A third act will be denied to the Dawn spacecraft, which has explored Vesta and Ceres, the two largest bodies in the asteroid belt. On 1 July, NASA headquarters said that the spacecraft should remain in orbit around Ceres, rather than using its ion propulsion engine to visit 145 Adeona, a 150-kilometer-wide asteroid, in 2019, as its mission leaders had wanted.
Learning of the decision, the Dawn team was surprised and disappointed. “We thought that everyone we had talked to about this plan was enthusiastic about it,” says Chris Russell, the mission principal investigator at the University of California, Los Angeles. “I had no negative vibes until this particular moment.”
The decision was based on the recommendations of a senior review panel, which evaluated all the agency’s ongoing planetary missions, NASA planetary division director Jim Green, who is based in Washington, D.C., said in a statement. “The long-term monitoring of Ceres, particularly as it gets closer to perihelion—the part of its orbit with the shortest distance to the sun—has the potential to provide more significant science discoveries than a flyby of Adeona,” Green said.
Instead, NASA will support a mission extension for New Horizons, the spacecraft that flew past Pluto in July 2015. The New Horizons team now has permission to pursue 2014 MU69, an icy Kuiper belt object about 30 kilometers across, for a rendezvous on 1 January 2019. Principal investigator Alan Stern, of the Southwest Research Institute in Boulder, Colorado, said earlier this year that that the spacecraft will zoom past four times closer than it did at Pluto.
Dawn had already accomplished more than many had thought possible, with two of its four spinning reaction wheels having failed. It had been using hydrazine fuel to stabilize itself, but those supplies were running low. The team needed to leave Ceres’s orbit this month for a chance to visit Adeona, and Russell says it’s a mistake to think that the science of staying outweighs the chance to visit a new asteroid. To have enough fuel to survive for more than a few months, the spacecraft will have to move up to a higher orbit—a vantage from which Ceres has already been mapped. And Russell says there are few signs of scientifically interesting changes. “Thus far we have no evidence that there are temporal changes in our data,” he says. “It’d have to be a fairly long time to identify changes, and we can’t operate for many months more.”
NASA did not release the report from the senior review, which is conducted every 2 years. Other missions that would receive continued support were: the Mars Reconnaissance Orbiter, Mars Atmosphere and Volatile Evolution, the Opportunity and Curiosity Mars rovers, the Mars Odyssey orbiter, and the Lunar Reconnaissance Orbiter.