Among the S-type asteroids, it ranks fifth in geometric mean diameter after Eunomia, Juno, Herculina and Amphitrite.
Discovery
Iris was the seventh asteroid discovered, on August 13, 1847, by John Russell Hind from London. Iris was Hind's first asteroid discovery.
Naming
Iris was named after the rainbow goddess Iris of Greek mythology, sister of the Harpies and messenger of the gods, especially Hera. Her quality of attendant of Hera was particularly appropriate to the circumstances of discovery, as Iris was spotted following 3 Juno (Juno is the Roman equivalent of Hera) by less than an hour of right ascension.
In Greek mythology, Iris is the personification of the rainbow and messenger of the gods. She is also known as one of the goddesses of the sea and the sky. Iris links the gods to humanity. She travels with the speed of wind from one end of the world to the other, and into the depths of the sea and the underworld.
Stats
Diameter (mean): 200 km
Aphelion: 2.937 AU
Perihelion: 1.824 AU
Semi-major axis: 2.387 AU
Orbital Period: 3.69 years
Rotation period: 7.139 hrs
Date discovered: 1847.8.13
Class: S
Type: Main-belt Asteroid
(data from JPL Small-Body Database)
Brightness
Iris's bright surface and small distance from the Sun make it the fourth brightest object in the asteroid belt after Vesta, Ceres, and Pallas. But at typical oppositions it marginally outshines the larger though darker Pallas.
Iris has a mean opposition magnitude of +7.8, comparable to that of Neptune, and can easily be seen with binoculars at most oppositions. At rare oppositions near perihelion Iris can reach a magnitude of +6.7 (next time on October 31st, 2017 reaching a magnitude of +6.9), which is as bright as Ceres ever gets. Reports of Iris being seen without optical aid are unverified.
Characteristics
Lightcurve analysis indicates a somewhat angular shape and that Iris' pole points towards ecliptic coordinates with a 10° uncertainty. This gives an axial tilt of 85°, so that on almost a whole hemisphere of Iris, the sun does not set during summer, and does not rise during winter. On an airless body this gives rise to very large temperature differences.
The Iridian surface likely exhibits albedo differences, with possibly a large bright area in the northern hemisphere. Overall the surface is very bright and is probably a mixture nickel-iron metals and magnesium- and iron-silicates. Its spectrum is similar to that of L and LL chondrites with corrections for space weathering, so it may be an important contributor of these meteorites. Planetary dynamics also indicates that it should be a significant source of meteorites.
Star Occultation
Iris was observed occulting a star on May 26, 1995, and later on July 25, 1997. Both observations gave a diameter of about 200 km.
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