[meteorite-list] Dawn Journal - July 27, 2008

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed, 30 Jul 2008 10:24:25 -0700 (PDT)
Message-ID: <200807301724.KAA04637_at_zagami.jpl.nasa.gov>

http://dawn.jpl.nasa.gov/mission/journal_7_27_08.asp

Dawn Journal
Dr. Marc Rayman
July 27, 2008

Dear Dawnminants,

Dawn continues its flight through the solar system with all systems
functioning well. It is vitally important that the spacecraft is
reliably staying on course and on schedule, gently and steadily
thrusting with the bluish glow of its ion propulsion system; yet that
doesn't lend itself to the sorts of spine-tingling, heart-pounding,
hair-raising, planet-shattering logs for which Dawn is famous (at least
among immigrants from brown dwarf systems reading these reports in the
vicinities of active galactic nuclei). So let's turn out attention to
consider a particular aspect of flying a mission with ion propulsion.

We crave power!!

Perhaps that requires a bit more detailed consideration...

Engineers are developing a method to determine how much power the solar
arrays can produce. It might seem odd
that with the spacecraft having been in interplanetary flight for 10
months, engineers don't already know the answer. (Other facts might seem
odd as well, such as the phrase "nihil ad rem" being in this sentence.
This log will address only one oddity however.)

When the spacecraft was at Earth's distance from the Sun, shortly after
launch, the solar arrays would have been able to supply more than 10
kilowatts, enough to operate about 10 average homes in the US (and
nearly as much as your correspondent's cat Regulus generates when Mr.
Vacuum Cleaner emerges from his closet). Dawn cannot use that much
electrical power, but as it pushes deeper into space, the weaker
illumination by the Sun will yield less power. The craft's two solar
array wings, each about 2.3 by 8.3 meters (more than 7 by 27 feet), were
designed to be large enough to meet the needs of the power-hungry ion
propulsion system plus all other spacecraft systems even in orbit about
dwarf planet Ceres. To thrust at nearly twice Mars' average distance
from the Sun, Dawn carries the most powerful solar arrays ever used on
an interplanetary mission.

The only way to measure the power of the arrays is for the spacecraft
actually to pull the power from them, and its ability to do that is
limited. When thrusting at full throttle and using all systems normally,
Dawn consumes 3.2 kilowatts. Even now, traveling farther from the Sun
than Mars ever ventures, the solar arrays
can provide about 4 kilowatts. If the spacecraft activated all of its
nonessential components, it still could not draw this much power. That
leaves engineers without an accurate determination of the full potential
of the arrays.

Of course, engineers thoroughly tested the electrical power system
before launch, including each of the 11,480 solar cells and all other
components, and from that they constructed a mathematical simulation of
the arrays. But laboratory measurements do not perfectly reproduce
conditions in space, so the computational model has some uncertainty.
In-flight measurements are needed to improve their simulation of how
much power the solar arrays can furnish at different distances from the Sun.

Who cares how much power is available? Well, first and foremost, our
readers do! After all, you've gotten this far (and even farther right
now) in this log, so you must have some reason for spending otherwise
good time reading about the solar arrays. The Dawn project appreciates
your interest, and we want to provide the information you apparently
seek, even though we have no idea why you suddenly are eager to
understand the solar array performance.

As it turns out though, there is another reason for establishing the
true capability of the solar arrays. As explained in many (but fewer
than 10,001) previous logs, Dawn's unique mission is possible only
through the persistent use of its ion propulsion system. Rather than
thrusting for minutes, as most spacecraft do, Dawn will thrust for years
As power diminishes in the dim depths of
space, Dawn must throttle its ion thruster to lower power (and lower
thrust) levels.

Because the throttle level depends on how much power is available, to
formulate the details of the craft's trajectory and other plans for the
mission, engineers require knowledge of how much power the arrays will
provide at any distance from the Sun. After all, it is misleading to
think of ion thrusting as an ion propulsion subsystem function; rather,
it is a spacecraft system function, requiring most subsystems to operate
together. Apart from the inevitable (and quite unpredictable) glitches
and anomalies on the spacecraft and appearances of cake in mission
control, and contrary to many people's preconceived notions, since well
before launch the greatest technical uncertainty in the planning of
Dawn's flight has been what the solar array power will be. So far,
mission engineers have incorporated a reasonable, but conservative,
estimate into the solar array simulation, but to refine the plans, they
need to verify or correct the numbers.

Although the arrays produce more power than can be measured now, they
would produce less power if they were not pointed directly at the Sun.
That could reduce their output low enough to allow the spacecraft to
draw as much power as the arrays could generate in that orientation,
providing the calibration measurement that is needed. (Engineers would
extrapolate to reveal how powerful the arrays would be when Sun-pointed
at different distances.) As is usually the case in controlling
interplanetary spacecraft, the details make such a test much less simple
than it might appear at first blush.

