[meteorite-list] Dawn Observing Ceres; 3rd Reaction Wheel Malfunctions

[Ron Baalke]

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 at jpl.nasa.gov

2017-125