307261 Máni
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307261 Máni ( provisional designation ) is a large
trans-Neptunian object A trans-Neptunian object (TNO), also written transneptunian object, is any minor planet in the Solar System that orbits the Sun at a greater average distance than Neptune, which has an orbital semi-major axis of 30.1 astronomical units (AU). ...
in the
Kuiper belt The Kuiper belt ( ) is a circumstellar disc in the outer Solar System, extending from the orbit of Neptune at 30 astronomical units (AU) to approximately 50 AU from the Sun. It is similar to the asteroid belt, but is far larger—20 times ...
, which is a region of icy
planetesimal Planetesimals () are solid objects thought to exist in protoplanetary disks and debris disks. Believed to have formed in the Solar System about 4.6 billion years ago, they aid study of its formation. Formation A widely accepted theory of pla ...
s beyond
Neptune Neptune is the eighth and farthest known planet from the Sun. It is the List of Solar System objects by size, fourth-largest planet in the Solar System by diameter, the third-most-massive planet, and the densest giant planet. It is 17 t ...
. It was discovered on 18 June 2002 by Chad Trujillo and Michael Brown during their search for
Pluto Pluto (minor-planet designation: 134340 Pluto) is a dwarf planet in the Kuiper belt, a ring of Trans-Neptunian object, bodies beyond the orbit of Neptune. It is the ninth-largest and tenth-most-massive known object to directly orbit the Su ...
-sized Kuiper belt objects at
Palomar Observatory The Palomar Observatory is an astronomical research observatory in the Palomar Mountains of San Diego County, California, United States. It is owned and operated by the California Institute of Technology (Caltech). Research time at the observat ...
. 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 Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...
ices. Máni has been observed through
stellar occultation Stellar means anything related to one or more stars (''stella''). The term may also refer to: Arts, entertainment, and media * Stellar (magazine), ''Stellar'' (magazine), an Irish lifestyle and fashion magazine * Stellar Loussier, a character fro ...
s, 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 A crater is a landform consisting of a hole or depression (geology), depression on a planetary surface, usually caused either by an object hitting the surface, or by geological activity on the planet. A crater has classically been described ...
-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 The Palomar Observatory is an astronomical research observatory in the Palomar Mountains of San Diego County, California, United States. It is owned and operated by the California Institute of Technology (Caltech). Research time at the observat ...
in
San Diego County, California San Diego County (), officially the County of San Diego, is a county (United States), county in the southwest corner of the U.S. state of California, north to its Mexico-United States border, border with Mexico. As of the 2020 United States Cen ...
, United States. The discovery formed part of their Caltech Wide Area Sky Survey for
Pluto Pluto (minor-planet designation: 134340 Pluto) is a dwarf planet in the Kuiper belt, a ring of Trans-Neptunian object, bodies beyond the orbit of Neptune. It is the ninth-largest and tenth-most-massive known object to directly orbit the Su ...
-sized Kuiper belt objects using the observatory's Samuel Oschin telescope with its wide-field
CCD camera A charge-coupled device (CCD) is an integrated circuit containing an array of linked, or coupled, capacitors. Under the control of an external circuit, each capacitor can transfer its electric charge to a neighboring capacitor. CCD sensors are a ...
, 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 Neptune is the eighth and farthest known planet from the Sun. It is the List of Solar System objects by size, fourth-largest planet in the Solar System by diameter, the third-most-massive planet, and the densest giant planet. It is 17 t ...
, which includes the
dwarf planet A dwarf planet is a small planetary-mass object that is in direct orbit around the Sun, massive enough to be hydrostatic equilibrium, gravitationally rounded, but insufficient to achieve clearing the neighbourhood, orbital dominance like the ...
s , , 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 Magnitude may refer to: Mathematics *Euclidean vector, a quantity defined by both its magnitude and its direction *Magnitude (mathematics), the relative size of an object *Norm (mathematics), a term for the size or length of a vector *Order of ...
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 The Minor Planet Center (MPC) is the official body for observing and reporting on minor planets under the auspices of the International Astronomical Union (IAU). Founded in 1947, it operates at the Smithsonian Astrophysical Observatory. Funct ...
on 21 November 2002 and the object was given the
minor planet According to the International Astronomical Union (IAU), a minor planet is an astronomical object in direct orbit around the Sun that is exclusively classified as neither a planet nor a comet. Before 2006, the IAU officially used the term ''minor ...
