A solar-powered spacecraft with arrays 65 ft. across is returning images of the largest object in the main asteroid belt with better resolution than the Hubble Space Telescope, and will soon be orbiting the dwarf planet Ceres close enough to generate topographical maps of its surface features.

Engineers at the Jet Propulsion Laboratory controlling the Dawn spacecraft began receiving “better-than-Hubble” imagery on Jan. 26,, as they use its German-built Framing Camera to help navigate the redesigned trajectory that will take the probe into orbit around its second asteroid-belt object on March 6. The view is only going to get better.

“By the end of February we’ll have around eight times the resolution of Hubble,” says Marc Rayman, the Dawn mission’s director and chief engineer. “We’re going to get in close and get a great view, and ultimately by the end of the mission, by the end of this year, we’ll have more than 800 times the resolution of Hubble because we’ll be down at an altitude comparable to the altitude of the International Space Station.”

Launched Sept. 27, 2007, on a Delta II from Cape Canaveral, Dawn already has explored the large main-belt object Vesta, giving unprecedented detail on the makeup of that protoplanet and demonstrating the utility of xenon-ion solar-electric propulsion in deep-space exploration (AW&ST Oct. 15, 2012, p. 24; May 21, 2012, p. 16).

Getting into position for a reprise at Ceres—a very different object that has planetary scientists practically salivating for a close-up look—has required a couple of major changes in the way the Dawn spacecraft is flown. Designed to operate with three reaction wheels for attitude control, with a fourth added as a spare, unit failures along the way have taken the spacecraft down to two. And a radiation hit on an avionics box last September required a last-minute redesign of the trajectory that Dawn will follow as it goes into polar orbit around Ceres.

The random cosmic-ray event shut down the spacecraft’s ion thruster for 95 hr. as it powered toward its rendezvous with Ceres. That threw off the navigation, forcing the experts at JPL to recalculate an approach that would work. “We redesigned the trajectory—I would say it’s significantly different at Ceres—in order to get into the same final orbit,” Rayman says.

Instead of arriving south of Ceres and letting the dwarf planet’s gravity essentially pull it around into the initial “RC3” scientific orbit 8,400 mi. above the surface, Dawn now will fly past Ceres to the north for its gravity capture and use its ion thruster to work with Ceres’s gravity pull to fly back into the RC3 orbit.

“Rather than simply ballistically flying down to the lower-altitude orbit, we actually propel Dawn down to the lower-altitude orbit, so we get down there more quickly than we otherwise would,” says Rayman. “The reason we do that is the ion propulsion system is so fantastically capable that it’s easy to maneuver, so we take advantage of that maneuverability to shorten the time between capture into orbit and our first targeted orbital altitude.”

At the end of the maneuver, the spacecraft will swing around to use the ion thruster as a brake for trimming the orbit. Rayman says the xenon-propellant penalty for the changed approach was “negligible”—less than a kilogram of the 937 lb. of xenon the spacecraft carried at launch. After the spacecraft reaches the RC3 orbit on April 23, the hydrazine it uses for attitude control will be the more serious propellant management concern because of the reaction-wheel failures.

“We use the ion thruster for attitude control when we’re flying from one orbit to another, and we used it in the interplanetary cruise when we were flying from Vesta to Ceres,” Rayman says. “But when we’re in the science orbits it will be done exclusively with hydrazine. When the second reaction wheel failed in 2012, we essentially re-architected the Ceres plan to make it much more hydrazine efficient.”

Instead of using the hypergolic propellant to reorient the spacecraft for the dark half of each orbit so its high-gain antenna can download imagery and other data from the sunlit half, at Ceres the spacecraft will store data from several orbits in its recorders before pivoting to point the antenna to Earth. That should be sufficient to carry the mission to its nominal end on June 30, 2016, with enough hydrazine remaining to continue operating for about another month.

During that time the spacecraft will image the surface as many as six times from the same altitude, with the Framing Camera pointing at a slightly different angle each time to give a 3-D map comparable to the topographic maps used on Earth. Its four spectrometers—visible, infrared, gamma-ray and neutron—will analyze the composition of the surface, including the mysterious white spot visible in telescope images and now in Dawn navigation images collected at better-than-Hubble resolutions (see photo). And although the instruments were not designed for the task, there is a chance they may be able to collect some data on the water vapor that the European Space Agency’s Herschel Space Observatory has detected above the surface.

“This is an uncharted alien world, and we’re going to explore it in great detail,” says Rayman. 

A version of this article appears in the February 2-15, 2015 issue of Aviation Week & Space Technology.