307261 Máni

307261 Máni ( 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 Pluto-sized Kuiper belt objects at Palomar Observatory. Máni is large enough that some astronomers believe that it might be a dwarf planet.

The surface of Máni is dark gray and is composed of water and carbon dioxide ices. Máni 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 tall and a crater-like depression that is wide and deep. Máni's topographic features are among the tallest and deepest known for Solar System bodies.

History

Discovery

Máni 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 of their Caltech Wide Area Sky Survey for Pluto-sized 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 objects beyond Neptune, which includes the dwarf planets , , and .

Máni 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, Máni 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 Máni'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. , Máni has been observed for over 68 years, or about 25% of its orbital period.

Numbering and naming

Máni received its permanent minor planet catalog number of 307261 from the Minor Planet Center on 10 December 2011. It remained unnamed until 9 June 2025, when it was officially named "Máni" by the International Astronomical Union's Working Group for Small Bodies Nomenclature. According to the naming citation, "Máni is a personification of the Moon from Old Norse as described in the Prose Edda. Máni is the son of Mundilfari and the brother of Sól, the Sun." The name Máni follows the official naming theme of mythological creation figures for classical Kuiper belt objects.

Orbit and classification

Máni 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, Máni 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. Máni last passed perihelion in April 1853, passed aphelion in February 1987, and will make its next perihelion passage in June 2123.

Máni 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. Máni'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. Máni'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 Máni'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 Máni's orbital inclination and eccentricity. Despite this, researchers do not consider Máni to be in resonance with Neptune.

Observability

In the night sky, Máni 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 Máni's faint apparent magnitude of 20.5 as seen from Earth, its crowded location can make observations difficult. On the other hand, Máni'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 Máni occur when it passes in front of a star and blocks out its light, causing the star to dim for several seconds until Máni emerges. Observing stellar occultations by Máni can provide accurate measurements for its position, shape, and size. Due to parallax between Earth, Máni, and the occulted star, occultations by Máni may only be observable to certain locations on Earth. For this reason, Máni's orbital trajectory and ephemeris must be accurately known before occultation predictions can be reliably made.

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

On 8 August 2020, the ''Lucky Star'' project organized a large observing campaign for Máni, 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 Máni. 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 Máni could be seen clearly for the first time.

File:2002 MS4 occultation map - Rommel et al. 2023 Fig 3.tif, Map showing the location of telescopes that participated in the 8 August 2020 occultation campaign. Telescopes within the path of Máni'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).
File:2002 MS4 occultation chords - Rommel et al. 2023 Fig 1.tif, Máni'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 Máni's limb in the northeast direction.

Physical characteristics

Results from the extensively observed 8 August 2020 occultation show that Máni has a shape close to that of an oblate spheroid, with an equatorial diameter of and a polar diameter of up to . Máni's mean diameter from these dimensions is , which places it between the diameters of the two largest asteroids, Ceres and Vesta. It is unknown whether Máni'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. Máni is the 10th (or 11th if counting Pluto's moon Charon) largest known TNO. Because of its large size, it is considered a candidate dwarf planet by some astronomers.

Máni 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. This thermal emission-derived diameter disagrees with the occultation-derived diameter; if both the thermal emission measurements and occultation-derived diameter are correct, then Máni would be emitting more thermal radiation than predicted if it were a non-rotating, simple airless body. It is not yet clear why Máni seems to be emitting excess thermal radiation; it could be possible that either there is an unknown satellite of Máni contributing to the excess thermal emission, or the predictions for Máni's thermal emission behavior are inaccurate.

The mass and density of Máni 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 Máni's spheroidal shape is due to hydrostatic equilibrium, which would qualify it as a dwarf planet. Inferring from its diameter and albedo, Máni 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. Otherwise, if Máni 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 Máni, so only its minimum and maximum possible densities could be estimated. Assuming a Maclaurin spheroid as the equilibrium shape for Máni, the ranges of possible densities are and for possible rotation periods of 7.44 and 10.44 hours, respectively.

Surface

Máni 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, Máni 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 Máni's surface. The large Kuiper belt object 120347 Salacia was observed by JWST to have a similar surface composition as Máni. Preliminary modeling of Máni'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 Máni's surface according to Cook et al.'s preliminary model, although they have not been detected in Máni's JWST spectrum. Volatile ices such as methane were also not detected in Máni's JWST spectrum. The lack of volatiles on Máni's surface agrees with its low geometric albedo of 0.1 determined from observations by the ''New Horizons'' spacecraft, which indicates Máni has a very dark and unevolved surface in contrast to the bright and volatile-rich dwarf planets like Pluto. ''New Horizons'' observations of Máni'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 Máni's northeastern outline, or limb, which notably includes a crater-like depression wide and 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 Varages, France during the occultation; this depression feature partially occulted the star as Máni 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 Máni's size, signifying that the object may have experienced a large impact in its past. It would be possible for Máni 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 deep.

The topographic peak on Máni has a height comparable to Mars's tallest mountain, Olympus Mons, and the central mound of the Rheasilvia crater on asteroid Vesta. If Máni'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 Máni 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 Máni's excess thermal emission.

If Máni'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 Máni'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 Máni 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 Máni's massive depression takes up 5.7% of Máni'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 and Vesta's Rheasilvia crater has a depth of up to .

Rotation and light curve

The rotation period of Máni is uncertain and its rotational axial tilt is unknown. It is difficult to measure Máni's rotation period photometrically with telescopes on Earth since the object is obscured in a dense field of background stars. Due to Máni'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 Máni'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 Máni 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.

Exploration

''New Horizons''

The ''New Horizons'' spacecraft observed Máni during 2016–2019, as part of its extended Kuiper belt mission after its successful Pluto flyby in 2015. Máni was away from the spacecraft when it began observations on 13 July 2016, and was away from the spacecraft when it ended observations on 1 September 2019. ''New Horizons'' had the unique vantage point of observing Máni 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 Máni'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 Máni's surface regolith. In addition to significantly improving the knowledge of Máni's phase curve, the observations by ''New Horizons'' also significantly improved the precision of Máni's orbit.

File:2002 MS4 New Horizons.gif, Máni imaged by the ''New Horizons'' spacecraft in July 2016, from a distance of
File:New Horizons KEM Trajectory.png, ''New Horizons'' trajectory through the Kuiper belt, with positions of nearby KBOs including Máni () labeled

Proposed

Máni 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 Máni 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 Máni 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 Máni over a minimum duration of 13 years. A 2038–2040 launch trajectory using a single Saturn gravity assist could bring a spacecraft to Máni 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 Máni over a minimum duration of 18.6–19.5 years.

See also

* List of Solar System objects by size

Notes

References

External links

Precovery Images
Andrew Lowe
*

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Discoveries by Michael E. Brown
Discoveries by Chad Trujillo
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Objects observed by stellar occultation
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Máni