis or average orbital distance from the Sun is 41.7astronomical 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 atperihelion and 47.8 AU ataphelion.[5] It has anorbital inclination of nearly 18° with respect to theecliptic.[5]2002 MS4 last passed perihelion in April 1853, passed aphelion in February 1987, and will make its next perihelion passage in June 2123.[23][24][6]
2002 MS4 is located in the classical region of theKuiper belt 37–48 AU from the Sun,[25]: 227 and is thus classified as aclassical Kuiper belt object or cubewano.[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 gravitationallyscattered out to its present location by Neptune's outwardplanetary migration in the Solar System's early history.[25]: 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][25]: 214
A dynamical study in 2007 simulated2002 MS4's orbital evolution over a 10-million-year timespan and found that it may be in an intermittent 18:11 mean-motionorbital resonance with Neptune,[25]: 218 which seems to cause irregular fluctations in2002 MS4's orbital inclination and eccentricity.[25]: 225 Despite this, researchers do not consider2002 MS4 to be in resonance with Neptune.[4][3]: 56 [10]: 2
In the night sky,2002 MS4 is located near theMilky Way'sGalactic Center in thesouthern celestial hemisphere. It has been passing through that region's dense field of background stars since its discovery.[10]: 9 Combined with2002 MS4's faintapparent magnitude of 20.5 as seen from Earth,[13] its crowded location can make observations difficult.[9]: 92 [10]: 9 On the other hand,2002 MS4's location makes it viable for observingstellar occultations as there are numerous stars for it to pass in front of.[10]: 9
| Date | Star apparent magnitude (V-band) | Positive detections | Negative detections[d] | Number of telescope locations[e] | 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 by2002 MS4 occur when it passes in front of a star and blocks out its light, causing the star to dim for several seconds until2002 MS4 emerges.[7]: 2 Observing stellar occultations by2002 MS4 can provide accurate measurements for its position, shape, and size.[7]: 1 [8]: 35 Due toparallax between Earth,2002 MS4, and the occulted star, occultations by2002 MS4 may only be observable to certain locations on Earth. For this reason,2002 MS4's orbital trajectory andephemeris must be accurately known before occultation predictions can be reliably made.[7]: 2 [8]: 35
To facilitate occultation predictions for2002 MS4, astronomers of theEuropean Research Council'sLucky Star project gatheredastrometric observations of2002 MS4 from 2009–2019 to reduce its orbital uncertainty and utilized theGaia catalogues for high-precision positions of stars.[26][7]: 2 From 2019 to 2022, theLucky Star project organized campaigns for astronomers worldwide to observe the predicted occultations by2002 MS4, yielding nine successfully-observed occultations by the end of the period.[7]: 1, 3 The first successfully-observed occultation by2002 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.[26][7]: 2, 18B.4 Additional successful observations of2002 MS4's occultations took place on 26 July and 19 August 2019, which provided more accurate astrometry that helped refine later occultation predictions.[27][7]: 2
On 8 August 2020, theLucky Star project organized a large observing campaign for2002 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, 18B.1–3 The observers of the occultation found no evidence ofrings,cometary jets, ornatural satellites around2002 MS4.[7]: 9 This is the most extensive participation in a TNO occultation campaign as of 2023[update].[28]: 1347 [7]: 9 Thanks to the large amount of positive detections across various locations, the global shape outline andtopography of2002 MS4 could be seen clearly for the first time.[29][7]
| Year of Publication | Diameter (km) | Method | Refs |
|---|---|---|---|
| 2008 | 726.2+123.2 −122.9 | thermal (Spitzer) | [30]: 173 |
| 2009 | 730+118 −120 | thermal (Spitzer, remodeled) | [31]: 291 |
| 2012 | 934±47 | thermal (Herschel) | [32]: 10 |
| 2020 | 770±2 | occultation (9 Jul 2019) | [26] |
| 2022 | <810±70 | occultation (26 Jul 2019) | [33] |
| 2023 | 796±24 | occultation (8 Aug 2020) | [7][f] |
Results from the extensively observed 8 August 2020 occultation show that2002 MS4 has a shape close to that of anoblate spheroid, with an equatorial diameter of 814 km (506 mi) and a polar diameter of up to 770 km (480 mi).[7]: 5 2002 MS4's mean diameter from these dimensions is 796 km (495 mi), which places it between the diameters of the two largest asteroids,Ceres andVesta.[7]: 5 It is unknown whether2002 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 greateroblateness, than observed in the August 2020 occultation.[7]: 8 2002 MS4 is the 10th (or 11th if counting Pluto's moonCharon) largest known TNO. Because of its large size, it is considered acandidate dwarf planet by some astronomers.[34]: 245 [10]: 2 [7]: 1 [8]: iii With measurement uncertainties considered, it is tied with2002 AW197 and2013 FY27 (diameters729–807 km[35] and659–820 km,[36] respectively) as thelargest unnamed object in the Solar System.