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 The Digitized Sky Survey (DSS) is a digital data, digitized version of several photography, photographic astronomical surveys of the night sky, produced by the Space Telescope Science Institute between 1983 and 2006. Versions and source materia ...
plates were identified by astronomer Andrew Lowe in 2007; the earliest of these was taken on 8 April 1954 by
Palomar Observatory The Palomar Observatory is an astronomical research observatory in the Palomar Mountains of San Diego County, California, United States. It is owned and operated by the California Institute of Technology (Caltech). Research time at the observat ...
. , Máni has been observed for over 68 years, or about 25% of its
orbital period The orbital period (also revolution period) is the amount of time a given astronomical object takes to complete one orbit around another object. In astronomy, it usually applies to planets or asteroids orbiting the Sun, moons orbiting planets ...
.


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 Máni (Old Norse: ; "Moon"Orchard (1997:109).) is the Lunar deity, Moon personified in Germanic mythology. Máni, personified, is attested in the ''Poetic Edda'', compiled in the 13th century from earlier traditional sources, and the ''Prose Edd ...
" by the
International Astronomical Union The International Astronomical Union (IAU; , UAI) is an international non-governmental organization (INGO) with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreach, education, and developmen ...
's Working Group for Small Bodies Nomenclature. According to the naming citation, "Máni is a personification of the Moon from
Old Norse Old Norse, also referred to as Old Nordic or Old Scandinavian, was a stage of development of North Germanic languages, North Germanic dialects before their final divergence into separate Nordic languages. Old Norse was spoken by inhabitants ...
as described in the
Prose Edda The ''Prose Edda'', also known as the ''Younger Edda'', ''Snorri's Edda'' () or, historically, simply as ''Edda'', is an Old Norse textbook written in Iceland during the early 13th century. The work is often considered to have been to some exten ...
. Máni is the son of
Mundilfari In Norse mythology Mundilfari (Old Norse: ; rendered variously ''Mundilfari'', ''Mundilföri'' and ''Mundilfœri'') (Old Norse, possibly "the one moving according to particular times"Simek (2007:222).) is the father of Sól, goddess associated ...
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 A trans-Neptunian object (TNO), also written transneptunian object, is any minor planet in the Solar System that orbits the Sun at a greater average distance than Neptune, which has an orbital semi-major axis of 30.1 astronomical units (AU). ...
(TNO) orbiting the
Sun The Sun is the star at the centre of the Solar System. It is a massive, nearly perfect sphere of hot plasma, heated to incandescence by nuclear fusion reactions in its core, radiating the energy from its surface mainly as visible light a ...
beyond Neptune with an orbital period of 269 years. Its
semi-major axis In geometry, the major axis of an ellipse is its longest diameter: a line segment that runs through the center and both foci, with ends at the two most widely separated points of the perimeter. The semi-major axis (major semiaxis) is the longe ...
or average orbital distance from the Sun is 41.7
astronomical unit The astronomical unit (symbol: au or AU) is a unit of length defined to be exactly equal to . Historically, the astronomical unit was conceived as the average Earth-Sun distance (the average of Earth's aphelion and perihelion), before its m ...
s (AU), with a moderate
orbital eccentricity In astrodynamics, the orbital eccentricity of an astronomical object is a dimensionless parameter that determines the amount by which its orbit around another body deviates from a perfect circle. A value of 0 is a circular orbit, values be ...
of 0.15. In its eccentric orbit, Máni comes within 35.7 AU from the Sun at
perihelion An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. The line of apsides (also called apse line, or major axis of the orbit) is the line connecting the two extreme values. Apsides perta ...
and 47.8 AU at
aphelion An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. The line of apsides (also called apse line, or major axis of the orbit) is the line connecting the two extreme values. Apsides perta ...
. It has an
orbital inclination Orbital inclination measures the tilt of an object's orbit around a celestial body. It is expressed as the angle between a reference plane and the orbital plane or axis of direction of the orbiting object. For a satellite orbiting the Earth ...
of nearly 18° with respect to the
ecliptic The ecliptic or ecliptic plane is the orbital plane of Earth's orbit, Earth around the Sun. It was a central concept in a number of ancient sciences, providing the framework for key measurements in astronomy, astrology and calendar-making. Fr ...
