When NASA's Dawn mission launched in 2007, it was on its way to breaking several records. When it entered orbit around the Vesta, it became the first to orbit a main-belt asteroid. After leaving Vesta, it journeyed on to Ceres, becoming the first spacecraft to visit and then orbit a dwarf planet and the first spacecraft to orbit two extraterrestrial targets.
The journey hasn't been smooth. Along the way, the spacecraft lost threeof its four reaction wheels that keep it oriented. It successfully concluded its primary mission to study both targets in 2016. Once it runs out of fuel near the end of 2018, it will continue to orbit Ceres for another 50 years.
"To me, the real story here is how cool it is that we're exploring what, in my view, are some of the last uncharted worlds in the inner solar system," chief engineer and Dawn mission director Marc Rayman told Space.com. "Most people think of asteroids as chips of rock, but these are whole new worlds."
Out of this world
Dawn launched from Cape Canaveral, Florida, on Sept. 27, 2007. Its arrived at the asteroid Vesta on July 16, 2011. The spacecraft spent almost a year in orbit around the second-largest object in the asteroid belt, departing on Sept. 5, 2012.
On March 6, 2015, Dawn entered orbit around Ceres. Ceres is by far the most massive object in the asteroid belt between Mars and Jupiter, weighing in at nearly two-thirds the total mass of the belt. Its massive size and roundness means it qualifies as a dwarf planet. Unlike a full-size planet, a dwarf planet is a round object that fails to clear out its orbit. Dawn was just barely the first mission to arrive at a dwarf planet; NASA's New Horizons mission zipped by Pluto only a few months later (though, in fairness, New Horizons launched first but had much farther to travel).
Dawn traveled 1.7 billion miles (2.8 billion kilometers) to reach Vesta, then another 3.1 billion miles (4.9 billion km) to Ceres. Boosted by an ion propulsion system, the spacecraft took four days to accelerate from 0 to 60 mph (0 to 97 km/h) at maximum throttle. Each engine produces about the same amount of force as a single piece of paper notebook paper presses against your hand.
Over time, however, that small force adds up. In 2010, it surpassed the previous record for velocity change held by NASA's Deep Space 1 when its accumulated acceleration over the mission exceeded 9,600 MPH (4.3 km per second).
"I am delighted that it will be Dawn that surpasses DS1's record," Rayman, who was a previous project manager for Deep Space 1, said in a statement. "It is a tribute to all those involved in the design and operations of this remarkable spacecraft.
Ion engines are extremely fuel-efficient — Dawn only carried 937 lbs. (425 kilograms) of xenon propellant at launch — but the fuel won't last forever. The spacecraft also carried 100.5 lbs. (45.6 kg) of hydrazine propellant used to change the spacecraft's orientation, and that tank is quickly running dry. Dawn is expected to run out of hydrazine fuel in the second half of 2018.
"When the last of the hydrazine is exhausted, the spacecraft will no longer be able to control its orientation, so it won't be able to point its solar arrays at the sun, its sensors at Ceres, nor its antenna at Earth," Rayman told Space.com in 2017. "That will be the end of Dawn's operational life."
Dawn will continue to orbit Ceres for decades. Planetary protection rules insisted on at least 20 years before the spacecraft crashed into the dwarf planet, to reduce the chances of contamination. The Dawn team opted to set the spacecraft on an orbit that would keep it aloft for at least 50 years.
Dawn's first stop was Vesta, a rocky asteroid and the second largest inhabitant of the asteroid belt, where it made several significant findings at Vesta. It discovered a geologic landscape that scientists called "exotic and diverse," and compiled the first map of the 330-mile (530 km) wide body. [Photos: Asteroid Vesta and NASA's Dawn Spacecraft]
Dawn discovered that Vesta has a layered structure like Earth. The spacecraft found that Vesta has a substantial nickel-iron core, just like Mercury, Earth and Mars. The core makes up about 18 percent of Vesta's total mass. The surface is entirely basalt, a type of frozen lava, and the asteroid once boasted a magnetic field.
The Dawn team reasoned that reaching those temperatures would have caused the heavy elements to melt and sink down to the core in a process known as differentiation. In fact, joked JPL’s Carol Raymond, Dawn's deputy principal investigator, "We like to call Vesta 'the smallest terrestrial planet.'"
