[meteorite-list] Dawn Journal - July 29, 2006

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Aug 3 12:11:03 2006
Message-ID: <200608031515.IAA10897_at_zagami.jpl.nasa.gov>

http://dawn.jpl.nasa.gov/mission/journal_07_29_06.asp

Dawn Journal
Dr. Marc D. Rayman
July 29, 2006

Dear Dawnthusiasts,

Dawn continues to keep its human handlers very busy as preparations
continue on schedule to meet the planned opening of the launch period on
June 20, 2007.

Much of June 2006 was devoted to conducting the comprehensive
performance tests (CPTs) described in the previous log. In these tests,
most of the hardware and software subsystems already on the spacecraft
were exercised to help uncover problems ranging from incomplete solder
connections on a microchip or a broken wire, to software bugs, to
unexpected interactions between subsystems that must work together. Of
course, each subsystem was tested extensively as it was being built, but
some afflictions may remain hidden until the subsystem is operating on
the spacecraft with other subsystems. As thorough as the CPTs are,
testing will become more and more demanding over the coming months as
the spacecraft is asked to perform in ways progressively more like what
it will encounter during its voyage to the asteroid belt and its
explorations of Ceres and Vesta, the most massive bodies in that region
of the solar system. The upcoming tests will be described in future logs.

The CPTs yield a tremendous volume of data, and engineers are still
analyzing the details of their subsystems' performance, but all
indications are that the tests went extremely well. As our humble human
readers know, some errors are inevitable in a design as intricate and
complex as one of Earth's interplanetary spacecraft. So far it appears
that all such flaws are easily correctable.

Some subsystems have not yet had their first CPT. The ion propulsion
subsystem and the instruments for collecting scientific data are
awaiting their tests in August. We will introduce each of Dawn's
subsystems in the next log.

Some of the ion propulsion subsystem's individual components received
some extra testing recently before being mated to the spacecraft in
June. The power processing units have to provide the properly controlled
voltages and currents to different elements of the ion thrusters, which
apply the electrical power to xenon gas to produce fantastically
efficient propulsion, without which Dawn's ambitious mission would be
quite impossible. Each unit will process up to 2500 watts (much more
than the average house consumes), and we wanted special assurance that
these devices would perform reliably on the mission. So in addition to
the testing they received at the company that manufactured them for
Dawn, each one was subjected to further trials in one of JPL's
laboratories. The devices were operated for about 20 days in vacuum
chambers. During most of that time, the units were pushed to the highest
temperature they will experience on the spacecraft of 35?C (95?F). Both
units passed with flying colors (what other kind of colors would you
expect for space hardware?), adding to the confidence that they are up
to the rigors of Dawn's mission.

While operating in their thermal vacuum chambers, the power processing
units were under the control of the same software that runs in the ion
propulsion control unit on the spacecraft. So this work provided a bonus
opportunity to test the software that operates this complex subsystem on
its travels through deep space.

Each of the three ion thrusters will be mounted on a mechanism that
allows its pointing direction to be fine tuned by other software on the
spacecraft. As we will see in the next log, this accurate aiming is
essential, so if one of these mechanisms fails, the attached thruster
would be useless. To verify the robustness of the design for the
mechanism, a test unit was subjected to 10 times the amount of work the
ones to be flown on Dawn will have to provide. The performance was flawless.

After the ion propulsion subsystem testing and the first set of CPTs
were completed in June, the focus of the Dawn team's activity in July
was on what nontechnical readers might think of as baking the
spacecraft. The technical term used by the engineering team was -- well,
baking out the spacecraft. This was not a test; rather, bake-out was
intended to heat the spacecraft to drive off contaminants it might have
collected, despite the assembly having been conducted in a specially
controlled "clean room" at Orbital Sciences Corporation.

The first step in the bake-out was to clean the oven in which the baking
would occur, a cylinder 3.7 meters high and 4.9 meters long. (Ever
poetic, team members fondly refer to this facility as the "12 by 16 foot
chamber.") For several days, the chamber was heated to 95?C (203?F).
This ensured that its interior would be free of chemical residue that
might contaminate Dawn.

