Dawn Journal
Dr. Marc Rayman
September 27, 2008
Dear Dawnniversaries,
On the first anniversary of its departure from Earth,
Dawn continues with what it has been doing
for most of its time in space: with the greatest patience it is
gently reshaping its orbit around the Sun with its ion propulsion
system.
In its first year of travels, the spacecraft has thrust for a
total of about 253 days, or 69% of the time. Dawn has been in
powered flight for 85% of the time since the beginning of its
interplanetary cruise phase in December
2007 and about 0.000000005% of the time since the Big Bang. While
for most spacecraft, firing a thruster to change course is a
special event, it is Dawn’s wont. All this thrusting has cost the
craft only 67 kilograms (148 pounds) of its supply of xenon
propellant, which was 425 kilograms (937 pounds) 1 year ago.
The thrusting so far in the mission has achieved the equivalent of
accelerating the probe by 1.68 kilometers per second (3760 miles
per hour). As the preceding log described,
because of the principles of motion for orbital flight, whether
around the Sun or any other gravitating body, Dawn is not actually
traveling this much faster than when it launched. But the
effective change in speed remains a useful measure of the effect
of any spacecraft’s propulsive work. Having accomplished only
one-eighth of the thrust time planned for its entire mission, Dawn
has already exceeded the velocity change required by many
spacecraft. (For a comparison with probes that enter orbit around
Mars, visit the red planet yourself or refer to a previous log.)
Since launch, our readers who have remained on or near Earth have
completed exactly 1 revolution around the Sun. (This log,
including the date it is filed, disregards that 2008 is a leap
year and that Earth actually takes almost 365.25 days to complete
one orbit. Oops – it isn’t being disregarded; in fact, it’s right
there in the previous sentence, and the longer this parenthetical
text goes on, the more attention is being drawn to it. As it makes
no significant difference, we request readers do a better job of
ignoring it than the writer is doing. Please return to the flow of
the log.) Orbiting farther from the Sun than Earth, and moving at
a more leisurely pace, Dawn has not traveled even two-thirds of
the way around the Sun. Of course, unlike Earth, when it has
completed 1 full circuit (in 2009), it will not be at the same
place it started. Earth’s orbit is quite repetitive, but the
combined effects of the powerful rocket launch, the extensive ion
thrusting, and the gravitational deflection from Mars next
February will cause the spacecraft to be farther from the Sun at
the end of its first revolution than it was at the beginning.
As readers who have followed the Dawn mission during 2008 know,
the spacecraft occasionally engages in activities other than
routine thrusting as its adventure progresses. On August 26,
mission controllers commanded the primary and backup cameras
to execute their calibration routines.
This not only served to confirm that both units remain healthy,
but it also let engineers verify one of the new features in the
software radioed to each camera in April that was not tested at
that time.
On September 22, an updated version of a method to establish how
much power Dawn’s extraordinary solar arrays can generate was
tested successfully. The first test
was conducted in July, and it yielded only some of the desired
information. The revised procedure was very similar to the earlier
one, principally differing in the timing of some instructions and
values of parameters based on the analysis of that initial run.
Because the entire activity, even including the 41-minute
round-trip travel time for radio signals, required less than 3
hours in the middle of the afternoon, among the most significant
changes that ever-observant mission controllers detected was that
no meals were incorporated into the carefully engineered plan.
As in July, the test included rotating both solar array wings 45
degrees, so they did not point directly at the Sun, thus reducing
how much light they received and converted to electrical power.
The test was carried out during the spacecraft’s routine weekly
interruption in thrusting to point its main antenna to Earth, but
the ion propulsion system was commanded into service when it
otherwise would have been idle. Its role then was not to provide
propulsion (although it did so); rather, it participated because
it is the greatest consumer of power onboard. Dawn’s enormous
solar arrays, even turned partially away from the Sun and more
than 1.66 times farther from the radiant orb than Earth is, were
able to provide the 2.5 kilowatts requested by the ion drive at
full power. Later in the mission, after all the data have been
analyzed thoroughly, the next step in the solar array calibration
will be to command the arrays to rotate farther, where they are
not expected to be able to deliver all the power requested.
As Dawn begins its second year (as measured back on Earth) of
interplanetary flight, the probe steadfastly continues its long
journey in the quiet solitude of space, quite isolated from events
on or near the distant planet that used to be its home. While no
spacecraft has left the vicinity of the Earth-moon system in the
year since Dawn’s departure, much has happened there, even as the
explorer has remained focused on accomplishing its voyage in deep
space. From the first circulation of protons at the Large Hadron
Collider 100 meters (330 feet) underground, to the beginning of
the Fermi Gamma-ray Space Telescope mission 550 kilometers (340
miles) overhead, to the arrival of SELENE (Kaguya) and Chang’e 1
at the moon, humankind’s thrilling work to understand nature has
continued. Apparently there have been some other kinds of news as
well, from shocking revelations about celebrities, to competitions
among athletes and among politicians, to still more shocking
revelations about celebrities, but such information is harder to
find, given the news media’s nearly exclusive focus on myriad
science topics. (News coverage may be different on your planet.)
Dawn is 374 million kilometers (232 million miles) from Earth, or
980 times as far as the moon and 2.49 times as far as the Sun.
Radio signals, traveling at the universal limit of the speed of
light, take 42 minutes to make the round trip.
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.