January 1998 Volume 7 Number 1
Mars Global Surveyor and TES Update
--- December 19, 1997 ---by Greg Mehall, Arizona State University
September 11-- We Are In Orbit!
After 14 years of hard work-- carried out by hundreds of people-- I am pleased to say that TES is finally orbiting Mars and doing very well!
On the evening of September 11, 1997, the Mars Global Surveyor (MGS) spacecraft performed a flawless Mars Orbit Insertion (MOI) and entered into a 45-hour elliptical orbit around the Red Planet. It had been over 20 years since the U.S. placed a spacecraft in orbit around Mars.
Two days later, the Thermal Emission Spectrometer (TES) team gathered at Arizona State University (ASU) to view the first TES data collected from Mars. The TES instrument is controlled and monitored from our mission operations facility here at ASU. From this facility we design the mapping strategies, generate TES command uploads, send TES commands to the spacecraft, and monitor the TES data and telemetry.
September 13-- TES Powered On!
The TES was turned-on just two hours after the start of the second orbit on September 13th. The first data received at ASU showed that the instrument was working perfectly. The TES team was thrilled to see the first thermal infrared spectra of Mars! The other MGS science instruments (camera, laser altimeter, magnetometer) were also turned on during this orbit and were checked-out successfully. At the start of the third orbit the "Nadir" deck of the spacecraft was pointed towards Mars near the orbit's closest approach (periapsis) to allow the science instruments to view Mars during the 30 minute pass.
Mid-September-- Aerobraking Begins!
Following the first science instrument check-out, the spacecraft began the "Aerobraking Phase" of the mission. MGS successfully performed six "walk-in" maneuvers on orbits 3 through 11 to slowly lower the orbit's periapsis altitude from about 155 miles to the desired aerobraking altitude of about 65-75 miles above the planet. Once the spacecraft started to pass through the desired atmospheric corridor, the drag of the atmosphere against the MGS solar panels began to slow the spacecraft by about 10 miles per hour on each pass. At this rate, the orbit period decreased by approximately 1 hour for each pass through the atmosphere. The spacecraft performed the first four aerobraking passes at this altitude perfectly.
Early October-- Mars and Solar Panel Don't Cooperate...
However, on orbit 15 the spacecraft encountered a 50% increase in the density of the atmosphere as it passed through for aerobraking. This increased density caused one of the two solar panels, called the "-Y solar array," to be bend more than it should have. This is the same solar panel that was damaged after launch (see TES News, March 1997) and was jammed 20 degrees short of its fully deployed, latched position (see diagram).
October-- Aerobrake Hiatus Provides Science Opportunity
During the aerobraking hiatus, the spacecraft was re-oriented so that the "Nadir" deck, containing all the science instruments, could look straight down (i.e., nadir) during each orbit's periapsis (the closest point to the planet during each elliptical orbit). The spacecraft performed these "peri-scan" observations from orbit 20 through 36, allowing the instruments to collect contingency science data in the event that the solar array problem could not be resolved.
October-- New Mission Scenarios Discussed and Examined
The initial concern with the solar panel problem was that any additional aerobraking might pose too much risk to the spacecraft. In this scenario, the orbit period could be partially reduced using the remaining propellant on board the spacecraft. However, the resulting mapping mission would have to be performed from a larger 18 hour elliptical orbit instead of the circular 2 hour mapping orbit that was originally desired. This would have resulted in a significant, negative impact on the scientific objectives of the MGS mission. During the aerobraking hiatus, thorough analysis of the aerobraking data along with additional data collected from both computer simulations and laboratory testing of the back-up solar arrays (at Lockheed-Martin in Denver, CO) were performed to better understand the problem.
October/November-- Solar Panel Damage Examined
After three weeks of analysis and testing, the MGS Project decided that the problem was sufficiently understood and characterized to resume aerobraking on orbit 36 (November 7, 1997). The decision of the MGS Project to resume aerobraking was based on the following conclusions made by the MGS engineering team:
The analysis and tests just concluded indicate that a secondary failure occurred when the -Y Solar Panel deployed undamped after launch as a result of the damper arm breaking off. At that time and throughout cruise, we only observed the result of the damper arm lodging in the panel's hinge point that prevented it from opening all the way. It wasn't until we saw the effects of the pressure of the Martian atmosphere on the panel that we recognized that something else was wrong.
The additional failure acts like an another flexure point or hinge, that allowed the panel to deflect more than we would have expected during high pressure drag passes. Although we don't know for sure, it is possible that the panel moved to its fully open position and latched during periapsis passes 11 and 12. Now we can go ahead with the mission because we understand why it flexes as it does, and we plan to learn how much it can flex before something catastrophic happens.
