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

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu, 2 Sep 2010 11:17:52 -0700 (PDT)
Message-ID: <201009021817.o82IHqMb016253_at_zagami.jpl.nasa.gov>

http://dawn.jpl.nasa.gov/mission/journal_08_30_10.asp

Dawn Journal
Dr. Marc Rayman
August 30, 2010

Dear Papardawnzzi,

Dawn's journey ever-deeper into the asteroid belt continues to go
well, as the spacecraft carries out its familiar routine of
thrusting gently with its ion propulsion system. But the
interplanetary traveler has changed some of its habits, performing
certain activities a little differently now from what its many
followers have been accustomed to.

Dawn is now so far from the sun, that even with its tremendous
solar arrays, the most powerful ever used on an interplanetary
mission, it does not receive enough sunlight to generate sufficient
electrical power to operate all systems and still achieve maximum thrust.

The largest consumer of power onboard the ship, the ion propulsion
system is power hungry. Indeed, the key to its remarkable
effectiveness is that, in concert with the solar arrays, it
converts the renewable energy from the omnipresent sunlight into
thrust with a high velocity beam of xenon ions, in contrast to
conventional propulsion systems, which only work with the more
limited energy stored within the chemical propellants. The
importance of high power to thrusting has been discussed in detail
in several previous logs, including in July 2008 and September 2009.
In preparation for the spacecraft's being as far from the sun as it
is now, the mission control team has conducted a great deal of work
(including the solar array calibration that was explained in the
first of those two logs and performed in November 2008. Now we
can see how some of the detailed planning has been put into effect
in the operation of the ship.

The electrical power generated by a solar cell depends on its
temperature. When a cell is warmer, it is a little less efficient
at transforming light into electricity, so it yields a little less
power, even if the intensity of light impinging on it is unchanged.

When Dawn is not thrusting, the onboard power demand is much
lower, so less power is drawn from the two huge wings of cells.
That means that less of the light captured by the solar arrays is
converted to electrical power. So where does the energy of that
light go? When the solar panels do not need to generate as much
electricity, the excess energy of the absorbed light simply turns
to heat, warming the wings. Therefore, the arrays are warmer when
Dawn is not thrusting; and when thrusting commences, the arrays
need to cool down before they can achieve their best performance.
Earlier in the mission, when Dawn was closer to the sun and could
produce far more power than it needed (even when the arrays were
warm), this small difference was entirely unimportant. Now it is
very important.

Each week (usually on a Monday), the probe stops thrusting for
around eight hours so that instead of pointing an ion thruster in
the direction needed to climb out to Vesta, it can aim the main
antenna to Earth in order to exchange radio signals with an
antenna in NASA's Deep Space Network. At the end of the
communication session, the spacecraft returns to the thrust
orientation and resumes its powered flight.
 
For the last few months, the solar arrays have been able to meet
the onboard demand when they were cool, but when they were warm,
the available light was too weak to yield the power required by
all systems. There were several solutions to this (some of which
probably would have lent themselves to simpler and more jocose
descriptions), and the one engineers chose was to initiate
thrusting at a somewhat reduced throttle level, demanding less
power than at full thrust. That drew enough power from the arrays
to bring their temperatures down, allowing them to approach their
highest efficiency. Then the sequence running in the main computer
commanded the ion propulsion system to throttle up, and the arrays
were able to provide the additional power. This strategy has been
in use every week since April 19 and has worked flawlessly.

As Dawn moved farther from the sun, the power diminished still
more. The team knew well in advance that by May, even when the
array temperatures were low, there would not be enough power for
all systems while at the maximum throttle level. So, beginning May
17, when the spacecraft completes its weekly communication session
with its main antenna, it powers off its radio transmitter. Prior
to that date, the transmitter had always been left on, even when
Dawn was not in contact with Earth. The radio signal had been
directed through an auxiliary antenna that broadcasts over a very
wide angle in exchange for making the signal much weaker at the
receiving antenna on Earth. (This is no different from the control
on a flashlight. A narrowly focused beam can easily illuminate a
small area, just as Dawn's main antenna allows it to transmit a
strong signal toward Earth. For the same power, a wide cone of
light from the flashlight provides fainter illumination over a far
broader area, but it does not require precise pointing; similarly,
Dawn can spread out a weak signal without choosing an orientation
specifically designed for communications.) With the transmitter
off, Dawn reduced its power needs, thereby ensuring it had enough
to allocate to ion thrusting.

