ESA’s Hera Probe Captures First Glimpse of Dimorphos Asteroid System

Four years after NASA crashed a spacecraft into the asteroid Dimorphos to test planetary defense, Europe’s Hera probe has captured its first distant glimpse of the target system. The European Space Agency spacecraft is now preparing for a critical 93-minute braking maneuver on October 15 to begin inspecting the historic impact site.

The binary asteroid system consisting of the larger rock Didymos and its smaller moonlet Dimorphos is finally coming into focus for European space engineers. Launched on a SpaceX Falcon 9 rocket on October 7, 2024, the European Space Agency planetary defense mission recently photographed its destination from an approximate distance of 236,500 kilometers away. In the composite image published by the space agency, the two asteroids appear as a single, slightly brighter pixel against the background star field.

That pixel marks the destination of a complex interplanetary journey. The Double Asteroid Redirection Test spacecraft slammed into Dimorphos on Sept. 26, 2022, in humanity’s first test of planetary defense. Ground-based and space telescopes later confirmed that the collision shortened the moonlet’s orbit around Didymos by about 33 minutes and even altered the entire binary system’s orbit around the Sun by 0.15 seconds. Yet basic questions about the wreckage remain unanswered.

October 15 Braking Maneuver at the Didymos System

Getting close enough to inspect those changes requires shedding immense speed. Moving at 12 kilometers per second relative to Earth, the mission is about to rendezvous with the binary pair. On the early morning of October 15, mission operators will command the spacecraft to fire its Orbit Control Thrusters continuously for approximately 93 minutes.

This major braking maneuver is designed to reduce the spacecraft’s relative velocity with respect to Didymos and Dimorphos. A second, shorter burn is scheduled for October 22 to fine-tune the adjusted orbit. Behind these engine sequences sits a team of top mathematicians, physicists, and aerospace engineers operating out of the European Space Operations Centre in Darmstadt, Germany. Approximately 10 specialists per shift from a division exceeding 50 members are managing the approach.

“The team performs Flight Dynamics operations for all ESA operated Science missions, including interplanetary missions and observatories, and now our agency’s first Space Safety mission as well!”

Michael Mueller, Flight Dynamics Engineer

“So as Hera arrives we will also be working on the BepiColombo mission’s approach to Mercury and during subsequent Hera proximity operations we will also have the launch of ESA’s Plato exoplanet-monitoring observatory, which will need to be shepherded to the Sun-Earth Lagrange point L2, 1.5 million kilometres from Earth.”

Michael Mueller, Flight Dynamics Engineer

Uncertainties Awaiting at the Impact Site

Although telescopes have monitored the system from afar, scientists emphasize that much of the post-impact reality is still a mystery. Researchers do not yet know how large a crater the impact left behind, whether it completely rearranged the surface boulders, or if material shifted between Dimorphos and Didymos.

There are also hints that Dimorphos is now tumbling, and the exact nature of its orbit change remains puzzling. Cristina Thomas, a planetary astronomer at Northern Arizona University affiliated with both missions, noted that the time Dimorphos took to circle Didymos was still changing some six months after impact for reasons that are not yet clear.

This thing that we thought was going to be very cut and dry turned out to be really interesting and complicated, Thomas said.

Other researchers are preparing for unexpected physical conditions around the targets. Angela Stickle, a planetary scientist at Johns Hopkins University’s Applied Physics Laboratory, pointed out that we know that there was a lot of material kicked up and we know that we probably left a pretty significant mark on Dimorphos. Mueller added that potential impact debris remaining in the vicinity has not been entirely ruled out, noting that there’s a lot of uncertainty, especially in terms of the asteroid’s degree of tumbling due to the collision.

Despite those unknowns, mission leadership expresses confidence.

“Nothing really worries me,” Michel said of the spacecraft’s arrival. “It’s really just excitement now.”

Patrick Michel, planetary scientist at the French National Centre for Scientific Research (CNRS) and principal investigator of the Hera mission

Probe Characterizes Binary System Before Deploying CubeSats

Once the spacecraft settles into the vicinity of the binary system, operations will unfold across several distinct phases. During November and December, the probe will conduct its early characterization phase from a safe distance of 25 to 30 kilometers to acquire baseline measurements of the bodies’ shapes and rotations. Later in December, it will achieve a minimum distance of 10 kilometers for detailed characterization.

In early 2027, the mission will deploy two small companion CubeSats named Milani and Juventas, which piggybacked out to the asteroid system. Milani will analyze dust fields and surface composition, while Juventas will use radar to probe the interior of Dimorphos directly—marking the first time scientists have examined the inside of a space rock. Both CubeSats may attempt landings on the asteroid moonlet at the end of their operational runs.

Meanwhile, the main spacecraft will draw as close as 4 kilometers during its close observation phase, eventually operating in tandem with its CubeSats through early 2027 while maintaining its primary duties at least through September 2027.

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