[meteorite-list] Dawn Journal - May 25, 2009

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed, 27 May 2009 17:42:59 -0700 (PDT)
Message-ID: <200905280042.RAA05907_at_zagami.jpl.nasa.gov>

http://dawn.jpl.nasa.gov/mission/journal_5_25_09.asp

Dawn Journal
Dr. Marc Rayman
May 25, 2009

Dear Astrodawns,
 
Dawn's mission continues to go very well, as the spacecraft nears
the end of the longest coasting period of its astronomical
journey. The deep-space member of the team has completed more
special activities under the helpful guidance of the terrestrial
contingent.
 
The previous log described the installation
of software in the spacecraft's main computer. (Known as flight
software 8.0, this name was chosen as part of Dawn's new outreach
effort to increase public awareness of the number 8. Please
consider yourself aware. The outreach plan for the designation of
the subsequent version of the software is expected to be 12.5% better.)
The software had been tested extensively in simulators on Earth,
but the ultimate test is its performance in keeping the spacecraft
running smoothly. Because Dawn devotes more time in space to
thrusting with its ion propulsion system than doing anything else
(this does not count the time it spends in eager anticipation of
the exciting rewards to be garnered at its destinations Vesta and
Ceres), it was important to verify that the capability to thrust
was not compromised with the new software. Despite the assiduity
with which 8.0 was developed, the possibility of a subtle bug
being introduced could not be discounted.
 
A test well ahead of the planned resumption of thrusting early in
June would allow time to rectify problems and still keep to the
itinerary. Just as with the tests in the initial checkout phase of
the mission, the objective was to show that all subsystems would
work together (under control of the new software) to sustain stable
thrusting.
 
On April 27, as it had done so many times earlier in the mission,
the spacecraft executed a set of commands to rotate to point a
thruster in a selected direction, perform the pre-start
procedures, and undertake powered flight. With tests during the
initial checkout phase plus all the thrusting in the
interplanetary cruise phase designed to reshape the probe's orbit
around the Sun, Dawn had accumulated 123 days of operation with
ion thruster #1 and 158 days with ion thruster #3. (The locations
of the thrusters are explained in a previous log.)
Thruster #2 has not had its turn yet for long-term thrusting, so it
had been operated only for 22 hours. While much longer than the
operating time for a propulsion system on a typical planetary
spacecraft, for Dawn this is still considered brand new. Therefore,
the 8.0 system thrust test used ion thruster #2, allowing it to
accelerate xenon for the first time since November 16, 2007.
 
After thrusting for 4 hours, the spacecraft stopped and reoriented
itself, aiming the main antenna to Earth again. On May 1, it
conducted a similar exercise, differing only in the direction of
thrust. These tests provided the final confirmation that 8.0 was
ready to take the helm when the time came to resume propelling the
craft to the asteroid belt.
 
These activities had a secondary benefit. During the initial
checkout phase, some tests were conducted of how accurately
thrusters #1 and #3 could be pointed. The equivalent test with #2
could not be run then because the spacecraft was too close to the
hot Sun to point that thruster in the required direction, so it
was planned for April 2008. That operation was interrupted by an
event that led to safe mode being invoked.
Because thruster #2 was not
scheduled to be used until much later in the mission, the test was
postponed to the coast period after Mars. The 8.0 system thrust
verification test was executed in the directions needed to
complete the thruster #2 pointing tests, thus accomplishing these
additional objectives.
 
Thrusting at this time in the mission does not help reach the
targets (if it did, Dawn would not spend so much time coasting),
but the short thrust trials did not modify the trajectory
appreciably. The ion propulsion system has changed the
spacecraft's speed by about 1.87 kilometers per second (4180 miles
per hour) so far in the mission, and this latest work added only
2.3 meters per second (5.1 miles per hour) to the total.
 
Following these successful tests as well as several weeks of other
operations, engineers were sufficiently pleased with 8.0's
admirable performance in both its software and its outreach
functions that they were ready to commit to using it. In April,
the software had been installed only in the primary location in
the primary computer. On May 11, mission controllers powered on
the backup computer and installed the primary and backup copies of
8.0. On May 13, they loaded the software to the backup area of the
primary computer, so now 4 copies of 8.0 are stored onboard.
Updating the software, even in a backup location, is a delicate
operation, but all the procedures went according to plan. That
brought the work on 8.0 to a successful and satisfying conclusion.
 
For the ion propulsion system to operate with its extraordinary
efficiency, a gentle flow of xenon
gas to the thruster needs to be accurately controlled. At full
throttle, this amounts to slightly more than 3 milligrams per
second (less than 10 ounces per day), and by the end of the
mission, when limited electrical power necessitates a low throttle
level, the flow will be reduced to less than 40% of that rate. A
sophisticated labyrinth of tubes, electronically controlled
valves, and other components feeds the propellant from the main
tank to the selected thruster. Along the way, xenon is held
temporarily in a pair of small, low-pressure tanks (each known as
a "plenum" and the pair as "plena"). The main tank is pressurized
to more 1000 pounds per square inch (psi, and note we will spare
readers the metric units for pressure), but one of the plena is
charged to less than 67 psi and the other is never raised above 37
psi. Pressures will be still lower at lower throttle levels.
 
The ion propulsion system's computer controller relies on pressure
and temperature sensors on each plenum as it regulates the flow of
xenon. The controller interprets any change in the pressure as a
change in the amount of gas moving through the system, so a shift
in the behavior of a sensor would cause the controller to adjust
the rate at which xenon is delivered to the operating thruster.
 
