|Discovered by||Norman Robert Pogson|
|Discovery date||May 16, 1866|
|MPC designation||(87) Sylvia|
|main belt · (outer)|
Sylvia · Cybele
|Epoch July 14, 2004 (JD 2453200.5)|
|Aphelion||3.768 AU (563.679 Gm)|
|Perihelion||3.213 AU (480.594 Gm)|
|3.490 AU (522.137 Gm)|
|6.52 a (2381.697 d)|
Average orbital speed
|Dimensions||(384 × 262 × 232) ±10 km|
(385 × 265 × 230) ± 10 km
286 km (mean)
|Mass||(1.478 ± 0.006) × 1019 kg|
|1.2 ± 0.1 g/cm3|
Equatorial surface gravity
Equatorial escape velocity
|0.2160 d (5.183642 h) |
Sylvia (minor planet designation: 87 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 core of the belt (see minor-planet groups). Sylvia is the first asteroid known to possess more than one moon.
Sylvia was discovered by N. R. Pogson on May 16, 1866, from Madras (Chennai), India. A. Paluzie-Borrell, writing in Paul Herget's The Names of the Minor Planets (1955), mistakenly states that the name honours 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 (MNRAS, 1866).
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 (around 1.2 times the density of water), 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 (giving an equatorial rotation velocity of about 230 km/h or 145 mph). The short axis is the rotation axis. Direct images indicate that Sylvia's pole points towards ecliptic coordinates (β, λ) = (+62.6°, 72.4°) with only a 0.5° uncertainty, which gives it an axial tilt of around 29.1°. Sylvia's shape is strongly elongated.
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.
|Name||Mass [kg]||Semi-major axis [km]||Orbital period [days]||Eccentricity|