The European Space Agency’s Hera spacecraft is about to start a twofold mission to a twofold asteroid. On November 4, after more than two years trekking through space, the spacecraft finally came within 30 kilometers of space rocks named Didymos and Dimorphos. The encounter marks the first time scientists will explore a pair of asteroids navigating the solar system together.
To date, spacecraft have stopped to investigate just seven other asteroids, every one of them a lonely speck in outer space. Hera will spend the coming weeks approaching Didymos and Dimorphos and will release two small companion spacecraft in January for an even closer look. Didymos, a mass of rocks about twice as wide as the Empire State Building is tall, and its small moonlet Dimorphos have had a rough time lately.
In 2022, NASA slammed its Double Asteroid Redirection Test (DART) spacecraft into Dimorphos, sending a plume of debris into space and shortening its orbit around Didymos by more than half an hour. The cosmic realignment wasn’t a mere lark. Such an impact is one of the few ways scientists can practice planetary defense, the field dedicated to searching for dangerous asteroids and developing ways to deal with them lest humans go the way of the dinosaurs.
DART and Hera are a carefully coordinated pair, with Hera’s top priority being to investigate the aftermath of DART’s crash and inform future planetary defense efforts. That process will also yield a treasure trove of observations asteroid scientists are determined to exploit. On the science side, she says, “we’re trying to figure out not so much how did we break the thing, but how was it put together in the first place.” DART purposefully targeted a binary system instead of a solo asteroid to make measuring the change in orbit it accomplished easier: Any difference would be much more obvious against the 12-hour baseline orbit of Dimorphos around Didymos than against the months- or yearslong orbit of an asteroid circling the sun. (Despite the difficulty, scientists were able to calculate that DART shortened the more-than-two-year orbit of the Didymos system around the sun by a fraction of a second.) Hera’s observations will provide a crucial glimpse at binary asteroids generally, which astronomers believe account for some 15 percent of all asteroids.
Scientists expect that most binary asteroids in the inner solar system, including the Didymos system, probably formed their moonlets from debris that sloughed off the larger object then clumped together in orbit. Didymos spins so quickly that its gravity may not be strong enough to hold all its rock together. As the asteroid spins, it may send boulders into space, where they could end up landing on Dimorphos, continuing that material transfer today.
Scientists will comb through Hera’s imagery for signs of rocks moving across each object’s surface, as well as between the two objects. Scientists expect that Dimorphos is a rubble pile composed of boulders and rocky debris. It’s similar in structure to other recent spacecraft targets Ryugu and Bennu, although those are both significantly larger.
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