Dawn Journal - November 13, 2007
- From: baalke@xxxxxxxxxxxxx
- Date: Thu, 15 Nov 2007 10:26:11 -0800 (PST)
http://dawn.jpl.nasa.gov/mission/journal_11_13_07.asp
Dawn Journal
Dr. Marc Rayman
November 13, 2007
Dear Dawnocentrics,
Now more than halfway through its 80-day initial checkout phase, the
Dawn spacecraft continues to please its fans in mission control and
throughout much of the rest of the universe. The project team has
maintained the intensive pace described in the last log, and the sole
team member in deep space has performed extremely well.
Dawn excelled in what may be the most important test of this part of
the
mission, an essential step in preparing for the rest of its voyage.
The
probe will spend most of 2008 - 2011 patiently using its ion
propulsion
system to change its orbit around the Sun to
match Vesta's solar orbit. After more than half a year orbiting the
enormous asteroid, the explorer will devote the majority of the
subsequent 3 years to ion-propelled travel to dwarf planet Ceres.
While
its interplanetary journey will include many other activities, gentle
thrusting will be the most common. (The most interesting of these
activities will be covered in future logs, so to advertise your
product
in one of those logs, contact our representative on your planet now!)
Dawn must be able to sustain thrusting week after week, month after
month, year after year, with only occasional interruptions, ranging
from a few hours to a few months at a time. In a typical week of its
interplanetary flight, Dawn will thrust for about 6 days 16 hours.
(The exact duration will vary from that by as much as
about a day. It will depend upon a number of details, including the
schedule for Dawn's use of the Deep Space Network, NASA's amazing
system
for communicating with
spacecraft throughout the solar system.) It will stop thrusting long
enough to rotate so that its main antenna points to distant Earth, to
transmit engineering information stored since the previous
communications session (to allow engineers to assess its health and
performance), to receive any new commands, and to rotate back to point
the ion thruster in the required direction. Then it will settle in for
nearly another week of thrusting.
While the craft was designed to be able to accomplish just such a
routine, one of the principal objectives of the 80-day initial
checkout
is to verify its readiness. On November 2, mission controllers radioed
all the instructions Dawn would need for a typical week of
interplanetary operations. On November 5, right on schedule at 4:00 pm
PST, Dawn began following the steps to start ion thruster #3. It
turned
to point that unit in the direction engineers had specified, and, with
all systems configured, began thrusting at 5:07 pm. Throughout the
subsequent week, as it emitted a high velocity beam of xenon ions, it
recorded information on the operation of its systems and executed
other
programmed maintenance activities. Following the plan perfectly, it
stopped thrusting on November 12 at 1:53 pm PST.
In the interest of full disclosure, 4 additional points should be
made:
1) During most of the time in the mission that Dawn will thrust, Earth
will not have contact with it, but this test included frequent
contact.
Dawn transmitted information on its health, so if a problem developed,
it could be diagnosed promptly. Still, the craft was instructed to
store
all the relevant data for transmission during one 6-hour period (on
November 12), just as it will have to for most of the mission, so
engineers could verify that all the data buffer sizes and recording
rates were adequate. This also shows the operators what their view of
spacecraft telemetry will be when contact really is only once each
week.
2) There are only 2 points that should be disclosed, not 4. 3) The
previous point is not correct.
After terminating ion thrust, Dawn turned to a new orientation and
transmitted the information it had been storing since November 5. The
mission operations team will not check out the main antenna until
later
this month, so this test used 1 of the 3 smaller antennas, each of
which
still provides an adequate signal this early in the mission (as the
probe has not receded too far from Earth).
Following a 6-hour communications session, the spacecraft repeated the
steps of a week before: it turned to the direction needed for
thrusting
as it initiated the steps required to start the ion thruster. Another
4
hours of thrusting was adequate to demonstrate that it could execute
the
repetitive pattern.
This test was not designed to verify the performance of any one
subsystem; rather, it was intended to show that all subsystems could
work together as one integrated system. All engineering subsystems
onboard played a role: command and data handling, electrical power,
attitude control, reaction control, ion propulsion, thermal control,
and
telecommunications. Overviews of these subsystems were presented in
the
September 17, 2006 log. (Upon reviewing that
material, and in preparing for the midterm exam, it may be worth
keeping
in mind that any system may be viewed as a subsystem at the next level
up. So the use of "subsystem" or "system" is often a matter of context
or personal preference.)
More than spacecraft subsystems were involved in this test. The full
set
of commands required to guide Dawn through a week would be too time
consuming for the small mission control team at JPL to formulate and
check without a broad suite of sophisticated software tools. Those
tools
-- another set of subsystems -- and the procedures for using them,
also
were part of this test.
While this test was an important success, much work remains in the
80-day checkout phase. Were it not for the need to characterize and
test
Dawn's systems, thrusting would not be helpful during this part of the
mission, as described on September 12.
(That's only 2 days after the wheel was invented on a planet in NGC
2099, a star cluster in Auriga. Although they won't see this until
after
they have made a little more technological progress, we offer our
congratulations now to those innovators, and we look forward to their
joining the legions of future readers fascinated by the history of
Dawn's adventure.) The thrusting prior to mid December does not help
propel Dawn to its destinations, but it does help prepare for the long
periods in the mission in which such thrusting is required.
In addition to the tests described in recent logs, the Dawn team has
been conducting many other activities, including testing reaction
wheel
desaturation during gyroless thrust vector control, changing hydrazine
catalyst bed thermal control set points, setting the safe mode battery
trickle charge rate in RAM to 175 milliamps, flying Ben's
radio-controlled blimp around Dawn mission control (boys will be
boys - and as engineers, we had fun figuring out how to improve it!),
and
loading Chebyshev thrust pointing files. While most such activities
don't lend themselves to more complete descriptions, they are part of
maintaining steady flight.
Dawn is 13.0 million kilometers (8.1 million miles) from Earth or 34
times farther than the moon. Radio signals, traveling at the universal
limit of the speed of light, take 87 seconds to make the round trip.
.
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