Juno was discovered on 1 September 1804, byKarl Ludwig Harding.[17] It was the thirdasteroid found, but was initially considered to be aplanet; it was reclassified as an asteroid andminor planet during the 1850s.[18]
Juno is named after the mythologicalJuno, the highest Roman goddess. The adjectival form is Junonian (from Latinjūnōnius), with the historical finaln 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: ItalianGiunone, FrenchJunon, RussianЮнона (Yunona), etc.[c]
The oldastronomical symbol of Juno, still used in astrology, is a scepter topped by a star,⟨⟩. There were many graphic variants with a more elaborated scepter, such as, sometimes tilted at an angle to provide more room for decoration.The generic asteroid symbol of a disk with its discovery number,⟨③⟩, was introduced in 1852 and quickly became the norm.[19][20] The scepter symbol was resurrected for astrological use in 1973.[21]
Juno is one of the larger asteroids, perhaps tenth by size and containing approximately 1% the mass of the entireasteroid belt.[22] It is the second-most-massive S-type asteroid after 15 Eunomia.[6] Even so, Juno has only 3% the mass ofCeres.[6] The orbital period of Juno is 4.36578 years.[23]
Amongst S-type asteroids, Juno is unusually reflective, which may be indicative of distinct surface properties. This high albedo explains its relatively highapparent 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 thanNeptune orTitan, and is the reason for it being discovered before the larger asteroidsHygiea,Europa,Davida, andInteramnia. At most oppositions, however, Juno only reaches a magnitude of around +8.7[24]—only just visible withbinoculars—and at smallerelongations a 3-inch (76 mm)telescope will be required to resolve it.[25] It is the main body in theJuno family.
Juno was originally considered a planet, along with1 Ceres,2 Pallas, and4 Vesta.[26] In 1811,Schröter estimated Juno to be as large as 2290 km in diameter.[26] All four were reclassified as asteroids as additional asteroids were discovered. Juno's small size and irregular shape preclude it from being designated adwarf planet.
Size comparison: the first 10 asteroids discovered, profiled against Earth'sMoon. Juno is third from the left.
Juno orbits at a slightly closer mean distance to theSun than Ceres or Pallas. Its orbit is moderately inclined at around 12° to theecliptic, but has an extremeeccentricity, greater than that ofPluto. This high eccentricity brings Juno closer to the Sun atperihelion than Vesta and further out ataphelion than Ceres. Juno had the most eccentric orbit of any known body until33 Polyhymnia was discovered in 1854, and of asteroids over 200 km in diameter only324 Bamberga has a more eccentric orbit.[27]
Juno rotates in aprograde direction with anaxial tilt of approximately 50°.[9] The maximum temperature on the surface, directly facing the Sun, was measured at about 293K on 2 October 2001. Taking into account theheliocentric distance at the time, this gives an estimated maximum temperature of 301 K (+28 °C) at perihelion.[11]
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 ofchondrites, a common type of stonymeteorite composed of iron-bearingsilicates such asolivine andpyroxene.[28]Infrared images reveal that Juno possesses an approximately 100 km-wide crater or ejecta feature, the result of a geologically young impact.[29][30]
Based on MIDAS infrared data using theHale Telescope, an average radius of 135.7±11 was reported in 2004.[31]
Juno was the first asteroid for which anoccultation was observed. It passed in front of a dimstar (SAO 112328) on 19 February 1958. Since then, several occultations by Juno have been observed, the most fruitful being the occultation ofSAO 115946 on 11 December 1979, which was registered by 18 observers.[32]Juno occulted the magnitude 11.3 starPPMX 9823370 on 29 July 2013,[33] and2UCAC 30446947 on 30 July 2013.[34]
Radio signals from spacecraft in orbit aroundMars 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.[35] Juno'sorbit appears to have changed slightly around 1839, very likely due to perturbations from a passing asteroid, whose identity has not been determined.[36]
In 1996, Juno was imaged by theHooker Telescope atMount Wilson Observatory at visible and near-IR wavelengths, usingadaptive optics. The images spanned a whole rotation period and revealed an irregular shape and a dark albedo feature, interpreted as a fresh impact site.[30]
Juno reachesopposition 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.[37][38] The next favorable opposition will be 30 October 2031, at a distance of 1.044 AU, magnitude 7.42.
^There are two exceptions: Greek, where the name was translated to its Hellenic equivalent,Hera (3 Ήρα), as in the cases of1 Ceres and4 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).
^abcdeP. Vernazza et al. (2021) VLT/SPHERE imaging survey of the largest main-belt asteroids: Final results and synthesis.Astronomy & Astrophysics 54, A56
^abJames Baer, Steven Chesley & Robert Matson (2011) "Astrometric masses of 26 asteroids and observations on asteroid porosity."The Astronomical Journal, Volume 141, Number 5
^abLim, 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.
^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.
^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.
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