[meteorite-list] Dawn Journal - June 28, 2009

[Ron Baalke]

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

Dawn Journal
Dr. Marc Rayman
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.
 
Prior to resuming thrust, the spacecraft carried out a routine
check of one its scientific instruments. All of the instruments
designed to uncloak the secrets Vesta and Ceres hold about the
dawn of the solar system spend most of the time during the
interplanetary cruise switched off, waiting for their
opportunities to go to work in orbit. Each instrument is powered
on occasionally to verify its health. On May 27, the visible and
infrared mapping spectrometer (VIR) was activated. On this occasion, 
VIR repeated the routines it first executed in space in October 2007.
All of its mechanisms were exercised,
and they operated smoothly. Instead of aiming at distant celestial
targets, its visible and infrared detectors measured emissions
from built-in lamps. VIR passed the 4-hour test with flying colors
(some of which are outside the range of human vision).
 
The VIR operation was one of many assignments for the coast
period,  most of which have been described in logs since November
2008.  With all activities completed
successfully, the spacecraft set about thrusting right on
schedule. On June 8, executing instructions already stored in its
main computer, Dawn rotated to point thruster #1 in the required 
direction. It powered on the ion beam shortly after 11:59 am PDT. 
Any readers who happened to be in the vicinity during their own 
deep-space excursions would immediately have recognized the familiar 
scene: Dawn majestically perched once again atop a blue-green pillar 
of xenon ions, as its ambitious journey of exploration continues.
 
Dawn is 291 million kilometers (181 million miles) from Earth, or
775 times as far as the moon and 1.91 times as far as the Sun.
Radio signals, traveling at the universal limit of the speed of
light, take 32 minutes to make the round trip.