[meteorite-list] Dawn Journal - October 24, 2007

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Tue, 30 Oct 2007 08:09:02 -0800 (PST)
Message-ID: <200710301609.IAA10441_at_zagami.jpl.nasa.gov>

http://dawn.jpl.nasa.gov/mission/journal_10_24_07.asp

Dawn Journal
Dr. Marc Rayman
October 24, 2007

Dear Extraordawnaires,

Dawn's checkout phase continues to go very well. The spacecraft is
healthy as it and Earth travel their separate ways, separating at
almost 1 light second (nearly 300,000 kilometers, or 186,000
miles) per day.

In our last report, thruster #3 of the ion
propulsion system had been operated for 25 hours. Dawn's mission
control team at JPL commanded the probe to conduct additional
tests on October 8 and 9, some focused on the thruster itself and
others on how the attitude control system operated during
thrusting. Both systems passed with flying colors (for the more
literal-minded readers: flying - away from Earth at about 3.3
kilometers/second, or 7400 miles/hour; colors - the blue-green of
a xenon ion beam).

Although operation of the ion thruster was an important success,
it did not guarantee that the other thrusters would perform as
planned. On October 10, the team's attention shifted to thruster
#1. (The thruster numbers are described in the previous log
and can be found in the white
pages in most binary star systems.) The system that feeds xenon
propellant to the thrusters was prepared for operational use
before the tests with thruster #3, but each thruster has to
undergo individual preparation as well. To begin, thruster #1 was
treated to the same ministrations #3 received on October 4 and 5.

Dawn will carry out most of its functions much farther from the
Sun than Earthlings reside (although closer than most of our
readers reside). On October 24, the spacecraft reached a greater
solar distance than Earth ever attains in its annual elliptical
orbit and will never again visit Earth's part of the solar system.
But still for a little while longer, it will be close enough to
the Sun that careful monitoring is required to ensure that
components remain at acceptable temperatures.

Although thruster #1 and #3 are mounted on different parts of the
spacecraft and point in different directions, engineers had
determined that to measure the thrust by using the Doppler shift,
just as thruster #3 should not be aimed at Earth, neither should
thruster #1 at this time in the mission. In both cases, it was
clear that pointing the thruster toward Earth would cause other
components to overheat. Pointing thruster #1 away from Earth,
however, raised the possibility that another component would reach
an undesirable temperature.

The plan was formulated to conduct the test with a software timer
running onboard to stop it after a certain time had elapsed. That
way, if the temperature of the component approached a limit that
engineers did not want it to reach, or any other issues arose, and
the team was unable to intervene because communications were
interrupted (as can happen occasionally with systems as complex as
deep-space communications), the spacecraft would stop the activity
and return to the original orientation. The timer was to be reset
by controllers at regular intervals until the data showed that the
temperature would stabilize at an acceptable value.

On October 11, after instructing the spacecraft to rotate so that
thruster #1 was directed away from Earth, mission control
transmitted the commands to initiate ion thrusting. Electrical
measurements transmitted from the spacecraft and Doppler
computations performed by the navigation team all soon confirmed
that the thruster was applying its well-known light touch.
Thruster #1's first operation in space was excellent.

After almost 8 hours of thrusting, the temperature of greatest
interest was below the limit engineers had established, although
it was still creeping up. While dealing with a temporary problem
with communications between systems at JPL and the Deep Space
Network, the small team monitoring the
spacecraft on the night shift did not have time to reset the
timer. The timer activated and, as designed, terminated the
activity rather than let it continue without approval from mission
control.

With some simple changes to parameters in the thermal control
system, the test was restarted on October
23. It completed flawlessly on October 24 about 6:30 pm after
operating thruster #1 at 5 different throttle levels for almost 27
hours.

The ion propulsion system delivers Dawn to its celestial
destinations, but it is the scientific investigations to be
conducted at Vesta and Ceres that make the journey worth
undertaking. Dawn's three science instruments were powered on and
tested to verify that they were in good condition. (Some of the
knowledge scientists will gain about the interior structure of the
bodies will derive from exquisitely sensitive measurements of the
gravitational pull they exert on the probe. These measurements
take advantage of capabilities built into the telecommunications
system, as we will see in a future log, and hence do not have a
dedicated scientific instrument.)

On October 16, the gamma ray and neutron detector
was activated. Despite its name, GRaND
is not at all pretentious, but its capabilities are quite
impressive. To infer the atomic composition of the outermost parts
of Vesta and Ceres, GRaND includes a sophisticated suite of 21
sensors to measure the energies of gamma rays (a highly energetic
form of light) and neutrons (subatomic particles). The instrument
converts some of the power it receives from the spacecraft's
electrical power system into about
1000 volts for its detectors, and all power supplies and other
electronics proved to be operating correctly. (Electrical power is
distributed to most systems onboard at about 30 volts, and devices
that need other voltages are responsible for making the conversion.)

While GRaND (and the other instruments) are much much too far from
any solar system bodies to verify how well they will work at Vesta
and Ceres, scientists have alternatives. Cosmic rays, high energy
radiation that pervades space, constantly impinge upon the
spacecraft. GRaND can detect some of that radiation directly. In
addition, it senses some of the byproducts of interactions between
cosmic rays and nuclei of atoms in the spacecraft. While the
signals that were observed were not as grand (yes, the pun is an
easy one, but readers should get accustomed to it, as it is
expected to recur throughout Dawn's mission) as those expected at
Vesta and Ceres, they were sufficient to demonstrate the
instrument is healthy and ready for further operations. GRaND
remained powered on and measuring cosmic rays through October 22,
when it was turned off in preparation for other spacecraft activities.

