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Juno (pronounced /ˈdʒuːnoʊ/,[9] or as in Latin: Iūno), formal designation 3 Juno in the Minor Planet Center catalogue system, was the third asteroid to be discovered and is one of the larger main belt asteroids, being one of the two largest stony (S-type) asteroids, along with 15 Eunomia. Juno is estimated to contain 1% of the total mass of the asteroid belt.[10] Juno was discovered on September 1, 1804, by German astronomer Karl L. Harding and named after the mythological figure Juno, the highest Roman goddess.


Juno is one of the larger asteroids, perhaps tenth by size and containing approximately 1.0% the mass of the entire Main asteroid belt.[11] It is the second most massive S-type asteroid after 15 Eunomia.[2] Though one of the most massive asteroids, Juno has only 3% the mass of Ceres.[2]
Size comparison: the first 10 asteroids discovered, profiled against Earth's Moon. Juno is third from the left.

Amongst S-type asteroids, Juno is unusually reflective, which may be indicative of distinct surface properties. This high albedo explains its relatively high apparent magnitude for a small object not near the inner edge of the asteroid belt. Juno can reach +7.5 at a favourable opposition, which is brighter than Neptune or Titan, and is the reason for it being discovered before the larger asteroids Hygiea, Europa, Davida, and Interamnia. At most oppositions, however, Juno only reaches a magnitude of around +8.7[12]—only just visible with binoculars—and at smaller elongations a 3-inch (76 mm) telescope will be required to resolve it.[13] It is the main body in the Juno family.

Juno was originally considered a planet, along with 1 Ceres, 2 Pallas, and 4 Vesta.[14] In 1811, Schröter estimated Juno to be as large as 2290 km in diameter.[14] All four were re-classified as asteroids as additional asteroids were discovered. Juno's small size and irregular shape preclude it from being a dwarf planet.

Juno orbits at a slightly closer mean distance to the Sun than Ceres or Pallas. Its orbit is moderately inclined at around 12° to the ecliptic, but has an extreme eccentricity, greater than that of Pluto. This high eccentricity brings Juno closer to the Sun at perihelion than Vesta and further out at aphelion than Ceres. Juno had the most eccentric orbit of any known body until 33 Polyhymnia was discovered in 1854, and of asteroids over 200 km in diameter only 324 Bamberga has a more eccentric orbit.[15]

Juno rotates in a prograde direction with an axial tilt of approximately 50°.[16] The maximum temperature on the surface, directly facing the Sun, was measured at about[clarification needed] 293 K on October 2, 2001. Taking into account the heliocentric distance at the time, this gives an estimated maximum temperature of 301 K (+28 °C) at perihelion.[5]

Spectroscopic studies of the Junonian surface permit the conclusion that Juno could be the progenitor of chondrites, a common type of stony meteorite composed of iron-bearing silicates such as olivine and pyroxene.[17] Infrared images reveal that Juno possesses an approximately 100 km-wide crater or ejecta feature, the result of a geologically young impact.[18][19]


Juno was the first asteroid for which an occultation was observed. It passed in front of a dim star (SAO 112328) on February 19, 1958. Since then, several occultations by Juno have been observed, the most fruitful being on December 11, 1979, which was registered by 18 observers.[20]

Radio signals from spacecraft in orbit around Mars and on its surface have been used to estimate the mass of Juno from the tiny perturbations induced by it onto the motion of Mars.[21] Juno's orbit appears to have changed slightly around 1839, "very likely" due to perturbations from a passing asteroid, whose identity has not been determined. An alternate but less likely explanation is an impact by a sizeable body.[22]

In 1996, Juno was imaged by the Hooker Telescope at Mount Wilson Observatory at visible and near-IR wavelengths, using adaptive optics. The images spanned a whole rotation period and revealed an irregular shape and a dark albedo feature, interpreted as a fresh impact site.[19]

