[meteorite-list] Engineers Assess Dawn's Reaction Wheel

[Ron Baalke]

http://www.jpl.nasa.gov/news/news.cfm?release=2010-218  

Engineers Assess Dawn's Reaction Wheel
Jet Propulsion Laboratory
June 29, 2010

Dawn Mission Status Update

Engineers are studying the reaction wheels on NASA's Dawn spacecraft
after automatic sensors detected excess friction building up in one of
them and powered it off early on the morning of June 17, 2010. Reaction
wheels spin to help a spacecraft maintain attitude control, and Dawn,
which is exploring the asteroid belt, uses three wheels in normal
operations.

The three other reaction wheels are functioning normally. Mission
managers said plans for Dawn to visit the asteroid Vesta in 2011 and
2012 and dwarf planet Ceres in 2015 will not be not affected.

"We're looking at the data carefully in order to understand what the
long-term prospects are for this reaction wheel," said Marc Rayman,
Dawn's chief engineer, based at NASA's Jet Propulsion Laboratory in
Pasadena, Calif. "In the meantime, we're continuing our planned
activities with the spacecraft." The spacecraft resumed ion thrusting on
schedule, on June 24.

Flight controllers had uploaded software to Dawn's primary flight
computer on June 15. As a planned part of that activity, all four
reaction wheels had been powered on. Two days later, while the
spacecraft was not performing any science or engineering activities, the
one reaction wheel built up excess friction. The spacecraft's fault
protection system acted as designed and turned the wheel off.

Engineers are analyzing what caused the friction buildup on the reaction
wheel. But they do not believe the new software, the reactivation of the
spacecraft, or a recent velocity change triggered the reaction wheel issue.

The Dawn mission to Vesta and Ceres is managed by JPL, a division of the
California Institute of Technology in Pasadena, for NASA's Science
Mission Directorate, Washington. The University of California, Los
Angeles, is responsible for overall Dawn mission science. Other
scientific partners include Planetary Science Institute, Tucson, Ariz.;
Max Planck Institute for Solar System Research, Katlenburg-Lindau,
Germany; DLR Institute for Planetary Research, Berlin, Germany; Italian
National Institute for Astrophysics, Rome; and the Italian Space Agency,
Rome. Orbital Sciences Corporation of Dulles, Va., designed and built
the Dawn spacecraft.

To read more about the software update and reaction wheels, see Rayman's
Dawn Journal entry for June:
http://www.dawn-mission.org/mission/journal_06_27_10.asp .

To learn more about Dawn and its mission to the asteroid belt, visit:
http://www.nasa.gov/dawn .

Jia-Rui C. Cook
Jet Propulsion Laboratory, Pasadena, Calif.
818-354-0850
jia-rui.c.cook at jpl.nasa.gov

2010-218