[meteorite-list] Dawn Journal - June 26, 2008

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Sat, 5 Jul 2008 20:53:05 -0700 (PDT)
Message-ID: <200807060353.UAA01563_at_zagami.jpl.nasa.gov>

http://dawn.jpl.nasa.gov/mission/journal_6_26_08.asp

Dawn Journal
Dr. Marc Rayman
June 26, 2008

Dear Dawnlettantes,

Now using an ion thruster that had been powered off since October, Dawn
continues to make steady progress on its journey deeper into space. In
this phase of the mission, each day of thrusting changes the probe's
speed by 6.7 meters/second (15 miles/hour). Dawn will operate its ion
thrusters for a total of more than 5 years, providing the extraordinary
boost required to orbit both main belt asteroid Vesta and dwarf planet
Ceres in the quest to understand the dawn of the solar system. Some
readers may be reminded of the prophetic sagacity of Tardigrade the
Celeritous, who, with uncanny prescience (and, at the time,
abstruseness), is believed to have said that the journey of 11
kilometers/second (25,000 miles/hour) begins with a single day of thrusting.

The spacecraft is outfitted with 3 ion thrusters but will never use more
than 1 at a time. In October and November, during the initial checkout phase of
the mission, all 3 thrusters were tested and confirmed to be healthy and
ready for operation. Following the flight plan, thruster #3 was the
first to propel the craft in its interplanetary cruise phase, which
began on December 17.

In the 184 days from the beginning of interplanetary cruise until ion
thruster #1 took over, thruster #3 operated for a total of 147 days,
flawlessly changing Dawn's orbit around the Sun. Each week the thrust
was turned off for a few hours when the spacecraft turned to point its
main antenna to Earth, and a few days every month or so have been
devoted to other, non-thrusting activities. (Readers are encouraged to
review the logs posted since December to remind themselves of such
activities. That might yield an unexpected reward, as it may now be
revealed that the text of those logs contains a highly encrypted message
with information of astonishing import. If you find it, please let this
writer know, as he has absolutely no idea what it is!)

The effect of thruster #3's operation during this mission phase was to
change the spacecraft's speed by about 0.96 kilometers/second (2150
miles/hour). Thanks to the exceptionally high efficiency of the ion
propulsion system, Dawn's solar system xenon footprint in accomplishing
this was only 39 kg (86 pounds). (Note also that its carbon footprint
was 0.)

Switching from one thruster to another is simple (to the extent that
anything is simple for early 21st century humans controlling a
spacecraft in deep space). The potential complication in this case was
explained in the previous log. Our readers
survey (conducted by Telepathic Business Services, Inc. when their
employees had time between major poker competitions) shows that 3
readers do not fully recall the details and will not refer to that log,
so the issue is summarized here. The 3 ion thrusters point in different
directions on the spacecraft. To provide thrust in the correct direction
in space, Dawn has to rotate to aim the designated thruster in that
direction. The use of thruster #1 now requires the craft to assume an
orientation quite different from any that had been experienced before,
and engineers were not confident certain components would remain within
their required temperature limits when the Sun shone on them.

Last month's test, in which the spacecraft spent a few hours pointing in
the required direction, provided some of the data needed to establish
when it would be safe to commit to the use of thruster #1 for long
periods of time. The results agreed with previous analyses, which had
shown that all the components would remain in their prescribed
temperature ranges if thruster #1 were put to use this month. Probably.

Probably? That "probably" was not good enough. Ever-cautious mission
controllers were not sufficiently confident to let the spacecraft remain
in the new orientation for a week at a time, because there were a few
components whose temperatures still could not be predicted well enough.
The analyses were conclusive that the temperatures would be safe for
more than 24 hours, as it takes a long time for that hardware to heat
up. Therefore, the team devised a new approach.

