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3 Juno

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3 Juno Juno symbol.svg
Juno 4 wavelengths.jpg
Juno seen at four wavelengths. A large crater appears dark at 934 nm.
Discovered by Karl Ludwig Harding
Discovery date September 1, 1804
Pronunciation / ˈ n /
Named after Juno (Latin: Iūno)
Alternative names none
Minor planet category Main belt ( Juno clump)
Adjective Junonian / ˈ n n i ə n /
Orbital characteristics
Epoch November 30, 2008 ( JD 2454800.5)
Aphelion 3.356 AU (502.050 Gm)
Perihelion 1.988 AU (297.40 Gm)
Semi-major axis 2.672 AU (399.725 Gm)
Eccentricity 0.2559
Orbital period 4.37 a (1595.4 d)
Average orbital speed 17.93 km/s
Mean anomaly 256.8°
Inclination 12.968°
Longitude of ascending node 169.96°
Argument of perihelion 247.93°
Proper orbital elements
Proper semi-major axis 2.6693661 AU
Proper eccentricity 0.2335060
Proper inclination 13.2515192°
Proper mean motion 82.528181 deg / yr
Proper orbital period 4.36215 yr
(1593.274 d)
Precession of perihelion 43.635655 arcsec / yr
Precession of the ascending node −61.222138 arcsec / yr
Physical characteristics
Dimensions 320×267×200 km
(233 km)
Mass 2.67 ×1019 kg
Mean density 2.98 ± 0.55 g/cm³
Equatorial surface gravity 0.12 m/s²
Escape velocity 0.18 km/s
Rotation period 7.21 hr (0.3004 d)
Albedo 0.238 ( geometric)
Temperature ~163 K
max: 301 K (+28°C)
Spectral type S-type asteroid
Apparent magnitude 7.4 to 11.55
Absolute magnitude (H) 5.33
Angular diameter 0.30" to 0.07"

Juno, minor-planet designation 3 Juno in the Minor Planet Centre catalogue system, was the third asteroid to be discovered and is one of the larger main-belt asteroids, being one of the two largest stony ( S-type) asteroids, along with 15 Eunomia. It is estimated to contain 1% of the total mass of the asteroid belt. Juno was discovered on September 1, 1804, by German astronomer Karl L. Harding.


3 Juno is named after the mythological Jūno, the highest Roman goddess. The adjectival form is Junonian (jūnōnius).

With two exceptions, 'Juno' is the international name, subject to local variation: Italian Giunone, French Junon, Russian Yunona, etc. Its planetary symbol is ③. An older symbol, still occasionally seen, is (Old symbol of Juno).


Juno is one of the larger asteroids, perhaps tenth by size and containing approximately 1.0% the mass of the entire asteroid belt. It is the second-most-massive S-type asteroid after 15 Eunomia. Though one of the most massive asteroids, Juno has only 3% the mass of Ceres.

Size comparison: the first 10 asteroids discovered, profiled against Earth's Moon. Juno is third from the left.

Amongst S-type asteroids, Juno is unusually reflective, which may be indicative of distinct surface properties. This high albedo explains its relatively high apparent 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 than Neptune or Titan, and is the reason for it being discovered before the larger asteroids Hygiea, Europa, Davida, and Interamnia. At most oppositions, however, Juno only reaches a magnitude of around +8.7—only just visible with binoculars—and at smaller elongations a 3-inch (76 mm) telescope will be required to resolve it. It is the main body in the Juno family.

Juno was originally considered a planet, along with 1 Ceres, 2 Pallas, and 4 Vesta. In 1811, Schröter estimated Juno to be as large as 2290 km in diameter. All four were re-classified as asteroids as additional asteroids were discovered. Juno's small size and irregular shape preclude it from being designated a dwarf planet.

Juno orbits at a slightly closer mean distance to the Sun than Ceres or Pallas. Its orbit is moderately inclined at around 12° to the ecliptic, but has an extreme eccentricity, greater than that of Pluto. This high eccentricity brings Juno closer to the Sun at perihelion than Vesta and further out at aphelion than Ceres. Juno had the most eccentric orbit of any known body until 33 Polyhymnia was discovered in 1854, and of asteroids over 200 km in diameter only 324 Bamberga has a more eccentric orbit.

Juno rotates in a prograde direction with an axial tilt of approximately 50°. The maximum temperature on the surface, directly facing the Sun, was measured at about 293 K on October 2, 2001. Taking into account the heliocentric distance at the time, this gives an estimated maximum temperature of 301 K (+28 °C) at perihelion.

Spectroscopic studies of the Junonian surface permit the conclusion that Juno could be the progenitor of chondrites, a common type of stony meteorite composed of iron-bearing silicates such as olivine and pyroxene. Infrared images reveal that Juno possesses an approximately 100 km-wide crater or ejecta feature, the result of a geologically young impact.


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 on its surface have been used to estimate the mass of Juno from the tiny perturbations induced by it onto the motion of Mars. 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.

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.

Juno mpl anim.gif
Juno moving across background stars.
Star field
Juno during opposition in 2009.
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