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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.

7 Iris 7 Iris Astronomical Symbol.svg
Star rich field showing asteroid Iris (apmag 10.1)
Discovered byJohn Russell Hind
Discovery dateAugust 13, 1847
MPC designation(7) Iris
Pronunciation/ˈaɪərɪs/ IRE-iss
Named after
Main belt
AdjectivesIridian /ˈrɪdiən/[1]
Orbital characteristics
Epoch November 26, 2005 (JD 2453700.5)
Aphelion2.937 AU (439.337 Gm)
Perihelion1.833 AU (274.259 Gm)
2.385 AU (356.798 Gm)
3.68 a (1345.375 d)
19.03 km/s
Proper orbital elements[2]
2.3862106 AU
97.653672 deg / yr
3.6865 yr
(1346.493 d)
Precession of perihelion
38.403324 arcsec / yr
Precession of the ascending node
−46.447128 arcsec / yr
Physical characteristics
Dimensions240×200×200 km[3]
225×190×190 km[4]
199.8 ± 10 km (IRAS)[5]
135 000 km2[6]
Volume4 630 000 km3[6]
Mass1.62 ± 0.09 ×1019 kg[3]
Mean density
3.21 ± 0.49 g/cm³[3]
Equatorial surface gravity
0.108 m/s²
Equatorial escape velocity
0.147 km/s
0.2975 d[7]
Equatorial rotation velocity
25.4 m/s[6]
Temperature~171 K
max: 275 K (+2°C)
S-type asteroid
6.7[8][9] to 11.4
0.32" to 0.07"


Discovery and nameEdit

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).


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


Iris is an S-type asteroid. Its 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]

Among the S-type asteroids, Iris ranks fifth in geometric mean diameter after Eunomia, Juno, Amphitrite and Herculina.


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),[8] which is as bright as Ceres ever gets.


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.[4] 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 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 alsoEdit


  1. ^ Oxford English Dictionary
  2. ^ "AstDyS-2 Iris Synthetic Proper Orbital Elements". Department of Mathematics, University of Pisa, Italy. Retrieved 2011-10-01.
  3. ^ a b c Jim Baer (2011). "Recent Asteroid Mass Determinations". Personal Website. Archived from the original on 2013-07-08. Retrieved 2011-09-14.
  4. ^ a b 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.
  5. ^ "JPL Small-Body Database Browser: 7 Iris". 2009-03-17. Retrieved 2009-03-17. last obs
  6. ^ a b c Calculated based on the known parameters
  7. ^ "Planetary Data System Small Bodies Node, lightcurve parameters". Archived from the original on 2006-06-14.
  8. ^ a b Donald H. Menzel & Jay M. Pasachoff (1983). A Field Guide to the Stars and Planets (2nd ed.). Boston, MA: Houghton Mifflin. p. 391. ISBN 0-395-34835-8.
  9. ^ "Bright Minor Planets 2006". Minor Planet Center. Retrieved 2008-05-21.[permanent dead link]
  10. ^ Hoffmann, M.; E. H. Geyer (1993). "Spots on 4-VESTA and 7-IRIS - Large Areas or Little Patches". Astronomy and Astrophysics Supplement. 101: 621. Bibcode:1993A&AS..101..621H.
  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 2007-07-16.

External linksEdit