3 Juno

3 Juno
Discovery
Who	Karl Ludwig Harding
When	September 1, 1804
Alternate Designation(s)
Orbital Characteristics
Category	Main belt (Juno family)
Semi-Major Axis	399.102 Gm (2.668 AU)
Perihelion	295.949 Gm (1.978 AU)
Aphelion	502.255 Gm (3.357 AU)
Eccentricity	0.258
Revolution Period	1591.609 d (4.36 yr)
Inclination	12.972°
Mean Orbital Speed	18.24 km/s
Physical Characteristics
Dimensions	233.9 km
Mass	2.824×1019 kg
Density	4.2 g/cm³
Surface Gravity	0.13 m/s²
Escape Velocity	0.18 km/s
Rotation Period	0.3004 d
Spectral Class	S-type asteroid
Absolute Magnitude	5.33
Albedo	0.238
Mean Surface Temperature	~225 K

3 Juno was discovered, on September 1, 1804 by German astronomer Karl L. Harding. Located in the main belt, it was the third asteroid to be discovered, leading to the prefixed "3" in the official name. It is named after the mythological figure Juno, the highest Roman goddess.

Juno, head of the Juno family of asteroids, is one of the largest main belt asteroids, measuring about 234 km in diameter. It is an S-type asteroid, meaning that it is highly reflective and composed of nickel-iron mixed with iron and magnesium silicates.

According to James L. Hilton (1999), Juno's orbit was observed to change (slightly) in 1839, "very likely" due to an "unmodeled encounter" — that is to say, some other object passed very close to it or actually hit it.

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.

In 2003 Juno was imaged by the Hooker Telescope at Mount Wilson Observatory, using adaptive optics. Its diameter was determined to be 267 km, as was its irregular shape, which resembles a potato. Infrared photographs revealed that Juno posesses an approx. 100-kilometre crater, the result of an impact that occured in the relatively recent past, geologically speaking. Spectroscopic studies permit the conclusion that Juno could be the body of origin of ordinary chondrites, a common group of stony meteors composed of iron-containing silicates such as olivine and pyroxene.

References

• James L. Hilton, U.S. Naval Observatory Ephemerides of the Largest Asteroids (1999)
• E. V. Pitjeva, Estimations of Masses of the Largest Asteroids and the Main Asteroid Belt From Ranging to Planets, Mars Orbiters And Landers (2004)

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