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Radiation strike slows down NASA's Dawn spacecraft

Posted: September 18, 2014

Next year's arrival of NASA's Dawn spacecraft at Ceres, an unexplored icy dwarf planet between Mars and Jupiter, has been pushed back about a month after a blitz of cosmic rays interrupted the probe's ion thrusting, officials said.

Artist's concept of the Dawn spacecraft on approach to Ceres. Credit: NASA/JPL-Caltech/UCLA/McREL
The solar-powered explorer was supposed to approach Ceres and enter orbit in March 2015. NASA says the spacecraft is now due to arrive a month later, some time in April.

Mission officials said the craft's ion thrusting suddenly stopped Sept. 11 as Dawn unexpectedly entered safe mode, a state where the probe stops non-essential activities to ensure it generates ample power for survival.

Ground teams at NASA's Jet Propulsion Laboratory restored Dawn to normal operations and resumed normal ion thrusting Sept. 15, according to a NASA press release.

But the stoppage of the spacecraft's ion propulsion system meant Dawn fell behind on its journey to Ceres, a dwarf world that failed to become a full-fledged planet after the formation of the solar system.

Ceres is the most massive body in the asteroid belt between Mars and Jupiter.

Scientists believe the growth of Ceres, which stretches about 590 miles across, was stymied by the gravitational influence of Jupiter as the building blocks of the solar system collided and coalesced to spawn planets.

Ceres is the Dawn mission's second stop after visting Vesta, the asteroid belt's second most massive object, from July 2011 to September 2012, when it started the trip to Ceres.

Launched in September 2007, Dawn will be the first spacecraft to go into orbit around two objects in the solar system, a feat enabled by three ion engines, which combine electrical power and xenon gas to generate modest levels of thrust.

Conventional thrusters fueled by chemical propellants provide a greater push, but ion thrusters are more efficient. By firing the less powerful ion thrusters for months at a time, Dawn is reshaping its trajectory through the solar system in ways chemically-powered spacecraft could not achieve.

Dawn suspended ion thrusting when it entered safe mode Sept. 11.

Although ground controllers reactivated the ion propulsion system four days later, it was switched off long enough to slow down Dawn's journey to Ceres.

Despite the delay in Dawn's arrival, its mission at Ceres will not be affected, NASA officials said in a press release.

Engineers say this month's glitch is similar to an event that struck Dawn three years ago on approach to Vesta. They blame the anomaly on a high-energy particle of radiation, which disabled an electrical component in the ion propulsion system.

"We followed the same strategy that we implemented three years ago to recover from a similar radiation strike -- to swap to one of the other ion engines and a different electronic controller so we could resume thrusting quickly," said Marc Rayman, Dawn mission director and chief engineer. "We have a plan in place to revive this disabled component later this year."

Dawn's main communications antenna was hit by a problem around the same time, affecting its ability to point toward Earth. The issue forced Dawn to use another antenna to communicate with its control team at slower speeds, complicating the recovery from safe mode, according to NASA.

"Although they have not yet specifically pinpointed the cause of this issue, it could also be explained by a high-energy particle corrupting the software running in the main computer," NASA said in a statement. "Ultimately the team reset the computer, which restored the pointing performance to normal."

Follow Stephen Clark on Twitter: @StephenClark1.