7 Iris

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7 Iris  

Convex hull model of 7 Iris
Discovery
Discovered by:	John Russell Hind
Discovery date:	August 13, 1847
Alternative names:	none
Minor planet category:	Main belt
Orbital characteristics
Epoch November 26, 2005 (JD 2453700.5)
Aphelion distance:	439.337 Gm (2.937 AU)
Perihelion distance:	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)
Avg. 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 [1]
Temperature:	~171 K max: 275 K (+2° C)
Spectral type:	S-type asteroid
Absolute magnitude:	5.51

7 Iris (IPA: [ˈaɪɹɪs]) is one of the largest main belt asteroids. It is the fourth brightest asteroid after Vesta, Ceres and Pallas, with a maximum brightness of about 6.8 under the most favourable conditions, when it can easily be viewed with binoculars.

[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

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

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.^[5]

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,^[6] so it may be an important contributor of these meteorites. Planetary dynamics also indicates that it should be a significant source of meteorites.^[7]

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

1. ^ ^{a b} Supplemental IRAS Minor Planet Survey.
2. ^ ^{a b} Kaasalainen, M.; et al (2002). "Models of twenty asteroids from photometric data". Icarus 159: 369. 
3. ^ Pitjeva, E. V. (2005). "High-Precision Ephemerides of Planets—EPM and Determination of Some Astronomical Constants" (PDF). Solar System Research 39 (3): 176. DOI:10.1007/s11208-005-0033-2. 
4. ^ Planetary Data System Small Bodies Node, lightcurve parameters.
5. ^ Hoffmann, M.; E. H. Geyer (1993). "Spots on 4-VESTA and 7-IRIS - Large Areas or Little Patches". Astronomy & Astrophysics Supplement 101: 621. 
6. ^ 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).
7. ^ 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 March 20, 2007. — Horizons can be used to obtain a current ephemeris.