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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.

This article is about the asteroid. For other uses, see Iris (disambiguation).
7 Iris
Iris imaged by the Very Large Telescope in 2017[1]
Discovery
Discovered byJohn Russell Hind
Discovery dateAugust 13, 1847
Designations
MPC designation
(7) Iris
Pronunciation/ˈaɪrɪs/[2]
Named after
Īris
Minor planet category
Main belt
AdjectivesIridian /ɪˈrɪdiən, aɪ-/[3]
Orbital characteristics[4]
Epoch 27 April 2019 (JD 2458600.5)
Aphelion2.937 AU (439.4 Gm)
Perihelion1.834 AU (274.4 Gm)
Semi-major axis
2.385 AU (356.8 Gm)
Eccentricity0.2312
Orbital period (sidereal)
3.68 a (1345.375 d)
Average orbital speed
19.03 km/s
Mean anomaly
140.420°
Inclination5.524°
Longitude of ascending node
259.563°
Argument of perihelion
145.265°
Proper orbital elements[5]
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
Physical characteristics
Dimensionsc/a = 0.58±0.07[6]
268 km × 234 km × 180 km
± (5 km × 4 km × 6 km)[1]
225 km × 190 km × 190 km[7]
Mean diameter
199±10 km[8][6]
214±5 km[1]
200±10 km (IRAS)[4]
Surface area
538460 km2[lower-alpha 1]
Volume37153500 km3[lower-alpha 1]
Mass(13.75±1.3)×1018 kg[6][1]
Mean density
3.2±0.74 g/cm3[6]
2.7±0.3 g/cm3[1]
Equatorial surface gravity
0.08 m/s²
Equatorial escape velocity
0.131 km/s
Synodic rotation period
7.138843 h (0.2974518 d)[1]
Equatorial rotation velocity
25.4 m/s[lower-alpha 1]
Geometric albedo
0.279[6]
0.277
Temperature~171 K
max: 275 K (+2°C)
Spectral type
S
Apparent magnitude
6.7[9][10] to 11.4
Absolute magnitude (H)
5.51
Angular diameter
0.32" to 0.07"

    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 Iris was spotted following 3 Juno by less than an hour of right ascension (Juno is the Roman equivalent of Hera).

    Iris's original symbol was a rainbow and a star: or more simply .


    Characteristics


    Size comparison: the first 10 asteroids profiled against Earth's Moon. Iris is fourth from the right.
    Size comparison: the first 10 asteroids profiled against Earth's Moon. Iris is fourth from the right.

    Geology


    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,[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]

    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.[1]


    Brightness


    Star rich field showing asteroid Iris (apmag 10.1)
    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 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.[13] 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),[9] which is as bright as Ceres ever gets.


    Surface features


    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 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 the IAU. The other 7 features are labeled A through G.[1]

    Named features on Iris
    FeaturePronunciationGreekMeaning
    Chloros/ˈkloʊrɒs/χλωρός'green'
    Chrysos/ˈkraɪsɒs/χρῡσός'gold'
    Cirrhos/ˈsɪrɒs/κιρρός'orange'[lower-alpha 2]
    Cyanos/ˈsaɪənɒs/κύανος'blue'
    Erythros/ˈɛrɪθrɒs/ἐρυθρός'red'
    Glaucos/ˈɡlɔːkɒs/γλαυκός'grey'[lower-alpha 3]
    Porphyra/ˈpɔːrfɪrə/πορφύρα'purple'
    Xanthos/ˈzænθɒs/ξανθός'yellow'

    Rotation


    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.


    Observations


    The orbit of 7 Iris compared with the orbits of Earth, Mars and Jupiter
    The orbit of 7 Iris compared with the orbits of Earth, Mars and Jupiter

    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.


    See also



    Notes


    1. Calculated based on parameters calculated by J. Hanuš et al.[1]
    2. κιρρός is variously translated. The OED has 'orange-tawny'.[14] The color coding of the proposers in their crater maps, however, is simply orange.
    3. Or greyish blue-green.

    References


    1. Hanuš, J.; Marsset, M.; Vernazza, P.; Viikinkoski, M.; Drouard, A.; Brož, M.; et al. (24 April 2019). "The shape of (7) Iris as evidence of an ancient large impact?". Astronomy & Astrophysics. 624 (A121). arXiv:1902.09242. Bibcode:2018DPS....5040406H. doi:10.1051/0004-6361/201834541.
    2. "iris". Oxford English Dictionary (Online ed.). Oxford University Press. (Subscription or participating institution membership required.)
    3. "iridian". Oxford English Dictionary (Online ed.). Oxford University Press. (Subscription or participating institution membership required.)
    4. "JPL Small-Body Database Browser: 7 Iris" (2018-03-27 last obs.). Jet Propulsion Laboratory. Retrieved 17 June 2019.
    5. "AstDyS-2 Iris Synthetic Proper Orbital Elements". Department of Mathematics, University of Pisa, Italy. Retrieved 1 October 2011.
    6. P. Vernazza et al. (2021) VLT/SPHERE imaging survey of the largest main-belt asteroids: Final results and synthesis. Astronomy & Astrophysics 54, A56
    7. Kaasalainen, M.; et al. (2002). "Models of twenty asteroids from photometric data" (PDF). Icarus. 159 (2): 369. Bibcode:2002Icar..159..369K. doi:10.1006/icar.2002.6907.
    8. Dudziński et al. (2020) Volume uncertainty of (7) Iris shape models from disk-resolved images. Monthly Notices of the Royal Astronomical Society, 499: 3, p. 4545-4560
    9. Donald H. Menzel & Jay M. Pasachoff (1983). A Field Guide to the Stars and Planets (2nd ed.). Boston, MA: Houghton Mifflin. pp. 391. ISBN 0-395-34835-8.
    10. "Bright Minor Planets 2006". Minor Planet Center. Retrieved 21 May 2008.[permanent dead link]
    11. Y. Ueda et al. Surface Material Analysis of the S-type Asteroids: Removing the Space Weathering Effect from Reflectance Spectrum, 34th Annual Lunar and Planetary Science Conference, March 17–21, 2003, League City, Texas, abstract no.2078 (2003).
    12. Migliorini, F.; et al. (1997). "(7) Iris: a possible source of ordinary chondrites?". Astronomy & Astrophysics. 321: 652. Bibcode:1997A&A...321..652M.
    13. Odeh, Moh'd. "The Brightest Asteroids". Jordanian Astronomical Society. Archived from the original on 13 August 2007. Retrieved 16 July 2007.
    14. "cirrhosis". Oxford English Dictionary (Online ed.). Oxford University Press. (Subscription or participating institution membership required.)



    На других языках

    [de] (7) Iris

    (7) Iris ist ein Asteroid des Hauptgürtels, der am 13. August 1847 von John Russel Hind am George-Bishop-Observatorium als siebter Asteroid entdeckt wurde.
    - [en] 7 Iris

    [es] (7) Iris

    (7) Iris es un asteroide perteneciente al cinturón de asteroides descubierto por John Russell Hind desde el observatorio George Bishop de Londres, Reino Unido, el 13 de agosto de 1847. Está nombrado por Iris, una diosa de la mitología griega.[2]

    [fr] (7) Iris

    (7) Iris est un astéroïde de la ceinture principale. Il a une surface claire et semble être composé de nickel, de fer et de silicates de magnésium.

    [ru] (7) Ирида

    (7) Ири́да (лат. Iris) — астероид главного астероидного пояса. Открыт 13 августа 1847 года английским астрономом Джоном Хиндом в обсерватории Бишопа, Великобритания. Астероид назван в честь древнегреческой богини радуги[5].


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