https://www.jpl.nasa.gov/news/news.php?feature=6830

Dawn Observing Ceres; 3rd Reaction Wheel Malfunctions
Jet Propulsion Laboratory
Arpil 26, 2017

Mission Status Report

NASA's Dawn spacecraft is preparing to observe Ceres on April 29 from
an "opposition" position, directly between the dwarf planet's mysterious Occator Crater and the sun. This unique geometry may yield new insights
about the bright material in the center of the crater.

While preparing for this observation, one of Dawn's two remaining reaction wheels stopped functioning on April 23. By electrically changing the speed
at which these gyroscope-like devices spin, Dawn controls its orientation
in the zero-gravity, frictionless conditions of space.

The team discovered the situation during a scheduled communications session
on April 24, diagnosed the problem, and returned the spacecraft to its
standard flight configuration, still with hydrazine control, on April
25. The failure occurred after Dawn completed its five-hour segment of
ion thrusting on April 22 to adjust its orbit, but before the shorter
maneuver scheduled for April 23-24. The orbit will still allow Dawn to
perform its opposition measurements. The reaction wheel's malfunctioning
will not significantly impact the rest of the extended mission at Ceres.

Dawn completed its prime mission in June 2016, and is now in an extended mission. It has been studying Ceres for more than two years, and before
that, the spacecraft orbited giant asteroid Vesta, sending back valuable
data and images. Dawn launched in 2007.

The Dawn operations team has been well prepared to deal with the loss
of the reaction wheel. The spacecraft is outfitted with four reaction
wheels. It experienced failures of one of the wheels in 2010, a year before
it entered orbit around Vesta, and another in 2012, as it was completing
its exploration of that fascinating world. (See issues with these devices). When a third reaction wheel stopped working this week, the spacecraft
correctly responded by entering one of its safe modes and assigning control
of its orientation to its hydrazine thrusters.

Today, Dawn's elliptical orbit will bring it from an altitude of 17,300
miles (27,900 kilometers) to 15,800 miles (25,400 kilometers) above Ceres.

The Dawn mission is managed by NASA's Jet Propulsion Laboratory in Pasadena, California, for NASA's Science Mission Directorate in Washington. Dawn
is a project of the directorate's Discovery Program, managed by NASA's
Marshall Space Flight Center in Huntsville, Alabama. UCLA is responsible
for overall Dawn mission science. Orbital ATK Inc., in Dulles, Virginia, designed and built the spacecraft. The German Aerospace Center, Max Planck Institute for Solar System Research, Italian Space Agency and Italian
National Astrophysical Institute are international partners on the mission team. For a complete list of mission participants, visit: https://dawn.jpl.nasa.gov/mission

More information about Dawn is available at the following sites:

https://www.nasa.gov/dawn

https://dawn.jpl.nasa.gov

News Media Contact
Elizabeth Landau
Jet Propulsion Laboratory, Pasadena, Calif.
818-354-6425
elizabeth.landau@jpl.nasa.gov

2017-125

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