[meteorite-list] Dawn Journal - April 22, 2008

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu, 24 Apr 2008 12:29:01 -0700 (PDT)
Message-ID: <200804241929.MAA10302_at_zagami.jpl.nasa.gov>

http://dawn.jpl.nasa.gov/mission/journal_4_22_08.asp

Dawn Journal
Dr. Marc Rayman
April 22, 2008

Dear Dawnocrats, Republidawns, and Indawnpendents,

Dawn continues its powered flight, having accumulated more than 100 days
of ion thrusting since its launch nearly 7 months ago. All systems are
healthy as the probe patiently and persistently propels itself through
the solar system.

In addition to its weekly hiatus in thrusting to point its main antenna
to Earth for about 6 hours, Dawn's flight plan includes occasional
longer intervals to conduct special activities. On March 31, the
spacecraft stopped its ion beam, turned to Earth, reported on its
activities from the previous week, and indicated its readiness (even
eagerness!) for whatever plans mission control had devised. This period,
scheduled well before launch, was planned to last 10 days.

To begin, the team loaded into the spacecraft's main computer updated
software that simplifies operation of the science instruments. Such
"science blocks" had already been used in the mission, but with the
experience gained from the tests of the instruments in the initial
checkout phase, the team made some improvements. After thorough testing
with instrument simulators, the modified science blocks had been deemed
ready for installation on the spacecraft. They were used during the rest
of the week, as each of Dawn's science instruments received special
attention.

With some of the updated blocks, operators powered on the Gamma Ray and
Neutron Detector (GRaND) (whose name belies
its unassuming demeanor) and let it collect data for about a week. GRaND
is designed to measure radiation from Vesta and Ceres to reveal the
chemical elements that compose the outermost material of these
protoplanets. As described during the first test of GRaND after launch,
gamma rays and neutrons will reach the
instrument not only from the targets it wants to investigate but from
elsewhere as well. Initially these signals were used to verify GRaND's
health. Now scientists want to collect more such data to begin
developing an accurate record of the effects of this omnipresent
radiation. One part of analyzing the signals from the asteroids will be
removing the "noise" that GRaND detects from cosmic rays, so it is
essential to know its characteristics.

While the science blocks streamline the process of sending instructions
through the main spacecraft computer to the instruments, the instruments
themselves have internal computers and software as well. Updated
versions of the software for the science cameras were passed through the
spacecraft computer and installed in the cameras' computers. The new
software includes capabilities that had been planned before launch but
were not needed for earlier tests, and it corrects minor bugs (yes, some
bugs are hardy enough even to survive extended periods in deep space)
discovered during those tests.

After the software was loaded into the primary camera, operators
commanded the instrument through a "minicalibration" to verify that the
installation was successful. Upon completion of the work with the
primary camera, they conducted all the same steps with the backup
camera. The two cameras are essentially identical, so they received the
same software. They are recognized by the spacecraft computer as
distinct devices though, and they are not operated simultaneously, so to
route software to both of them required executing the loading procedure
twice.

The team also conducted new calibration tests of the visible and
infrared mapping spectrometer (VIR). The
unit displayed excellent performance in the initial checkout phase, but
as with most complex instruments, many tests are required in order to
characterize its performance fully. For this calibration, the spacecraft
pointed VIR to the star Canopus. From Earth, the only stars that appear
brighter are the Sun and Sirius, but Canopus is a familiar sight to many
observers besides those who are far enough south to see it from Earth.
Canopus is one of the intrinsically brightest stars for hundreds of
light years, shining brilliantly in the skies of many planets in this
neighborhood of the Milky Way galaxy. When the measurements of Canopus
were complete, Dawn rotated to aim VIR at Mars. At a distance of more
than 55 million kilometers (34 million miles), that was the closest
planet to the spacecraft. Too distant to be observed with any detail,
the red planet provided a good infrared signal for testing the instrument.

Turning their attention away from the instruments, operators loaded the
latest version of software to the backup main computer. Software version
7.0.3 was installed in the main computer on February 15,
and the same update now resides in the
primary and backup locations of the backup computer, ready to be used if
the spacecraft detects a serious problem with the primary computer.