2002 MS4 was previously thought to have a larger diameter of 934 km (580 mi), according toinfraredthermal emission measurements made by theSpitzer andHerschel space telescopes in 2006 and 2010.[32]: 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, then2002 MS4 would be emitting more thermal radiation than predicted if it were a non-rotating, simple airless body.[8]: 68, 70, 73 It is not yet clear why2002 MS4 seems to be emitting excess thermal radiation; it could be possible that either there is an unknown satellite of2002 MS4 contributing to the excess thermal emission,[7]: 9 or the predictions for2002 MS4's thermal emission behavior are inaccurate.[8]: 73
The mass and density of2002 MS4 is unknown since it has no known moons; otherwise, estimation of its mass would have been possible byKepler's third law.[8]: 35 Without a known mass and density, it is not possible to determine whether2002 MS4's spheroidal shape is due tohydrostatic equilibrium, which would qualify it as adwarf planet.[37]: 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 highlyporous interior structures that have not gravitationally compressed into solid bodies.[38]: 1, 8 Otherwise, if2002 MS4 is in hydrostatic equilibrium, then its density could be estimated from its oblateness and rotation period.[7]: 8 However, both of these properties are poorly known for2002 MS4, so only its minimum and maximum possible densities could be estimated.[7]: 8 Assuming aMaclaurin spheroid as the equilibrium shape for2002 MS4, the ranges of possible densities are0.72–8.0 g/cm3 and0.36–3.9 g/cm3 for possible rotation periods of 7.44 and 10.44 hours, respectively.[7]: 8
2002 MS4 has a gray or spectrally neutral surface color, meaning it reflects similar amounts of light forwavelengths across thevisible spectrum.[12]: 6 In Barucci et al.'s classification scheme forTNO color indices,2002 MS4 falls under the BB group of TNOs with neutral colors, whose surface compositions characteristically have a high fraction of water ice andamorphous carbon but low amounts oftholins.[39]: 1294, 1296 Near-infraredspectroscopy by theJames Webb Space Telescope (JWST) in 2022 revealed the presence ofcrystalline water ice,amorphous water ice, andcarbon dioxide ice in2002 MS4's surface.[40][11] The large Kuiper belt object120347 Salacia was observed by JWST to have a similar surface composition as2002 MS4.[11] Preliminary modeling of2002 MS4'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.[11] Tholins should also exist on2002 MS4's surface according to Cook et al.'s preliminary model, although they have not been detected in2002 MS4's JWST spectrum.[11]Volatile ices such asmethane were also not detected in2002 MS4's JWST spectrum.[40] The lack of volatiles on2002 MS4's surface agrees with its lowgeometric albedo of 0.1 determined from observations by theNew Horizons spacecraft, which indicates2002 MS4 has a very dark and unevolved surface in contrast to the bright and volatile-rich dwarf planets like Pluto.[10]: 2, 18–19 New Horizons observations of2002 MS4'sphase curve indicate that the icyregolith grains on the object's surface are rough and irregularly shaped.[10]: 19
The 8 August 2020 occultation revealed massive topographic features along2002 MS4's northeastern outline, orlimb, 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 a25+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 fromVarages, France during the occultation; this depression feature partially occulted the star as2002 MS4 emerged, which resulted in the star brightening gradually instead of instantly.[7]: 7 Theelevations of these observed topographic features lie beyond the maximum elevation of 6–7 km (3.7–4.3 mi) expected for an icy body of2002 MS4's size, signifying that the object may have experienced a large impact in its past.[7]: 6, 9 It would be possible for2002 MS4 to support its massive topographic features if itsmaterial strength increases toward its core.[7]: 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.[41][42]
The topographic peak on2002 MS4 has a height comparable toMars's tallest mountain,Olympus Mons, and the central mound of theRheasilvia crater on asteroid Vesta.[42][43] If2002 MS4's topographic peak is a mountain, then it would qualify as one of thetallest known mountains in the Solar System.[42] It is possible that this topographic peak may actually be an unknown 213 km (132 mi)-diameter satellite that waspassing in front or behind2002 MS4 during the occultation, but this scenario is unlikely according to Bruno Sicardy, one of the occultation team members.[7]: 9, 25 [42] A satellite of this size would not be large enough to explain2002 MS4's excess thermal emission.[7]: 25
