704 Interamnia

**704 Interamnia** is a large F-type asteroid. With an estimated mean diameter of 330 kilometres, it is the fifth-largest asteroid after Ceres, Vesta, Pallas, and Hygiea. Its mean distance from the Sun is 3.067 (AU). It was discovered on 2 October 1910 by Vincenzo Cerulli, and named after the Latin name for Teramo, Italy, where Cerulli worked. It is probably the fifth-to-eighth-most-massive asteroid, with a mass estimated to be 1.2% of the mass of the entire asteroid belt. Observations by the Very Large Telescope's SPHERE imager in 2017–2019, combined with occultation results, indicate that the shape of Interamnia may be consistent with hydrostatic equilibrium for a body of its density with a rotational period of 7.6 hours. (Its current period is 8.7 hours.) This suggests that Interamnia may have formed as an equilibrim body, and that impacts changed its rotational period after it fully solidified.

## Characteristics

Although Interamnia is the largest asteroid after the "big four", it is a very little-studied body. It is easily the largest of the F-type asteroids, but until 2017-2019 there existed very few details of its internal composition or shape, and no light curve analysis has yet been done to determine the ecliptic coordinates of Interamnia's poles (and hence its axial tilt). Studies by the Very Large Telescope give an average diameter of about 332 km and found an ellipsoidal shape for Interamnia, similar to 4 Vesta; the resulting density calculation (1.98 ± 0.68 g · cm^3) is not precise enough to definitely infer Interamnia's composition, but the presence of hydrated materials at the surface and its overall spectral similarities to Ceres suggest that it is likely an icy body. The absence of an affiliated asteroid family implies that Interamnia has not suffered a giant impact within the past 3 billion years, in contrast to 4 Vesta and 10 Hygeia.

Its very dark surface and relatively large distance from the Sun means Interamnia can never be seen with 10x50 binoculars. At most oppositions its magnitude is around +11.0, which is less than the *minimum* brightness of Vesta, Ceres or Pallas. Even at a perihelic opposition its magnitude is only +9.9, which is over four magnitudes lower than Vesta.

Its orbit is slightly more eccentric than that of Hygiea (15% versus 12%) but differs from Hygiea's in its much greater inclination and slightly shorter period. Another difference is that Interamnia's perihelion is located on the opposite side from the perihelia of the "big four", so that Interamnia at perihelion is actually closer to the Sun than Ceres and Pallas are at the same longitude. It is unlikely to collide with Pallas because their nodes are located too far apart, whilst although its nodes are located on the opposite side from those of Ceres, it is generally clear of Ceres when both cross the same orbital plane and a collision is again unlikely.

## Surface

There are no deep basins visible in the VLT images. Any large craters must have flat floors, consistent with an icy C/F-type composition.

n 2001, Michalak estimated Interamnia to have a mass of (7±2)×1019 kg. Michalak's estimate depends on the masses of 19 Fortuna, 29 Amphitrite, and 16 Psyche; thus this mass was obtained assuming an incomplete dynamical model.

In 2011, Baer calculated Interamnia had a mass of (3.9±0.2)×1019 kg.

Goffin's 2014 astrometric reanalysis gives an even lower mass of (2.7±0.1)×1019 kg.

In 2019, Hanuš et al. consolidated 21 selected prior mass estimates, dating from 1992 to 2017, with a metastatistical result of (3.8±1.3)×1019 kg (that is, (2.5 to 5.1)×1019 kg to within 1 sigma uncertainty.