|Discovered by||Annibale de Gasparis|
|Discovery date||September 19, 1852|
|Minor planet category||Main belt (Massalia family)|
|Epoch October 22, 2004 (JD 2453300.5)|
|Aphelion||411.911 Gm (2.753 AU)|
|Perihelion||308.699 Gm (2.064 AU)|
|Semi-major axis||360.305 Gm (2.408 AU)|
|Orbital period||1365.261 d (3.74 a)|
|Average orbital speed||19.09 km/s|
|Longitude of ascending node||206.530°|
|Argument of perihelion||255.578°|
145 km (mean)
160×145×130 km 
|Mean density||3.54±0.85 g/cm³|
|Equatorial surface gravity||0.054 m/s²|
|Escape velocity||0.093 km/s|
|Rotation period||0.3374 d (8.098 h) |
max: 265 K (-8°C)
|Spectral type||S |
|Apparent magnitude||8.3 to 12.0|
|Absolute magnitude (H)||6.50|
|Angular diameter||0.186" to 0.058"|
20 Massalia is a large and fairly bright main-belt asteroid. It is also the largest member of the Massalia family of asteroids. Its name is the Greek name for Marseille, the city from which one of the two independent co-discovers, Jean Chacornac, first sighted it.
Massalia is an S-type asteroid. It orbits at very low inclination in the intermediate main belt, and is by far the largest asteroid in the Massalia family. The remaining family members are fragments ejected 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, a rarity among asteroids of its size. Apart from the few largest bodies over 400 km in diameter, such as 1 Ceres and 4 Vesta, most asteroids appear to have been significantly fractured, or are even rubble piles. In 1998, Bange estimated Massalia to have a mass of 5.2×1018 kg assuming that 4 Vesta has 1.35×10−10 solar mass. The mass of Massalia is dependent on the mass of 4 Vesta and perturbation of 44 Nysa.
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 discovered by A. de Gasparis on September 19, 1852 in Naples, and also found independently the next night by J. Chacornac in Marseilles. It was Chacornac's discovery that was announced first. In the nineteenth century the variant spelling Massilia was often used. Asteroids discovered prior to Massalia were assigned iconic symbols, like the ones traditionally used to designate the planets. However, astronomers had begun to phase out this practice with the discovery of 16 Psyche in March 1852, and 20 Massalia (being the first object in the Solar System with a non-mythological name) was the first asteroid that was not assigned an iconic symbol.
- Jim Baer (2008). "Recent Asteroid Mass Determinations". Personal Website. Retrieved 2008-12-11.
- Supplemental IRAS Minor Planet Survey
- M. Kaasalainen et al. (2002). "Models of Twenty Asteroids from Photometric Data" (PDF). Icarus 159 (2): 369. Bibcode:2002Icar..159..369K. doi:10.1006/icar.2002.6907.
- J. Bange (1998). "An estimation of the mass of asteroid 20-Massalia derived from the HIPPARCOS minor planets data". Astronomy & Astrophysics 340: L1. Bibcode:1998A&A...340L...1B.
- PDS lightcurve data
- PDS spectral class data
- Donald H. Menzel and Jay M. Pasachoff (1983). A Field Guide to the Stars and Planets (2nd ed.). Boston, MA: Houghton Mifflin. p. 391. ISBN 0-395-34835-8.
- D. Vokrouhlický et al. (2006). "Yarkovsky/YORP chronology of asteroid families". Icarus 182: 118. Bibcode:2006Icar..182..118V. doi:10.1016/j.icarus.2005.12.010.
- Gradie, J.; Flynn, L. (March 1988), "A Search for Satellites and Dust Belts Around Asteroids: Negative Results", Abstracts of the Lunar and Planetary Science Conference 19: 405–406, Bibcode:1988LPI....19..405G.
- shape model deduced from lightcurve
- Elements and Ephemeris for (20) Massalia from the Minor Planet Center.
- Orbital simulation from JPL (Java) / Ephemeris