ESA Launches Probe to Revisit Asteroid Crime Scene

The European Space Agency's Hera spacecraft is en route to assess how NASA's 2022 DART mission impacted the asteroid Dimorphos. DART smashed into the asteroid at a little over 6 kilometers per second, sending out a cloud of debris and slowing it slightly. Hera's detailed post-collision survey aims to tell us just how efficient the kinetic impact was. This will inform the design of planetary defense missions, should we need to defend Earth from an asteroid threat.

A SpaceX Falcon 9 rocket carrying Hera lifted off from Cape Canaveral on 7 October and is now on a journey, via a flyby of Mars in 2025, to asteroid Didymos and its minute moon Dimorphos. Arrival at the binary asteroid system is scheduled for December 2026.

DART impacted the 151-meter-diameter Dimophos back in 2022, reducing its velocity by 2.7 millimeters per second and thus, over time, changing its orbit around the Sun. There are, however, still many things to learn about the DART impact and its target before its kinetic impactor technique could be used in earnest against an incoming asteroid. We need to measure the mass of Dimorphos to know how efficient the impact technique actually is, to offer any future use," says Michael Kueppers, a project scientist with ESA, said during a press briefing on 2 October.

It's the first time that CubeSats will operate autonomously around another body." -Michael Kueppers, ESA

The Hera mission cost is 363 million (US $398 million). The spacecraft was built within a tight four-year timeline, despite the COVID-19 pandemic, and involved 18 European countries. It carries a suite of 12 payloads and will investigate any crater formation or deformation of the asteroid caused by DART and provide insights into how energy from the impact was distributed and better understand the impact process. It will also assess the asteroid's internal structure and the local environment and debris field. This debris could, incidentally, later result in meteor showers at Mars and even reach Earth.

Hera is then going back to the crime site and getting all of the scientific and technical information about these asteroids to validate our physical models," Ian Carnelli, Hera Project Manager at ESA, said during the briefing.

How Hera Will Study an Asteroid

Getting up close and assessing the asteroids will not be straightforward, however. As Dimorphos's gravity is negligible, at approximately 200,000 times weaker than Earth's, the spacecraft will not orbit the asteroid in the usual sense. Hera will operate around the binary system's combined center of gravity, or barycenter, leveraging techniques from the Rosetta mission, which visited Comet 67P/Churyumov-Gerasimenko a decade ago. This will see it use a series of hyperbolic arcs 20 to 30 kilometers out, to get a global picture, determine mass, and understand thermal and dynamic properties. Then, matters will get more complex.

Hera will deploy two CubeSats, Milani and Juventas, to complement the study after about four weeks at the Didymos system. For the first time, we will investigate such a body with three spacecraft in parallel," says Kueppers. And it's the first time that CubeSats will operate autonomously around another body."

Milani carries a hyperspectral imager and a dust and volatiles detector. Juventas carries a radar which will map the asteroid's internal structure. Both use cold gas propulsion. The trio will embark on ever-closer flybys of Dimorphos, eventually returning images of selected areas with a resolution of 10 centimeters.

After achieving their main science objectives, Milani and Juventas will attempt landings on Dimorphos. The pair are not engineered for landing, but will approach with relative velocities of centimeters per second and, hopefully, continue operations once on the surface. Likewise, Hera could land on the larger Didymos asteroid at the end of its six-month primary mission. The details of this are still being worked out, says Kueppers.

The orbital challenges will also present opportunities. While designed to be fully operated by ground teams, Hera will have a high degree of onboard autonomy, using a range of data to determine the environment and react in a way similar to a self-driving car. One navigation mode is autonomous feature tracking, which will see Hera image boulders and craters to judge the spacecraft's attitude and trajectory relative to the asteroid. This will be tested out fully once Hera's primary objectives are complete.

Hera will pave the way for future endeavors in more ways than just its science return. The mission came in under budget, Carnelli noted, and the savings have, fittingly, been redirected for more asteroid adventures. The funds have allowed the start of the Ramses mission, a stripped-down version of Hera, that will fly out to meet infamous asteroid Apophis when it makes an approach to within 32,000 km of Earth in 2029. Together, Hera, DART, and future missions will help us develop the tools to protect Earth from threats in our cosmic neighborhood.