[FPSPACE] Dawn Journal for June 28, 2009

[LARRY KLAES]

June 28, 2009
 
Dear Dawnterested Readers,
 
Having completed the longest planned coasting period of its entire mission, Dawn is now back to its familiar routine. On June 8, the ion propulsion system was called back into action to propel the probe to its rendezvous with asteroid Vesta.
 
The spacecraft began its 7-month coast period on October 31, 2008. Since then, it had used its ion thrusters for a measurement of the solar array power, a small adjustment in its course to Mars (the gravitational effect of which provided a boost to its distant destination), and tests of the software remotely installed on the main computer in April. The accumulated thrusting during all of those activities added only about 10 hours to the mission’s log of 282 days when coasting commenced.
 
Now that the ship has resumed its powered flight, the spacecraft devotes most of its time to thrusting. With the utmost patience, like an artist perfecting each delicate detail in a grand masterpiece, Dawn gradually reshapes its orbit around the Sun. A full day of thrusting is enough only to change its speed by a modest 7 meters/second (less than 16 miles/hour). 
 
Dawn thrusts all but about 6 to 8 hours per week, providing only a brief opportunity to turn away from the direction it needs to aim its ion thruster in order to point its main antenna at Earth. That weekly radio communications session affords the robotic explorer its sole contact with mission controllers. While it is thrusting, Dawn is programmed to broadcast signals from one of its small, auxiliary antennas, spreading its radio signal in a wide swath that encompasses distant Earth. Usually one of the exquisitely sensitive receivers of NASA’s Deep Space Network will listen in on the spacecraft for a few hours halfway through the week, capturing the extraordinarily faint transmission showing the spacecraft is sailing smoothly.
 
Spending so much time thrusting is possible thanks to the extremely frugal use of xenon propellant. The ion thruster expels only about 0.26 kilograms (10 ounces) in a day. So while Dawn would need nearly 4 days to accelerate from 0 to 60 miles/hour, it would consume little more than 1 kilogram (about 2.3 pounds) of its supply of xenon during that time.
 
As the probe climbs away from the Sun to reach the cold depths of the asteroid belt, the multiyear thrust profile is designed to make its solar orbit match that of Vesta. The current flight plan has it arriving at the massive protoplanet in September 2011, requiring it to thrust for more than 700 days along the way, the significant majority of the time. 

Full journal entry here:

 

http://dawn.jpl.nasa.gov/mission/journal_6_28_09.asp

 

 

 

 
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