If2002 MS4's massive depression is a crater, then it would be the first observation of a massive crater on a TNO.[7]: 9 The depression's width takes up about 40% of2002 MS4's diameter, which is comparable to the largest crater-to-diameter ratios seen in Saturn's moonsTethys andIapetus. For context, Tethys's largest craterOdysseus takes up about 43% of its diameter, while Iapetus's largest craterTurgis takes up about 40% of its diameter, but they are much shallower than the purported2002 MS4 crater.[7]: 9 The trans-Neptunian dwarf planets Pluto and Charon do not exhibit such large craters on the other hand,[g] as their largest crater-to-diameter ratios are 10.5% and 18.9%, respectively.[7]: 9 The depth of2002 MS4's massive depression takes up 5.7% of2002 MS4's diameter and exceeds those seen in the largest craters of other Solar System bodies of comparable size: the largest crater of Saturn's moonMimas has a depth of up to 10–12 km (6.2–7.5 mi)[44]: 424 and Vesta's Rheasilvia crater has a depth of up to 25 km (16 mi).[43]
Therotation period of2002 MS4 is uncertain and its rotationalaxial tilt is unknown. It is difficult to measure2002 MS4's rotation periodphotometrically with telescopes on Earth since the object is obscured in a dense field of background stars.[9]: 118 [7]: 7 Due to2002 MS4's spheroidal shape and possible surface albedo variations, itslight curve only exhibits very small fluctuations in brightness (amplitude 0.05–0.12mag[8]: 85 ) over time as it rotates.[7]: 7 [8]: 73 The first attempts at measuring2002 MS4's rotation were made with theSierra 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.[9]: 31, 92 Subsequent observations by theGalileo National Telescope in June–July 2011 took advantage of2002 MS4 passing in front of adark nebula, which enabled it to determine possible periods of either 7.33 hours or 10.44 hours.[9]: 94 On the other hand, observations by theCanada–France–Hawaii Telescope in July–August 2013 measured a rotation period of 14.251 hours, with other less probable rotation periodaliases of 8.932 and 5.881 hours.[8]: 43, 53, 74
TheNew Horizons spacecraft observed2002 MS4 during 2016–2019, as part of its extended Kuiper belt mission after its successful Pluto flyby in 2015.[10]: 8 2002 MS4 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.[10]: 8 New Horizons had the unique vantage point of observing2002 MS4 and other TNOs while it was inside the Kuiper belt, which allowed the spacecraft to observe these objects at highphase angles (>2°) that are not observable from Earth.[10]: 1 By observing how2002 MS4's brightness changes as a function of phase angle, the object's phase curve could be determined, which can reveal the lightscattering properties of2002 MS4's surface regolith.[10]: 1 In addition to significantly improving the knowledge of2002 MS4's phase curve, the observations byNew Horizons also significantly improved the precision of2002 MS4's orbit.[45]
2002 MS4 has been considered as a possible exploration target for future missions to the Kuiper belt and beyond, such as NASA'sInterstellar Probe concept.[46] A 2019 study by Amanda Zangari and collaborators identified several possible trajectories to2002 MS4 for a spacecraft that would be launched in 2025–2040.[47] For a spacecraft launched in 2027–2031, a singlegravity assist from Jupiter could bring a spacecraft to2002 MS4 over a minimum duration of 9.1–12.8 years, depending on theexcess launch energy of the spacecraft.[47]: 922 Another trajectory using a single Jupiter gravity assist for a 2040 launch date could bring a spacecraft to2002 MS4 over a minimum duration of 13 years.[47]: 922 A 2038–2040 launch trajectory using a single Saturn gravity assist could bring a spacecraft to2002 MS4 over a minimum duration of 16.7 years,[47]: 925 while a 2038–2040 launch trajectory using two gravity assists from Jupiter and Saturn could bring a spacecraft to2002 MS4 over a minimum duration of 18.6–19.5 years.[47]: 923
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| Candidate (for TNOs, D+1σ ≥ 700 km or H ≤ 4.2) |
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