TES NEWS, Volume 5, Number 2, June 1996
TES Meets Mars Global Surveyor
It's "Bolted On!"
by Greg Mehall
TES Systems Engineer,
Arizona State University
TES In Denver.
The past four months have been very exciting, as well as hectic,
for the Thermal Emission Spectrometer (TES) project. Since
we've worked around the clock to complete the final testing on the
instrument so that it could be installed on the Mars Global Surveyor
(MGS) spacecraft. The original delivery date to the spacecraft contractor,
Lockheed Martin Astronautics (LMA) in Denver, Colorado, was April 1, 1996.
However, as we entered into the final phase of instrument testing at the
beginning of March, we discovered several serious problems with the TES.
As a result, we missed our planned delivery date. Since then, we have
worked frantically to meet an ever-shrinking schedule. I'll get into the
problems in the next few paragraphs-- however, I am happy to report that
we were finally able to solve the problems and we delivered the TES to
LMA on May 28, 1996. MGS is scheduled to launch Nov. 6, 1996.
Shaken, not Stirred.
In my last update, the TES was entering the final phases of the
instrument testing and characterization. At the end of February,
the TES was subjected to vibration testing. This test simulates
the vibrations that the TES will experience during launch on the
Delta II rocket. TES was attached to a vibration table at Hughes
Santa Barbara Remote Sensing (SBRS, in Goleta, California) and was
vibrated at various levels. These levels exceed the launch vibration,
in order to ensure that the TES won't break during launch. It was very
nerve-racking to watch the TES being shaken on the table!! It's amazing
that such a sophisticated optical and electronic instrument is able to
survive such a violent event! After the testing was completed, TES's
performance and optical alignment were checked for any signs of
degradation. We were all relieved when the TES passed all of these
tests with no problems.
Pump it Down.
After vibration testing, the TES was ready for the thermal/vacuum
chamber by the end of February. This was the most important phase
of instrument testing and had to be completed successfully prior to
delivery. The TES instrument was placed in the vacuum chamber on
February 29, 1996, and was to be tested for three weeks. This test
helps us understand how well the TES will perform under the various
conditions it will experience during the mission in space. The data
we get during this test allows us to "calibrate" the instrument so
that the scientists who use the TES data from Mars can interpret
it accurately. The chamber was "pumped down" to simulate the vacuum
of space over a temperature range of -20 C to +40 C (-4 F to 104 F).
Image, below: TES engineer Greg Mehall (front) working at Hughes
Santa Barbara Remote Sensing, Goleta, Calif., during Thermal-Vac
testing. The thermal vacuum chamber is behind Greg. Photo by
P. Christensen, March 1996.
At the start of the test we warmed the chamber to +40 C for two days
in order to "bake out" all of the contaminants (like water, carbon
dioxide) that might be in the chamber and in the TES. Although the
performance of the TES
dropped slightly at this elevated temperature, it still was within
the acceptable range. Next, we started lowering the temperature of
the chamber and the TES towards -20 C. As the temperature dropped,
we noticed that the infrared spectra, which are the primary data from
the TES, began to degrade. The colder we went, the worse it became.
We could not figure out why the instrument was performing so poorly
at these cold temperatures. The bad luck continued when we decided
to warm the instrument back up to room temperature and "break vacuum".
As we started warming the TES, the signals on the other two detector
arrays began to behave strangely. They continued to behave this way
until the TES approached room temperature. In addition, during this
cold-to-warm transition, the primary neon lamp failed and the TES
switched over to its backup. This was alarming since the neon lamp
is a critical element of the TES spectrometer. When the TES finally
warmed up, we turned it off and opened the thermal-vac chamber.
It was frustrating. We assembled a team of experts to try and
determine what might be happening. We spent the next week analyzing
the ~500 Mbytes of data that we collected in an attempt to develop
some hypotheses. We also started working on adding additional test
connectors to the instrument to help analyze the problem the next time
we would cool the instrument down. On the second try, we tested out
several of our hypotheses but were unable to determine the exact cause
of our problem. However, we were able to eliminate most of these
hypotheses, thereby narrowing it to a couple of possibilities. We
determined that it had to be a temperature-dependent misalignment of
one or more of the optical elements in the TES interferometer. The
interferometer (also known as the spectrometer) is the subsystem of
the TES that generates the signals (interferograms) that will be used
to measure the spectra of the rocks and minerals on Mars. After we
removed the TES from the chamber the second time, we took it down the
hall-- back to the optics lab-- and opened up the TES aft optics cover.
Image, below: Neon Lamp (like a light bulb) from the TES instrument.
We were able to measure the positions of the various optical elements
in the lab over a smaller temperature range. We determined that the
degradation that we had been seeing in the chamber was due to a very
slight misalignment of the Cesium Iodide beamsplitter over the range
of temperatures. The beamsplitter is the primary optical element in
We were able to determine that it was moving about 5 millionth's of a
meter over the ~60 C (140 F) temperature range. Amazingly, this was
enough to seriously degrade the performance of the TES at the cold
temperatures. We finally discovered that the movement of the beamsplitter
was caused by the epoxy ("glue") that we used to hold the beamsplitter
Once we determined the cause, we spent the next month and a half
redesigning, refabricating and installing a new beamsplitter. We also
fixed the problem with the neon lamps, and we modified the two detector
arrays that were behaving strangely (they were "self-oscillating").
