[meteorite-list] Dawn Journal - October 30, 2008

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Fri, 31 Oct 2008 10:48:43 -0700 (PDT)
Message-ID: <200810311748.KAA27413_at_zagami.jpl.nasa.gov>

http://dawn.jpl.nasa.gov/mission/journal_10_30_08.asp

Dawn Journal
Dr. Marc Rayman
October 30, 2008

Dear Presidawntial Candidawnts,
 
The Dawn spacecraft continues on course and on schedule for its
bold campaign to unexplored worlds. The probe is thrusting gently
with its ion propulsion system, as it has been for most of its
time in space, gradually modifying its path around the Sun.
 
New research in the well-named Department of Recent Earthling
Communications and Knowledge at the increasingly popular Galactic
University of Fatuity and Frivolity (GUFF) has revealed that the
significant majority of these logs written since Dawn's
interplanetary cruise phase commenced on December 17, 2007,
have begun with something similar to that
introductory paragraph. That may not be very surprising, as
humankind would not be able to accomplish this ambitious and
exciting mission without a reliable, ion-propelled spacecraft.
(Note to other readers: for bureaucratic reasons, earthlings have
chosen not to collaborate with more technologically advanced
species on this mission. Rest assured, though, that it's nothing
personal!) Nevertheless, as you will see in a second (assuming you
can read about 800 words per second), this familiar story will
change quite soon, as the typical content of our opening remarks
will no longer be fully applicable. First, let's review what Dawn
has accomplished since the last log besides 28 days of thrusting.
 
On September 29, as its own silent but joyous celebration of its
first anniversary of being in space was
winding down, the spacecraft stopped thrusting so mission
controllers could conduct routine maintenance on components in 2
of its subsystems: attitude control and ion propulsion. (Thrusting
is suspended during these activities principally because the
orientation in which the main antenna is aimed at Earth is
different from the orientation required to point an ion thruster
in the direction needed for changing the craft's course through
space.) Attitude control is responsible for the orientation (known
to engineers as "attitude") of the probe in the zero-gravity of
spaceflight. Despite its name, this subsystem is as pleasant a
member of the onboard crew as any other. Ion propulsion, of
course, reshapes the spacecraft's orbit so it will rendezvous with
distant Vesta and Ceres and maneuver at each to obtain the
precious scientific secrets they hold.
 
Some of the work during this week was to verify that the contents
of the computer memory in certain components remained intact. On
September 30, engineers confirmed that the memory in each of the 2
ion propulsion computer control units was in good condition. On
October 2, the backup star tracker was tested, and it also remains
healthy and ready for use whenever needed. A star tracker helps
the attitude control system determine the orientation of the
spacecraft by imaging groups of stars and recognizing patterns,
much as you might orient yourself on a dark, cloudless night if
you were familiar with the constellations. (Readers who travel
frequently, and hence must keep track of where they are in their
galaxy in order to know what the arrangement of stars should be,
have a more difficult problem than Dawn's star trackers face. The
solar system is so tiny compared to interstellar distances that
the views of the stars remain essentially unaffected by where the
spacecraft is, just as the shapes of constellations are the same
for observers anywhere on Earth.)
 
In addition to performing maintenance on software, the mission
control team needs to keep Dawn's hardware in peak condition. The
3 ion thrusters are mounted on separate mechanical apparatuses
that allow each 8.9-kilogram (19.5-pound) thruster to be pointed
accurately. These thruster gimbal assemblies, known as TGAs to
team members who find themselves too busy to use entire words
(such people are themselves known as being TBTUEW), need to have
lubricant in their bearings redistributed occasionally. Even when
a TGA is in use for an operating thruster (thruster #1 has been
the active one since June, the usual motion
is not enough to accomplish the needed spreading of lubricant.
Therefore, all 3 TGAs were moved through a prescribed pattern,
ensuring that they will be able to continue to operate smoothly
and point correctly.
 
