87 Sylvia has a low density, which indicates that it is probably a
rubble pile formed when debris from a collision between its parent body and another asteroid re-accreted gravitationally. Therefore, it is likely that both Romulus and
Remus, the second of Sylvia's moons, are smaller rubble piles which accreted in orbit around the main body from debris of the same collision. In this case their albedo and density are expected to be similar to Sylvia's. Romulus's orbit is expected to be quite stable − it lies far inside Sylvia's
Hill sphere (about 1/50 of Sylvia's
Hill radius), but also far outside the
synchronous orbit. From Romulus's surface, Sylvia takes up an angular region 16°×10° across, while Remus's apparent size varies between 0.62° and 0.19° (for comparison, Earth's
Moon has an apparent size of about 0.5°). ==See also==