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Euphrosyne (minor planet designation: 31 Euphrosyne) is the 12th-largest and the 5th-most-massive asteroid in the asteroid belt, discovered by James Ferguson on September 1, 1854. It was the first asteroid found from North America. It is named after Euphrosyne, one of the Charites in Greek mythology. In 2019 a small companion was discovered, making Euphrosyne a binary asteroid, secondary diameter 6.7 km.[1]

31 Euphrosyne
Three-dimensional model of 31 Euphrosyne created based on light-curve
Discovered byJ. Ferguson
Discovery dateSeptember 1, 1854
MPC designation(31) Euphrosyne
Pronunciation/jˈfrɒzɪni/ yoo-FROZ-i-nee
Named after
A907 GP; A918 GB
Main belt
Orbital characteristics[2]
Epoch April 27, 2019 (JD 2458600.5)
Aphelion3.8523 AU
(576.296 Gm)
Perihelion2.4585 AU
(367.786 Gm)
3.1554 AU
(472.041 Gm)
5.61 yr (2041.585 d)
Known satellites1[1]
Physical characteristics
Dimensions267.08±2.611 km (IRAS)[2]
Mass(5.81±1.97)×1019 kg[3]
Mean density
6.61 ± 2.41 g/cm³[3]
0.2305 d (5.531 h)[2]
10.16[6] to 13.61

It is a fairly dark body near the belt's outer edge. Consequently, Euphrosyne is never visible with binoculars, having a maximum magnitude at the best possible opposition of around +10.2, as in November 2011, which is actually fainter than any of the thirty asteroids previously discovered.[7]

It is a very little-studied body despite being one of the largest asteroids. It is a C-type asteroid with a primitive surface. Its orbit, however, is quite unusual and bears a considerable resemblance to that of 2 Pallas in its high inclination and eccentricity. Whereas Pallas and Eris—the only larger bodies with comparably tilted orbits—have nodes near perihelion and aphelion, Euphrosyne's perihelion lies at the northernmost point of its orbit. During a rare perihelic opposition Euphrosyne is very high in the sky from northern latitudes, but invisible from such countries as New Zealand and Chile.

The mass estimate of Euphrosyne in Baer (2011) makes it apparently the 5th-most-massive asteroid, coming after only the "big four". It also has the highest estimated density, indicating that it is a solid body like the other largest asteroids. However, all large asteroids with comparable densities (16 Psyche and 532 Herculina) have very large uncertainties, so both the mass and density are likely to be lower than the median estimate.

Its rotation period is typical for large asteroids.[citation needed] Nothing is known of its axial tilt.

Euphrosyne has been studied by radar.[8]

This object is the namesake of a family of 323–2066 asteroids that share similar spectral properties and orbital elements; hence they may have arisen from the same collisional event. All members have a relatively high orbital inclination.[9]

Asteroid Euphrosyne—time-lapse view by WISE (May 17, 2010)


  1. ^ a b Johnston, Wm. Robert (27 May 2019). "Asteroids with Satellites Database – (31) Euphrosyne". Johnston's Archive. Retrieved 4 June 2019.
  2. ^ a b c d e "JPL Small-Body Database Browser: 31 Euphrosyne" (2018-06-15 last obs). Retrieved 2019-04-28.
  3. ^ a b Baer, James; Chesley, Steven R; Matson, Robert D (2011). "Astrometric Masses of 26 Asteroids and Observations on Asteroid Porosity". The Astronomical Journal. 141 (5): 143. Bibcode:2011AJ....141..143B. doi:10.1088/0004-6256/141/5/143.
  4. ^ Albedo table
  5. ^ Astrometric and Geodetic Properties of Earth and the Solar System
  6. ^ "Bright Minor Planets 2000". Minor Planet Center. Retrieved 2008-05-23.[permanent dead link]
  7. ^ Brightest asteroids
  8. ^ "Radar-Detected Asteroids and Comets". NASA/JPL Asteroid Radar Research. Retrieved 2011-10-30.
  9. ^ Novaković, Bojan; et al. (November 2011), "Families among high-inclination asteroids", Icarus, 216 (1), pp. 69–81, arXiv:1108.3740, Bibcode:2011Icar..216...69N, doi:10.1016/j.icarus.2011.08.016.

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