7 Iris
From Wikipedia, the free encyclopedia
|
Discovery
|
|
|---|---|
| Discovered by | John Russell Hind |
| Discovery date | August 13, 1847 |
|
Designations
|
|
| Alternative names | none |
| Minor planet category |
Main belt |
| Epoch November 26, 2005 (JD 2453700.5) | |
| Aphelion | 439.337 Gm (2.937 AU) |
| Perihelion | 274.259 Gm (1.833 AU) |
| Semi-major axis | 356.798 Gm (2.385 AU) |
| Eccentricity | 0.231 |
| Orbital period | 1345.375 d (3.68 a) |
| 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° |
|
Physical characteristics
|
|
| Dimensions | 225×190×190 km[1][2] |
| Mass | 1.25×1019 kg[3] |
| Mean density | 2.9 g/cm³ |
| Equatorial surface gravity | 0.055 m/s² |
| Escape velocity | 0.11 km/s |
| Rotation period | 0.2975 d[4] |
| Albedo | 0.277 (geometric)[1] |
| Temperature | ~171 K max: 275 K (+2° C) |
| Spectral type | S-type asteroid |
| Apparent magnitude | 6.7[5][6] to 11.4 |
| Absolute magnitude | 5.51 |
| Angular diameter | 0.32" to 0.07" |
7 Iris (pronounced /ˈaɪrɪs/, Greek: Ίρις) is a large main belt asteroid. Among 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[7]. 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,[5] which is as bright as Ceres ever gets, and reports of it being seen without optical aid are unverified.
Contents |
[edit] Discovery and name
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.
According to the OED, the correct adjectival form of the name is Iridian.
[edit] Characteristics
Lightcurve analysis indicates a somewhat angular shape and that Iris' pole points towards ecliptic coordinates (β, λ) = (10°, 20°) with a 10° uncertainty.[2] 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.
Iris' surface likely exhibits albedo differences, with possibly a large bright area in the northern hemisphere.[8]
The surface of Iris is overall 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,[9] so it may be an important contributor of these meteorites. Planetary dynamics also indicates that it should be a significant source of meteorites.[10]
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.
[edit] References
- ^ a b Supplemental IRAS Minor Planet Survey.
- ^ a b Kaasalainen, M.; et al (2002). "Models of twenty asteroids from photometric data". Icarus 159: 369.
- ^ Pitjeva, E. V. (2005). "High-Precision Ephemerides of Planets—EPM and Determination of Some Astronomical Constants" (PDF). Solar System Research 39 (3): 176. doi:.
- ^ Planetary Data System Small Bodies Node, lightcurve parameters.
- ^ a b Donald H. Menzel and Jay M. Pasachoff (1983). A Field Guide to the Stars and Planets, 2nd edition, Boston, MA: Houghton Mifflin, p. 391. ISBN 0395348358.
- ^ Bright Minor Planets 2006. Minor Planet Center. Retrieved on 2008-05-21.
- ^ Odeh, Moh'd. The Brightest Asteroids. Jordanian Astronomical Society. Retrieved on 2007-07-16.
- ^ Hoffmann, M.; E. H. Geyer (1993). "Spots on 4-VESTA and 7-IRIS - Large Areas or Little Patches". Astronomy & Astrophysics Supplement 101: 621.
- ^ 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).
- ^ Migliorini, F.; et al (1997). "(7) Iris: a possible source of ordinary chondrites?". Astronomy & Astrophysics 321: 652.
[edit] External links
- shape model deduced from lightcurve
- "Discovery of Iris", MNRAS 7 (1847) 299
- Yeomans, Donald K.. Horizons system. NASA JPL. Retrieved on 2007-03-20. — Horizons can be used to obtain a current ephemeris.
|
|||||
