87 Sylvia


Sylvia is the 8th-largest asteroid in the asteroid belt. It is the parent body of the Sylvia family and member of Cybele group located beyond the main asteroid belt. Sylvia was the first asteroid known to possess more than one moon.

Discovery and naming

Sylvia was discovered by N. R. Pogson on May 16, 1866, from Madras, India. Antonio Paluzie-Borrell, writing in Paul Herget's The Names of the Minor Planets, mistakenly states that the name honors Sylvie Petiaux-Hugo Flammarion, the first wife of astronomer Camille Flammarion. In fact, in the article announcing the discovery of the 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, indicating that the asteroid is porous to very porous; from 25% to as much as 60% of it may be empty space, depending on the details of its composition. However, the mineralogy of the X-type asteroids is not known well enough to constrain this further. Either way, this suggests a loose rubble pile structure. Sylvia is also a fairly fast rotator, turning about its axis every 5.18 hours. The short axis is the rotation axis. Direct images indicate that Sylvia's pole points towards ecliptic coordinates = with only a 0.5° uncertainty, which gives it an axial tilt of around 29.1°. Sylvia's shape is strongly elongated.

Satellite system

Sylvia has two orbiting satellites. They have been named Sylvia I Romulus and Sylvia II Remus, after Romulus and Remus, 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. Remus, the second moon, was discovered over three years later on August 9, 2004, by Franck Marchis of UC Berkeley, and Pascal Descamps, Daniel Hestroffer, and Jérôme Berthier of the Observatoire de Paris, France.
The orbital properties of the satellites are listed in this table. The orbital planes of both satellites and the equatorial plane of the primary asteroid are all well-aligned; all planes are aligned within about 1 degree of each other, suggestive of satellite formation in or near the equatorial plane of the primary.
NameMass Semi-major axis Orbital period Eccentricity
Remus7.3×1014706.51.370.027
Romulus9.3×101413573.650.006