|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
200±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 and perhaps remnant planetesimal 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. The surface is 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. Its shape is consistent with an oblate spheroid with a large equatorial excavation, suggesting it is a remnant planetesimal. No collisional family can be associated with Iris, likely because the excavating impact occurred early in the history of the Solar System, and the debris has since dispersed.
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 20 to 40 km in diameter, and seven recurring features who remain nameless due to a lack of consistency and their occurrence on the edge of Iris. The names are Greek names of colors, corresponding to the rainbow as the sign of Iris. It is unknown whether these names are under consideration by the IAU. All 8 features are craters, the natures of the remaining 7 features are unknown, and they are labeled A through G.
Iris has a rotational period of 7.14 hours. Iris's north pole points towards the ecliptic coordinates (λ, β) estimated to be (18°, +19°) with a 4° uncertainty (Viikinkoski et al. 2017) or (19°, +26°) with a 3° uncertainty (Hanuš et al. 2019). This gives an axial tilt of xx°,[fix] so that on much of each hemisphere, 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.