3 Juno is a large asteroid in the asteroid belt. Juno was the third asteroid discovered, in 1804, by German astronomer Karl Harding.[15] It is one of the twenty largest asteroids and one of the two largest stony (S-type) asteroids, along with 15 Eunomia. It is estimated to contain 1% of the total mass of the asteroid belt.[16]

3 Juno Juno symbol.svg
Juno orbit 2018.png
The orbit of Juno is significantly elliptical with a small inclination, moving between Mars and Jupiter.
Discovered byKarl Ludwig Harding
Discovery date1 September 1804
(3) Juno
Named after
Juno (Latin: Iūno)
Main belt (Juno clump)
AdjectivesJunonian /ˈnniən/[2]
Orbital characteristics[3]
Epoch JD 2457000.5 (9 December 2014)
Aphelion3.35293 AU
Perihelion1.98847 AU
2.67070 AU
4.36463 yr
17.93 km/s
Proper orbital elements[4]
2.6693661 AU
82.528181 deg / yr
4.36215 yr
(1593.274 d)
Precession of perihelion
43.635655 arcsec / yr
Precession of the ascending node
−61.222138 arcsec / yr
Physical characteristics
Dimensions(320×267×200)±6 km[5]
Mean diameter
246.596±10.594 km[3]
Mean radius
135.7±11 [6]
216 000 km2[7]
Volume8 950 000 km3[7]
Mass(2.86±0.46)×1019 kg[a][8]
Mean density
3.20±0.56 g/cm3[8]
Equatorial surface gravity
0.12 m/s2
Equatorial escape velocity
0.18 km/s
7.21 hr[3] (0.3004 d)[9]
Equatorial rotation velocity
31.75 m/s[7]
Temperature~163 K
max: 301 K (+28°C)[11]
7.4[13][14] to 11.55
0.30" to 0.07"



Juno was discovered on 1 September 1804, by Karl Ludwig Harding.[3] It was the third asteroid found, but was initially considered to be a planet; it was reclassified as an asteroid and minor planet during the 1850s.[17]


Juno is named after the mythological Juno, the highest Roman goddess. The adjectival form is Junonian (from Latin jūnōnius), with the historical final n of the name (still seen in the French form, Junon) reappearing, analogous to Pluto ~ Plutonian.[2]

'Juno' is the international name for the asteroid, subject to local variation: Italian Giunone, French Junon, Russian Юнона (Yunona), etc.[b] Its planetary symbol is ③. An older symbol, still seen in astrology, is ⚵ ( ).[citation needed]


Juno is one of the larger asteroids, perhaps tenth by size and containing approximately 1% the mass of the entire asteroid belt.[18] It is the second-most-massive S-type asteroid after 15 Eunomia.[5] Even so, Juno has only 3% the mass of Ceres.[5]

Size comparison: the first 10 asteroids discovered, profiled against Earth's Moon. Juno is third from the left.

The orbital period of Juno is 4.36578 years.[19]

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[20]—only just visible with binoculars—and at smaller elongations a 3-inch (76 mm) telescope will be required to resolve it.[21] It is the main body in the Juno family.

Planets 1807–1845
1 Mercury 
2 Venus 
3 Earth  
4 Mars 
5 Vesta  
6 Juno  
7 Ceres  
8 Pallas  
9 Jupiter 
10 Saturn  
11 Uranus 

Juno was originally considered a planet, along with 1 Ceres, 2 Pallas, and 4 Vesta.[22] In 1811, Schröter estimated Juno to be as large as 2290 km in diameter.[22] All four were reclassified as asteroids as additional asteroids were discovered. Juno's small size and irregular shape preclude it from being designated 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.[23]

Juno rotates in a prograde direction with an axial tilt of approximately 50°.[24] The maximum temperature on the surface, directly facing the Sun, was measured at about 293 K on 2 October 2001. Taking into account the heliocentric distance at the time, this gives an estimated maximum temperature of 301 K (+28 °C) at perihelion.[11]

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.[25] Infrared images reveal that Juno possesses an approximately 100 km-wide crater or ejecta feature, the result of a geologically young impact.[26][27]

Based on MIDAS infrared data using the Hale telescope, an average radius of 135.7±11 was reported in 2004.[6]


Juno was the first asteroid for which an occultation was observed. It passed in front of a dim star (SAO 112328) on 19 February 1958. Since then, several occultations by Juno have been observed, the most fruitful being the occultation of SAO 115946 on 11 December 1979, which was registered by 18 observers.[28] Juno occulted the magnitude 11.3 star PPMX 9823370 on 29 July 2013,[29] and 2UCAC 30446947 on 30 July 2013.[30]

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.[31] 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.[32]

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.[27]

Juno seen at four wavelengths with a large crater in the dark (Hooker telescope, 2003)
Juno moving across background stars
Star field
Juno during opposition in 2009

Video of Juno taken as part of ALMA's Long Baseline Campaign


Juno reaches opposition from the Sun every 15.5 months or so, with its minimum distance varying greatly depending on whether it is near perihelion or aphelion. Sequences of favorable oppositions occur every 10th opposition, i.e. just over every 13 years. The last favorable oppositions were on 1 December 2005, at a distance of 1.063 AU, magnitude 7.55, and on 17 November 2018, at a minimum distance of 1.036 AU, magnitude 7.45.[33][34] The next favorable opposition will be 30 October 2031, at a distance of 1.044 AU, magnitude 7.42.

See alsoEdit


  1. ^ 1.44 ± 0.23) × 10−11 M
  2. ^ There are two exceptions: Greek, where the name was translated to its Hellenic equivalent, Hera (3 Ήρα), as in the cases of 1 Ceres and 4 Vesta; and Chinese, where it is called the 'marriage-god(dess) star' (婚神星 hūnshénxīng). This contrasts with the goddess Juno, for which Chinese uses the transliterated Latin name (朱諾 zhūnuò).


