8th Largest Asteroid, 87 Sylvia

Left: Adaptive Optics observations of (87) Sylvia, showing its two moonlet satellites, Remus and Romulus










87 Sylvia is one of the largest asteroids in the outer-main asteroid belt, having a mean diameter of 286 km. It is the 8th largest asteroid currently known.

Sylvia is also remarkable for being the first asteroid known to possess more than one moon. Sylvia has 2 known satellites.

Discovery

Sylvia was discovered by Norman Robert Pogson on May 16, 1866 from Madras (Chennai), India.

Naming

In the article announcing the discovery of the asteroid, Pogson explained that he selected the name for the asteroid in reference to Rhea Silvia, mother of Romulus and Remus.

Rhea Silvia (also written as Rea Silvia), and also known as Ilia, was the mythical mother of the twins Romulus and Remus, who founded the city of Rome.

Stats

Diameter (mean): 286 km
Semi-major axis: 3.499 AU
Orbital Period: 6.55 years
Rotation period: 5.184 hrs
Date discovered: 1866.5.16
Class: X
Group: Cybele group
Type: Outer Main-belt Asteroid
Satellite: 2

Orbit

Sylvia is a member of the Cybele group located beyond the core of the asteroid belt.

Cybele asteroids are a group of asteroids in the outer main belt with a semi-major axis between 3.27 AU and 3.7 AU, an eccentricity less than 0.3, and an inclination less than 25°. The group is named for the asteroid 65 Cybele. As of 2010, the group is thought to have been formed by the breakup of larger object in the distant past.

Physical characteristics

Sylvia's shape is strongly elongated and 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.

Sylvia's 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.