Dawn was a NASA mission that operated between 2007 and 2018, breaking several records along the way.
When it entered orbit around the asteroid Vesta, it became the first spacecraft to orbit an asteroid in the zone where most solar system asteroids reside, the main belt, between Mars and Jupiter. After leaving Vesta, the satellite journeyed on to the dwarf planet Ceres, becoming the first spacecraft to visit and then orbit a dwarf planet and the first spacecraft to orbit two extraterrestrial targets.
But the journey wasn't smooth. Along the way, the spacecraft lost three of the four reaction wheels that kept it oriented in space. Nonetheless, Dawn successfully concluded its primary mission to study both extraterrestrial targets in 2016. Dawn ran out of fuel on Nov. 1, 2018, NASA announced, but the spacecraft will continue to orbit Ceres for another 50 years.
"Today, we celebrate the end of our Dawn mission — its incredible technical achievements, the vital science it gave us and the entire team who enabled the spacecraft to make these discoveries," Thomas Zurbuchen, associate administrator of NASA's Science Mission Directorate in Washington, D.C., said in a statement when the mission ended. "The astounding images and data that Dawn collected from Vesta and Ceres are critical to understanding the history and evolution of our solar system."
Out of this world
Dawn launched from Cape Canaveral, Florida, on Sept. 27, 2007. After traveling 1.7 billion miles (2.8 billion kilometers), the spacecraft arrived at the asteroid Vesta on July 16, 2011. Dawn spent almost a year in orbit around Vesta, the second-largest object in the asteroid belt. On Sept. 5, 2012, Dawn left Vesta and began its track toward the dwarf planet, Ceres, another 3.1 billion miles (4.9 billion km) away.
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 mean 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 of smaller objects. 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 (New Horizons launched before Dawn but had much farther to travel).
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 produced only about the same amount of force as a single piece of notebook paper pressed against your hand. Over time, however, that small force adds up. In 2010, Dawn 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," Marc Rayman, the chief engineer for the Dawn mission and 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."
Dawn's first stop was Vesta, a rocky asteroid and the second largest inhabitant of the asteroid belt. Images captured by Dawn allowed scientists to discover the "exotic and diverse" geologic landscape of Vesta. Researchers used data collected by Dawn to compile the first map of the asteroid, which is 330 miles (530 km) wide. [Photos: Asteroid Vesta and NASA's Dawn Spacecraft]
Dawn's research team discovered that Vesta has a layered structure like Earth. The spacecraft also revealed 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.
Dawn also confirmed that Vesta is the source of the howardite-eucrite-diogenite (HED) meteorites found on Earth and Mars. The HEDs likely came from an impact basin the Dawn team named Rheasilvia. The basin itself is around 1 billion years old 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.
Research from Dawn also suggests that Vesta may hide ice beneath its surface. Originally, scientists suspected that roughness on the asteroid's surface came from impacts, but Dawn's data suggests that some of those 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," Christopher Russell, Dawn's principal investigator, 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 were spotted by Dawn. [Photos: The Changing Bright Spots of Dwarf Planet Ceres]
On the surface, Ceres appears relatively bland. Aside from a few craters, 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.
"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."From a distance, Dawn caught sight of bright spots 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 carbonate salts, which only form in the presence of water. Since water skips to gas almost immediately on the dwarf planet's surface, the discovery of carbonates suggested that there was liquid beneath the dwarf planet's crust.
The flowing ice that formed Ahuna Mons and the presence of salts suggest that an ancient ocean once lay beneath the crust of Ceres.
"We believe these bright spots are a sign that Ceres once had a global ocean," Lynnae Quick, a planetary geologist at the Smithsonian Institution 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.
Dawn's twilight phase
Although the spacecraft is out of fuel, Dawn will continue to orbit Ceres for decades. Planetary-protection rules require that at least 20 years go by before the spacecraft is allowed to crash 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.
This article was updated on Jan. 25, 2019 by Space.com contributor Elizabeth Howell.