Interamnia’s first real portrait
How VLT images and occultations finally gave the fifth-largest asteroid a shape — and hinted it may be icy.
Published 2026-10-23
For most of the twentieth century, Interamnia was a point of light with a rough size and almost no shape information.[2]
Why it stayed mysterious
Large asteroids are usually studied with light curves, the rise and fall in brightness as they rotate.[7] For Interamnia, no light-curve analysis had determined the direction of its poles, and until 2017–2019 there were very few details of its shape or internal composition.[2]
Enter SPHERE
Between 2017 and 2019, astronomers imaged Interamnia with SPHERE, an adaptive-optics instrument on ESO’s Very Large Telescope in Chile, and combined the images with stellar occultations.[2][6] The result was an average diameter of about 332 km and an ellipsoidal shape similar to 4 Vesta.[2]
Nearly in equilibrium
That shape may be consistent with hydrostatic equilibrium — the shape a body takes when gravity pulls it into a fluid-like form — for its density and a 7.6-hour spin.[2][5] Interamnia spins in about 8.7 hours today, suggesting it may have formed as an equilibrium body and was later spun down by impacts.[2][1]
Hints of ice
- A density of 1.98 ± 0.68 g/cm³ — too uncertain on its own to settle composition.[2]
- Hydrated materials on the surface and a spectrum like Ceres’.[2]
- No deep basins; large craters would need flat floors, as on icy bodies.[2]
Together these suggest Interamnia is likely an icy body, though the case is not closed.[2]
Numbered citations link to the sources list on the Interamnia overview page.