The little we already know about Didymos and Dimorphos is due to observations made by ground-based optical and radio telescopes. In fact, the only way astronomers can say that Didymos even has a moon is that its brightness decreases at regular intervals, suggesting that there is an object in orbit around it. “Much of what we know about the Didimos system comes from observations in 2003,” said Christina Thomas, an astronomer at the University of Northern Arizona and head of the DART monitoring task force. “The Didymos system has a monitoring window approximately every two years, and since DART was an idea, we started monitoring Didymos on a regular basis.”
DART originates from Don Quixote, an asteroid impact element proposed by the European Space Agency in the early 2000s. The idea was to send two spacecraft ̵
Andy Cheng, now a chief scientist in the applied physics lab and one of the DART’s lead researchers, trained one morning shortly after the report was published, when he hit a way to crash into an asteroid at a cheap price. “The idea came to me that we had to do this with a binary asteroid, because then you wouldn’t need a second spaceship to measure the deviation,” Cheng said. “You can 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 pass close enough to Earth to be observed by ground-based telescopes as a spaceship crashes into them. Still fewer are small enough for a spacecraft to make a noticeable difference in its orbit. By the time Cheng and his crew split the list of possible targets, there were only two viable options – and one of them was Didimos. “It was the best choice,” Cheng said. So he and a small group prepared a proposal and passed the idea to NASA in late 2011. The agency did not take long to bite. Until 2012, DART was officially included in the books.
After Didymos was chosen as a target, astronomers began observing the asteroid system when it appeared every two years. “We realized we needed to understand the pre-impact system as best we could before we changed it forever,” Rivkin said. Didimos’ first surveillance campaign in 2003 began in 2015 and has been held 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 other than that, it’s a mystery. Before Didimos became DART’s target, there was simply no reason to keep an eye on him because he didn’t pose a major 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 that’s a lot. The most important unknowns for the DART team to model before launch are the shape of Dimorphos and its composition, as these factors play a huge role in determining how the spacecraft’s impact will affect its trajectory. For example, an asteroid shaped like a dog’s bone will react differently than a spherical asteroid, and it will also be more difficult for a spacecraft to identify and strike its exact center. The data show that many asteroids are not solid, but are in fact large piles of debris 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 the rocks near the crash site will blow into space. When they repel the asteroid, they will further increase the change in the asteroid’s trajectory.