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"|
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.
Discovery and name
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 she was spotted following 3 Juno by less than an hour of right ascension (Juno is the Roman equivalent of Hera).
Iris is an S-type asteroid. Its 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.
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. But 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.
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. 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.
- Oxford English Dictionary
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- 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.; Manara; Cellino; Di Martino; Zappala (1997). "(7) Iris: a possible source of ordinary chondrites?". Astronomy & Astrophysics 321: 652. Bibcode:1997A&A...321..652M.
- Odeh, Moh'd. "The Brightest Asteroids". Jordanian Astronomical Society. Archived from the original on 13 August 2007. Retrieved 2007-07-16.
- 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
- JPL Ephemeris
- "Elements and Ephemeris for (7) Iris". Minor Planet Center. (displays Elong from Sun and V mag for 2011)