Dawn is outfitted with 4 reaction wheels, devices whose spin is
controlled electrically. Changing a wheel's spin rate allows the
attitude control system to rotate the spacecraft. The wheels are
mounted in different orientations, but any 3 are sufficient for
normal operations. Wheel #3 has been off since May
On October 2, it was powered on again
and wheel #2 was deactivated, beginning its turn as the backup.
 
Gyroscopes, which will help attitude control perform the accurate
pointing of science instruments at the 2 protoplanetary
destinations, normally are turned off, as they are not needed for
most of Dawn's assignments along the way. A few times each year
they do need to be operated to ensure they remain in good
condition. The last such time was in May.
On September 29, the units were
activated again, and they remained powered on until October 3.
 
With all maintenance completed successfully, normal interplanetary
thrusting resumed on October 3. Soon however, interplanetary
thrusting will no longer be the norm. Some of the unusual
principles of an interplanetary journey driven with ion propulsion
were considered in a log written while
Dawn was still gravitationally anchored to Earth. One essential
characteristic of such missions is the long periods of thrusting,
familiar now to those fortunate enough to have followed Dawn's
progress since the beginning of the interplanetary cruise phase.
But, thrusting is not required for the entire voyage; indeed, at
some times thrusting is helpful to the mission and at other times
it would be detrimental. Extensive analysis is devoted to
computing the thrusting schedule, based on factors ranging from
the physical characteristics of the solar system (e.g., the masses
and orbits of Earth, Mars, Vesta, Ceres, and myriad other bodies)
to the capabilities of the spacecraft (e.g., electrical power
available to the ion thrusters to
constraints on when thrusting is not permitted (e.g., during
spacecraft maintenance periods).
 
As hinted obscurely only a second ago, the period in which
thrusting is beneficial for reaching Vesta on schedule is drawing
to a temporary close. For nearly all of the next 7 months, Dawn
will coast in its orbit around the Sun (just as do most objects in
the solar system, including other spacecraft and planets), no
longer mounted atop a bluish-green pillar of xenon ions. Still,
its orbit will change dramatically during this interval, as its
flight by Mars in February will deflect its path through the solar
system. As we shall see in the next log, to achieve exactly the
gravitational bending needed, the spacecraft will execute some
special thrusting in November and again in January, but very
little indeed.
 
The interplanetary cruise phase has gone so smoothly that the
completion of thrusting is being reached somewhat sooner than had
been expected earlier in the mission. Commands already stored in
Dawn's central computer will terminate the thrust on October 31 at
3:22 pm PDT. In the next log, we will discuss a bit about the
process the team used to determine that time, as it bears on
another activity planned for November; contrary to what you might
conclude however, leaving enough time for team members to don
their costumes in preparation for going door to door to collect
Halloween treats was not a factor. (Your correspondent, who
disguises himself in costumes at JPL most days, won't need any
extra time at all tomorrow to outfit himself for perfectly
frightening appearances on Halloween.)
 
Although thrusting will be uncommon over the coming months, there
will be plenty of other news to look forward to in these logs,
including the reversal of Dawn's departure from Earth, the first
attempt to measure the total power generating capability of the
solar arrays, passage of the spacecraft nearly behind the Sun,
plans for and results of the brief visit to Mars, a dramatic
increase in the quality of writing [Note from writer to sponsor:
Now that I've made such a promise to our readers, I hope you'll
come through with that generous raise I've been requesting. Note
from sponsor to writer: OK, you win. We agree to a 2% raise from
the current $0.00 per log, and we will pay 1% of your tuition if
you can buckle down, gain readmission to GUFF, and finally receive
your degree.], and much more.
 
Dawn is 384 million kilometers (238 million miles) from Earth, or
950 times as far as the moon and 2.58 times as far as the Sun.
Radio signals, traveling at the universal limit of the speed of
light, take 43 minutes to make the round trip.
Received on Fri 31 Oct 2008 01:48:43 PM PDT