3 Juno (jew'-noe (Pronunciation respelling 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

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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° uncertaintyM. Kaasalainen et al Models of Twenty asteroids from photometric data, Icarus, Vol. 159, p. 369 (2002).. 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 pyroxeneM. J. Gaffey Mineralogical variations within the S-type asteroid class, icarus, Vol. 106, pp. 573 (1993).. 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 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)..

Infrared images reveal that it possesses an approximately 100 km wide crater or ejecta feature, the result of a geologically young impactMt 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)..

Observations

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Some notable observation milestones for Juno include:

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 observersL. Millis et al The diameter of Juno from its occultation of AG+0°1022, The Astronomical Journal, Vol. 86, pp. 306 (1981).