|Discovered by||John Russell Hind|
|Discovery date||August 13, 1847|
|Epoch 27 April 2019 (JD 2458600.5)|
|Aphelion||2.937 AU (439.4 Gm)|
|Perihelion||1.834 AU (274.4 Gm)|
|2.385 AU (356.8 Gm)|
|3.68 a (1345.375 d)|
Average orbital speed
|Proper orbital elements|
Proper semi-major axis
Proper mean motion
|97.653672 deg / yr|
Proper orbital period
Precession of perihelion
|38.403324 arcsec / yr|
Precession of the ascending node
|−46.447128 arcsec / yr|
|Dimensions||268 km × 234 km × 180 km|
± (5 km × 4 km × 6 km)
225 km × 190 km × 190 km
199.8±10 km (IRAS)
Equatorial surface gravity
Equatorial escape velocity
|7.138843 h (0.2974518 d)|
Equatorial rotation velocity
max: 275 K (+2°C)
|6.7 to 11.4|
|0.32" to 0.07"|
Iris (minor planet designation: 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.
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 Iris was spotted following 3 Juno by less than an hour of right ascension (Juno is the Roman equivalent of Hera).
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.
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 (last time on October 31, 2017 reaching a magnitude of +6.9), which is as bright as Ceres ever gets.
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.
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.