The pressure sensors are designed to remain stable throughout the
mission, but in order to keep the ship in good shape,
ever-skeptical engineers want to determine how much (if at all)
the devices have drifted in their measurements since they were
calibrated well before launch. So here they are confronted with a
conundrum: how can they verify the sensors on the distant
spacecraft are giving accurate readings?
 
We solicit here ideas from readers for how to accomplish this
"plenum pressure transducer calibration." Please submit your ideas
to the Dawn project before the next paragraph is written, and the
best idea will be implemented. What a thrilling reward that will
be for some lucky reader! Be sure to include thorough
calculations, assessment of risks to the spacecraft and
mitigations for those risks, a complete list of commands,
predictions for subsystem telemetry, and any other pertinent details.
 
Thank you very much to all those who proffered suggestions; we
received a surprising number of innovative methods, a few of which
are even compatible with physical laws and the capabilities of the
Dawn spacecraft. We now have an effective solution. (The winning
entry came from a reader on Earth who does not want the prestige
of being named here, but we offer our most sincere congratulations!)
 
By bringing the plena to a pressure that is ascertained without
the use of the pressure sensors, engineers can compare the sensor
readings with the independently determined pressures. So they
commanded certain valves to open, thus allowing the xenon in these
tanks to vent into space. Each plenum will evacuate to nearly 0
psi, so the report from the pressure sensors will provide a direct
measurement of how much their behavior has changed, and any
discrepancy can be accounted for by straightforward updates to the
software.
 
The procedure of connecting the plena to the cosmic vacuum is
known as a blowdown, perhaps connoting a gale-force wind of xenon
as it begins rushing out of the system. The reality is somewhat
different. When the valves were opened on May 4, the force from
the zephyr of xenon leaving the spacecraft was less than 3% of the
famously light thrust achieved when the
xenon is ionized and accelerated at full throttle. The unionized
xenon pushed on the spacecraft with about one tenth of the force
you would feel holding a penny. As the plena pressures fell, the
force diminished still further.
 
At the conclusion of the 8.0 system thrust test, one plenum held
90 grams (about 3 ounces) of gas and the other held 50 grams (less
than 2 ounces). (For comparison, the main tank carried 425
kilograms, or 937 pounds, at launch and now holds 350 kilograms,
or 772 pounds.) The expenditure of this small amount (about what
is used in half a day of thrusting at maximum throttle level) is
well within the mission's overall xenon budget. The valves will be
left open for several weeks to allow plenty of time for the xenon
to find its way from the plena through the maze of hardware to
space. Well before thrusting begins on June 8, the valves will be
closed and the plena repressurized.
 
In the meantime, Dawn will pass another milestone in its solar
system adventure. As we have seen in previous logs,
the nature of orbits is that at greater
distances, objects travel more slowly. Dawn and Earth both orbit
the Sun, but the spacecraft, being more remote than the planet,
travels more slowly. On June 3, Dawn will complete its first
revolution around the Sun (a "Dawn year") since its September 27,
2007 launch. Earth, along with its residents
(constituting a small but enthusiastic fraction of our readership)
as well as the moon and all other natural and human-made
satellites in orbit around it, needs about 365 days for a
revolution, so it will have made 1.68 loops while Dawn was in its
first.
 
You may be tempted to think that Dawn takes longer to round the
Sun than Earth because it has to travel a greater distance to do
so in its larger orbit, but you are encouraged not to yield to
this simple explanation. Earth has moved more than 1.58 billion
kilometers (984 million miles) since letting go of the probe. In
the same time, Dawn has traveled only 1.32 billion kilometers (817
million miles). When farther from the Sun, objects travel more
slowly because the gravitational pull they need to resist is
weaker. As it climbs atop its ion beam
to still greater heights, Dawn will go even more slowly as it
seeks to match orbits with its more remote destinations. Since the
spacecraft departed from Earth, Vesta has completed only 0.44
revolutions, traveling 1.00 billion kilometers (622 million
miles), and even more distant Ceres has made only 0.40 loops,
covering 994 billion kilometers (617 million miles).
 
To keep our subscription fees low, we include here another subtle
product placement for one of the many clocks in the Dawn gift
shop, several of which have been featured in logs over the past 5
months. On this model, the Sun is at the center and there is a
separate hand for each of these orbiting objects. Although they
were not aligned at launch, suppose they were, with each hand
pointed toward the 12. (Because of the northern hemisphere
perspective with which astronomy was developed on Earth, our
standard view of the solar system depicts the orbits going
counterclockwise. Here we will ignore that familiar perspective,
as our focus is on how many rounds each body completes, not the
direction of their travel.) Since launch, Earth would have gone
all the way around once and continued on clockwise to just past
the 8. The spacecraft will return exactly to the 12 on June 3.
Vesta will have leisurely advanced just beyond the 5, and Ceres
will not quite have made it even to that number yet.
 
As Dawn continues to push outward from the Sun, with its sights
set on alien worlds deep in the asteroid belt, we hope readers
will continue to follow its progress. With (some of) the profits
from the first Dawn Ponzi fund, we have added RSS feeds for these
logs and other features at the Dawn website. To learn more, and to
be among the first of your species to subscribe to automatic
updates, visit: http://dawn.jpl.nasa.gov/RSS/index.asp.
 
Dawn is 305 million kilometers (189 million miles) from Earth, or
845 times as far as the moon and 2.01 times as far as the Sun.
Radio signals, traveling at the universal limit of the speed of
light, take 34 minutes to make the round trip.
 
Received on Wed 27 May 2009 08:42:59 PM PDT