13th Largest Asteroid, 3 Juno

Juno seen at four wavelengths. A large crater appears dark at 934 nm

3 Juno is the second-most-massive S-type asteroid after 15 Eunomia, and the 13th largest asteroid overall currently known.


Discovery

Juno was discovered on September 1, 1804, by German astronomer Karl Ludwig Harding.

Juno is unusually reflective and can reach +7.5, which is brighter than Neptune or Titan, at a favourable opposition. This is the reason for Juno being discovered before the larger asteroids.

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.

Naming

Juno the asteroid was named after the mythological figure Juno, the highest Roman goddess.

Juno is an ancient Roman goddess, the protector and special counselor of the state. She is a daughter of Saturn and sister (but also the wife) of the chief god Jupiter and the mother of Mars and Vulcan. Juno also looked after the women of Rome.

Stats

Diameter (mean): 234 km
Semi-major axis: 2.671 AU
Orbital Period: 4.37 years
Rotation period: 7.21 hrs
Date discovered: 1804.9.1
Class: S
Type: Main-belt Asteroid
Group: Juno group
(data from JPL Small-Body Database)

Orbit

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'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.

Physical Characteristics

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.

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.