The GRASS instrument onboard Hera for Dimorphos

GRASS, the GRAvimeter for Small Solar System objects, will be the first gravimeter to perform in situ gravimetric measurements on an asteroid. It flies onboard ESA’s Hera spacecraft, currently en route to the binary asteroid system Didymos. Hera will arrive in late 2026 and deploy two CubeSats in early 2027, including Juventas, which carries the GRASS instrument.

After deployment, the Juventas CubeSat will land on the surface of Dimorphos, the secondary body of the system, where GRASS will measure the three-dimensional surface acceleration. The acceleration vector in its magnitude and direction, as well as its possible temporal variation, will help to constrain the local geological structure (mass anomalies, regolith depth and lateral variations) and the surface geophysical environment (tides, dynamic slopes, centrifugal forces).

We present here the latest status of the GRASS calibration, ground tests  and in‑flight operations, including the checkout results obtained during the cruise phase. We also discuss the planned activities following the Juventas release, in the orbit of Dimorphos.  CubeSat before landing as well as, expected measurement scenarios and signal characteristics on the surface of Dimorphos.