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. 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 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 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.
Size
measurements in 1983 estimated the asteroid to be 317 ± 5 km in diameter. An occultation in 1996 produced a diameter of 329 km. Observations of a favorable occultation of a bright 6.6 magnitude star on March 23, 2003, produced thirty-five chords indicating an ellipsoid of 350×304 km, thus giving the asteroid a geometric mean diameter of 326 km.
Mass
In 2001, Michalak estimated Interamnia to have a mass of 6.9 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 2007, Baer and Chesley estimated Interamnia to have a mass of kg., Baer suggests Interamnia has a mass of only kg. This makes it more massive than 511 Davida, though the error bars overlap. Goffin's 2014 astrometric reanalysis gives an even lower mass of 2.725 ± 0.12 kg.
