7 Iris
 Star rich field showing asteroid Iris (apmag 10.1) | |
| Discovery | |
|---|---|
| Discovered by | John Russell Hind |
| Discovery date | August 13, 1847 |
| Designations | |
Named after | Iris |
| none | |
| Main belt | |
| Adjectives | Iridian[1] |
| Symbol |  |
| Orbital characteristics | |
| Epoch November 26, 2005 (JD 2453700.5) | |
| Aphelion | 439.337 Gm (2.937 AU) |
| Perihelion | 274.259 Gm (1.833 AU) |
| 356.798 Gm (2.385 AU) | |
| Eccentricity | 0.231 |
| 1345.375 d (3.68 a) | |
Average orbital speed | 19.03 km/s |
| 269.531° | |
| Inclination | 5.527° |
| 259.727° | |
| 145.440° | |
| Physical characteristics | |
| Dimensions | 240×200×200 km[2] 225×190×190 km[3] 199.8 ± 10 km (IRAS)[4] |
| Mass | 1.36×1019 kg[2] |
Mean density | 2.72±0.43 g/cm³[2] |
| 0.055 m/s² | |
| 0.11 km/s | |
| 0.2975 d[5] | |
| Albedo | 0.277 (geometric) |
| Temperature | ~171 K max: 275 K (+2° C) |
Spectral type | S-type asteroid |
| 6.7[6][7] to 11.4 | |
| 5.51 | |
| 0.32" to 0.07" | |
7 Iris (Template:Pron-en EYE-ris, or as in Template:Lang-el) 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.[8] 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,[6] which is as bright as Ceres ever gets; reports of it being seen without optical aid are unverified.
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.
Characteristics
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.[9] 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,[10] so it may be an important contributor of these meteorites. Planetary dynamics also indicates that it should be a significant source of meteorites.[11]
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.[12]
References
- ^ Oxford English Dictionary
- ^ a b c Jim Baer (2008). "Recent Asteroid Mass Determinations". Personal Website. Retrieved 2008-12-04.
- ^ a b Kaasalainen, M. (2002). "Models of twenty asteroids from photometric data" (PDF). Icarus. 159: 369. doi:10.1006/icar.2002.6907.
{{cite journal}}: Unknown parameter|coauthors=ignored (|author=suggested) (help) - ^ "JPL Small-Body Database Browser: 7 Iris". 2009-03-17 last obs. Retrieved 2009-03-17.
{{cite web}}: Check date values in:|date=(help) - ^ "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 ed.). Boston, MA: Houghton Mifflin. p. 391. ISBN 0-395-34835-8.
- ^ "Bright Minor Planets 2006". Minor Planet Center. Retrieved 2008-05-21.
- ^ Odeh, Moh'd. "The Brightest Asteroids". Jordanian Astronomical Society. Retrieved 2007-07-16.
- ^ Hoffmann, M. (1993). "Spots on 4-VESTA and 7-IRIS - Large Areas or Little Patches". Astronomy & Astrophysics Supplement. 101: 621.
{{cite journal}}: Unknown parameter|coauthors=ignored (|author=suggested) (help) - ^ 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. (1997). "(7) Iris: a possible source of ordinary chondrites?". Astronomy & Astrophysics. 321: 652.
{{cite journal}}: Unknown parameter|coauthors=ignored (|author=suggested) (help) - ^ "JPL Close-Approach Data: 7 Iris". 2008-05-30 last obs. Retrieved 2009-05-06.
{{cite web}}: Check date values in:|date=(help)
External links
- shape model deduced from lightcurve
- "Discovery of Iris", MNRAS 7 (1847) 299
- Yeomans, Donald K. "Horizons system". NASA JPL. Retrieved 2007-03-20. — Horizons can be used to obtain a current ephemeris.
- "Elements and Ephemeris for (7) Iris". Minor Planet Center. (displays Elong from Sun and V mag for 2011)