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3 Juno

Juno (or as Iūno), designated 3 Juno in the Minor Planet Center catalogue 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.

Characteristics

Juno is one of the largest asteroids, containing about 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 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 explains it discovery predating the larger asteroids Hygiea, Europa, Davida and Interamnia. At most oppositions, however, Juno reaches around +8.7—only just visible with binoculars—and at smaller elongations a 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. 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 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 eccentricity higher 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 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 Hilton 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 at visible and near-IR wavelengths, 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.