A spacecraft is circling the two asteroids NASA nudged in 2022, taking the first close measurements of what that collision left behind. The European Space Agency said Wednesday that its Hera spacecraft had begun surveying Didymos and its smaller moon, Dimorphos, from about 16 to 19 miles away. Neither asteroid posed a threat to Earth. NASA chose the pair to test whether a spacecraft could change an asteroid’s course.
The impact changed the moon’s orbit, a result telescopes could measure from Earth. They could see the cloud of debris that followed, too. What they could not see in detail was the surface that remained. Hera will look for a crater or larger changes in the moon’s shape and measure its mass, an input scientists need to judge how much of the deflection came from the spacecraft and how much came from the material it knocked loose.
What NASA changed in 2022
NASA’s Double Asteroid Redirection Test, or DART, struck Dimorphos on Sept. 26, 2022. Dimorphos went around Didymos roughly every 11 hours and 55 minutes before the collision. Afterward, it completed an orbit about 33 minutes sooner. Didymos is the larger body, about half a mile across. ESA puts Dimorphos at roughly 500 feet across.
That change in the moon’s orbit around its companion is separate from the change in the pair’s path around the Sun. In a NASA account of later research, researchers estimated that the pair’s roughly 770-day trip around the Sun shifted by a fraction of a second. They used radar observations and timed the brief dips in starlight when the asteroids crossed in front of distant stars. A small shift can add up over time, which is why early detection matters for a possible future deflection.
Why the debris matters
The spacecraft transferred momentum when it hit. Rock and dust leaving Dimorphos carried momentum in the opposite direction, adding to the push. Hubble observed that debris spreading after the collision. But the size of the cloud alone cannot tell scientists how the asteroid responded. An intact, dense body and a loose aggregate can react differently to a strike. Measuring the moon’s mass and mapping its surface will help researchers test which explanation best fits the change telescopes recorded.
Some debris may have landed on Didymos, ESA says. That possibility gives Hera a reason to map both bodies. Looking at the larger asteroid alongside the impact target may help scientists distinguish material displaced by DART from features that were already there.
Why Hera flies a rectangle
A conventional circular orbit is impractical in the pair’s weak gravity, ESA says. Hera travels at about five inches per second relative to the pair. It flies roughly straight legs, then uses its thrusters to turn about 90 degrees. ESA says that if a planned firing fails, the spacecraft would drift away from the asteroids. The pattern also lets its cameras keep both bodies in view for part of the survey.
What Hera will measure next
ESA plans to present detailed images of both asteroids on Nov. 25. The spacecraft is due to start a closer survey in December. According to the Japanese space agency’s observation plan, Hera is scheduled to release two small spacecraft in January. Juventas will use radar to probe Dimorphos’ interior, and Milani will study the materials at its surface. Those measurements could help explain how the same type of impact might play out on an asteroid with a different structure.
The December flight path is designed to give cameras different angles of sunlight on the asteroids. The changing light helps the team measure terrain and other surface properties. Later flybys, planned for spring 2027, could bring Hera within about a mile of Dimorphos for its sharpest look at the impact site.
