3 Juno

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3 Juno

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Discovery A
Discoverer	Karl Ludwig Harding
Discovery date	September 1, 1804
Alternate designations B	none
Category	Main belt ( Juno clump)
Orbital elements C
Epoch November 25, 2005 ( JD 2453699.5)
Eccentricity (e)	0.2583
Semi-major axis (a)	399.155 G m (2.668 AU)
Perihelion (q)	296.03 Gm (1.979 AU)
Aphelion (Q)	502.276 Gm (3.358 AU)
Orbital period (P)	1591.93 d (4.36 a)
Mean orbital speed	17.93 km/ s
Inclination (i)	12.971 °
Longitude of the ascending node (Ω)	170.125°
Argument of perihelion (ω)	247.839°
Mean anomaly (M)	7.879°
Physical characteristics D
Dimensions	290×240×190 km
Mass	3.0×1019 kg
Density	3.4 g/ cm³
Surface gravity	0.12 m/s²
Escape velocity	0.18 km/s
Rotation period	0.3004 d
Spectral class	S-type asteroid
Absolute magnitude	5.33
Albedo ( geometric)	0.238
Mean surface temperature	~163 K max: 301 K (+28° C)

Juno (jew'-noe ( key)), designated 3 Juno in the Minor Planet Centre catologue system, was the third asteroid to be discovered and is one of the largest main belt asteroids, being the second heaviest of the stony S-type. It was discovered on September 1, 1804 by German astronomer Karl L. Harding and named after the mythological figure Juno, the highest Roman goddess. The adjectival form of the name is Junonian.

Characteristics

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 tenth. 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. This high reflectivity along with Juno's high eccentricity (the highest of any known object until Polyhymnia was discovered in 1854) explains its relatively high magnitude and its discovery predating that of the larger asteroids Hygiea, Europa, Davida and Interamnia. It is the main body in the Juno family.

Juno was originally considered a planet, along with 1 Ceres, 2 Pallas, and 4 Vesta. It was re-classified as an asteroid, with the other three, when many more additional asteroids were discovered. Juno's small size and irregular shape preclude it from being considered a dwarf planet under the IAU classification.

Juno rotates in a prograde direction, with the north pole pointing towards ecliptic coordinates (β, λ) = (27°, 103°) with a 10° uncertainty. 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. 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, this gives an estimated maximum of 301 K (+28° C) at perihelion .

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

Observations

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 observers.

Radio signals from spacecraft in orbit around Mars and/or on its surface have been used to estimate the mass of Juno from the tiny perturbations induced by it onto the motion of Mars.

A study by James L. Hilton (1999) suggests that Juno's orbit changed (slightly) around 1839, "very likely" due to perturbations from a passing asteroid, whose identity has not been determined yet. An alternate yet unlikely explanation is an impact by a sizeable body.

In 1996, Juno was imaged by the Hooker Telescope at Mount Wilson Observatory, using adaptive optics. The images spanned a whole rotation period and revealed an irregular (lumpy) shape with a dark feature, interpreted as a fresh impact site.

Aspects