Each week (usually on a Thursday or Friday), the Deep Space
Network listens in to Dawn's radio whisper while the spacecraft is
thrusting. Faint though it is, the reception is adequate to
confirm that the probe is generally healthy. Although the
transmitter has been powered off for thrusting since May, the
sequence turns it back on for two hours to permit this
verification of the craft's status midway between the main
communications sessions on Mondays. The onboard battery was
available to help cover the increased power demand for that short
time. (So if you were wondering why you haven't been hearing from
the spacecraft at other times during the week recently, now you
know. Now the probe travels in radio silence except when
communication is scheduled.)
 
By July 26, that strategy was no longer sufficient to accommodate
the ever-decreasing power. Since then the ion propulsion system
has been throttled down during the mid-week health check. Reducing
power for the ion thrust allows power to be devoted to the
transmitter.

Based on extensive analyses performed in 2009, engineers had
estimated that Dawn would no longer be able to sustain the highest
throttle level by the last week of July 2010, even with the radio
off. As it turned out, however, the craft exceeded their
expectations and persisted through August 23. Finally, at a
distance of 2.02 AU from the sun, it was time to reduce the power
to the ion drive. From now on, Dawn will gradually decrease thrust
as it travels still farther from the brilliant star. Even at lower
throttle levels, however, the ion propulsion system's efficiency
is far beyond what is achievable with chemical propulsion.

In February we took a detailed look at Dawn's daily change in velocity
and recognized that it would continue to increase (thanks to the
decrease in the total mass) until throttled operation would be
necessary. As predicted then, the space traveler has now reached its
peak acceleration of 7.6 meters per second per day (17 miles per hour
for a day of thrusting). As this had been accounted for long ago in
the design of the trajectory, and motivated some of the tests performed
shortly after launch, the future gradual reductions in thrust have
already been incorporated into the plan for keeping the ship on a
steady course to Vesta and then to Ceres.
 
As the spacecraft continues its ambitious expedition through the
asteroid belt, engineers have recently changed another aspect of
its operation as well. On August 23, following instructions that
had been stored in the main computer the previous week, Dawn
powered off all four of its reaction wheels. (It's only
coincidental that that is also both the date the ion drive was
throttled down to save power and the date there was a power
failure in the ice cream shop at the Tribute to Coincidence.)
This was the first time since the day it was launched that all the
wheels were off. For most of its mission so far, Dawn has used
three of these units at a time to help hold its orientation or to
turn to a different orientation in the zero-gravity, frictionless
environment of spaceflight by electrically changing the speed at
which they spin. Wheel no. 4 developed increased friction on June
17, so it was turned off, and wheels 1 - 3 have been in use since
then. Mission controllers subsequently elected to turn all the
wheels off in order to help preserve them for use at Vesta and Ceres.

With the wheels being given a rest, the reaction control system
takes over their function. This system fires conventional rocket
propellant (perhaps even the same hydrazine formulation you use in
your rocket) through small thrusters aimed in different directions
to provide the required control of the craft's orientation.

Since smoothly accomplishing the transition, Dawn has maintained
its usual schedule of devoting 95% of the time to thrusting,
gradually changing its orbit so that it will match Vesta's orbit
in about 11 months.
 
The occasional brief pulses from the hydrazine thrusters are very
effective at keeping the spacecraft stable, but they are too weak
to contribute much to reshaping the trajectory around the sun.
Even if the entire 45.6-kilogram (101-pound) supply of hydrazine
Dawn carried into space had been devoted to changing the probe's
velocity, the effect would be less than 0.1 kilometers per second
(220 miles per hour), quite insignificant compared to the 4.81
kilometers per second (10,800 miles per hour) the xenon ion
thrusting has already achieved (and even that is less than half of
what is planned for the entire mission). The ion propulsion system
is so much more efficient that it remains the only system capable
of propelling the ship to its distant ports, Vesta and then Ceres.
 
Dawn continues to make excellent progress in its voyage to those
ancient and unexplored worlds. In the next log, we will consider
how far it has come and look ahead to more of what is store for it
at the first of these enticing targets.
 
Dawn is 0.19 AU (29 million kilometers or 18 million miles) from
Vesta, its next destination. It is also 2.82 AU (421 million
kilometers or 262 million miles) from Earth, or 1070 times as far
as the moon and 2.79 times as far as the sun. Radio signals,
traveling at the universal limit of the speed of light, take 47
minutes to make the round trip.
Received on Thu 02 Sep 2010 02:17:52 PM PDT