Context : With an estimated diameter in the 320 to 350 km range , ( 704 ) Interamnia is the fifth largest main belt asteroid and one of the few bodies that fills the gap in size between the four largest bodies with D > 400 km ( Ceres , Vesta , Pallas and Hygiea ) and the numerous smaller bodies with diameter \leq 200 km .
However , despite its large size , little is known about the shape and spin state of Interamnia and , therefore , about its bulk composition and past collisional evolution .

Aims : We aimed to test at what size and mass the shape of a small body departs from a nearly ellipsoidal equilibrium shape ( as observed in the case of the four largest asteroids ) to an irregular shape as routinely observed in the case of smaller ( D \leq 200 km ) bodies .

Methods : We observed Interamnia as part of our ESO VLT/SPHERE large program ( ID : 199.C-0074 ) at thirteen different epochs .
In addition , several new optical lightcurves were recorded .
These data , along with stellar occultation data from the literature , were fed to the All-Data Asteroid Modeling ( ADAM ) algorithm to reconstruct the 3D-shape model of Interamnia and to determine its spin state .

Results : Interamnia ’ s volume-equivalent diameter of 332 \pm 6 km implies a bulk density of \rho =1.98 \pm 0.68 g \cdot cm ^ { -3 } , which suggests that Interamnia – like Ceres and Hygiea – contains a high fraction of water ice , consistent with the paucity of apparent craters .
Our observations reveal a shape that can be well approximated by an ellipsoid , and that is compatible with a fluid hydrostatic equilibrium at the 2 \sigma level .

Conclusions : The rather regular shape of Interamnia implies that the size/mass limit , under which the shapes of minor bodies with a high amount of water ice in the subsurface become irregular , has to be searched among smaller ( D \leq 300km ) less massive ( m \leq 3x10 ^ { 19 } kg ) bodies .