With the chamber certified to be clean, the spacecraft was moved in. As
it is not completely assembled yet, some of the flight equipment was
simply placed in the chamber with the spacecraft. After the chamber was
sealed, it took 7 hours to reduce the pressure to about 100 million
times lower than atmospheric pressure. Then the temperature was raised
gradually over more than a day and a half to bring the spacecraft to a
toasty 53?C (127?F). As with most other details of the design, assembly,
test, and operations in flight, the temperature was selected after a
substantial amount of careful analysis. It had to be high enough to
force the contaminants off in a reasonable amount of time without the
heat endangering the spacecraft. As Dawn will go farther from the hot
Sun than Earth is, accommodating very high temperatures was not among
its design criteria.

Accompanying the flight hardware in the chamber were sensors to allow
the operators to monitor temperatures, and team members babysat the
spacecraft around the clock to ensure its safety. Contamination monitors
permitted the operators to observe the rate at which material was being
driven from the spacecraft, and the bake-out was scheduled to continue
until the rates reached a predetermined low value. Although it had been
expected this would require nearly a week of baking, it turned out to
require less than two days at the maximum temperature. The team was
pleased to conclude that the spacecraft must have started out cleaner
than had been anticipated.

In addition to the sensors that provided feedback during the bake-out,
other contamination monitors were included that will be analyzed in
laboratories to understand more about the kinds of contaminants that
were present. Ultraclean silicon wafers were intended to collect tiny
fibers, many times thinner than human hair, and aluminum plates can
reveal evidence of films just a few molecules thick.

Both before and after the bake-out, the spacecraft was inspected
carefully under illumination with ultraviolet light and separately with
old-fashioned (but still effective!) visible light. Such inspections
will be repeated many more times before launch. Any debris that is found
is removed with a brush and small vacuum or with special materials
soaked with purified alcohol. As one might imagine, using an inspector's
thumb or the corner of his T-shirt to wipe off unwanted material is not
part of such a delicate procedure.

While the careful assembly and demanding testing of the spacecraft
continues, there remains one piece of essential hardware that will not
even begin fabrication until late this year. It is not needed for Dawn
to carry out its assignment to explore alien worlds and to reveal clues
to the dawn of the solar system, but it is a vital ingredient in another
aspect of Dawn's mission. And despite the engineering and scientific
breadth and depth of the Dawn team, this component is beyond our means
to produce without the help of hundreds of thousands of people,
including you, dear reader.

In many ways, we are still in the early stages of humankind's journeys
through the solar system. This is evident when we recognize that Dawn is
the first spacecraft designed to orbit a target in the main asteroid
belt. This vast region of space has been traversed by a few probes that
were flung from the inner solar system to reach the outer solar system,
but no spacecraft has yet been assigned to stop there and develop an
intimate portrait of some of the residents. Indeed, until Dawn, with its
ion propulsion system, no spacecraft has had such a capability.

As other explorers, Dawn's mission is about more than seeing what is out
there and helping us unravel secrets of the cosmos. Dawn carries with it
the spirit of those it represents on its homeworld: a yearning to extend
beyond the bounds of our terrestrial experience, a grand quest for ever
greater knowledge, an irrepressible drive to understand how our
celestial neighborhood fits together in space and time. Missions like
Dawn's help fuel the passionate and noble fires that burn in the hearts
and minds of people everywhere. It is these shared feelings that make
this more than simply another attempt to record scientific data.

To emphasize the nature of this joint participation in the mission and
make it more real, more rewarding, and more personal, we invite you to
include your name on the spacecraft. Later this year, we will fabricate
a chip to be carried on the spacecraft, imprinted with the name of each
person who wishes to have a part in this collective adventure of
humankind. While the Dawn project can make the chip, it would mean
nothing without all the names.

So be among the first on your planet to submit your name by going to
http://www.dawn-mission.org/DawnCommunity/Sendname2asteroid/nameEntry.asp .
Received on Thu 03 Aug 2006 11:15:46 AM PDT