20 Massalia is a big and fairly bright Main belt asteroid. It is also the biggest member of the Massalia family of asteroids.
Massalia is an S-type asteroid. It orbits at very low inclination in the intermediate main belt, and is by far the biggest asteroid in the Massalia family. The remaining family members are pieces blown off by a cratering event on Massalia 
Massalia has an above-average density for S-type asteroids, similar to the density of silicate rocks. As such, it appears to be a solid un-fractured body, which is rare among asteroids of its size. Apart from the few biggest bodies over 400 km in diameter, such as 1 Ceres and 4 Vesta, most asteroids appear to have been fractured a lot.
Lightcurve analysis indicates that Massalia's pole points towards either ecliptic coordinates (β, λ) = (45°, 10°) or (β, λ) = (45°, 190°) with a 10° uncertainty . This gives an axial tilt of 45°in both cases. The shape reconstruction from lightcurves has been described as quite spherical with large planar, nonconvex parts of the surface.
Massalia was found by A. de Gasparis on September 19, 1852, and also found independently the next night by J. Chacornac. It was Chacornac's discovery that was announced first.
Massalia is the Greek name for Marseille, where Chacornac made his discovery (de Gasparis was observing from Naples).
Other websites [change]