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Dawn's Vesta Departure Delayed

The Dawn spacecraft, which is currently orbiting the large asteroid Vesta, has had issues with its reaction wheel crop up. This will cause its departure from the tiny world to be delayed. Image Credit: NASA / JPL-Caltech/UCLA/McREL

NASA’s Dawn spacecraft will be staying at the large asteroid Vesta a little longer than originally planned. Part of the spacecraft’s guidance system, a reaction wheel, which allows Dawn to point itself precisely was powered off by software onboard Dawn back on Aug. 8. Dawn has four of these reaction wheels.

Dawn’s controllers switched the spacecraft back to its normal flight mode Tuesday Aug. 14. Dawn is set to resume utilizing its ion engine on Friday, Aug. 17. It will break orbit from Vesta and head toward its next destination, the dwarf planet Ceres, on Sept. 5. As it stands right now, Dawn’s reaction wheels are turned off, leaving the orbiting spacecraft to rely on its attitude control thrusters to “point” the spacecraft in the correct direction. Despite this issue, NASA is staying focused on the flight so far as well as the mission ahead.

Dawn was launched to conduct a scientific investigation of two of the largest bodies located in the main asteroid belt. Image Credit: NASA/JPL-Caltech/UCLA/McREL

“The Vesta mission has been spectacularly successful, and we are looking forward to the exciting Ceres mission ahead of us,” said Robert Mase, Dawn project manager, at NASA’s Jet Propulsion Laboratory located in Pasadena, Calif.

Typically Dawn only uses three of the reaction wheels. These spinning devices control Dawn’s orientation within the zero-gravity environment of space. By electrically controlling the speed at which these wheels spin Dawn can move however is needed. This is not the first time that an issue has arisen with one of these wheels. Then, as now, the software powered off the affected wheel.

Now that Dawn has concluded its scientific mission at Vesta, it will travel to the dwarf planet Ceres. Image Credit: NASA/JPL-Caltech/UCLA/McREL

In June of 2010 friction was detected in reaction wheel no. 4. Dawn detected the problem in that wheel and powered it down. The mission moved forward, relying on the three remaining wheels. 

Controllers will occasionally exercise Dawn’s reaction wheel throughout the cruise phase to Ceres. The fact that the spacecraft will take two and a half years to reach its next destination will provide the Dawn team plenty of time to figure out what happened to the affected wheel.

Dawn is slated to arrive at Ceres in early 2015 – making it the first spacecraft ever to fly to one world, break orbit and then head to another destination. This is possible because Dawn uses an ion engine which provides slowly accumulating (but long term) propulsion. Traditional chemical-based rockets generally provide rapid acceleration over a short time frame – making the ion engine of Dawn somewhat of a revolution.

Dawn uses an ion engine, which is very different than conventional rocket engines in that it provides a steady, near-constant thrust. Photo Credit: Credit: NASA/JPL

Dawn was launched to study the geology and geochemistry of both Vesta and Ceres. These are the two largest objects located in the main asteroid belt.

NASA learned about the friction on the reaction wheel during a planned communications pass on Aug. 9. Same as what took place in 2010, the data received from Dawn suggested that the wheel developed excessive friction – and was powered down. The Dawn team has proven in the past that they can control the spacecraft without the use of the reaction wheels and this should not be an issue during the cruise phase.

Dawn has been orbiting Vesta since July of last year and wrapped up the main science mission on July 25, 2012. The spacecraft has been using its ion propulsion system to slowly pull it away from the large asteroid.

Besides the reaction wheel issue, Dawn is otherwise healthy.

For more info about Dawn: http://www.nasa.gov/dawn and http://dawn.jpl.nasa.gov

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