  1. ^ "Juno". Dictionary.com Unabridged. Random House.
  2. ^ a b "Junonian". Oxford English Dictionary (Online ed.). Oxford University Press. (Subscription or participating institution membership required.)
  3. ^ a b c d e f g "JPL Small-Body Database Browser: 3 Juno" (2017-11-26 last obs). Retrieved 17 November 2014.
  4. ^ "AstDyS-2 Juno Synthetic Proper Orbital Elements". Department of Mathematics, University of Pisa, Italy. Retrieved 1 October 2011.
  5. ^ a b c Baer, Jim (2008). "Recent Asteroid Mass Determinations". Personal Website. Archived from the original on 2 July 2013. Retrieved 3 December 2008.
  6. ^ a b Lim, L; McConnochie, T; Belliii, J; Hayward, T (2005). "Thermal infrared (8?13 ?m) spectra of 29 asteroids: The Cornell Mid-Infrared Asteroid Spectroscopy (MIDAS) Survey" (PDF). Icarus. 173 (2): 385. Bibcode:2005Icar..173..385L. doi:10.1016/j.icarus.2004.08.005. Archived from the original (PDF) on 3 March 2016. Retrieved 26 August 2019.
  7. ^ a b c Calculated based on the known parameters
  8. ^ a b James Baer, Steven Chesley & Robert Matson (2011) "Astrometric masses of 26 asteroids and observations on asteroid porosity." The Astronomical Journal, Volume 141, Number 5
  9. ^ 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. Archived from the original on 9 April 2009. Retrieved 15 March 2007.
  10. ^ a b Davis, D. R.; Neese, C., eds. (2002). "Asteroid Albedos. EAR-A-5-DDR-ALBEDOS-V1.1". NASA Planetary Data System. Archived from the original on 17 December 2009. Retrieved 18 February 2007.
  11. ^ 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. Bibcode:2005Icar..173..385L. doi:10.1016/j.icarus.2004.08.005.
  12. ^ Neese, C., ed. (2005). "Asteroid Taxonomy.EAR-A-5-DDR-TAXONOMY-V5.0". NASA Planetary Data System. Archived from the original on 5 September 2006. Retrieved 24 December 2013.
  13. ^ "AstDys (3) Juno Ephemerides". Department of Mathematics, University of Pisa, Italy. Retrieved 26 June 2010.
  14. ^ "Bright Minor Planets 2005". Minor Planet Center. Archived from the original on 29 September 2008.
  15. ^ Cunningham, Clifford J (2017), Bode's Law and the discovery of Juno : historical studies in asteroid research, Springer, ISBN 978-3-319-32875-1
  16. ^ Pitjeva, E. V. (2005). "High-Precision Ephemerides of Planets—EPM and Determination of Some Astronomical Constants" (PDF). Solar System Research. 39 (3): 176. Bibcode:2005SoSyR..39..176P. doi:10.1007/s11208-005-0033-2. Archived from the original (PDF) on 31 October 2008.
  17. ^ Hilton, James L. "When did the asteroids become minor planets?". U.S. Naval Observatory. Archived from the original on 24 March 2008. Retrieved 8 May 2008.
  18. ^ 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
  19. ^ "Comets Asteroids". Find The Data.org. Archived from the original on 14 May 2014. Retrieved 14 May 2014.
  20. ^ Odeh, Moh'd. "The Brightest Asteroids". The Jordanian Astronomical Society. Archived from the original on 11 May 2008. Retrieved 21 May 2008.
  21. ^ "What Can I See Through My Scope?". Ballauer Observatory. 2004. Archived from the original on 26 July 2011. Retrieved 20 July 2008. (archived)
  22. ^ a b Hilton, James L (16 November 2007). "When did asteroids become minor planets?". U.S. Naval Observatory. Archived from the original on 24 March 2008. Retrieved 22 June 2008.
  23. ^ "MBA Eccentricity Screen Capture". JPL Small-Body Database Search Engine. Archived from the original on 27 March 2009. Retrieved 1 November 2008.
  24. ^ 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. Bibcode:2002Icar..159..369K. doi:10.1006/icar.2002.6907.
  25. ^ 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. Bibcode:1993Icar..106..573G. doi:10.1006/icar.1993.1194.
  26. ^ "Asteroid Juno Has A Bite Out Of It". Harvard-Smithsonian Center for Astrophysics. 6 August 2003. Archived from the original on 8 February 2007. Retrieved 18 February 2007.
  27. ^ 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. Bibcode:2003Icar..163..135B. doi:10.1016/S0019-1035(03)00049-6.
  28. ^ Millis, R. L.; Wasserman, L. H.; Bowell, E.; Franz, O. G.; White, N. M.; Lockwood, G. W.; Nye, R.; Bertram, R.; et al. (February 1981). "The diameter of Juno from its occultation of AG+0°1022" (PDF). Astronomical Journal. 86: 306–313. Bibcode:1981AJ.....86..306M. doi:10.1086/112889.
  29. ^ Asteroid Occultation Updates – 29 Jul 2013
  30. ^ Asteroid Occultation Updates – 30 Jul 2013.
  31. ^ 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. p. 2014. Bibcode:2004cosp...35.2014P.
  32. ^ Hilton, James L. (February 1999). "US Naval Observatory Ephemerides of the Largest Asteroids". Astronomical Journal. 117 (2): 1077–1086. Bibcode:1999AJ....117.1077H. doi:10.1086/300728. Retrieved 15 April 2012.
  33. ^ The Astronomical Almanac for the year 2018, G14
  34. ^ Asteroid 3 Juno at opposition 16 Nov 2018 at 11:31 UTC

External linksEdit