7 Iris is a largemain-belt asteroid and possible remnantplanetesimal orbiting the Sun between Mars and Jupiter. It is the fourth-brightest object in theasteroid belt. 7 Iris is classified as anS-type asteroid, meaning that it has a stony composition.
Iris was discovered on 13 August 1847, byJ. R. Hind fromLondon,UK. It was Hind's first asteroid discovery and the seventh asteroid to be discovered overall. It was named after therainbow goddessIris inGreek mythology, who was a messenger to the gods, especiallyHera. Her quality of attendant of Hera was particularly appropriate to the circumstances of discovery, as Iris was spotted following3 Juno by less than an hour ofright ascension (Juno is theRoman equivalent of Hera).
Iris's original symbol was a rainbow and a star: or more simply. It is in the pipeline forUnicode 17.0 as U+1CEC1 ().[11][12]
Iris is an S-type asteroid. The surface is bright and is probably a mixture ofnickel-iron metals andmagnesium- and iron-silicates. Its spectrum is similar to that ofL andLL chondrites with corrections forspace weathering,[13] so it may be an important contributor of thesemeteorites. Planetary dynamics also indicates that it should be a significant source of meteorites.[14]
Among theS-type asteroids, Iris ranks fifth inmean diameter afterEunomia,Juno,Amphitrite andHerculina. 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.[1]
Star rich field showing asteroid Iris (apmag 10.1)
Iris's bright surface and small distance from the Sun make it the fourth-brightest object in theasteroid belt afterVesta,Ceres, andPallas. It has amean oppositionmagnitude of +7.8, comparable to that ofNeptune, and can easily be seen withbinoculars at most oppositions. At typicaloppositions it marginally outshines the larger though darkerPallas.[15] But at rare oppositions near perihelion Iris can reach a magnitude of +6.7 (last time on 31 October 2017, reaching a magnitude of +6.9),[9] which is as bright as Ceres ever gets.
A study by Hanus et al. using data from theVLT's SPHERE instrument names eight craters 20 to 40 km in diameter, and seven recurring features of unknown nature that 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 theIAU. The other 7 features are labeled A through G.[1]
Iris has a rotational period of 7.14 hours. Iris’ north pole points towards theecliptic 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 anaxial tilt of 85°,[17] 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.
The orbit of 7 Iris compared with the orbits of Earth, Mars and Jupiter
Iris was observedocculting astar on 26 May 1995, and later on 25 July 1997. Both observations gave adiameter of about 200 km.
In February 2024, water molecules were discovered on 7 Iris, alongside20 Massalia, marking the first time water molecules were detected on asteroids.[18][19]
^Ueda, Y.; Miyamoto, M.; Mikouchi, T.; Hiroi, T. (March 2003).Surface Material Analysis of the S-type Asteroids: Removing the Space Weathering Effect from Reflectance Spectrum. 34th Annual Lunar and Planetary Science Conference. p. 2078.Bibcode:2003LPI....34.2078U.
^Migliorini, F.; et al. (1997). "(7) Iris: a possible source of ordinary chondrites?".Astronomy & Astrophysics.321: 652.Bibcode:1997A&A...321..652M.
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