What little we already know about Didymos and Dimorphos is due to observations made by optical and radio telescopes on the ground. In fact, the only way for astronomers to know that Didymos even has a moon is because its luminosity decreases at regular intervals, suggesting that there is an object orbiting around it. “Much of what we know about the Didymos system comes from observations in 2003,” says Cristina Thomas, astronomer at Northern Arizona University and head of the DART Observation Working Group. “The Didymos system has an observation window every two years or so, and once DART came up with an idea, we started to observe Didymos regularly.”
DART has its origins in Don Quijote, an asteroid impactor proposed by the European Space Agency in the early 2000s. The idea was to send two spaceships – one to hit an asteroid while the other watched. – and to study how the strike changed the trajectory of the asteroid 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 Sciences, Engineering and Medicine, which set the priorities of various scientific disciplines, published a report which strongly recommended an impacteur mission. The question was how to reduce the cost.
Andy Cheng, now the chief scientist of the Applied Physics Laboratory and one of the principal investigators of the DART mission, was working a morning shortly after the report was released when he found a way to crash an asteroid on the cheap. . “The idea came to me that we should do this on a binary asteroid, because then you wouldn’t need a second spacecraft to measure the deflection,” Cheng explains. “You can do it from Earth with telescopes on the ground.”
All it took was a target. There aren’t many double asteroids floating around, and only a few of them pass close enough to Earth to be observed by ground-based telescopes as a spacecraft crashes into them. Fewer are still small enough that a spacecraft can make a noticeable difference in their orbit. By the time Cheng and his crew narrowed 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,” Cheng says. So he and a small group came up with a proposal and presented it to NASA in late 2011. It didn’t take long for the agency to bite. In 2012, DART was officially on the books.
Once Didymos was chosen as a target, astronomers began to observe the asteroid system when it appeared every two years. “We realized that we had to understand the pre-impact system as well as possible before changing it forever,” says Rivkin. The first Didymos observation campaign since 2003 began in 2015 and has taken place every two years since.
Based on previous observations, astronomers know that Dimorphos circles 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 was no real reason to watch her, as she did not pose a great threat to Earth, at least not for the foreseeable future. “We have no idea what Dimorphos looks like at all,” Adams says. “We only saw 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 a lot of them. The most important unknowns the DART team must model before launch are Dimorphos’ shape and makeup, as these factors play a major role in determining how the impact of the spacecraft will affect its trajectory. A dog bone-shaped asteroid, for example, will react differently to a spherical asteroid, and it will also be more difficult for the spacecraft to identify and reach its exact center. Evidence suggests that many asteroids are not solid but are actually large piles of rubble held together by the gravity of their individual rocks. The size and distribution of these rocks will determine the effects of the DART impact, as rocks near the crash site will explode in space. When they push back the asteroid, they will further increase the change in course of the asteroid.