. 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 The Kuiper belt ( ) is a circumstellar disc in the outer Solar System, extending from the orbit of Neptune at 30 astronomical units (AU) to approximately 50 AU from the Sun. It is similar to the asteroid belt, but is far larger—20 times ...
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 Planetary migration occurs when a planet or other body in orbit around a star interacts with a disk of gas or planetesimals, resulting in the alteration of its orbital parameters, especially its semi-major axis. Planetary migration is the most ...
in the Solar System's early history. Máni's present orbit is far enough from Neptune (
minimum orbit intersection distance Minimum orbit intersection distance (MOID) is a measure used in astronomy to assess potential close approaches and collision risks between astronomical objects. It is defined as the distance between the closest points of the osculating orbits o ...
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 In celestial mechanics, orbital resonance occurs when orbiting bodies exert regular, periodic gravitational influence on each other, usually because their orbital periods are related by a ratio of small integers. Most commonly, this relation ...
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 The Milky Way or Milky Way Galaxy is the galaxy that includes the Solar System, with the name describing the #Appearance, galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars in other arms of the galax ...
's
Galactic Center The Galactic Center is the barycenter of the Milky Way and a corresponding point on the rotational axis of the galaxy. Its central massive object is a supermassive black hole of about 4 million solar masses, which is called Sagittarius A*, a ...
in the
southern celestial hemisphere The southern celestial hemisphere, also called the Southern Sky, is the Southern Hemisphere, southern half of the celestial sphere; that is, it lies south of the celestial equator. This arbitrary sphere, on which seemingly fixed stars form cons ...
. It has been passing through that region's dense field of background stars since its discovery. Combined with Máni's faint
apparent magnitude Apparent magnitude () is a measure of the Irradiance, brightness of a star, astronomical object or other celestial objects like artificial satellites. Its value depends on its intrinsic luminosity, its distance, and any extinction (astronomy), ...
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 occultation Stellar means anything related to one or more stars (''stella''). The term may also refer to: Arts, entertainment, and media * Stellar (magazine), ''Stellar'' (magazine), an Irish lifestyle and fashion magazine * Stellar Loussier, a character fro ...
s 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 Parallax is a displacement or difference in the apparent position of an object viewed along two different sightline, lines of sight and is measured by the angle or half-angle of inclination between those two lines. Due to perspective (graphica ...
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 In astronomy and celestial navigation, an ephemeris (; ; , ) is a book with tables that gives the trajectory of naturally occurring astronomical objects and artificial satellites in the sky, i.e., the position (and possibly velocity) over tim ...
must be accurately known before occultation predictions can be reliably made. To facilitate occultation predictions for Máni, astronomers of the
European Research Council The European Research Council (ERC) is a public body for funding of scientific and technological research conducted within the European Union (EU). Established by the European Commission in 2007, the ERC is composed of an independent Scientific ...
's ''Lucky Star'' project gathered
astrometric Astrometry is a branch of astronomy that involves precise measurements of the positions and movements of stars and other celestial bodies. It provides the kinematics and physical origin of the Solar System and this galaxy, the Milky Way. History ...
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 satellite A natural satellite is, in the most common usage, an astronomical body that orbits a planet, dwarf planet, or small Solar System body (or sometimes another natural satellite). Natural satellites are colloquially referred to as moons, a deriv ...
s 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 Topography is the study of the forms and features of land surfaces. The topography of an area may refer to the landforms and features themselves, or a description or depiction in maps. Topography is a field of geoscience and planetary sci ...
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 Chord or chords may refer to: Art and music * Chord (music), an aggregate of musical pitches sounded simultaneously ** Guitar chord, a chord played on a guitar, which has a particular tuning * The Chords (British band), 1970s British mod ...
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 A spheroid, also known as an ellipsoid of revolution or rotational ellipsoid, is a quadric surface obtained by rotating an ellipse about one of its principal axes; in other words, an ellipsoid with two equal semi-diameters. A spheroid has circu ...
, 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 Flattening is a measure of the compression of a circle or sphere along a diameter to form an ellipse or an ellipsoid of revolution (spheroid) respectively. Other terms used are ellipticity, or oblateness. The usual notation for flattening is f ...