See also

* Juno in fiction
* List of Solar System bodies formerly considered planets


1. ^ a b c d e f "JPL Small-Body Database Browser: 3 Juno". 2008-07-30 last obs. Retrieved 2008-11-01.
2. ^ a b c d e Jim Baer (2008). "Recent Asteroid Mass Determinations". Personal Website. Retrieved 2008-12-03.
3. ^ Harris, A. W.; Warner, B. D.; Pravec, P.; Eds. (2006). "Asteroid Lightcurve Derived Data. EAR-A-5-DDR-DERIVED-LIGHTCURVE-V8.0.". NASA Planetary Data System. Retrieved 2007-03-15.
4. ^ a b Davis, D. R.; Neese, C., Eds. (2002). "Asteroid Albedos. EAR-A-5-DDR-ALBEDOS-V1.1.". NASA Planetary Data System. Retrieved 2007-02-18.
5. ^ a b Lim, Lucy F.; McConnochie, Timothy H.; Bell, James F.; Hayward, Thomas L. (2005). "Thermal infrared (8-13 µm) spectra of 29 asteroids: the Cornell Mid-Infrared Asteroid Spectroscopy (MIDAS) Survey". Icarus 173 (2): 385–408. doi:10.1016/j.icarus.2004.08.005.
6. ^ Neese, C.; Ed. (2005). "Asteroid Taxonomy.EAR-A-5-DDR-TAXONOMY-V5.0.". NASA Planetary Data System. Retrieved 2007-03-15.
7. ^ "AstDys (3) Juno Ephemerides". Department of Mathematics, University of Pisa, Italy. Retrieved 2010-06-26.
8. ^ "Bright Minor Planets 2005". Minor Planet Center. Retrieved 2008-05-21.
9. ^ In US dictionary transcription, us dict: jū′·nō.
10. ^ Pitjeva, E. V. (2005). "High-Precision Ephemerides of Planets—EPM and Determination of Some Astronomical Constants" (PDF). Solar System Research 39 (3): 176. doi:10.1007/s11208-005-0033-2.
11. ^ Pitjeva, E. V.; Precise determination of the motion of planets and some astronomical constants from modern observations, in Kurtz, D. W. (Ed.), Proceedings of IAU Colloquium No. 196: Transits of Venus: New Views of the Solar System and Galaxy, 2004
12. ^ Odeh, Moh'd. "The Brightest Asteroids". The Jordanian Astronomical Society. Retrieved 2008-05-21.
13. ^ "What Can I See Through My Scope?". Ballauer Observatory. 2004. Retrieved 2008-07-20.
14. ^ a b Hilton, James L (2007-11-16). "When did asteroids become minor planets?". U.S. Naval Observatory. Retrieved 2008-06-22.
15. ^ "MBA Eccentricity Screen Capture". JPL Small-Body Database Search Engine. Retrieved 2008-11-01.
16. ^ The north pole points towards ecliptic coordinates (β, λ) = (27°, 103°) within a 10° uncertainty. Kaasalainen, M.; Torppa, J.; Piironen, J. (2002). "Models of Twenty Asteroids from Photometric Data" (PDF). Icarus 159 (2): 369–395. doi:10.1006/icar.2002.6907.
17. ^ Gaffey, Michael J.; Burbine, Thomas H.; Piatek, Jennifer L.; Reed, Kevin L.; Chaky, Damon A.; Bell, Jeffrey F.; Brown, R. H. (1993). "Mineralogical variations within the S-type asteroid class". Icarus 106 (2): 573. doi:10.1006/icar.1993.1194.
18. ^ "Asteroid Juno Has A Bite Out Of It". Harvard-Smithsonian Center for Astrophysics. 2003-08-06. Retrieved 2007-02-18.
19. ^ a b Baliunas, Sallie; Donahue, Robert; Rampino, Michael R.; Gaffey, Michael J.; Shelton, J. Christopher; Mohanty, Subhanjoy (2003). "Multispectral analysis of asteroid 3 Juno taken with the 100-inch telescope at Mount Wilson Observatory" (PDF). Icarus 163 (1): 135–141. doi:10.1016/S0019-1035(03)00049-6.
20. ^ Millis, R. L.; Wasserman, L. H.; Bowell, E.; Franz, O. G.; White, N. M.; Lockwood, G. W.; Nye, R.; Bertram, R.; Klemola, A.; Dunham, E.; Morrison, D. (February 1981). "The diameter of Juno from its occultation of AG+0°1022". Astronomical Journal 86: 306–313. doi:10.1086/112889.
21. ^ Pitjeva, E. V. (2004). "Estimations of masses of the largest asteroids and the main asteroid belt from ranging to planets, Mars orbiters and landers". 35th COSPAR Scientific Assembly. Held 18–25 July 2004, in Paris, France. pp. 2014.
22. ^ Hilton, James L. (February 1999). "US Naval Observatory Ephemerides of the Largest Asteroids". Astronomical Journal 117: 1077–1086. doi:10.1086/300728.

External links

* Yeomans, Donald K.. "Horizons system". NASA JPL. Retrieved 2007-03-20. – Horizons can be used to obtain a current ephemeris
* Well resolved images from four angles taken at Mount Wilson observatory
* NASA Planetary Data System asteroid data sets
* Shape model deduced from light curve
* Asteroid Juno Grabs the Spotlight

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