A typical set of commands for 5 weeks of operation with thruster #1 was
formulated. In addition, engineers prepared instructions for storage
onboard to stop thruster #1, rotate to the thruster #3 orientation,
resume thrusting, and perform all the other associated functions, the
description of which is precluded by laws on profoundly incomprehensible
prose. (While such laws are applicable only in the vicinity of
supermassive stars, we obey them out of consideration for such regions
of our distribution.) The instructions were structured so that only a
single, brief message from Earth would be needed to trigger the switch
back to thruster #3. On June 18, the spacecraft turned from pointing its
antenna to Earth to aim thruster #1 in the correct direction and
initiated thrusting.

A Deep Space Network antenna that was available was scheduled to listen
in to the spacecraft on June 19. Dawn was programmed to use one of its
small antennas, with a very broad radio beam, to
transmit temperature measurements.

Dawn's terrestrial team members receiving the data found the results to
be much as expected. As predicted, the temperatures had not yet
stabilized, and all were within the desired ranges. When they had about
two hours of measurements in hand, engineers were able to predict with
high confidence what the final temperatures would be. This confirmed
that continued operation was safe, so there was no need to switch back
to thruster #3. (Providing the spacecraft with the capability to make
that decision, while that might seem pretty neat, would have required
more work than the neatness would have merited.)

As some may recall from long, long ago (to be specific, 8 paragraphs
ago), several days of coasting are included in the flight plan
occasionally. Activities for some of those times are planned long in
advance. Other such periods are held in reserve in case mission
controllers identify the need for some previously unplanned work that
could not be accommodated in the normal schedule. June 16 - 18 was one
such interval. The mission has been going so smoothly, however, that no
special activities were required then. The team did take advantage of
the extra time that the primary antenna was pointed to Earth to clean up
some file buffers and perform other maintenance on some of the
spacecraft's computers.

With the mission continuing so well, the Dawn team can devote much of
its attention to preparing for future events. Although the Dawn project
has no specific plans, readers may rest assured that the team members,
as with their fellow residents of Earth, are completing personal plans
to commemorate the centennial of the Tunguska event on June 29 (the
event occurred on June 30 in Siberia's time zone).

The next item of interest occurs on June 30, when the spacecraft exceeds
the outermost reaches of the orbit of Mars; the probe will be farther
from the Sun than that planet ever travels. Earth reaches its greatest
distance from the Sun on July 4, when it will be almost 1.7% farther
than its average distance. (On January 2, it was about 1.7% closer than
average.) Then even as Earth begins a slow fall toward the Sun (a trend
that will continue until next January), Dawn will continue its climb
outward. On July 10, the robotic explorer will be twice as far from
Earth as Earth will be from the Sun. At that time it will be 304 million
kilometers (189 million miles) from the planet it left on a lovely dawn
in September 2007.

At these extraordinary distances, humankind (and even some of our other
readers) does not have the technology to see the spacecraft. Indeed,
Dawn is barely discernible in a pair of portraits taken when it was
more than 300 times
closer to Earth. Yet some who follow the mission might enjoy gazing in
the direction of the probe as they contemplate its journey deeper into
space and the ambitious and exciting mission that lies ahead. For those
in the continental United States, the spacecraft will be between 3? and
5? northeast of the moon in the evening of July 6 as the moon is
approaching the western horizon. (In other words, Dawn will appear to be
6 to 10 times the moon's diameter away, north and higher in the sky.)
Although quite invisible to your eyes, in that direction your mind may
be able to see with great clarity one of your planet's envoys to the
cosmos. With a blue-green trail of xenon ions behind it and appointments
with distant, uncharted, alien worlds ahead of it, Dawn will be silently
and contentedly carrying out its mission to extend our reach into space
and to help fulfill our passionate search for knowledge and our yearning
for adventure.

Dawn is 286 million kilometers (178 million miles) from Earth, or more
than 760 times as far as the moon and 1.88 times as far as the Sun.
Radio signals, traveling at the universal limit of the speed of light,
take 32 minutes to make the round trip.
Received on Sat 05 Jul 2008 11:53:05 PM PDT