Dawn also confirmed that Vesta is the source of the howardite-eucrite-diogenite (HED) meteorites found on Earth and Mars. The Dawn team thinks that the HEDs came from an impact basin the team named Rheasilvia. The basin itself has an age of about 1 billion years, and formed from a massive collision that stripped the away the bulk of the asteroid's southern hemisphere. With a diameter of 310 miles (500 km), Rheasilvia is nearly as large as Vesta itself.
"Vesta likely came close to shattering," said Raymond.
A second basin showcases another dangerous impact beneath Rheasilvia. Named Veneneia, the basin is nearly as big and nearly a billion years younger than Rheasilvia and may be another potential source for HED meteorites.
Research from Dawn also suggests that Vesta may hide ice beneath its surface. Originally, the scientists suspected that roughness on the asteroid's surface came from impacts, but Dawn's data suggests that some of the features are caused by ice buried beneath the surface.
"We suggest that modifications of the surface by melting of buried ice could be responsible for smoothing those areas," Essam Heggy, a planetary scientist at the University of Southern California in Los Angeles, told Space.com. "Buried ice could have been brought to the surface after an impact, which caused heated ice to melt and travel up through the fractures to the surface."
The findings mean that ice could have played a more dominant role in shaping Vesta than previously thought.
"We went to Vesta to fill in the blanks of our knowledge about the early history of our solar system," said Dawn principal investigator Christopher Russell, of UCLA, said in a statement.
"Dawn has filled in those pages, and more, revealing to us how special Vesta is as a survivor from the earliest days of the solar system. We can now say with certainty that Vesta resembles a small planet more closely than a typical asteroid."
While Vesta is rocky, Ceres is surprisingly icy. Before Dawn arrived, scientists estimated that water could make up as much as a quarter of the dwarf planet, though that water would be tucked beneath the surface. Observations made by the Hubble Space Telescope revealed a cloud of vapor that suggested the dwarf planet might be degassing, though no strong signs of such activity has been spotted by Dawn. [Photos: The Changing Bright Spots of Dwarf Planet Ceres]
On the surface, Ceres appears relatively bland. Aside from a few craters — though less than scientists anticipated — the only outstanding feature is a single mountain, Ahuna Mons. Researchers suspected the mountain was a cryovolcano, oozing ice instead of hot lava. Further studies revealed that, while it may be thought of as a "lonely mountain" today, it could have had companions in the past. Made of ice, these mountains may have slowly flowed back onto the surface.
"We think we have a very good case that there have been lots of cryovolcanoes on Ceres but they have deformed," Dawn researcher Michael Sori of the University of Arizona in Tucson said in a statement.
The same fate may await the lonely volcano.
"Ahuna Mons is at most 200 million years old. It just hasn't had time to deform," Sori said.
From a distance, Dawn caught sight of bright spots from a distance that soon resolved into more than 130 bright patches, most of them tied to craters. Initially thought to be Epsom salt, the patches turned out to be a version of salts that require water to form. Since water skips to gas almost immediately on the dwarf planet's surface, that suggests that the liquid must lie beneath the crust.
"That was something we had not expected," Russell told Space.com. "The carbonates are a very strong indication of the processes now that we believe took place in the interior, that makes it more Earthlike, when it can alter the chemistry inside."
"It's not something that's just lying around out there in space," he added.
The flowing ice that formed Ahuna Mons and the presence of salts suggest that an ancient ocean once lie beneath the crust of Ceres.
"We believe these bright spots are a sign that Ceres once had a global ocean," planetary geologist Lynnae Quick, of the Smithsonian Institute in Washington, D.C., told Space.com.
That ocean may continue to feed activity on Ceres today.
"It's possible there is still brine coming up to the surface," Nathan Stein, a planetary scientist at the California Institute of Technology in Pasadena, told Space.com. "It's certainly intriguing."
Researchers also spotted ammonia-rich clays on the dwarf planet. Ammonia is more commonly found in the outer solar system. The material could have been delivered to Ceres by comets, or its presence could be a sign that the dwarf planet formed in the outer solar system.
These and other discoveries by Dawn have revealed that Ceres is a rich, evolving world.
"The IAU [International Astronomical Union] has defined what a planet is in a particular way," Russell said, "but I think of a planet more as a body which, when it's big enough and has enough activity … is now making things, producing things in its interior that are not just sitting there for the eons but in fact that the body evolves with time inside."