Dawn was the first spacecraft to orbit two different extraterrestrial objects. The mission was technically canceled twice before the spacecraft got off the ground, but Dawn launched in September 2007 with its sights set on the asteroid Vesta and the dwarf planet Ceres, chosen for how little they resembled each other. While they are only two of the millions of objects in the asteroid belt, they contain a whopping 45 percent of its mass. NASA announced the end of Dawn's asteroid mission's end on Thursday (Nov. 1).
"Both Vesta and Ceres had previously only been viewed mostly as just faint smudges of light amidst the stars," Marc Rayman, Dawn's mission director at NASA, told Space.com. "Now we have these richly detailed, intimate portraits of alien terrains and complex geology and just a wealth of detail that we had never really even imagined before, unveiling secrets that these bodies have held for billions of years." [Photos: Asteroid Vesta and NASA's Dawn Spacecraft]
In order to succeed, the Dawn spacecraft needed a careful trajectory and a secret weapon — an ion propulsion system, which had only powered one previous NASA mission. As the mission played out, that system gave engineers the flexibility to spend twice as long at Vesta as originally planned and nearly five times as long at Ceres.
Scientists were oddly familiar with Dawn's first destination, Vesta, thanks to a cosmic quirk: the majority of meteorites that have fallen to Earth with an identified home are actually chunks of this distant object. But seeing planetary rubble that pummels our planet is not the same as seeing an asteroid in its full glory — and that's exactly what Dawn let us do, beginning with its arrival in 2011.
"All of the Vesta images were super cool," Kristina Larson, a systems engineer on the mission at NASA, told Space.com. She began working on Dawn as an undergraduate summer intern and gradually took on more and more responsibility with the spacecraft, and the first command she sent to it brought some of those images back to Earth. "Vesta is such a funky-looking body, it's not very spherical and it's just super heavily cratered."
Vesta's funky appearance is scientifically interesting as well: It experienced two cratering events so large that the shocks created a network of more than 90 gashes. "The entire planet-like body reverberated," Rayman said. During Dawn's time at Vesta, he said that day after day he was continually impressed by how successful the mission was and how much scientists took away from the visit. "I always felt like if the spacecraft just mysteriously died that day, at least we would have a valuable return for what we had invested."
The spacecraft did have a near-death experience as it left Vesta, when the second of four reaction wheels it used to steer broke, jeopardizing Dawn's ability to send data home. The engineers on the team hustled their way to a solution substituting fuel maneuvers, which saved the mission.
In 2015, Dawn reached its second destination, the dwarf planet Ceres. Here, its discoveries include particularly reflective regions now dubbed bright spots, which scientists now believe represent salts sprayed onto Ceres' surface from below. But for Rayman, they're also stunning. "How can you not be mesmerized by these things?" he said. "The way I like to think of it is it's as if Ceres is casting its light out like a lighthouse shining its light across the interplanetary ocean." [Photos: Dwarf Planet Ceres, the Solar System's Largest Asteroid]
Dawn has also revealed that Ceres isn't a dead world — as recently as 250 million years ago, a giant ice volcano dubbed Ahuna Mons gushed liquid water, with dozens more compatriots, making the dwarf planet's composition and geology a particularly complex puzzle. "It's kind of a hybrid world," Lynnae Quick, a planetary scientist at the National Air and Space Museum who works with Dawn, told Space.com. "Because Ceres has this kind of exotic composition, you can't just think about water and salts you have to think about water and salts and mud, and that makes it quite interesting."
As the spacecraft has inched closer — when it ran out of fuel, it was skimming as close as 22 miles (35 kilometers) above the dwarf planet's surface — its views of Ceres have only become more incredible. Dawn's time at Ceres has reshaped scientists' opinions of the object, team members said, and now it's a viable candidate for a return mission. In large part, that's because of the boxes scientists checked off their search-for-life criteria.
"Before, it was just an asteroid, and now it plays with the big boys," Julie Castillo-Rogez, a planetary scientist on the mission at NASA, told Space.com, referring to places like Saturn's moon Enceladus and Jupiter's moon Europa, which have been astrobiological contenders for much longer. In order to protect any life that may be hiding on Ceres, the spacecraft's demise is carefully arranged to keep it away from the dwarf planet for at least 20 years, in case NASA decides to build a follow-up mission.
NASA is already working on a successor of sorts to the $467 million Dawn mission: Psyche. Like Dawn, it will visit an asteroid — and like Dawn, it will do so thanks to an ion propulsion system. And the scientists behind Dawn hope they have paved the way for more excursions to tiny corners of our solar system.
"I hope there's going to be many, many missions like [Dawn] that follow," Carol Raymond, the mission's principal investigator at NASA, told Space.com. "We have a new way of exploring farther into the solar system and going to smaller objects, which maybe weren't appreciated in the past for how much information they hold."
Editor's note: This story has been updated to qualify characterizations about the Dawn mission and its targets.