WASHINGTON — A senior review of NASA’s planetary science missions has concluded the Dawn spacecraft should remain in orbit around the dwarf planet Ceres rather than venture to another asteroid as project officials proposed.
NASA announced July 1 the outcome of a review of extended planetary science missions, including the approval of plans to send New Horizons past a distant Kuiper Belt object and the extension of seven other missions at the moon and Mars.
Dawn, which completed its primary mission June 30, had proposed an extended mission where the spacecraft would leave its current orbit around Ceres and travel to another asteroid. Project officials declined to name that asteroid, saying that they would identify it if NASA approved the extended mission.
However, in a mission update posted to the Dawn web site late June 30, only to be removed within minutes, the mission did identify that destination: the asteroid 145 Adeona, a main belt asteroid about 150 kilometers across that Dawn would fly by in May 2019. The NASA statement July 1 about the mission extension confirmed the planned target was Adeona.
Jim Green, director of NASA’s planetary science division, said in a statement that the decision to keep Dawn at Ceres was driven by the science it would provide in the opinion of the senior review panel. “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,” he said.
Carol Raymond, deputy principal investigator for Dawn, said in a June 28 presentation at a meeting of the Small Bodies Assessment Group (SBAG) at the Johns Hopkins University Applied Physics Laboratory that the project proposed the Adeona flyby because they believed visiting a different asteroid would be of more interest than remaining at Ceres.
Dawn, she noted, had completed all of its main scientific priorities at Ceres earlier this year. “I think we’ve gotten so much already that the incremental amount of knowledge that we would gain would be maybe not as great as one would have thought,” she said of an extended mission at Ceres.
The possibility of an extended mission to another asteroid was possible because of steps the project took to conserve the spacecraft’s supply of hydrazine used for stationkeeping and attitude control thrusters. The project started those efforts in 2012 while still in orbit around another asteroid, Vesta, because problems with two of its reaction control wheels threatened to use up all of its hydrazine before completing its mission at Ceres.
“The team worked very hard to execute an aggressive and persistent campaign to preserve the hydrazine,” Raymond said, crediting “exceptionally smooth” operations of the spacecraft that limited consumption of the fuel. “In 2016, we ended up not only achieving our intended mission, but with a surplus which we can now use to do something else.”
Dawn launched in 2007 and orbited the main belt asteroid Vesta from July 2011 to September 2012. It then departed for Ceres, arriving in March 2015. Dawn used an electric propulsion system to travel through the asteroid belt, and planned to use it again to fly past Adeona.
The remaining hydrazine, Raymond said, should allow Dawn to operate at Ceres until March or April 2017. That date could be moved up if the spacecraft suffers new problems with its reaction wheels that require additional use of its thrusters.
The Dawn project had been pressing for an answer from NASA on its extended mission proposal because of a tight schedule. “We’re on a very tight timeline to leave Ceres by July 12 in order not to expend more hydrazine at Ceres, because we’ll rapidly run out of the ability to leave Ceres and go any place else,” she said at the SBAG meeting.
While the senior review was a disappointment for Dawn, it was good, although not unexpected, news for New Horizons. NASA approved plans for an extended mission to send the spacecraft past the distant Kuiper Belt object 2014 MU69 on Jan. 1, 2019. The project had already performed a series of maneuvers after its July 2015 flyby of Pluto, its primary mission, to set up that flyby if NASA approved its extended mission.
“The New Horizons mission to Pluto exceeded our expectations and even today the data from the spacecraft continue to surprise,” Green said in a statement. “We’re excited to continue onward into the dark depths of the outer solar system to a science target that wasn’t even discovered when the spacecraft launched.”
The senior review also approved an extension of the Lunar Reconnaissance Orbiter and six Mars missions: Mars Reconnaissance Orbiter (MRO), the Mars Atmosphere and Volatile Evolution (MAVEN) orbiter, the Opportunity and Curiosity Mars rovers, the Mars Odyssey orbiter and NASA’s support for the European Space Agency’s Mars Express mission.
