GRASS Gravimeter Ready for Integration on ESA Spacecraft

The GRASS Gravimeter, the first instrument designed to directly measure gravity on the surface of an asteroid, has undergone testing in ESA’s Mechanical Systems Laboratory and is ready for integration for the Hera mission.

“It may be small but GRASS is packed with complex mechanical parts and electronics.”

Jose Carrasco, EMXYS

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 defense – and is designed to measure an expected gravity level of less than a millionth of Earth’s own.

The Gravimeter for Small Solar System Objects, GRASS, has been developed by the Royal Observatory of Belgium (ROB) with Spain’s EMXYS company.

To prove it was ready for integration with Hera’s Juventas CubeSat, the GRASS gravimeter was subjected to spacelike vacuum and temperature extremes within a thermal vacuum chamber, then underwent sustained shaking to mimic the violence of a rocket launch.

“It may be small but GRASS is packed with complex mechanical parts and electronics,” said Jose Carrasco, overseeing the instruments at EMXYS. “We’re very happy with its endurance during the test campaign. We will now follow up with a full integrity test to ensure it has sustained no damage during the testing, after which it will be passed to the GomSpace company in Luxembourg for integration aboard Juventas.”

The L-shaped instrument, the size of two smartphones stuck together, is designed to measure miniscule gravity levels because the Dimorphos asteroid it will touch down on is the smallest planetary object ever to be visited by a spacecraft. At around 160 m in diameter it is about the same size as Egypt’s Great Pyramid of Giza.

Dimorphos became famous last year when it was impacted by NASA’s DART spacecraft, sending out plumes of debris thousands of kilometers across space and measurably shifting its orbit around the larger Didymos asteroid.

“ESA’s Hera mission is Europe’s contribution to an international experiment in planetary defense,” said Hera system engineer Hannah Goldberg. “Following the DART impact, Hera will gather close-up data on the Dimorphos asteroid to turn this kinetic impact experiment into a well-understood and in principle repeatable method of planetary defense. GRASS’s surface gravity measurements will help researchers learn the precise mass of the asteroid, along with radio science experiments performed by the main Hera spacecraft.”

Grass Gravimeter Design

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 micrometer – or thousandth of a millimeter.

“Obviously the real-world performance of the gravimeter cannot be demonstrated directly here on Earth, but our lab testing coupled with numerical simulations showed we have achieved sufficiently high sensitivity to detect such low gravity,” said Özgür Karatekin of ROB.

The GRASS team also had to design an instrument small enough to fit into the Juventas CubeSat, itself only the size of a shoebox, along with Juventas’ main radar instrument. In the event their final design is just 330 grams in mass and requires only half a watt of power.

The GRASS gravimeter will be turned on as Juventas gradually falls to the surface of Dimorphos following the conclusion of its subsurface radar imaging campaign. The instrument will record the impact on the asteroid, any subsequent bounces and then any shifts in surface gravity over time, due to the influence of its parent Didymos asteroid.

(Source: ESA news release. Images provided)