704 Interamnia

704 Interamnia is a large F-type asteroid orbiting in the main belt. With an average diameter of about 330 kilometers, it ranks as the fifth-largest asteroid known, trailing only Ceres, Vesta, Pallas, and Hygiea. Its mass likely falls between the fifth and tenth highest in the belt, accounting for roughly 1.2% of the total mass of all asteroids there. It orbits the Sun at an average distance of 3.067 AU.

Discovered on October 2, 1910, by Vincenzo Cerulli, the asteroid was named after the Latin name for Teramo, Italy, where Cerulli worked. Observations made with the Very Large Telescope’s SPHERE imager between 2017 and 2019, combined with occultation data, suggest that Interamnia’s shape may be consistent with hydrostatic equilibrium for a body of its density if it had a rotation period of 7.6 hours. However, its current rotation period is 8.7 hours. This mismatch implies that Interamnia may have originally formed as an equilibrium body, but later impacts altered its spin after it fully solidified.

Despite being the largest asteroid after the “big four,” Interamnia remains poorly studied. It is by far the biggest F-type asteroid, yet before the 2017–2019 VLT observations, little was known about its internal composition or shape, and no light curve analysis has determined the ecliptic coordinates of its poles or its axial tilt.

The VLT data give an average diameter of about 332 kilometers and reveal an ellipsoidal shape similar to that of 4 Vesta. The resulting density estimate of 1.98 ± 0.68 g/cm³ is too imprecise to pin down its composition, but the presence of hydrated materials on its surface and its spectral resemblance to Ceres suggest it is likely an icy body. The lack of an associated asteroid family indicates that Interamnia has not experienced a major impact in the last 3 billion years, unlike Vesta and Hygiea.

Its very dark surface and relatively large distance from the Sun make Interamnia invisible through 10x50 binoculars. At most oppositions, its apparent magnitude is around +11.0, fainter than the minimum brightness of Vesta, Ceres, or Pallas. Even at a perihelic opposition, it only reaches magnitude +9.9—over four magnitudes dimmer than Vesta.

Surface

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

Mass estimates for Interamnia have varied widely.