May 1997 Volume 6 Number 2

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-- April 8, 1997 Status --

Mars Global Surveyor and ASU's Thermal Emission Spectrometer

by Greg Mehall, TES Systems Engineer, Arizona State University

"Wave bye bye" as Mars Pathfinder Goes Past. Since my last report in the March issue of TES News, things have been fairly quiet with Mars Global Surveyor (MGS). The spacecraft continues to operate normally during this "cruise" phase of the mission. On March 15, the Mars Pathfinder spacecraft finally "passed by" (within 4.7 million kilometers; 2.9 million miles) MGS on the way to its July 4th landing on Mars. Even though Mars Pathfinder launched a month after MGS, it is on a faster and more direct flight path to Mars. The longer MGS flight path will allow the spacecraft to enter Mars orbit using much less fuel than if we used the same trajectory as Mars Pathfinder. This is necessary because the MGS spacecraft must carry additional fuel to put it into its final two hour mapping orbit and to maintain this orbit for the two year mapping mission.

MGS is cruisin'. From March 20 through April 21, Mars Global Surveyor executed the "C-6 Cruise Command Sequence". This sequence started with the second trajectory correction maneuver (TCM-2) on March 20. At that time, the on-board flight computer commanded the spacecraft's main rocket engine to fir for six seconds in order to make minor corrections in MGS's flight path. This was the second in a series of four trajectory correction maneuvers that are designed to refine the spacecraft's flight path to Mars. The third (TCM-3) and fourth (TCM-4) rocket firings are currently scheduled for April 21st and August 25th.

Half Way There! On March 31st, MGS passed the mid-point of its flight path to Mars. For the first time, Mars Global Surveyor was closer to Mars than to Earth. At that point, MGS was approximately 57 million kilometers (35 million miles) between the two planets. The half-way point measured in terms of days to reach Mars occurred on April 10th. At that point, MGS was traveling with a velocity of 25.1 kilometers per second (15.6 miles per second) relative to the Sun. At the half-way point, the time for a signal from Earth to reach MGS was about 3 minutes, 42 seconds.

Solar Panel Jammin'. Siginificant effort, including testing and analysis, has been performed over the past few months to attempt to clear the obstruction that's jamming the "-Y" (pronounced "minus-why") solar array. Although these tests helped to characterize the problem, they did not succeed in clearing the obstruction (see March 1997 TES News for a diagram of the stuck solar panel). The Mars Global Surveyor Project Office at the Jet Propulsion Laboratory (Pasadena, CA) has decided to suspend further testing and to go ahead and redesign the aerobraking phase of the mission by assuming that the panel will remain jammed. The redesigned aerobraking sequence will place the front (solar cell) side of the -Y panel into the flow of the atmosphere on each aerobraking pass. Initially, engineers were concerned that this would overheat the solar cells and damage the array, but further analysis has shown that this is not the case. This redesign poses very little risk and will still allow us to reach the correct mapping orbit by March 1998.

ASU's TES-In-Space. The Thermal Emission Spectrometer (TES) has remained powered off since its November 1996 check-out. When the TES is off we can only monitor the temperature of the instrument at the TES Mission Operations Facility at Arizona State University. As the MGS travels further from the Sun the temperature of the TES, as well as other parts of the spacecraft, continue a general cooling trend. We monitor these data as well as other spacecraft telemetry daily to ensure the health of the TES instrument. When the TES is powered on at Mars, we will design mapping strategies, generate TES command uploads, send TES commands to the spacecraft, and monitor the TES data and telemetry here at the ASU facility.

TES Planning For September. The TES team at ASU is currently in the process of hiring the mission operations staff that will control and monitor the TES on a daily basis. We are also working on the software that will be required to complete these daily tasks. The next powered-on test of the TES is scheduled for the week of August 4. We will be testing several potential modifications to the on-board TES flight software during this test. In addition to these tests we hope to be powered on, along with the Mars Orbiter Camera (MOC), when MOC will be obtaining approach images about 20 days before reaching Mars.

During the remaining months of cruise we will also be focusing our efforts on preparing for aerobraking and mapping activities that will occur after the successful Mars Orbit Insertion (MOI) on September 12, 1997 (Universal Time; or just before 6:30 p.m. on September 11, Arizona time). We are currently planning to make numerous measurements of Mars during the six month aerobraking period. I will update you on these plans and the remaining instrument cruise activities in the next edition of TES News. Until then... to Mars!

TES News is published quarterly by the Arizona Mars K-12 Education Program. This newsletter may be copied for EDUCATIONAL PURPOSES ONLY.

EDITED BY Kenneth S. Edgett, Arizona Mars K-12 Education Program, Arizona State University, Tempe, Arizona, USA.