[meteorite-list] Dawn Journal - November 30, 2010

[Ron Baalke]

http://dawn.jpl.nasa.gov/mission/journal_11_30_10.asp

Dawn Journal
Dr. Marc Rayman
November 30, 2010

Dear Dawnizens of the Solar System,

Dawn is maintaining its smooth and steady course through the solar
system as it gradually closes in on Vesta. With the utmost patience and
persistence, it continues thrusting with its ion propulsion system,
heading toward its July rendezvous with the second most massive member
of the main asteroid belt. Even as the spacecraft climbs farther from
the sun, Earth's orbit is beginning to bring the planet closer to the
probe.

Having thrust for two-thirds of its time in space, Dawn has now achieved
the velocity equivalent of about 5.5 kilometers per second (more than
12,000 miles per hour). We have seen before that this does not represent the
/actual/ change in speed, but it is still a very useful measure of the
effect of the thrusting. Although it has long since surpassed the record
for propulsive change in velocity, Dawn is only now at the halfway point in 
the planned profile of thrusting for its ambitious eight-year expedition. 
By the time it completes its mission at dwarf planet Ceres (the asteroid 
belt's most massive resident) in 2015, it will have accomplished twice the 
effective velocity change it has achieved so far.

At the beginning of this month, mission controllers installed new
parameters in the software used to control the spacecraft's orientation
(which engineers refer to as "attitude") in the zero-gravity conditions
of spaceflight. The attitude control system has four methods of keeping 
Dawn stable or turning to point in a new direction. When the spacecraft 
is not thrusting with the ion propulsion system, it has two techniques: 
it can rely on its reaction wheels or on its reaction control system. 
The wheels are gyroscope-like devices which, when electrically spun faster 
or slower, rotate (or stop the rotation of) the spacecraft. As we saw early 
last year, the reaction wheels alone are not sufficient, so when they are 
in operation, the reaction control jets also are used, although only 
occasionally. (The jets also are known as "thrusters," nomenclature used in 
previous logs, but to avoid confusion with the ion thrusters in this 
discussion, we will refer to them only as "jets.") The conventional rocket 
propellant hydrazine is fired through the jets to impart a small force to 
the ship, causing it to turn or to stop turning.

Not content simply to coast through the solar system, as most
interplanetary probes do, Dawn devotes the majority of the time to using
one of its ion thrusters to change its course, constantly applying a
light pressure to its orbit to bring it closer and closer to that of its
destination. In addition to accelerating the ship, an ion thruster can
be used to rotate it by slightly changing the angle of the thrust. This
provides the attitude control system two other means of control when the
spacecraft is in powered flight: it can use either the thruster plus the
wheels (again with the occasional help of the reaction control system)
or the thruster plus the more frequent use of the jets.

Over the summer, engineers powered the reaction wheels off,
preserving them for use in orbit around Vesta and Ceres. The reaction 
control system took over quite smoothly and has been keeping the craft 
stable ever since, most of the time in concert with the ion propulsion 
system.

Even before deciding to deactivate the wheels for the rest of the
interplanetary phases of the mission, engineers began working on a
technique to use the hydrazine more efficiently, ensuring that the
supply would last to the end of the long journey. When it left Earth
more than three years ago, Dawn carried 45.6 kilograms (101 pounds)
of hydrazine. It still has more than 38 kilograms (nearly 84 pounds) 
onboard, and frugal operators want to continue to use the precious 
resource sparingly. They devised a means to reduce the rate of 
propellant consumption when the reaction control
system is the primary control method. (The expenditure of hydrazine
during ion thrusting was already so low that there was no need for an
improvement in that control mode.)

The new control parameters were finalized after extensive analysis and
simulation at Orbital Sciences Corporation and JPL. Without needing to
change the software, operators radioed the values to Dawn in October and
timed them to go into effect on Nov. 1 during the normal weekly hiatus
in thrusting (and thus when attitude control is reliant exclusively on
the jets) to point the main antenna to Earth. Thanks to this successful
upgrade, the system now uses only about one-eighth as much hydrazine
during those periods that it is holding steady and not applying ion thrust.

To rotate from one attitude to another requires firing some jets to
start the huge ship (the largest NASA has ever sent on an interplanetary
voyage) turning and then others to stop it. To achieve a further savings
in the hydrazine, controllers reduced the rate at which Dawn executes
its turns. The standard speed had been a whiplash-inducing 0.1 degrees
per second; that's the same pace at which the minute hand of a clock
moves (except for some of the clocks sold in Dawn gift shops, and we're
still processing your refund requests on those). On Nov. 1, the speed
was lowered to half that, meaning less propellant is needed to initiate
a rotation and less is needed to terminate it. The only turns in a
typical week are those required to shift between the attitude required
for ion thrusting and the attitude required for pointing the main
antenna to Earth, and a little extra time spent turning is easily
affordable.

On Nov. 8, all four reaction wheels were powered on for a short time.
For wheels 1, 2, and 3, this served as routine maintenance, keeping them
in top condition so they will be ready to return to duty as the Vesta
phase of the mission begins next summer. Running wheel 4 provided
additional data on its condition so that engineers could assess its
long-term prospects.

Dawn is 0.083 AU (12 million kilometers or 7.7 million miles) from
Vesta, its next destination. It is also 3.05 AU (456 million kilometers
or 283 million miles) from Earth, or 1235 times as far as the moon and
3.09 times as far as the sun. Radio signals, traveling at the universal
limit of the speed of light, take 51 minutes to make the round trip.