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113 Amalthea (/æməlˈθə/) is a stony Florian asteroid and binary system from the inner regions of the asteroid belt, approximately 50 kilometers (31 miles) in diameter. It was discovered on 12 March 1871, by German astronomer Robert Luther at the Bilk Observatory in Düsseldorf, Germany. The elongated S-type asteroid has a rotation period of 9.95 hours. It was named after Amalthea from Greek mythology. The discovery of its 5-kilometer satellite was announced in July 2017.

113 Amalthea
Discovered byR. Luther
Discovery siteBilk Obs.
Discovery date12 March 1871
MPC designation(113) Amalthea
Named after
main-belt · Flora
Orbital characteristics[2]
Epoch 31 July 2016 (JD 2457600.5)
Uncertainty parameter 0
Observation arc144.90 yr (52926 d)
Aphelion2.5819 AU (386.25 Gm)
Perihelion2.17010 AU (324.642 Gm)
2.37598 AU (355.442 Gm)
3.66 yr (1337.7 d)
0° 16m 8.832s / day
Known satellites1[1]
Physical characteristics
Mean diameter
46.14±1.4 km
9.950 h (0.4146 d)


Amalthea is thought to be a fragment from the mantle of a Vesta-sized, 300–600 km diameter parent body that broke up around one billion years ago, with the other major remnant being 9 Metis.[3] The spectrum of Amalthea reveals the presence of the mineral olivine, a relative rarity in the asteroid belt.[4][5]

Based on observations made during a stellar occultation by Amalthea of a 10th-magnitude star on 14 March 2017, it was announced in July 2017 that the asteroid has a small, 5-kilometer-sized satellite, provisionally designated S/2017 (113) 1. The observations also indicated that Amalthea has a distinctly elongated shape.[1]

One of Jupiter's inner small satellites, unrelated to 113 Amalthea, is also called Amalthea, as is an (apparently fictional) small Arjuna asteroid in Neal Stephenson's 2015 novel Seveneves.


  1. ^ a b Beatty, Kelly. "Amateur Observers Find an Asteroid's Moon". Sky & Telescope. Retrieved 15 July 2017.
  2. ^ Yeomans, Donald K., "113 Amalthea", JPL Small-Body Database Browser, NASA Jet Propulsion Laboratory, retrieved 12 May 2016.
  3. ^ Kelley, Michael S.; Gaffey, Michael J. (March 2000), "9 Metis and 113 Amalthea: A Genetic Asteroid Pair", Icarus, 35 (144), pp. 27–38, Bibcode:2000Icar..144...27K, doi:10.1006/icar.1999.6266.
  4. ^ Cloutis, E. A. (March 1993), "Olivine-rich asteroids, pallasitic olivine and olivine-metal mixtures: Comparisons of reflectance spectra", Lunar and Planetary Institute, Twenty-fourth Lunar and Planetary Science Conference. Part 1: A-F, pp. 317–318, Bibcode:1993LPI....24..317C.
  5. ^ Burbine, T. H.; et al. (July 2000), "The Nature of Olivine Asteroids", Meteoritics & Planetary Science, 35, pp. A35, Bibcode:2000M&PSA..35R..35B, doi:10.1111/j.1945-5100.2000.tb01796.x.

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