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Star rich field showing asteroid Iris (apmag 10.1)
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Discovery
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| Discovered by | John Russell Hind |
| Discovery date | August 13, 1847 |
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Designations
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| Named after | Iris |
| Alternate name(s) | none |
| Minor planet category |
Main belt |
| Adjective | Iridian[1] |
| Epoch November 26, 2005 (JD 2453700.5) | |
| Aphelion | 2.937 AU (439.337 Gm) |
| Perihelion | 1.833 AU (274.259 Gm) |
| Semi-major axis | 2.385 AU (356.798 Gm) |
| Eccentricity | 0.231 |
| Orbital period | 3.68 a (1345.375 d) |
| Average orbital speed | 19.03 km/s |
| Mean anomaly | 269.531° |
| Inclination | 5.527° |
| Longitude of ascending node | 259.727° |
| Argument of perihelion | 145.440° |
| Proper semi-major axis | 2.3862106 AU |
| Proper eccentricity | 0.2125516 |
| Proper inclination | 6.3924857° |
| Proper mean motion | 97.653672 deg/yr |
| Proper orbital period | 3.6865 yr (1346.493 d) |
| Precession of perihelion | 38.403324 arcsec/yr |
| Precession of the ascending node | −46.447128 arcsec/yr |
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Physical characteristics
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| Dimensions | 240×200×200 km[2] 225×190×190 km[3] 199.8 ± 10 km (IRAS)[4] |
| Mass | 1.62 ± 0.09 ×1019 kg[2] |
| Mean density | 3.21 ± 0.49 g/cm³[2] |
| Equatorial surface gravity | 0.109 m/s² |
| Escape velocity | 0.147 km/s |
| Rotation period | 0.2975 d[6] |
| Albedo | 0.277 (geometric) |
| Temperature | ~171 K max: 275 K (+2° C) |
| Spectral type | S-type asteroid |
| Apparent magnitude | 6.7[7][8] to 11.4 |
| Absolute magnitude (H) | 5.51 |
| Angular diameter | 0.32" to 0.07" |
7 Iris ( /ˈaɪrɨs/ eye-ris; Greek: Ίρις) is a large main-belt asteroid. Among the S-type asteroids, it ranks fifth in geometric mean diameter after Eunomia, Juno, Amphitrite and Herculina.
Its bright surface and small distance from the Sun make Iris 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.[9] 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 rare oppositions near perihelion Iris can reach a magnitude of +6.7 (next time on October 31st, 2017 reaching a magnitude of +6.9),[7] which is as bright as Ceres ever gets; reports of it being seen without optical aid are unverified.
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It was the seventh asteroid discovered, on August 13, 1847, by J. R. Hind from London, UK. It was Hind's first asteroid discovery.
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 she was spotted following 3 Juno (Juno is the Roman equivalent of Hera) by less than an hour of right ascension.
Lightcurve analysis indicates a somewhat angular shape and that Iris' pole points towards ecliptic coordinates (β, λ) = (10°, 20°) with a 10° uncertainty.[3] 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.[10] 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,[11] so it may be an important contributor of these meteorites. Planetary dynamics also indicates that it should be a significant source of meteorites.[12]
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.
Iris regularly comes within 0.4AU of Mars and will next do so on November 2, 2054.[13]
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