Sylvia is very dark in color and probably made of primitive. The discovery of its moons made possible an accurate measurement of the asteroid's mass and density. Its density is very low (around 1.2 times the density of water), indicating that the asteroid has a lot of empty spaces; From 25% to as much as 60% of it may be empty space, depending on the details of what it's made of. However, the mineralogy of the X-type asteroids is not known well enough to force this further. Either way, this suggests a loose rubble pile (an asteroid that has been broken up in a collision and pulled back together by gravity) structure. Sylvia is also a fairly fast rotator, turning about its axis every 5.18 hours (giving an equatorial rotation speed 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 very stretched.