GRASS gravimeter

The first instrument to directly measure gravity on the surface of an asteroid has undergone testing in ESA’s Mechanical Systems Laboratory. The GRASS gravimeter will be landed on the surface of the Dimorphos asteroid aboard the Juventas CubeSat – which will itself be deployed from ESA’s Hera mission for planetary defence.

The L-shaped instrument, the size of two smartphones stuck together, is designed to measure an expected gravity level of less than a millionth of Earth’s own. The GRASS gravimeter design involves two sets of thin blades which are anchored within cradles, both of which are continuously rotating. The slightest motion of each blade changes the overall voltage of the blade itself and its surrounding walls. This capacitance-based measuring technique gives the gravimeter a sensitivity equivalent of a single micrometre – or thousandth of a millimetre, by measuring capacitances in the order of ato-Farads. Because the instrument’s two gravimeters are positioned at right angles from each other, and go on rotating, the instrument can construct the 3D gravity vector and monitor its variations from any landing position.