With the normal switching of heaters on and off throughout the
spacecraft, the total power consumption fluctuates, and that could add
"noise" to the data, making the results harder to interpret and less
accurate. If the spacecraft tried to draw more power than the solar
arrays could produce, the battery would temporarily make up the
difference but, depending upon the circumstances, protective software
onboard would intervene to turn some systems off and place the
spacecraft in safe mode. While that would
not threaten the health of the spacecraft, it would threaten the solar
array calibration. (By the way, the battery can store only enough energy
to operate the spacecraft for about an hour. The solar arrays keep it
charged for its occasional use.)

The solar array calibration working group (a runner up in the highly
competitive Least Cool Dawn Team Name Contest) devised a method to
calibrate the solar arrays that accounted for all these and many other
considerations, including the solar panel thermal equilibration time and
the dependence on temperature of the power vs. voltage curve, high
voltage down converter phase margin, the solar array voltage set point,
power processor unit undervoltage trips, the voltage-temperature control
loop for the battery on the low voltage bus, and spacecraft safety even
in the event of an unrelated anomaly during the test.

While conceptually simple (rotate the solar arrays by a certain angle
and measure how much power the spacecraft can draw), the calibration
proved complex enough that a somewhat simplified test was deemed
appropriate. The objective was to verify how the spacecraft would
operate in the test conditions before committing to the full
calibration. The plan was to execute the test on July 21, and if
everything went perfectly, the final version would be attempted the next
day. Last year, when the planning for this began, it was decided to
schedule a backup opportunity late in 2008 in case the first time did
not yield the desired data. (In addition, the calibration will be
repeated occasionally over the course of the mission to monitor changes
in the solar array characteristics, ensuring the power predictions
remain accurate.)

Because electrical power is essential to the operation of all
subsystems, a test of this nature calls for all subsystem personnel to
scrutinize spacecraft telemetry for symptoms of unpredicted and
infelicitous behavior. All commands were contained in a single file
transmitted to the spacecraft, and immediate intervention would not be
physically possible, as radio signals revealing the condition of the
spacecraft would take nearly 18 minutes to reach Earth, and commands
sent in response would require the same time to travel back to the
spacecraft. Nevertheless, the team needed to be prepared to take action
in the very unlikely case a problem developed, so two key measures were
put in place: all stations in mission control were at the ready, and
pizza was provided to help fill the gaps in this early-evening test
while radio signals raced across the solar system.


The result: overall the test went well, although there was unexpected
spacecraft behavior and unexpected toppings on the pizza. For the
former, no response was required by the flight team, as the spacecraft
executed all the commands correctly and returned to its normal
configuration at the end. The test yielded only a partial set of
calibration data however, apparently because some of the
reconfigurations of the electrical power system and the ion propulsion
system for the purposes of the test led to a few responses that were not
anticipated. The spacecraft transmitted a large volume of supporting
data, which will take longer to digest than the pizza, and when the
satiated engineers have finished, they will determine what modifications
to make for a new test. A future log will describe the next test and any
corresponding changes in the food delivered to mission control.

Turning their attention on July 22 to a different topic, the team
modified software in one of the many computers onboard. In January,
with neither permission nor warning, a
subatomic particle traveling through the solar system hit a sensitive
electronic component on the spacecraft, triggering a quick sequence of
events that culminated in the spacecraft entering its safe mode. Since
then, programmers have developed a way to prevent space radiation that
reaches that particular circuit from having the same effect. With the
updated software, now the only consequence would be a notice to
controllers that the device was hit, and the spacecraft would not need
to enter safe mode or interrupt its activities.

The solar array test and the software change were conducted during a
planned 2-day pause in thrusting. On schedule on July 23, Dawn resumed
propelling itself with xenon ions. Once again the special lights
adorning a wall in mission control were turned on, emitting a blue glow
to remind everyone who visits or works there of the probe's patient
pursuit of intriguing and unexplored worlds in the asteroid belt.

As Dawn travels through space, Earth and the Sun grow more remote.
Although the journey will never bring it near the part of the solar
system it used to consider home, we will see in the next log that its
path to Vesta and then to Ceres is not as direct as some might expect.
As part of the explanation, the log also may reveal something about this
mispelling.

Dawn is 324 million kilometers (202 million miles) from Earth, or more
than 885 times as far as the moon and 2.14 times as far as the Sun.
Radio signals, traveling at the universal limit of the speed of light,
take 36 minutes to make the round trip.
Received on Wed 30 Jul 2008 01:24:25 PM PDT