, 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 Infrared (IR; sometimes called infrared light) is electromagnetic radiation (EMR) with wavelengths longer than that of visible light but shorter than microwaves. The infrared spectral band begins with the waves that are just longer than those ...
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 In fluid mechanics, hydrostatic equilibrium, also called hydrostatic balance and hydrostasy, is the condition of a fluid or plastic solid at rest, which occurs when external forces, such as gravity, are balanced by a pressure-gradient force. I ...
, which would qualify it as a
dwarf planet A dwarf planet is a small planetary-mass object that is in direct orbit around the Sun, massive enough to be hydrostatic equilibrium, gravitationally rounded, but insufficient to achieve clearing the neighbourhood, orbital dominance like the ...
. 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 Porosity or void fraction is a measure of the void (i.e. "empty") spaces in a material, and is a fraction of the volume of voids over the total volume, between 0 and 1, or as a percentage between 0% and 100%. Strictly speaking, some tests measure ...
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
wavelength In physics and mathematics, wavelength or spatial period of a wave or periodic function is the distance over which the wave's shape repeats. In other words, it is the distance between consecutive corresponding points of the same ''phase (waves ...
s across the
visible spectrum The visible spectrum is the spectral band, band of the electromagnetic spectrum that is visual perception, visible to the human eye. Electromagnetic radiation in this range of wavelengths is called ''visible light'' (or simply light). The optica ...
. 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 Amorphous carbon is free, reactive carbon that has no crystalline structure. Amorphous carbon materials may be stabilized by terminating dangling-π bonds with hydrogen. As with other amorphous solids, some short-range order can be observed. Amo ...
but low amounts of
tholin Tholins (after the Greek (') "hazy" or "muddy"; from the ancient Greek word meaning "sepia ink") are a wide variety of organic compounds formed by solar ultraviolet or cosmic rays, cosmic ray irradiation of simple carbon-containing compounds su ...
s.
Near-infrared Infrared (IR; sometimes called infrared light) is electromagnetic radiation (EMR) with wavelengths longer than that of visible light but shorter than microwaves. The infrared spectral band begins with the waves that are just longer than those of ...
spectroscopy Spectroscopy is the field of study that measures and interprets electromagnetic spectra. In narrower contexts, spectroscopy is the precise study of color as generalized from visible light to all bands of the electromagnetic spectrum. Spectro ...
by the
James Webb Space Telescope The James Webb Space Telescope (JWST) is a space telescope designed to conduct infrared astronomy. As the largest telescope in space, it is equipped with high-resolution and high-sensitivity instruments, allowing it to view objects too old, Lis ...
(JWST) in 2022 revealed the presence of crystalline water ice, amorphous water ice, and
carbon dioxide Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...
ice in Máni's surface. The large Kuiper belt object
120347 Salacia Salacia (minor-planet designation: 120347 Salacia) is a large trans-Neptunian object (TNO) and possible dwarf planet in the Kuiper belt, approximately in diameter. It was discovered on 22 September 2004, by American astronomers Henry G. Roe, Hen ...
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 Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The abundance of methane on Earth makes ...
were also not detected in Máni's JWST spectrum. The lack of volatiles on Máni's surface agrees with its low
geometric albedo In astronomy, the geometric albedo of a celestial body is the ratio of its actual brightness as seen from the light source (i.e. at zero phase angle (astronomy), phase angle) to that of an ''idealized'' flat, fully reflecting, diffuse reflection, d ...
of 0.1 determined from observations by the ''
New Horizons ''New Horizons'' is an Interplanetary spaceflight, interplanetary space probe launched as a part of NASA's New Frontiers program. Engineered by the Johns Hopkins University Applied Physics Laboratory (APL) and the Southwest Research Institut ...
'' 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 Regolith () is a blanket of unconsolidated, loose, heterogeneous superficial deposits covering solid rock. It includes dust, broken rocks, and other related materials and is present on Earth, the Moon, Mars, some asteroids, and other terrestria ...
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
elevation The elevation of a geographic location (geography), ''location'' is its height above or below a fixed reference point, most commonly a reference geoid, a mathematical model of the Earth's sea level as an equipotential gravitational equipotenti ...
s 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 A material is a matter, substance or mixture of substances that constitutes an Physical object, object. Materials can be pure or impure, living or non-living matter. Materials can be classified on the basis of their physical property, physical ...
