Star rich field showing asteroid Iris (apmag 10.1)
|Discovered by||John Russell Hind|
|Discovery date||August 13, 1847|
|Minor planet category||Main belt|
|Epoch November 26, 2005 (JD 2453700.5)|
|Aphelion||2.937 AU (439.337 Gm)|
|Perihelion||1.833 AU (274.259 Gm)|
|Semi-major axis||2.385 AU (356.798 Gm)|
|Orbital period||3.68 a (1345.375 d)|
|Average orbital speed||19.03 km/s|
|Longitude of ascending node||259.727°|
|Argument of perihelion||145.440°|
|Proper orbital elements|
|Proper semi-major axis||2.3862106 AU|
|Proper mean motion||97.653672 deg / yr|
|Proper orbital period||3.6865 yr
|Precession of perihelion||38.403324 arcsec / yr|
|Precession of the ascending node||−46.447128 arcsec / yr|
199.8 ± 10 km (IRAS)
|Mass||1.62 ± 0.09 ×1019 kg|
|Mean density||3.21 ± 0.49 g/cm³|
|Equatorial surface gravity||0.109 m/s²|
|Escape velocity||0.147 km/s|
|Rotation period||0.2975 d|
max: 275 K (+2°C)
|Spectral type||S-type asteroid|
|Apparent magnitude||6.7 to 11.4|
|Absolute magnitude (H)||5.51|
|Angular diameter||0.32" to 0.07"|
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. 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 (next time on October 31, 2017 reaching a magnitude of +6.9), which is as bright as Ceres ever gets; reports of it being seen without optical aid are unverified.
Discovery and name
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.
Lightcurve analysis indicates a somewhat angular shape and that Iris' pole points towards ecliptic coordinates (β, λ) = (10°, 20°) with a 10° uncertainty. 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. 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, so it may be an important contributor of these meteorites. Planetary dynamics also indicates that it should be a significant source of meteorites.
- Oxford English Dictionary
- "AstDyS-2 Iris Synthetic Proper Orbital Elements". Department of Mathematics, University of Pisa, Italy. Retrieved 2011-10-01.
- Jim Baer (2011). "Recent Asteroid Mass Determinations". Personal Website. Retrieved 2011-09-14.
- Kaasalainen, M.; et al. (2002). "Models of twenty asteroids from photometric data". Icarus 159 (2): 369. Bibcode:2002Icar..159..369K. doi:10.1006/icar.2002.6907.
- "JPL Small-Body Database Browser: 7 Iris". 2009-03-17 last obs. Retrieved 2009-03-17.
- "Planetary Data System Small Bodies Node, lightcurve parameters".
- 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. Archived from the original on 13 August 2007. Retrieved 2007-07-16.
- Hoffmann, M.; Geyer; E. H. Geyer (1993). "Spots on 4-VESTA and 7-IRIS - Large Areas or Little Patches". Astronomy & Astrophysics Supplement 101: 621. Bibcode:1993A&AS..101..621H.
- 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.; Manara; Cellino; Di Martino; Zappala; et al. (1997). "(7) Iris: a possible source of ordinary chondrites?". Astronomy & Astrophysics 321: 652. Bibcode:1997A&A...321..652M.
- "JPL Close-Approach Data: 7 Iris". 2008-05-30 last obs. Retrieved 2009-05-06.
- Shape model deduced from lightcurve (M. Kaasalainen 2002)
- 2011-Feb-19 Occultation (Durech Model) / (2011 Asteroidal Occultation Results for North America)
- "Discovery of Iris", MNRAS 7 (1847) 299
- Yeomans, Donald K. "Horizons system". NASA JPL. Archived from the original on 28 March 2007. 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)