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21 Lutetia is a large main belt asteroid of an unusual spectral type. It measures about 100 kilometers in diameter (130km along its major axis) and is the largest asteroid to have been visited by a spacecraft, having been the subject of a flyby by the Rosetta probe in July 2010. This record will hold until the Dawn mission arrives at the asteroid Vesta in July of 2011. The name Lutetia is the Latin name for Paris.

Discovery and exploration

Lutetia was discovered on November 15, 1852 by Hermann Goldschmidt from the balcony of his apartment in Paris.[7]

There have been two reported stellar occultations by Lutetia, observed from Malta in 1997 and Australia in 2003, with only one chord each, roughly agreeing with IRAS measurements.[citation needed]

On July 10, 2010, the European Rosetta space probe flew by Lutetia at a minimum distance of 3160 km and a velocity of 15 kilometres per second on its way to the comet 67P/Churyumov-Gerasimenko.[8] Data will help determine if Lutetia is a primitive undifferentiated C-type asteroid, or an M-type asteroid from the core of a larger differentiated asteroid. Asteroid 253 Mathilde was the previous largest asteroid to be explored by space probe.


The composition of Lutetia has puzzled astronomers for some time, and investigation picked up in anticipation of the 2010 Rosetta flyby. While classified among the M-type asteroids,[1] most of which are metallic, Lutetia is one of the anomalous members that do not display much evidence of metal on their surface. Indeed, there are various indications of a non-metallic surface: a flat, low frequency spectrum similar to that of carbonaceous chondrites and C-type asteroids and not at all like that of metallic meteorites,[9] a low radar albedo unlike the high albedos of strongly metallic asteroids like 16 Psyche,[6] evidence of hydrated materials on its surface,[10] abundant silicates,[11] and a thicker regolith than most asteroids.[12] Lutetia is suspected of having a high density of around 5 g/cm³.[3]

Lightcurve analysis indicates that Lutetia is a rough sphere with "sharp and irregular shape features" and that its pole points toward either ecliptic coordinates (β, λ) = (3°, 40°) or (β, λ) = (3°, 220°) with a 10° uncertainty.[13] This gives an axial tilt of 85°, or 89°, respectively, meaning that the axis of rotation is approximately parallel to the ecliptic, similar to the planet Uranus.


1. ^ a b c d e "JPL Small-Body Database Browser: 21 Lutetia". 2010-06-13 last obs. http://ssd.jpl.nasa.gov/sbdb.cgi?sstr=21. Retrieved 2008-12-07.
2. ^ Belaskaya et al., 2010
3. ^ a b c Jim Baer (2008). "Recent Asteroid Mass Determinations". Personal Website. http://home.earthlink.net/~jimbaer1/astmass.txt. Retrieved 2008-11-28.
4. ^ M. Mueller et al. (2006). "The size and albedo of Rosetta fly-by target 21 Lutetia from new IRTF measurements and thermal modeling". Astronomy & Astrophysics 447: 1153. doi:10.1051/0004-6361:20053742. http://www.edpsciences.org/articles/aa/pdf/2006/09/aa3742-05.pdf?access=ok.
5. ^ "AstDys (21) Lutetia Ephemerides". Department of Mathematics, University of Pisa, Italy. Retrieved 2010-06-28.
6. ^ a b C. Magri et al. (1999). "Mainbelt Asteroids: Results of Arecibo and Goldstone Radar Observations of 37 Objects during 1980-1995". Icarus 140: 379. doi:10.1006/icar.1999.6130. http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1999Icar..140..379M&db_key=AST&data_type=HTML&format=&high=444b66a47d20870.
7. ^ Lardner, Dionysius (1867). "The Planetoides". Handbook of astronomy. James Walton. p. 222. http://books.google.com/?id=A-5HAAAAIAAJ&pg=PA222.
8. ^ M. A. Barucci, M. Fulchignoni and A. Rossi (2007). "Rosetta Asteroid Targets: 2867 Šteins and 21 Lutetia". Space Science Reviews 128 (1-4): 67–78. doi:10.1007/s11214-006-9029-6.
9. ^ Birlan, M., et al. (2004). "Near-IR spectroscopy of asteroids 21 Lutetia, 89 Julia, 140 Siwa, 2181 Fogelin and 5480 (1989YK8) [sic], potential targets for the Rosetta mission; remote observations campaign on IRTF". New Astronomy 9: 343. doi:10.1016/j.newast.2003.12.005. http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2004NewA....9..343B&db_key=AST&data_type=HTML&format=&high=444b66a47d20870.
10. ^ M. Lazzarin et al. (2004). "Visible spectral properties of asteroid 21 Lutetia, target of Rosetta Mission". Astronomy and Astrophysics 425: L25. doi:10.1051/0004-6361:200400054. http://www.edpsciences.org/articles/aa/pdf/2004/38/aagf171.pdf?access=ok.
11. ^ M. A. Feierberg et al. (1983). "Detection of silicate emission features in the 8- to 13 micrometre spectra of main belt asteroids". Icarus 56: 393. doi:10.1016/0019-1035(83)90160-4.
12. ^ A. Dollfus & J. E. Geake (1975). "Polarimetric properties of the lunar surface and its interpretation. VII - Other solar system objects". Proceedings of the 6th Lunar Science Conference, Houston, Texas, March 17–21 3: 2749. http://adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1975LPSC....6.2749D&link_type=ARTICLE&db_key=AST.
13. ^ J. Torppa et al. (2003). "Shapes and rotational properties of thirty asteroids from photometric data" (PDF). Icarus 164: 346. doi:10.1016/S0019-1035(03)00146-5. http://www.rni.helsinki.fi/~mjk/thirty.pdf.

External links

* Shape model deduced from lightcurve
* Orbital simulation from JPL (Java) / Ephemeris
* Rosetta blog
* Rosetta snaps views of asteroid Lutetia
* Size comparison of asteroids and comets visited by space probe

Astronomy Encyclopedia

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