The last planned activity of this period was a test of how accurately
ion thruster #2 can be pointed. (The naming convention for the thrusters
is explained in text in a previous log. For an enjoyable explanation in
another medium, see a performance of the new and popular Dawn pas de
trois.) Such tests were conducted for the other 2 thrusters when they
were checked out during the month after launch. This test was not run
with #2 because the spacecraft was still too close to the hot Sun even
in November when thruster #2 was put through
all of its other tests. (The different positions of the thrusters on the
spacecraft means they experience different temperatures when they are
operated.) There was no urgency in making this measurement, so it was
postponed to this convenient opportunity.

On April 8, the spacecraft oriented itself as required for the test.
Executing the same steps it always does to start a thruster, this time
the ion propulsion system's computer controller detected a potential
problem and halted thruster operation. Because of the orientation of the
spacecraft, the radio signal received on Earth was so weak that data
could be returned only very very slowly. Mission control saw an
indication that the onboard controller had stopped the thruster, so they
radioed new instructions to end the test and turn to point the main
antenna to Earth. Meanwhile, when an onboard system found that there was
no thrust, it issued different instructions to accomplish the same ends:
stop the rest of the test and aim the antenna at Earth. Either set of
instructions would have worked, but the computer trying to process both
sets led to a conflict, so it responded by entering "safe mode."

Safe mode is a standard response designed into the software to deal with
uncertain, unexpected, or difficult conditions. Nonessential systems are
powered off, and essential systems are reconfigured according to a plan
stored in software. The details of that plan were modified in October
and again in January to account for the spacecraft's growing
distance from Earth and the Sun.

In safe mode, just as in the orientation for the test, the use of an
auxiliary antenna greatly limits the amount of data that can be
returned. Although controllers soon recognized the conflict that
triggered safe mode, many steps in a carefully planned and methodical
process were required to reconfigure the spacecraft to point the main
antenna to Earth. To expedite this work, colleagues working on the Mars
Odyssey and Mars Reconnaissance Orbiter projects agreed to exchange
their scheduled use of one of the Deep Space Network's largest antennas,
a 70-meter (230-foot) dish at Tidbinbilla, Australia, with Dawn's use of
a 34-meter (112-foot) antenna at the same communications complex. The
larger antenna allowed the Dawn team to send and receive data at greater
speed; this significantly reduced the time it would have taken to return
to normal operations. Such cooperative use of the shared resources of
the Deep Space Network is one of the many ways missions work together to
the benefit of all space exploration.

By April 11 the main antenna was pointing to Earth and all the data
stored during the aborted thruster test had been returned and analyzed.
Engineers recognized that there had not been a problem after all, and
the thruster could have operated perfectly well. The ion propulsion
control software sets and verifies many electrical parameters and checks
many others to ensure the thruster is performing correctly. In this
case, the software was conducting a check that was unnecessary, so there
was no need for it to interrupt the thruster operation. The test was
built into the software before the ion propulsion system received its
exhaustive test flight on Deep Space 1. With the knowledge gained on
that mission, this software check was determined to be unimportant, but
given the overall complexity of the software, it had not been removed
from the ion controller. The controller dutifully carried out its
programming, not knowing that it was performing an unwanted function.

In all the thrusting conducted so far in the mission (and the far
greater duration of thrusting on Deep Space 1), this unnecessary test
had never tripped. Now engineers were able to calculate that the
conditions required to indicate a (false) problem would arise later in
the Dawn mission with the use of any of the thrusters, but the
conditions would not occur for some time with thruster #3, which has
been the one in use since December.
Therefore, the "go" was given to resume thrusting, and on April 14 the
spacecraft began its powered flight once again. (The entry into safe
mode did not interfere with any special activities other than the
pointing test of thruster #2, as that was the last planned event of this
period.)

Now that the necessity of making a change in the ion controller software
was identified, the fix itself was determined to be quite simple. In
just a few days it was thoroughly tested in a simulator at JPL and was
transmitted to the spacecraft during the next weekly communications
session on April 21.

Dawn is 185 million kilometers (115 million miles) from Earth, or 480
times as far as the moon and 1.24 times as far as the Sun. Radio
signals, traveling at the universal limit of the speed of light, take
almost 21 minutes to make the round trip.
Received on Thu 24 Apr 2008 03:29:01 PM PDT