These modifications were completed by the end of April and the instrument
was "buttoned up" and prepared for a second vibration test.
More Tests, Fixes.
After successful completion of the second vibration test, we prepared
for our third try at the thermal/vacuum test. We started the third
round on May 5, 1996. Five days into this test we noticed that two of
the three temperature sensors mounted on the TES internal calibration
surface were not reading out the correct temperature. Without these
sensors it would be very difficult to accurately calibrate the data
collected by the TES at Mars. We decided to "break vacuum" one last
time to repair these sensors. They were replaced within three days.
After completing the repairs we started the fourth and final round
of the thermal/vacuum testing on May 14, 1996. The testing lasted
twelve days and the TES finally performed as expected over the entire
temperature range. We were quite relieved! We were able to collect
all of the data we needed to be able to calibrate the observations
that we will eventually get from Mars.
Image, below: Hughes aircraft getting ready to ferry the TES from
Santa Barbara, California, to Denver, Colorado. Photo by P. Christensen,
May 28, 1996.
Flight to Denver.
After the successful completion of thermal/vacuum testing, the chamber
was opened and the TES was removed. The "Pre-shipment Bench Acceptance
Test" was performed to get a benchmark of the instrument's performance
prior to shipment. After this test was completed, the TES and its test
equipment were packed and prepared for shipment to LMA in Denver. On
May 28, 1996, five members of the TES team carried the TES instrument
aboard the Hughes Aircraft corporate jet and flew with it to Denver.
The TES team was met at the airport by LMA personnel who took the TES
to the Mars Global Surveyor assembly facility. The following
day, the test equipment arrived by truck from Goleta, California, and
the "Post-shipment Bench Acceptance Test" was performed on the TES in
order to verify it made the trip safely and without damage. The
instrument passed this test and was officially transferred to the
responsibility of the Jet Propulsion Laboratory and Lockheed Martin
for integration with the rest of the MGS spacecraft.
TES Meets MGS.
The TES was installed on the spacecraft "nadir deck" on May 31, 1996.
The nadir deck is the panel that holds the MGS scientific instruments
and is the side of the spacecraft that will face the planet when it
orbits Mars. The TES was the final instrument to be installed on the
spacecraft. The Mars Orbiter Laser Altimeter (MOLA) was installed two
days earlier, and the other instruments had been there for several
During the TES installation, the TES was hoisted into its position
on the nadir deck and then bolted in-place. Next, the electrical
interface between TES and the spacecraft was verified by the "Initial
Power-On Test" (IPTO). This was an extremely important test, because
any mistakes in the electrical connections could severely damage the
spacecraft and/or the TES. When the spacecraft finally applied power
to the TES, the instrument's computer initialized itself and the TES
pointing mirror began to turn just as it was supposed to! Shortly
after that, our test equipment began to receive the data from the TES!
This was a major milestone for us: the TES was now an integrated
part of the MGS spacecraft!
Image, below: TES Principal Investigator, Phil Christensen, with the
new MGS Thermal Emission Spectrometer, May 1996.
TES-ting the Spacecraft.
The next day, the TES "Functional Electrical Test" (FET) was performed.
The purpose of this test was to verify that we could successfully run
our instrument using the new spacecraft commanding interface. We
re-ran the TES "Bench Acceptance Test" and the instrument functioned
properly. On June 3-5, 1996, we participated in the "Spacecraft Functional
Test," which was the first test in which all of the spacecraft sub-systems
and payloads were installed and operating. The purpose of this test was
to verify that the spacecraft subsystems and the science instruments did
not degrade each others' data by interacting with each other. This test
also simulated the mapping mode we will be in when we start systematically
observing Mars in early 1998. The test was only partially successful,
because several problems were encountered that need to be addressed.
This test will be fine-tuned and run again before the spacecraft gets
shipped to Florida.
The Weeks to Come.
The final preparations for the spacecraft thermal/vacuum testing are
being performed during the first two weeks of June. Thermal blankets
(the gold covering that looks like foil) are being put on the spacecraft,
and temperature sensors are being placed on the spacecraft and throughout
the vacuum chamber. On June 16, 1996, the entire spacecraft will be
placed in the 30 m (100 ft) tall by 15 m (50 ft) diameter chamber.
The chamber will then be "pumped down," and the entire spacecraft will
be tested over various temperatures and configurations, just as we did
with the TES in May.
The thermal/vacuum testing is scheduled for the last two weeks of June.
The TES will be turned on during the first half of the test when the
spacecraft is in the mapping simulation
mode. This will be the best opportunity for us to verify the performance
of the TES with all elements of the spacecraft operating prior to launch.
After spacecraft thermal/vacuum testing is completed, the spacecraft will
be removed from the chamber and the final alignments and preparations for
shipment will be performed.
The MGS team is aiming for an August 16, 1996, delivery of the spacecraft
to Cape Canaveral, Florida. The spacecraft will be loaded aboard a
military transport plane and flown to the Air Force base at Cape Canaveral.
Once there, it will be tested some more, then placed on top the Delta II
launch vehicle. I will update you on final stages of the spacecraft
integration and testing as they are completed over the next few months in
the next edition of the
Until then... To Mars!!
Back to Contents of TES News June 1996