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 Mars is the fourth planet from the Sun. It is also known as the "Red Planet", because of its orange-red appearance. Mars is a desert-like rocky planet with a tenuous carbon dioxide () atmosphere. At the average surface level the atmosph ...
's tallest mountain,
Olympus Mons Olympus Mons (; ) is a large shield volcano on Mars. It is over high as measured by the Mars Orbiter Laser Altimeter (MOLA), about 2.5 times the elevation of Mount Everest above sea level. It is Mars's tallest volcano, its tallest planetary mou ...
, 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 In Greek mythology, Iapetus (; ; ), also Japetus, is a Titan, the son of Uranus and Gaia and father of Atlas, Prometheus, Epimetheus, and Menoetius. He was also called the father of Buphagus and Anchiale in other sources. Iapetus was linked ...
. For context, Tethys's largest crater
Odysseus In Greek mythology, Greek and Roman mythology, Odysseus ( ; , ), also known by the Latin variant Ulysses ( , ; ), is a legendary Greeks, Greek king of Homeric Ithaca, Ithaca and the hero of Homer's Epic poetry, epic poem, the ''Odyssey''. Od ...
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 In astronomy, the rotation period or spin period of a celestial object (e.g., star, planet, moon, asteroid) has two definitions. The first one corresponds to the '' sidereal rotation period'' (or ''sidereal day''), i.e., the time that the objec ...
of Máni is uncertain and its rotational
axial tilt In astronomy, axial tilt, also known as obliquity, is the angle between an object's rotational axis and its orbital axis, which is the line perpendicular to its orbital plane; equivalently, it is the angle between its equatorial plane and orbita ...
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 In astronomy, a light curve is a graph (discrete mathematics), graph of the Radiance, light intensity of a celestial object or region as a function of time, typically with the magnitude (astronomy), magnitude of light received on the ''y''-axis ...
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 A dark nebula or absorption nebula is a type of interstellar cloud, particularly molecular clouds, that is so dense that it obscures the visible wavelengths of light from objects behind it, such as background stars and emission or reflection ...
, 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 A pseudonym (; ) or alias () is a fictitious name that a person assumes for a particular purpose, which differs from their original or true meaning (orthonym). This also differs from a new name that entirely or legally replaces an individual's ow ...
of 8.932 and 5.881 hours.


Exploration


''New Horizons''

The ''
New Horizons ''New Horizons'' is an Interplanetary spaceflight, interplanetary space probe launched as a part of NASA's New Frontiers program. Engineered by the Johns Hopkins University Applied Physics Laboratory (APL) and the Southwest Research Institut ...
'' 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 In physics, scattering is a wide range of physical processes where moving particles or radiation of some form, such as light or sound, are forced to deviate from a straight trajectory by localized non-uniformities (including particles and radiat ...
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 ''New Horizons'' is an Interplanetary spaceflight, interplanetary space probe launched as a part of NASA's New Frontiers program. Engineered by the Johns Hopkins University Applied Physics Laboratory (APL) and the Southwest Research Institut ...
'' 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 An interstellar probe is a space probe that has left—or is expected to leave—the Solar System and enter interstellar medium, interstellar space, which is typically defined as the region beyond the Heliopause (astronomy), heliopause. It also r ...
'' 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 A gravity assist, gravity assist maneuver, swing-by, or generally a gravitational slingshot in orbital mechanics, is a type of spaceflight flyby (spaceflight), flyby which makes use of the relative movement (e.g. orbit around the Sun) and gra ...
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 This article includes a list of the most massive known objects of the Solar System and partial lists of smaller objects by observed mean radius. These lists can be sorted according to an object's radius and mass and, for the most massive objects, ...


Notes


References


External links


Precovery Images
Andrew Lowe * {{DEFAULTSORT:Máni 307261 Discoveries by Michael E. Brown Discoveries by Chad Trujillo 307261 Objects observed by stellar occultation 20020618
Máni Máni (Old Norse: ; "Moon"Orchard (1997:109).) is the Lunar deity, Moon personified in Germanic mythology. Máni, personified, is attested in the ''Poetic Edda'', compiled in the 13th century from earlier traditional sources, and the ''Prose Edd ...