3 Juno

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Juno (minor-planet designation: 3 Juno) is a large asteroid in the asteroid belt. Juno was the third asteroid discovered, in 1804, by German astronomer Karl Harding.[1] It is tied with three other asteroids as the thirteenth largest asteroid, and it is 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.[2]

History

Discovery

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.[4]

Name and symbol

Juno is named after the mythological Juno, the highest Roman goddess. The adjectival form is Junonian (from Template:Langx), with the historical final n of the name (still seen in the French form, Script error: No such module "Lang".) reappearing, analogous to Pluto: Plutonian.[5] Juno is the international name for the asteroid, subject to local variations, such as Italian Script error: No such module "Lang"., French Script error: No such module "Lang"., and Russian Script error: No such module "Lang". (Script error: No such module "Lang".).[lower-alpha 1]

The old astronomical symbol of Juno, still used in astrology, is a scepter topped by a star: ⚵. There are many graphic variants with more elaborate scepters, such as orbed symbol of Juno, sometimes tilted at an angle to provide more room for decoration. The generic asteroid symbol of a disk with its discovery number (③ for Juno) was introduced in 1852 and quickly became the norm.[6][7] The scepter symbol was resurrected for astrological use in 1973.[8]

Characteristics

Juno is one of the larger asteroids, perhaps tenth by size and containing approximately 1% the mass of the entire asteroid belt.[9] It is the second-most-massive S-type asteroid after 15 Eunomia.[10] Even so, Juno has only 3% the mass of the largest asteroid, Ceres.[10] The orbital period of Juno is 4.36578 years.[11]

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 Template:Convert telescope is 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.[4] In 1811, Johann Schröter estimated Juno to be as large as 2290 km in diameter.[4] 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.

File:Moon and Asteroids 1 to 10.svg
Size comparison: the first 10 asteroids discovered, profiled against Earth's Moon. Juno is third from the left.

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. It 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.[14]

Juno rotates in a prograde direction with an axial tilt of approximately 50°.[15] 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.[16]

File:Juno orbit 2018.png
The orbit of Juno is significantly elliptical with a small inclination, moving between Mars and Jupiter

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]

Based on Mid-Infrared Asteroid Spectroscopy (MIDAS) data using the Hale Telescope, an average radius of 135.7 ± 11 km was reported in 2004.[20]

Observations

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.[21] It occulted the magnitude 11.3 star PPMX 9823370 on 29 July 2013,[22] and 2UCAC 30446947 on 30 July 2013.[23]

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.[24] 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.[25]

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]

Oppositions

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.[26][27] The next favorable opposition will be 30 October 2031, at a distance of 1.044 AU, magnitude 7.42.

See also

Notes

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References

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External links

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  1. ↑ Script error: No such module "citation/CS1".
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  3. ↑ Script error: No such module "citation/CS1".
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  5. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named OED
  6. ↑ Script error: No such module "Citation/CS1".
  7. ↑ Script error: No such module "Citation/CS1".
  8. ↑ Eleanor Bach (1973). Ephemerides of the asteroids: Ceres, Pallas, Juno, Vesta, 1900–2000. Celestial Communications.
  9. ↑ Pitjeva, E. V.; Precise determination of the motion of planets and some astronomical constants from modern observations Template:Webarchive, in Kurtz, D. W. (Ed.), Proceedings of IAU Colloquium No. 196: Transits of Venus: New Views of the Solar System and Galaxy, 2004
  10. ↑ a b Cite error: Invalid <ref> tag; no text was provided for refs named Baer
  11. ↑ Script error: No such module "citation/CS1".
  12. ↑ Script error: No such module "citation/CS1".
  13. ↑ Script error: No such module "citation/CS1". (archived)
  14. ↑ Script error: No such module "citation/CS1".
  15. ↑ The north pole points towards ecliptic coordinates (β, λ) = (27°, 103°) within a 5° uncertainty. Script error: No such module "Citation/CS1".
  16. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named lim2005
  17. ↑ Script error: No such module "Citation/CS1".
  18. ↑ Script error: No such module "citation/CS1".
  19. ↑ a b Script error: No such module "Citation/CS1".
  20. ↑ Script error: No such module "Citation/CS1".
  21. ↑ Script error: No such module "Citation/CS1".
  22. ↑ Asteroid Occultation Updates – 29 Jul 2013
  23. ↑ Asteroid Occultation Updates – 30 Jul 2013.
  24. ↑ Script error: No such module "citation/CS1".
  25. ↑ Script error: No such module "Citation/CS1".
  26. ↑ The Astronomical Almanac for the year 2018, G14
  27. ↑ Asteroid 3 Juno at opposition Template:Webarchive 16 Nov 2018 at 11:31 UTC


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