The visible and infrared mapping spectrometer
(VIR) was the center of attention
on October 17. (The instrument's acronym was chosen because, as
its Italian developers of both sexes recognized, "vir" is Latin
for "man." [Editors in Virgo, please take note: To maintain the
good relations we have worked so hard to establish with locals,
you may substitute the explanation that the name was chosen
because stars in your constellation often are known by
abbreviations that include "Vir," such as Alpha Vir, R Vir, and
109 Vir.]) Objects as warm as the spacecraft (or, for that matter,
as warm as many readers) emit so much infrared that if VIR's
detector were at that temperature, their own infrared radiation
would interfere with planned measurements. Therefore, VIR
incorporates an electronic cooling system which brought the
infrared detector to about
-191?C (-312?F). In the absence of nearby targets to observe, VIR
tests use internal lamps that produce both visible and infrared
light.

VIR's cover was opened and closed, demonstrating that the
mechanism controlling it works correctly. In addition, the
instrument has an internal mirror that can be moved to make small
changes in the direction VIR looks, and that scan mechanism was
verified.

With all tests showing VIR to be fully healthy and ready for
future operations, the flight team conducted the first checkout of
a science camera on October 18. Two
such cameras are onboard, and following standard practice on JPL's
planetary spacecraft, only one is checked out at a time. While it
does not need to operate as cold as VIR's detector, the camera
requires its detector to be well below normal spacecraft
temperatures, and the device cooled to below -69?C (about -93?F).
The camera's filter wheel, which allows pictures to be taken in
color and even in near infrared, was exercised, and the instrument
cover was opened and closed.

The camera took 102 images during the test. For this first set of
instrument tests, no special spacecraft pointing was used;
instead, the orientation of the spacecraft chosen for other
purposes was maintained. Nevertheless, the camera captured images
of the star field in the constellation Cancer that happened to be
in its line of sight, and that was sufficient to demonstrate that
it was working extremely well. The instrument's view of the sky is
shown in the figures at
http://dawn.jpl.nasa.gov/technology/fc.asp. (As readers in the
vicinity of the star HIP 42243 may recall, the images were
acquired during the Dark Matter Bacchanalia. As a further
demonstration that the camera works as expected, and to the
probable relief of the participants, no evidence of the
festivities is apparent.)

As with VIR, all of the camera's optics, electronics, detectors,
and mechanisms are in excellent condition and ready for the
mission ahead. More tests will be conducted with each of the
instruments during the coming years, but all indications from
their first opportunities to operate in space are that they are in
fine fettle and will yield the exciting scientific data for which
they were designed.

Following a wonderfully successful week of instrument tests, the
Dawn team returned to propulsion tests the week of October 22. As
well over 0.20% of readers know, Dawn relies on ion propulsion for
reaching its destinations. Indeed, ions will propel the craft past
Mars, on to Vesta, into orbit around that massive asteroid, from
one orbit to another to allow the science instruments to gather
data from different perspectives, out of Vesta orbit and back into
orbit around the Sun, through more of the asteroid belt to dwarf
planet Ceres, into orbit there, and through a series of orbits as
at Vesta. Such a mission is far beyond the capability of
conventional chemical propulsion; yet Dawn also has a reaction
control system powered by the conventional propellant hydrazine.

The reaction control system is planned to be used only to help in
stabilizing or changing the spacecraft's orientation. Even if the
entire 45.6-kilogram (101-pound) supply of hydrazine were devoted
to changing Dawn's velocity, the effect would be less than 0.1
kilometers per second (220 miles per hour), quite insignificant
compared to the ion thrusting plan of about 11 kilometers per
second (nearly 25,000 miles per hour). Despite its much lower
efficiency, there are some dire (but highly unlikely!) cases in
which the higher thrust of the reaction control system may prove
critical. In those contingencies, patience may not be a virtue. As
well over 0.19% of readers know, ion propulsion, efficient though
it is, changes the trajectory only gradually. Should serious
spacecraft control problems arise during the final approach to Mars
or while in orbit at one of its destinations, engineers want to be
prepared to execute a rapid change in the trajectory.

On October 22, the spacecraft's capability to execute a
hydrazine-based maneuver was verified. Dawn turned to a new
orientation and fired 2 of its hydrazine thrusters simultaneously
for 2 minutes. (Other hydrazine thrusters fired in other
directions during this time as needed to keep the spacecraft
stable.) The maneuver expended about 51 grams (1.8 ounces) of
rocket propellant and changed the probe's speed by about 8
centimeters per second (0.18 miles per hour). Although the boost
in speed was quite modest, it was sufficient to demonstrate that
the system could perform an emergency maneuver. While having
confidence in such a capability, your correspondent expects (and
invites you to share his hopes) that future occasions to write
about the use of the reaction control system to change the
trajectory will arise only while rhapsodizing about the
differences between ion propulsion and conventional propulsion.

The spacecraft has been in space almost 4 weeks now. Three days into its
flight, when it was between 964,000 and 968,000 kilometers (599,000 and
601,000 miles) from Earth, Bill Dillon managed to capture portraits of
Dawn <http://www.sierrastars.com/gp/What's_New.aspx> among the stars.
The faint smudge visible at 2.5 times the distance to the moon is
Earth's last glimpse of its robotic ambassador to the cosmos. Although
barely visible, indistinct, and unimpressive, what it represents is so
much more: humankind reaching out from its tiny home into the vastness
of space. Somehow this simultaneously recalls both our insignificance in
the universe and our yearning to do our noble best.

Dawn is 7.90 million kilometers (4.91 million miles) from Earth or
almost 21 times farther than the moon. Radio signals, traveling at the
universal limit of the speed of light, take nearly 53 seconds to make
the round trip.
Received on Tue 30 Oct 2007 12:09:02 PM PDT