[meteorite-list] Dawn Journal - November 30, 2012

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed, 5 Dec 2012 09:15:14 -0800 (PST)
Message-ID: <201212051715.qB5HFETw012204_at_zagami.jpl.nasa.gov>

http://dawn.jpl.nasa.gov/mission/journal_11_30_12.asp

Dawn Journal
Dr. Marc Rayman
November 30, 2012

Dear Dawndroids,

Dawn is continuing to gently and patiently change its orbit around the
sun. In September, it left Vesta, a complex and fascinating world it
had accompanied for 14 months, and now the bold explorer is traveling
to the largest world in the main asteroid belt, dwarf planet Ceres.

Dawn has spent most of its time since leaving Earth powering its way
through the solar system atop a column of blue-green xenon ions emitted
by its advanced ion propulsion system. Mission controllers have made
some changes to Dawn's operating profile in order to conserve its supply
of a conventional rocket propellant known as hydrazine. Firing it
through the small jets of the reaction control system helps the ship
rotate or maintain its orientation in the zero-gravity of spaceflight.
The flight team had already taken some special steps to preserve this
precious propellant, and now they have taken further measures. If you
remain awake after the description of what the changes are, you can read
about the motivation for such frugality.

Dawn's typical week of interplanetary travel used to include ion
thrusting for almost six and two-thirds days. Then it would stop and
slowly pirouette to point its main antenna to Earth for about eight
hours. That would allow it to send to the giant antennas of NASA's Deep
Space Network a full report on its health from the preceding week,
including currents, voltages, temperatures, pressures, instructions it
had executed, decisions it had made, and almost everything else save its
wonderment at operating in the forbidding depths of space so
fantastically far from its planet of origin. Engineers also used these
communications sessions to radio updated commands to the craft before it
turned once again to fire its ion thruster in the required direction.

Now operators have changed the pace of activities. Every turn consumes
hydrazine, as the spacecraft expels a few puffs of propellant through
some of its jets to start rotating and through opposing jets to stop.
Instead of turning weekly, Dawn has been maintaining thrust for two
weeks at a time, and beginning in January it will only turn to Earth
once every four weeks. After more than five years of reliable
performance, controllers have sufficient confidence in the ship to let
it sail longer on its own. They have refined the number and frequency of
measurements it records so that even with longer intervals of
independence, the spacecraft can store the information engineers deem
the most important to monitor.

Although contact is established through the main antenna less often,
Dawn uses one of its three auxiliary antennas twice a week. Each of
these smaller antennas produces a much broader signal so that even when
one cannot be aimed directly at Earth, the Deep Space Network can detect
its weak transmission. Only brief messages can be communicated this way,
but they are sufficient to confirm that the distant ship remains healthy.

In addition to turning less often, Dawn now turns more slowly. Its
standard used to be the same blinding pace at which the minute hand
races around a clock (fasten your seat belt!). Engineers cut that in
half two years ago but returned to the original value at the beginning of
the Vesta approach phase. Now they have lowered it to one quarter of a
minute hand's rate. Dawn is patient, however. There's no hurry, and the
leisurely turns are much more hydrazine-efficient.

With these two changes, the robotic adventurer will arrive at Ceres in
2015 with about half of the 45.6-kilogram (101-pound) hydrazine supply
it had when it rocketed away from Cape Canaveral on a lovely September
dawn in 2007. Mission planners will be able to make excellent use of it
as they guide the probe through its exploration of the giant of the main
asteroid belt.

Any limited resource should be consumed responsibly, whether on a planet
or on a spaceship. Hydrazine is not the only resource that Dawn's
controllers manage carefully, but let's recall why this one has grown in
importance recently.

The spacecraft can stabilize or change its orientation using the
hydrazine powered jets or reaction wheels. By electrically changing a
wheel's spin rate, Dawn can start or stop rotating. When it is relying
principally on these gyroscope-like devices, it still occasionally has
to expend a little hydrazine to keep them from spinning too fast, as
explained nearly four years ago. While thrusting (which is most of the
time), the ion thruster works in concert with one of those other actuators
to control the orientation.

For an ambitious and complex eight-year interplanetary expedition,
Dawn's builders equipped it with backup systems. The craft was designed
to use three reaction wheels at a time for normal operations, so it is
outfitted with four. One of them encountered increased friction in June.
To preserve the life of the remaining wheels, engineers flew the spacecraft
with all the wheels turned off from August 2010 until the Vesta
approach phase began in May 2011, and they are doing the same during the
flight from Vesta to Ceres.

As soon as the wheel had difficulty in 2010, Orbital Sciences
Corporation and JPL began working on a method to operate with fewer than
three, in case another one faltered. They developed software to operate
in a "hybrid" mode with two wheels plus the hydrazine jets and installed
it in the robot's main flight computer in April 2011
so it would be available at Vesta if needed.

The exploration of that alien orb, which exceeded all expectations not
only for productivity but also for pure awesomeness, went very smoothly
with the three operational wheels. As Dawn was spiraling away
from the rocky behemoth in August 2012,
however, another one experienced the same peculiar friction. Because the
wheels had already been scheduled to be powered off shortly thereafter,
the flight team continued the departure with them turned off, and it
proceeded without further interruptions. With their typical swift
professionalism, they immediately began working on the long-term
ramifications of two wheels being unavailable in case the devices could
not be recovered.

Because the hybrid control scheme uses more hydrazine than three wheels
would, and using the hydrazine jets by themselves with no wheels
consumes still more, operators undertook the new campaign to conserve
the propellant during the journey to Ceres. Ever resourceful, engineers
now anticipate that regardless of how healthy the wheels are, the probe
will be able to conduct an exciting and rewarding exploration there.

Dawn will arrive at the distant and mysterious Ceres in 2015, and that
allows plenty of time for the terrestrial members of the team to
complete the exquisitely detailed plans for its adventures there. While
that work is underway, the intrepid ship continues forging silently
through the vast emptiness of space, distant and alone, patient and
persistent. Despite its remoteness, the robot remains tightly bound to
its human colleagues, for it is on their behalf and under the power of
their ingenuity, thirst for knowledge, and hunger for adventure that it
sails deeper into uncharted cosmic seas.

Dawn is 1.5 million kilometers (960 thousand miles) from Vesta and 57
million kilometers (36 million miles) from Ceres. It is also 1.59 AU
(238 million kilometers or 148 million miles) from Earth, or 590 times
as far as the moon and 1.61 times as far as the sun today. Radio
signals, traveling at the universal limit of the speed of light, take 26
minutes to make the round trip.
Received on Wed 05 Dec 2012 12:15:14 PM PST