307261 Máni
A large Kuiper belt object with extreme topography.
307261 Máni, originally designated 2002 MS4, is a sizable object in the Kuiper belt—the icy ring of debris beyond Neptune. Its diameter of roughly 800 km puts it in the running for dwarf planet status, though an irregular shape (similar to that of the asteroid Vesta) casts doubt on that classification. The surface is a dark gray mix of water and carbon dioxide ices. Observations of stellar occultations have revealed dramatic terrain: a mountain-like peak rising 25 km high and a crater-like depression 320 km across and 45 km deep, ranking among the most extreme topographic features found anywhere in the Solar System. Astronomers also suspect it has a large moon, but this has not yet been confirmed through direct imaging.
Discovered on 18 June 2002 by Chad Trujillo and Michael Brown at Palomar Observatory, Máni was part of their Caltech survey hunting for Pluto-sized bodies in the Kuiper belt—the same project that turned up Eris, Sedna, and Quaoar. The team’s automated software flagged potential moving objects, which were then checked by hand; Máni was one of the fainter finds, with an apparent magnitude of 20.9, just at the survey’s limit. Follow-up observations came two months later, on 8 August 2002, using Palomar’s 1.52-meter telescope. The Minor Planet Center announced the discovery on 21 November 2002, assigning the provisional designation 2002 MS4.
After that initial follow-up, Máni went unobserved for over nine months until Trujillo recovered it on 29 May 2003 at Palomar, with additional observations by Wolf Bickel in Germany the following month. These recovery efforts tightened the orbit enough to identify precovery images: seven plates from the Digitized Sky Survey, the earliest taken on 8 April 1954 at Palomar, were found by Andrew Lowe in 2007. By 2025, the object had been tracked for more than 68 years, covering about a quarter of its orbital period.
The permanent minor planet number 307261 was assigned on 10 December 2011. On 9 June 2025, the International Astronomical Union’s Working Group for Small Bodies Nomenclature officially named it Máni, after the personification of the Moon in Old Norse mythology (son of Mundilfari and brother of Sól, the Sun), following the group’s theme of mythological creation figures for classical Kuiper belt objects.
Quick Facts
- Minorplanet
- yes
- Discovered
- 18 June 2002
- Discoverer
- Chadwick A. Trujillo / Michael E. Brown
- Discovery Site
- Palomar Obs.
- Mpc Name
- (307261) Máni
- Pronounced
- ˈ · m · ɑː · n · i
- Named After
- Máni
- Epoch
- 25 February 2023 (JD 2460000.5)
- Uncertainty
- 1
- Observation Arc
- 68.24 yr (24,924 d)
- Earliest Precovery Date
- 8 April 1954
- Aphelion
- 47.801 AU
Facts from the source article.
Lore & Background
Máni was discovered on 18 June 2002 by astronomers Chad Trujillo and Michael Brown at Palomar Observatory as part of their Caltech Wide Area Sky Survey for Pluto-sized Kuiper belt objects. It was found through manual vetting of potential moving objects identified by automatic image-searching software, with an observed brightness of magnitude 20.9. The discovery was announced by the Minor Planet Center on 21 November 2002, and the object received the provisional designation 2002 MS4. It remained unnamed until 2025, when it was named after the personification of the Moon in German mythology, following the WGSBN's recommended naming theme of mythological creation figures for classical Kuiper belt objects.
Máni is a classical Kuiper belt object (cubewano) with a high orbital inclination of nearly 18°, classifying it as dynamically 'hot.' Its orbit has a semi-major axis of 41.7 AU, an eccentricity of 0.15, and an orbital period of 269 years. It last passed perihelion in April 1853, passed aphelion in February 1987, and will next reach perihelion in June 2123. A dynamical study in 2007 suggested it may be in an intermittent 18:11 mean-motion orbital resonance with Neptune, though researchers do not consider it to be in resonance.
Physically, Máni has a dark gray surface composed of water and carbon dioxide ices. Stellar occultations observed from 2019 to 2022, particularly a large campaign on 8 August 2020 involving 116 telescope locations, revealed its shape as an oblate spheroid with an equatorial diameter of 824 ± 20 km and a polar diameter of up to 770 ± 34 km. The occultations also revealed massive topographic features: a mountain-like peak 25 km tall and a crater-like depression 320 km wide and 45 km deep. Máni is suspected to have one large moon, though it has not been confirmed with direct observations.
Reader's Guide
Máni's significance lies in its status as one of the largest known trans-Neptunian objects, ranking 10th (or 11th if counting Pluto's moon Charon) among TNOs. Its size makes it a candidate dwarf planet, though its irregular shape—similar to that of the asteroid Vesta—leaves this uncertain. The extensive stellar occultation campaign on 8 August 2020, which involved 116 telescope locations across Europe, North Africa, and Western Asia, yielded 61 positive detections and provided the first clear global shape outline and topography of Máni. This campaign is the most extensive participation in a TNO occultation as of 2023. The topographic features revealed—a 25 km peak and a 320 km wide, 45 km deep depression—are among the tallest and deepest known for Solar System bodies, highlighting Máni's geological complexity. Its location near the Milky Way's Galactic Center, while making observations difficult due to dense background stars, also makes it viable for observing stellar occultations. The object's mass and density remain unknown due to the lack of confirmed moons, preventing determination of its internal composition.
Did You Know?
- Its surface is dark gray and composed of water and carbon dioxide ices.
- The 8 August 2020 occultation campaign involved 116 telescope locations, the most extensive for a TNO as of 2023.
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