87 Sylvia (sil'-vee-a) is one of the largest main belt asteroids. It orbits beyond most of the main belt asteroids, so it is classed as one of the Cybeles (see Minor planet groups). Sylvia is remarkable for being the first known asteroid to possess more than one moon.

Discovery and naming

Sylvia was discovered by N. R. Pogson on May 16, 1866 from Madras (Chennai), India. Paul Herget, in his The Names of the Minor Planets (1955), attributes the name as honouring the first wife of astronomer Camille Flammarion, Sylvie Petiaux-Hugo Flammarion (this entry is signed by A. Paluzie-Borrell), but in the article (MNRAS, 1866) announcing the discovery of this asteroid, Pogson explained that he selected the name in reference to Rhea Silvia, mother of Romulus and Remus.

Physical characteristics

Sylvia is very dark in color and probably has a very primitive composition. The discovery of its moons made possible an accurate measurement of the asteroid's mass and density. Its density was found to be very low (in the 1.2 to 1.6 range), indicating that the asteroid is probably very porous; as much as 60% of it may be empty space. Sylvia is also a fairly fast rotator, turning about its axis every 5.18 hours (giving an equatorial rotation velocity of about 160 km/h or 98 mph).


Sylvia is orbited by two small moons. They have been named Romulus and Remus, after the children of the mythological Rhea Silvia.

Romulus, the first moon, was discovered on February 18, 2001 from the Keck II telescope by Michael E. Brown and Jean-Luc Margot. Its full designation is (87) Sylvia I Romulus; before receiving its name, it was known as S/2001 (87) 1. It is 18±4 km in diameter and orbits at a distance of 1356±5 km, taking 3.6496±0.0007 days (87.59 h) to complete an orbit of Sylvia. It should not be confused with the asteroid 10386 Romulus.

Remus, the second moon, was discovered on images taken starting on August 9, 2004 and announced on August 10, 2005. It was discovered by Franck Marchis of UC Berkeley, and Pascal Descamps, Daniel Hestroffer, and Jérôme Berthier of the Observatoire de Paris, France, using the Yepun telescope of the European Southern Observatory (ESO) in Chile. Marchis, the project leader, was waiting for the completion of the image acquisition programme before starting to process the data. Just as he was set to go on vacation in March 2005, Descamps sent him a brief note entitled "87 Sylvia est triple ?" pointing out that he could see two moonlets on several images of Sylvia. The entire team then focused quickly on analysis of the data, wrote a paper, submitted an abstract to the August meeting in Rio de Janeiro and submitted a naming proposal to the IAU.

Remus' full designation is (87) Sylvia II Remus; before receiving its name, it was known as S/2004 (87) 1. It is 7±2 km in diameter and orbits at a distance of 706±5 km, taking 1.3788±0.0007 days (33.09 h) to complete an orbit of Sylvia.

Astronomers believe that these moons were broken off Sylvia by an impact in the past, and that other, smaller moons may also be found.

From the surface of Sylvia, Romulus and Remus would appear roughly the same size. Romulus, the outermost moon, would be about 0.89° across, slightly bigger than the closer but smaller Remus, which would be about 0.78° across. Because Sylvia is far from spherical, these values may vary by a little more than 10%, depending on where the observer is on Sylvia's surface. Since the two asteroidal moons appear to orbit (as best we can tell) in the same plane, they would occult each other once every 2.2 days. When the season is right, twice during Sylvia's 6.52 year orbital period, they would eclipse the Sun, which, at 0.15° across, is much smaller than when seen from Earth (0.53°). From Remus, the inner moon, Sylvia appears huge, roughly 30°×18° across, while its view of Romulus varies between 1.59 and 0.50° across. From Romulus, Sylvia measures 16°×10° across, while Remus varies between 0.62° and 0.19°.

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For a complete listing, see: List of asteroids. See also Pronunciation of asteroid names and Meanings of asteroid names.
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