The little we already know about Didymos and Dimorphos is thanks to observations done by ground-based optical and radio telescopes. In fact, the only way astronomers can tell Didymos even has a moon is because its brightness dims at regular intervals, suggesting that there is an object in orbit around it. “Much of what we know about the Didymos system comes from observations in 2003,” says Cristina Thomas, an astronomer at Northern Arizona University and the leader of DART’s observation working group. “The Didymos system has an observing window approximately every two years, and once DART was an idea, we started observing Didymos regularly.”
DART traces its origins to Don Quijote, an asteroid impactor proposed by the European Space Agency in the early 2000s. The idea was to send out two spacecraft—one to hit an asteroid while the other watched—and study how the strike changed the asteroid’s trajectory around the sun. ESA officials ultimately determined that the mission would be too expensive and killed the idea. But a few years later, the National Academies of Science, Engineering, and Medicine, which sets priorities for various scientific disciplines, published a report that strongly recommended an impactor mission. The question was how to lower the cost.
Andy Cheng, now the chief scientist at the Applied Physics Laboratory and one of the lead investigators on the DART mission, was working out one morning shortly after the report was published when he hit on a way to crash into an asteroid on the cheap. “The idea came to me that we should do this at a binary asteroid, because then you wouldn’t need a second spacecraft to measure the deflection,” says Cheng. “You could do it from Earth with ground-based telescopes.”
All that was needed was a target. There aren’t many double asteroids floating around, and only a few of those pass close enough to Earth to be observed by ground-based telescopes while a spacecraft rams into them. Fewer still are small enough that a spacecraft could make a noticeable difference in their orbit. By the time Cheng and his crew had whittled down the list of possible targets, there were only two viable options—and one of them was Didymos. “It was by far the best choice,” says Cheng. So he and a small group put together a proposal and pitched the idea to NASA in late 2011. It didn’t take long for the agency to bite. By 2012, DART was officially on the books.
Once Didymos was selected as a target, astronomers began observing the asteroid system when it came around every two years. “We realized that we needed to understand the pre-impact system as well as we could before we changed it forever,” says Rivkin. The first Didymos observation campaign since 2003 began in 2015 and has occurred every two years since.
Based on previous observations, astronomers know that Dimorphos orbits Didymos about once every 12 hours and is about 500 feet wide. But beyond that, it’s a mystery. Before Didymos became the DART target, there just wasn’t that much of a reason to keep an eye on it, because it didn’t pose much of a threat to Earth—at least not for the foreseeable future. “We don’t know what Dimorphos looks like at all,” says Adams. “We’ve only seen Didymos.”
So how do you plan a mission to crash into an asteroid when you don’t even know what it looks like? Simulations—and lots of them. The most important unknowns for the DART team to model before launch are the shape of Dimorphos and its composition, since these factors play a huge role in determining how the spacecraft’s impact will affect its trajectory. An asteroid shaped like a dog bone, for example, will react differently than a spherical asteroid, and it will also be harder for the spacecraft to identify and hit its exact center. Evidence suggests that many asteroids aren’t solid but are actually big rubble piles held together by the gravity of their individual rocks. The size and distribution of these rocks will determine the effects of DART’s impact, since the rocks near the crash site will blow off into space. When they push off the asteroid, they will further increase the change in the asteroid’s trajectory.