Gravimeter for ESA’s Hera mission is ready to launch to Dimorphos asteroid


A tiny instrument that can measure the gravity levels of an asteroid has completed its ground tests, pushing the European Space Agency’s (ESA) Hera mission closer to launch. Hera will complete the first post-impact survey of the asteroid that NASA’s Double Asteroid Redirection Test (DART) collided with in September 2022, and the gravimeter for small solar system objects (GRASS) is the small device that will be carried by Hera to measure the minuscule gravity levels of Dimorphos, the orbiting moonlet of the 65803 Didymos binary system.

GRASS is one of the core parts of Hera that will work with the other instruments to provide a clearer picture of the impact made by DART, and is scheduled to launch in October 2024, for a December 2026 rendezvous with Dimorphos.

This follow-up mission forms part of the asteroid impact and deflection assessment, an international collaboration focused on experimenting with kinetic impact as a means of planetary defense against near-Earth objects (NEO) through DART and providing accurate data from the impact through Hera.

GRASS is designed to measure such miniscule gravity levels as the #Dimorphos asteroid #HeraMission will deliver it to is the smallest planetary object ever to be visited by spacecraft. At 160 m in diameter it is about the same size as #Rome’s #Colosseum

— ESA Technology (@ESA_Tech) May 22, 2023

What is GRASS?

The tiny GRASS payload, which has a mass of just under 400 grams, is designed to measure the expected gravity levels and the precise mass of the Dimorphos asteroid. The payload is carried in one of Hera’s two cubesats, Juventas, which itself is the size of a shoebox.

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ESA describes the gravimeter as the size of two smartphones stuck together, shaped like the letter L. The payload was developed by the Royal Observatory of Belgium (ROB) and Spain-based EMXYS, which was responsible for manufacturing its electronics and mechanical integration.

“It may be small, but GRASS is packed with complex mechanical parts and electronics,” Jose Carrasco, the chief executive officer at EMXYS said.

After Hera completes the subsurface radar imaging campaign of Dimorphos, Juventas will be released and gradually fall onto the surface of the asteroid. During this descent, GRASS will record the impact on the asteroid, subsequent bounces, and any shifts in surface gravity over time from the influence of Didymos, ESA explains.

“It will be in the final falling phase that GRASS will start working to measure the gravitational attraction of the asteroid and thus picturing its mass distribution while it falls, lands [after some rebounds], and sits still on the surface of the asteroid,” Carrasco told NSF.

The GRASS instrument is the size of two smartphones stuck together. (Credit: ESA)

The gravimeter can detect the slightest motion, with the sensitivity equivalent of a single micrometer, or a thousandth of a millimeter.

“GRASS has two small masses attached to two beams placed orthogonally to each other that rotate at around one revolution per minute. By measuring the small displacements of these beams and the mass, it is possible to reconstruct the gravity in three dimensions,” continued Carrasco.

“As the measurements are made during the free fall of Juventas over Dimorphos, that will follow a bent trajectory before the impact, it will measure the gravity field from several different positions and therefore may reconstruct the mass distribution of the asteroid.”

The Hera mission is particularly interesting as it will mark the first probe to rendezvous with a binary asteroid and will be the smallest planetary object a spacecraft has ever visited. While the asteroid may be small compared to other planetary objects measuring at 160 meters in diameter, it is the same size as Egypt’s Great Pyramid of Giza.

One of the most significant impacts of DART’s collision with the asteroid was that it reduced Dimorphos’ orbital period around Didymos by 32 minutes. The Hera probe will be able to characterize even more changes like this and turn “the kinetic impact experiment into a well-understood and in principle repeatable method of planetary defense,” according to Hera system engineer Hannah Goldberg.

The GRASS gravimeter of the @esa #HeraMission has recently undergone a successful environmental test campaign. The @ORB_KSB developed GRASS with @emxys.

— (@ORB_KSB) May 23, 2023

Hera hosts several instruments such as the asteroid framing camera, A thermal infrared imager, the planetary altimeter, the hyperscout hyperspectral imager, and the two cubesats, Milani and Juventas.

Testing the Device for Space

Before launching, GRASS was put under fire to ensure it can withstand the harsh environment of space at ESA’s mechanical systems laboratory in the Netherlands, where the agency tests its smaller-scale equipment.

“When it’s released, we expect the instrument to work as it’s been intended because ESA has made an intensive test campaign, including vibration, radiation, and thermal excursions from minus 35 degrees up to 55 degrees [celsius] to ensure that the instrument is able to withstand the environment that exists around the asteroid,” Carrasco said.

GRASS was subjected to spacelike vacuum and temperature extreme tests within a thermal vacuum chamber and underwent sustained shaking to “mimic the violence of a rocket launch,” ESA explained.

“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,” Özgür Karatekin from ROB explained.

GRASS undergoes testing in a vacuum chamber. (Credit: ESA)

While Carrasco claimed there were only minor tweaks the company had to make since 2016 when EMXYS was chosen for the project, there were still some challenges to face ahead.

“The main difficulty was to fit everything in such a small volume,” Carrasco said. “The very small displacement is measuring very, very small electrical currents. So, the most challenging thing was to do it very small mechanically and to [make it] measure extremely small electrical currents.”

During Hera’s journey to the asteroid, GRASS will be sleeping the entire time. Carrasco said every few months ground teams will move the device, specifically its motors which can get stuck if untouched for long periods of time, to make sure the system is still working properly.

Karatekin from ROB also explained that because Juventas could land at any part of Dimorphos, its solar arrays could be blocked from generating more power, therefore GRASS will run on the cubesat’s internal battery for up to 20 hours.

“In addition, 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,” Karatekin added.

The Juventas cubesat that GRASS will be integrated into. (Credit: ESA)

Deflecting Asteroids for Planetary Defense

While NASA says no asteroid larger than 140 meters in size will pose any threat to the Earth within the next 100 years, only 40 percent of asteroids nearby have been cataloged.

In February 2013, a meteor about 17 to 20 meters in diameter collided near Chelyabinsk in the Ural Federal District in Russia. The shockwave caused around 1,500 injuries but did not result in any deaths.

This event catalyzed a wake-up call to establish stronger planetary defense strategies, and only three years later in 2016, the planetary defense coordination office was created to catalog NEOs and coordinate efforts to respond to threats, such as DART.

This led to the first-ever full-scaled asteroid deflection launch of DART in November 2021. The collision produced an impact that was equivalent to that of about three tonnes of TNT, which inevitably destroyed the spacecraft.

However, the United States and Europe are not the only nations developing strategies against potential planetary threats. In April this year, China selected NEO 2019 VL5 for a combined deflection and observation test, launching in 2025. The mission is now scheduled to launch a year earlier than previous plans, to a much smaller asteroid compared to Dimorphos.

Illustration of Hera and the two cubesats, Milani and Juventas. (Credit: ESA)

Following Hera’s critical design review at the end of 2022, next up for GRASS is its integration with Juventas by the end of this year.

“This mission represents a huge step for EMXYS because we have been able to develop an extremely sensitive instrument that will travel into deep space to study a small body of the solar system,” Carrasco said.

“This instrument may well be used in future missions to study Solar Systems bodies and make a model of its mass and composition”.

(Lead image: Juventas CubeSat prepares for landing onto Dimorphos. Credit: ESA)

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