NASA’s Dawn spacecraft is in the process of spiralling down into its final, lowest-ever orbit around the dwarf planet Ceres, aiming for an elliptical trajectory that will carry it to within 50 kilometres (30 miles) of the surface at its low point – 10 times lower than its ever been before.
Starting in early June, Dawn will begin collecting gamma ray and neutron spectra to better understand the composition of the topmost rock and soil layers while taking high-resolution photographs of the cratered terrain below.
“The team is eagerly awaiting the detailed composition and high-resolution imaging from the new, up-close examination,” said Carol Raymond, Dawn principal investigator at NASA’s Jet Propulsion Laboratory. “These new high-resolution data allow us to test theories formulated from the previous data sets and discover new features of this fascinating dwarf planet.”
Launched in 2007, the Dawn entered orbit around Ceres in March 2015 after wrapping up similar studies at the dwarf planet Vesta. The spacecraft had not fired its ion thrusters since last June, orbiting Ceres once every 30 days, but it’s now using engine No. 2 to slowly lower its orbit.
The descent began on 16 April. When the manoeuvre is complete, “Dawn will swoop down to an incredibly low 22 miles (35 kilometres) above the exotic terrain of ice, rock and salt,” Marc Rayman, the mission director, writes in his blog. “The last time it was that close to a solar system body was when it rode a rocket from Cape Canaveral over the Atlantic Ocean more than a decade ago.”
The final orbit will have a high point of 4,000 kilometres (2,500 miles) and a period of 27 hours and 13 minutes.
The goal is to synchronise Dawn’s orbit with the nine-hour four-minute rotation of Ceres to ensure the spacecraft will repeatedly fly over a specific point on the surface – Occator Crater, where highly reflective salt deposits are visible – during the low point of each orbit.
“The flight team will synchronise the orbit so that each time Dawn swoops down to low altitude, it does so at just the right time so that Ceres’ rotation will place the Occator geological unit under the probe’s flight path,” Rayman writes.
But it will not be easy, and it will not last.
The spinning reaction wheels that once helped Dawn control its orientation no longer work, forcing the spacecraft to rely on small thrusters instead. Those thrusters have a small but noticeable impact on the trajectory, as do areas of Ceres that have slightly higher or lower densities, resulting in slight changes in the total gravitational pull experienced by the spacecraft.
JPL flight planners studied more than 45,000 possible trajectories before choosing the one now being implemented.
“The low point of Dawn’s orbit will gradually shift southward on each successive revolution,” Rayman writes. “That means we will have only a limited number of opportunities to fly over Occator before the low point is too far south. Given the complexity of the operations, the planned measurements are not at all assured.”
Even if Dawn achieves and maintains the desired orbit, taking sharp, focused pictures will pose yet another challenge. The spacecraft will be racing across Ceres, from south to north, at 1,690 kph (1,050 mph) at the low point of the ellipse. Occator crater is 92 kilometres across (57 miles) and from Dawn’s perspective it will be moving to the right at more than 310 kph (190 mph).
Dawn’s camera will image an area 3.4 kilometres (2.1 miles) across.
“Even if the probe arrived at Occator’s latitude a mere 20 seconds off schedule, a spot on the ground that was expected to be in the center of the camera would have moved entirely out of view and so would not even be glimpsed,” Rayman explained. “If Dawn were four minutes too early or too late, the ground beneath the spacecraft would shift west or east by 13 miles (21 kilometres), and the terrain that’s photographed could be entirely different from what was expected.”