The European Space Agency's Hera spacecraft has returned its first close-up photo of Didymos and Dimorphos, the asteroid pair NASA hit with its DART spacecraft in 2022. It shows a side of Didymos that DART never imaged before impact.
The black-and-white frame was taken from about 37 miles away. Didymos, roughly a half-mile wide, fills most of it. Shadow hides most of Dimorphos, the 500-foot moonlet that took the hit. The team said the "backside" of Didymos looks remarkably different from the side seen earlier, with boulders, craters and grooves, plus the continuation of a ridge that wraps around its middle. "This feels like looking at a whole new world," said Patrick Michel, Hera's principal investigator. Mission scientist Michael Kueppers said some of those features may come from debris the crash threw across the half-mile gap between the two bodies. Detailed images of both asteroids are due November 25.
The photo is a good start, but the real value of Hera is measurement. DART, a 1,300-pound spacecraft, shortened Dimorphos's orbit by 33 minutes, more than scientists predicted. The extra push came from rock and dust blasted into space, which acted like rocket exhaust. Researchers call that bonus the "beta factor." Without a firm number for it, nobody can say how hard a real threat would need to be hit.
That makes DART a single data point, and one hit on one rock is thin evidence for a defense strategy. Hera will map the crash site to see whether it left a deep crater, a shallow dent or a reshaped asteroid. It will also measure mass and density, and its companion probe Juventas will scan Dimorphos's interior with radar. Gravity there is about 200,000 times weaker than on Earth, so whether Dimorphos is solid rock or a loose pile of boulders could change how it responds to an impact.
There is also a question about motion. Dimorphos once kept one face toward Didymos, as the moon does with Earth, but telescope data now hint that it may be tumbling. If the impact did that, any future mission would have to plan for the aftermath as well as the hit.