ESA's Hera spacecraft returned its first close-up images of Didymos on November 5, kicking off six months of work to explain why DART deflected Dimorphos twice as hard as predicted.
ESA/Hera — Asteroid Framing Camera image of Didymos (780 m) and Dimorphos (151 m) from about 60 km, acquired 31 October 2026
ESA’s Hera spacecraft sent back its first close-up image of the Didymos binary asteroid system on November 5, two years after launch and 196.8 million km from Earth. The image shows Didymos, 780 m across, lit clearly in the frame, with Dimorphos, its 151 m moonlet, mostly in shadow about 1 km away.
It is a modest picture by the standards of what is coming. Hera’s Asteroid Framing Camera took it from roughly 60 km out, and ESA has already scheduled a release of sharper imagery for November 25. But the picture marks the start of the phase this mission exists for: a six-month investigation into what NASA’s DART spacecraft actually did to Dimorphos in 2022, and why it did more of it than anyone predicted.
What the First Image Shows
The frame captures the far side of Didymos from the one DART’s own camera saw just before impact, so this is new terrain for mission scientists. A ridge runs along Didymos’s equator, a feature already associated with how fast, rubble-pile asteroids shed material toward their equator as they spin. Scattered across the surface are boulders, craters, and what look like grooves or lineaments, some consistent with layers of material slumping downslope under near-zero gravity.
Dimorphos is harder to read in this first pass. Most of its surface sits in shadow, and at 60 km distance from a 151 m target, individual boulders are barely at the edge of resolution. That resolution improves fast. Hera is flying an autonomous navigation system that tracks surface features to guide its own approach, rather than waiting on light-delayed commands from Earth, and the November 25 imagery should show the moonlet directly.
The Momentum Mystery DART Left Behind
DART hit Dimorphos on September 26, 2022. The spacecraft weighed 580 kg and struck at about 6.1 km/s, a kinetic impact meant to test whether slamming a spacecraft into an asteroid could measurably change its orbit. It worked. Dimorphos’s orbital period around Didymos shortened by 33 minutes, give or take one minute. That is far more than the pre-impact models predicted.
The reason matters more than the headline number. A simple momentum transfer, where the spacecraft’s momentum just adds to the asteroid’s, would have produced a smaller orbital change. The excess comes from ejecta: material blasted off Dimorphos’s surface by the impact that carried additional momentum away in the opposite direction, effectively giving the asteroid a second kick. Mission scientists currently estimate that DART transferred about 2.1 times its own momentum to Dimorphos once ejecta is accounted for. That ratio is called the beta factor, and it is the single number that determines whether kinetic impactors are a viable planetary defense tool against a real threatening asteroid, or a far less predictable one.
Ten Open Questions, One Close-Up Mission
ESA has framed Hera’s science campaign around ten specific unknowns, and nearly all of them trace back to that beta factor. Did DART leave a crater on Dimorphos large enough to have reshaped the whole moonlet, or did the impact only disturb a local patch of surface. Is there still ejecta in the system that poses a collision risk to Hera itself, though most of it has likely already escaped. Does Dimorphos now tumble instead of keeping one face toward Didymos, and if so, how long before tidal forces resettle it. Did Didymos’s own fast rotation, through the YORP effect that lets sunlight torque small bodies over time, originally spin off the debris ring that became Dimorphos in the first place.
The questions that matter most for interpreting the beta factor are about what Dimorphos is actually made of. Is it strong enough to hold a crater’s shape, or so weakly bound by its own near-zero gravity that the whole moonlet is better described as a loosely cohered rubble pile. Why did earlier images show no visible dust between its boulders, when a body that fragile should be shedding fine material constantly. Are the boulders themselves porous and fragile, more like pumice than rock, or solid.
Measuring a Mass by Watching It Fall Around You
Underlying nearly every one of those questions is a number nobody has pinned down with confidence: Dimorphos’s mass, currently uncertain by tens of percent. DART’s beta factor cannot be computed precisely without it, since the same orbital change could come from more ejecta acting on a lighter moonlet or less ejecta acting on a heavier one.
Hera resolves this the same way geodesists measure a planet’s gravity field, just at asteroid scale. As the spacecraft orbits increasingly close to Dimorphos, tiny deviations in its trajectory, caused by the moonlet’s actual gravitational pull rather than an assumed one, show up as Doppler shifts in its radio signal back to Earth. Those measurements are planned for the end of the six-month campaign, when Hera will be flying closest. A shoebox-sized CubeSat riding with Hera, called Juventas, adds a second method: a radar instrument with roughly 10 m resolution, the smallest ever flown in space, built to probe what Dimorphos looks like inside rather than just at the surface. A second CubeSat is slated to attempt an actual touchdown on Dimorphos before the mission ends, assuming its weak gravity and uncertain surface strength allow for one.
Why This Number Decides the Next Deflection Mission
A beta factor of roughly 2 instead of 1 is not a rounding error. It is the difference between needing one kinetic impactor to deflect a genuinely hazardous asteroid and needing two, or needing a warning time measured in years rather than months. Every future planetary defense mission that chooses between a kinetic impactor and a slower option, such as a gravity tractor that nudges a target by hovering nearby for an extended period, depends on knowing whether DART’s result at Dimorphos was typical of rubble-pile asteroids in general or a lucky outcome specific to one small, loosely bound body. For the orbital mechanics behind how such small momentum changes compound into large positional shifts over time, see how delta-v budgets drive deep-space mission design. Hera spent two years flying to get close enough to answer that question properly, and its first picture is just the proof that it arrived.