Pronunciation /ˈaɪərᵻs/ YR-is Minor planet category Main belt Orbital period 1,345 days Discoverer John Russell Hind | Discovery date August 13, 1847 Alternative names none Adjectives Iridian /aɪˈrɪdiən/ Discovered 13 August 1847 Orbits Sun | |
Similar John Russell Hind discoveries, Other celestial objects | ||
7 Iris is a large main-belt asteroid orbiting the Sun between Mars and Jupiter. It is the fourth-brightest object in the asteroid belt. It is classified as an S-type asteroid, meaning that it has a stony composition.
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Discovery and name
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 in Greek 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 she was spotted following 3 Juno by less than an hour of right ascension (Juno is the Roman equivalent of Hera).
Orbit
Iris regularly comes within 0.4AU of Mars and will next do so on November 2, 2054.
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 mean diameter 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 mean opposition magnitude 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 (next time on October 31, 2017 reaching a magnitude of +6.9), which is as bright as Ceres ever gets.
Rotation
Lightcurve analysis indicates a somewhat angular shape and that Iris's pole points towards the ecliptic coordinates (β, λ) = (10°, 20°) 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 occulting a star on May 26, 1995, and later on July 25, 1997. Both observations gave a diameter of about 200 km.