20 Massalia

Massalia is the namesake and the parent body of the Massalia family (404), a very large inner belt asteroid family consisting of stony asteroids with very low inclinations.^{[6][15]: 23 } It is by far the largest body in this family. The remaining family members are fragments ejected by a cratering event on Massalia.^[16]

It orbits the Sun in the inner main-belt at a distance of 2.1–2.8 AU once every 3 years and 9 months (1,366 days; semi-major axis of 2.41 AU). Its orbit has an eccentricity of 0.14 and an inclination of 1° with respect to the ecliptic.^[5]

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 unfractured 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.^[10] The calculation of the mass of Massalia is dependent on the mass of 4 Vesta and perturbation of 44 Nysa.^[10]

Light curve analysis indicates that Massalia's pole points towards ecliptic coordinates either (β, λ) = (45°, 10°) or (β, λ) = (45°, 190°) with a 10° uncertainty.^[8] This gives an axial tilt of 45° in both cases. The shape reconstruction from light curves has been described as quite spherical with large planar, nonconvex parts of the surface.

In 1988 there was a search for satellites or dust orbiting this asteroid using the UH88 telescope at the Mauna Kea Observatories, but none were found.^[17]

In February 2024, water molecules were discovered on 20 Massalia, alongside 7 Iris, marking the first time water molecules were detected on asteroids.^[18][19]

Massalia was discovered on 19 September 1852, by Annibale de Gasparis at Naples Observatory in Italy, and also found independently the next night by Jean Chacornac at Marseilles Observatory, France. 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)^[3] was the first asteroid that was not assigned an iconic symbol: indeed the circled number 20 was explicitly proposed as the symbol instead.^[13]