Ten years ago, NASA’s Dawn spacecraft launched atop a Delta II Heavy booster from Cape Canaveral Air Force Station, Fla., bound for one of the most audacious missions of exploration ever undertaken in human history. For the first time, it would utilize xenon-ion propulsion operationally on an exploratory foray, deep into the Solar System, performing not one, but two orbital tours of Vesta and Ceres, dwarf planets of the asteroid belt, between Mars and Jupiter. In doing so, the 2,680-pound (1,210 kg) spacecraft—laden with instruments from the United States, Italy, the Netherlands and Germany—became the first unmanned spacecraft to conduct extended orbital operations around two celestial bodies.
As outlined last week by AmericaSpace, Dawn underwent a tortured development process, after its initial approval as a member of NASA’s medium-cost Discovery program in late 2001. It was twice canceled and twice reinstated, before eventually rising from Earth on 27 September 2007. Built by Orbital ATK, the spacecraft picked up a gravitational boost from Mars in early 2009, which sent it deep into the asteroid belt and at 12:47 a.m. EDT on 16 July 2011 it entered orbit around Vesta, becoming the first human-made machine to do so. “Today, we celebrate an incredible exploration milestone, as a spacecraft enters orbit around an object in the main asteroid belt for the first time,” said then-NASA Administrator Charlie Bolden. “Dawn’s study of the asteroid Vesta marks a major scientific accomplishment and also points the way to the future destinations where people will travel in the coming years.”
Following its capture into orbit around the walnut-shaped Vesta—whose name honors the ancient Roman goddess of home and hearth—Dawn quickly set to work reconfiguring its orbit to allow for three weeks of “survey” operations in August 2011. A month later, it assumed a high-altitude “mapping” orbit, some 420 miles (680 km) from the Vestan surface, during which it circled its host once every 12.3 hours. Eventually, by December, Dawn had spiraled “downwards” into a lower mapping orbit, at an altitude of just 130 miles (210 km). From this vantage point, its framing camera and visible and infrared mapping spectrometers imaged Vesta at high resolution and its gamma-ray and neutron detector and gravity experiment set to work looking for the tell-tale signatures of cosmic rays radiated by the 326-mile-diameter (510 km) world, as an indicator of its surface composition characteristics.
Early data from Dawn suggested that Vesta is sufficiently dark and cold that water-ice could have survived close to its north and south poles, perhaps for billions of years. Breccia-type rocks, fused during debris impacts, were apparent in the imaging data, with iron- and magnesium-rich minerals widespread. Exposed “layering” in the vicinity of the south pole indicated that the Vestan crust was in a state of constant renewal and Dawn revealed surface temperatures ranging from as warm as -23 degrees Celsius (-10 degrees Fahrenheit) to as cold as -100 degrees Celsius (-150 degrees Fahrenheit).
The vast Rheasilvia basin in the southern hemisphere, spanning 314 miles (505 km) across—more than 95 percent of the diameter of Vesta itself—was shown to boast a central mountain peak, some 14 miles (22 km) tall, more than twice as high as Mount Everest. Whatever impact created Rheasilvia, around a billion years ago, caused large amounts of material to rain down onto the Vestan surface, meaning that the southern hemisphere is much younger than the more heavily-cratered north. The impact also produced dozens of equatorial gorges, including Divalia Fossa, which is believed to be a compression fracture and stretches across the terrain for 290 miles (465 km).
In April 2012, NASA announced that Dawn’s time at Vesta would be extended by an additional six weeks, allowing for additional observations with the gamma-ray and neutron detector. “Dawn has beamed back to us such dazzling Vestan vistas that we are happy to stay a little longer and learn more about this special world,” said Principal Investigator Christopher Russell of the University of California at Los Angeles. “While we have this one-of-a-kind opportunity to orbit Vesta, we want to make the best and most complete data-sets that we can.” From 1 May, the spacecraft began a final mapping cycle, before gradually spiraling “outwards” from the dwarf planet, with the intent of departing its gravitational clutches on 26 August.
Then the gremlins struck. In early August, excessive friction to one of Dawn’s reaction wheels—part of its pointing mechanism—caused it to be powered-off and as efforts to restore the spacecraft to full operational capability continued, its departure from Vesta was postponed and eventually took place at 2:26 a.m. EDT on 5 September. Thus began a 30-month trek to its second dwarf-planet destination, Ceres. Almost 600 miles (950 km) in diameter, Ceres is the largest known body in the Mars-Jupiter asteroid belt and is named for the ancient Roman goddess of agriculture.
Although it was forced briefly into “safe mode” in September 2014, probably due to the effect of high-energy particle radiation on its xenon-ion system, Dawn’s voyage to Ceres was uneventful. In December, the spacecraft entered an approach phase at a mean distance of 400,000 miles (640,000 km) and at 7:39 a.m. EST on 6 March 2015 slipped smoothly into orbit. Initially occupying a polar orbit, the spacecraft gradually refined its circuitous path around Ceres, dipping as low as 233 miles (375 km) during the ten-period between December 2015 and September 2016.
From the start, Ceres begrudgingly yielded its secrets. A strange white “spot” had been apparent from Dawn’s long-distance imagery, which appeared to gleam against an otherwise ubiquitous asphalt-gray surface. This extremely bright, salty material occupied Occator Crater and turned out to be a smattering of multiple “faculae”. Its presence indicated that Ceres had remained geologically active until relatively recently, when briny water may have risen to the surface and deposited salts, including a kind of magnesium sulphate, known as hexahydrite. During the course of Dawn’s time at Ceres, more than 300 small bright areas have been observed, some hosting ice at northern latitudes.
Ahuna Mons, a 13,000-foot-tall (4,000-meter) peak protruding from otherwise smooth terrain, is Ceres’ largest mountain, and the presence of top-to-bottom bright streaks along its flanks have raised suspicions of subsurface cryovolcanism. It had been seen in the weeks before Dawn’s arrival, as a “bump” protruding from the dwarf planet’s limb, but by early 2016 data indicated that it was dome-shaped, rather than pyramidal, and that its walls were exceptionally steep. During its 2.5 years in orbit around Ceres, the data from Dawn has tantalizingly indicated at the presence of large quantities of water-ice, a possibility of life-sustaining conditions in the distant past, a temporary, tenuous atmosphere and a differentiated interior. Evidence of organic material on the surface was announced in early 2016.
And in spite of another reaction-wheel malfunction last spring, Dawn continues to return valuable data. It is expected to remain operational through at least the end of 2018, although an effort to remove the spacecraft from Ceres and visit another member of the asteroid belt—perhaps Adeona—was briefly tabled last year. Ultimately, in July 2016, NASA opted to keep Dawn at Ceres. “The long-term monitoring of Ceres, particularly as it gets closer to perihelion,” explained Jim Green, NASA’s director of planetary science, “has the potential to provide more significant science discoveries than a flyby of Adeona.”