3 Juno

3 Juno []

392px-Juno 4 wavelengths — Juno seen at four wavelengths. A large crater appears dark at 934 nm.

3 Juno (jew'-noe (key)) was the third asteroid to be discovered and is one of the largest main belt asteroids, being the heaviest of the stony S-type. It was discovered on September 1, 1804 by German astronomer Karl L. Harding, using a humble 2-inch telescope. It was named after the mythological figure Juno, the highest Roman goddess. The adjectival form of the name is Junonian.

Characteristics[]

Moon and Asteroids 1 to 10 at 10 km per px — Size comparison: the first 10 asteroids profiled against Earth's Moon. Juno is third from the left.

Juno is one of the largest asteroids, containing approximately 1.0% the mass of the entire asteroid belt. In a ranking by size, it is in the lower teens. It vies with 15 Eunomia for the honour of being the largest of the stony S-type asteroids, although the newest estimates put Juno in second place. Amongst S-types it is unusually reflective, which may be indicative of different surface properties. It is the main body in the Juno family.

Juno rotates in a prograde direction, with the north pole pointing towards ecliptic coordinates (β, λ) = (27°, 103°) with a 10° uncertainty^[1]. This gives an axial tilt of 51°.

Spectroscopic studies of the Junonian surface permit the conclusion that Juno could be the body of origin of ordinary chondrites, a common group of stony meteorites composed of iron-containing silicates such as olivine and pyroxene^[2]. The maximum temperature on the surface, when the sun is overhead, was measured at about 293 K on October 2, 2001. Taking into account also the heliocentric distance at the time, gives an estimated maximum of 301 K (+28°C) at perihelion ^[3].

Infrared images reveal that it possesses an approximately 100 km wide crater or ejecta feature, the result of a geologically young impact^[4].

Observations[]

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.^[5]

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.^[6] 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.^[7]

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.^[4]

Animation Juno moving across background stars.	Star field Juno during opposition in 2009.

Aspects[]

Stationary, retrograde	Opposition	Distance to Earth (AU)	Maximum brightness (mag)	Stationary, prograde	Conjunction to Sun
November 2, 2005	December 9, 2005	1.06025	7.5	January 16, 2006	February 24, 2005
February 19, 2007	April 10, 2007	2.13320	9.7	June 5, 2007	September 2, 2006
April 18, 2008	June 12, 2008	2.28071	10.1	August 10, 2008	November 14, 2007
August 15, 2009	September 21, 2009	1.18972	7.6	October 31, 2009	January 18, 2009
January 24, 2011	March 13, 2011	1.78236	8.9	May 1, 2011	July 10, 2010
March 27, 2012	May 20, 2012	2.37727	10.2	July 20, 2012	October 23, 2011
June 14, 2013	August 4, 2013	1.67506	8.9	September 21, 2013	December 23, 2012
December 17, 2014	February 1, 2015	1.33782	8.2	March 12, 2015	April 13, 2014
March 7, 2016	April 28, 2016	2.30641	10.0	June 27, 2016	September 28, 2015
May 9, 2017	July 3, 2017	2.07904	9.7	August 27, 2017	November 30, 2016
October 20, 2018	November 22, 2018	1.03276	7.4	December 31, 2018	February 16, 2018
February 14, 2020	April 4, 2020	2.07425	9.6	May 29, 2020	August 25, 2019
April 14, 2021	June 8, 2021	2.31368	10.1	August 6, 2021	November 9, 2020

References[]

↑ M. Kaasalainen et al Models of Twenty asteroids from photometric data, Icarus, Vol. 159, p. 369 (2002).
↑ M. J. Gaffey Mineralogical variations within the S-type asteroid class, icarus, Vol. 106, pp. 573 (1993).
↑ L. F. Lim et al Thermal infrared (8-13μm) spectra of 29 asteroids: the Cornell Mid-Infrared Asteroid Spectroscopy (MIDAS) Survey, Icarus, Vol. 173, pp. 385 (2005).
↑ ^4.0 ^4.1 Mt Wilson Observatory S. Baliunas et al Multispectral analysis of asteroid 3 Juno taken with the 100-inch telescope at Mount Wilson Observatory, Icarus, Vol. 163, pp 135 (2003).
↑ 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.
↑ 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, 2014.
↑ Hilton, James L. (February 1999). "US Naval Observatory Ephemerides of the Largest Asteroids". Astronomical Journal 117 (2): 1077–1086. DOI:10.1086/300728. Retrieved on 2012-04-15.

External links[]

Well resolved images from four angles taken at Mount Wilson observatory
NASA Planetary Data System asteroid data sets
Shape model deduced from light curve

Minor planets
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List of asteroids