Iris was discovered on August 13, 1847, by J. R. Hind from London, UK. It was Hind's first asteroid discovery and the seventh asteroid to be discovered overall. Iris was named after the rainbow goddess Iris inGreek mythology, who was a messenger to 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 by less than an hour of right ascension.
Characteristics
Geology
Iris is an S-type asteroid. Its 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. Among the S-type asteroids, Iris ranks fifth in geometric meandiameter after Eunomia, Juno, Amphitrite and Herculina.
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. It has a meanoppositionmagnitude of +7.8, comparable to that of Neptune, and can easily be seen with binoculars at most oppositions. At typical oppositions it marginally outshines the larger though darker Pallas. But at rare oppositions near perihelion Iris can reach a magnitude of +6.7, which is as bright as Ceres ever gets.
Surface features
A study by Hanus et al. using data from the VLT's SPHERE instrument names eight craters, and seven recurring features who remain nameless due to a lack of consistency and their occurrence on the edge of Iris. The tentative naming scheme is that of colors in Latin. It is unknown whether these names are accepted by the IAU. All 8 features are craters, the remaining 7 features are unknown, and are named A through G.
Feature
Named after
Chloros
Chrysos
Cirrhos
Cyanos
Erythros
Glaucos
Porphyra
Xanthos
Rotation
Lightcurve analysis indicates a somewhat angular shape and that Iris's pole points towards the 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.
Observations
Iris was observed occultinga star on May 26, 1995, and later on July 25, 1997. Both observations gave a diameter of about 200 km.