November--- Aerobraking Resumes.... Carefully
Since aerobraking resumed on November 7th, the spacecraft and -Y solar array have been performing quite well. In order to avoid further damage to the panel, the pressure that the panel has been subjected to has been about one third of the pressure that was experienced during orbit 15. This reduced pressure corresponds to a periapsis altitude of about 75 to 85 miles. As of mid-December, the spacecraft had executed 22 successful passes through the martian atmosphere and the orbital period had been reduced to under 30 hours. At the current rate, the orbit period decreases by 10-15 minutes during each pass through the atmosphere.
New Aerobrake Plan Means New Mission Plan
Before the latest solar panel problem was encountered, MGS was scheduled to start the mapping phase of the mission in mid-March 1998. The goal was to put MGS in a nearly circular, 2-hour, polar "Mapping Orbit". In this orbit the MGS would pass over the North and South poles of Mars once every 2 hours. The mapping orbit would also ensure that the spacecraft passes over the equator at the same local time of day on every orbit (2 p.m. on the day side). This is so that the science instruments will always collect data under similar viewing conditions on each orbit. The original "Mapping Mission" duration was to be 687 Earth days, to allow MGS to observe the planet for an entire Martian year.
However, due to the solar array problem and resulting aerobraking hiatus, the mission plan had to be revised. In order to meet these objectives, the entire aerobraking plan had to be modified to take into account the reduced aerobraking capability and for the time lost during the hiatus. The more gradual aerobraking rate has resulted in a one-year delay in the start of the 687-day Mapping Phase.
New Schedule for 1998
Under the current plan, Mars Global Surveyor will carefully aerobrake up to about May 1998. Then it will stop for a while because Mars will be on the opposite side of the Sun relative to Earth-- this is called "Solar Conjunction". By May, the spacecraft's orbit period will be down to approximately 12 hours if no further problems are encountered. At that time the science instruments on MGS will powered off and the spacecraft will be placed in a safe orbit for two weeks while Mars passes behind the Sun.
Following Solar Conjunction, the "Science Phasing Orbits" will begin. During this period, the spacecraft will spend six months without doing any aerobraking. Instead, the science instruments will be pointed at Mars during each periapsis pass. This period will be used to collect additional contingency science data as well as allowing time for Mars to move into the correct alignment with the Sun for global mapping. After this six month hiatus, the science instruments will be turned off (due to power constraints), and the final aerobraking phase of the mission will begin around November 1998.
New Mapping Orbit For 1999
The May to November 1998 hiatus is needed to delay the start of the final aerobraking phase so that an acceptable Mapping Orbit can be reached. The last aerobraking phase will take the spacecraft to its final mapping orbit in March 1999. In this new mapping orbit, the spacecraft will pass over the Martian equator from north to south at 2 a.m. local Mars time during each orbit. In the original plan, the mapping orbit would have passed over the equator from north to south at 2 p.m. From the perspective of the science instruments, the new orbit will look just like the original orbit. This new plan was designed to minimize the risk of damaging the spacecraft while maximizing the probability of the achieving the mission's science goals. Once the spacecraft reaches its Mapping Orbit in March 1999, the plan is to go ahead with the original 687 Earth-day mission. The spacecraft will map Mars in detail for one full martian year, up through about January 2001.
TES-- Collecting Data, Adjusting to New Plans
Since the TES was turned on in September, it has collected large volumes of unique data of both the martian surface and atmosphere. The instrument has already acquired more than one million spectra of Mars. So far, TES has worked beyond our expectations and is giving the scientific community valuable new insights into martian geology and atmosphere.
Our present plan is for TES to continue collecting data until the final aerobrake phase in late 1998. At that point it will probably be turned off until the mapping mission begins. Although the TES wasn't designed to collect data in the elliptical aerobraking orbits, it has proven very versatile in collecting useful data in these orbits. Since TES has a rotating pointing mirror, it as able to collect Mars data throughout the entire aerobraking orbit. This allows TES to "see" Mars during all three phases of the aerobraking orbit: the aerobraking drag pass, the post-drag pass "roll-out", and the array normal spin (called "ANS"). The spacecraft is spinning (ANS) during most of its orbit. It stops spinning before it goes into the "drag pass" through the martian atmosphere (during aerobraking), then "rolls out" to resume spinning after each aerobrake pass.
What TES Is Doing
The data collected during the drag pass are acquired when the spacecraft is closest to Mars (at periapsis). During aerobrake drag passes, the instrument deck on MGS is pointed opposite to the direction of travel, so the TES mirror has to be pointed 90 degrees down in order to see the martian surface. The TES is also able to scan its mirror in such a way that it can get a temperature and pressure profile of the martian atmosphere.
The data collected during the "roll-out" are acquired when the spacecraft has finished its drag pass through the atmosphere and is maneuvering back to its ANS attitude. In this mode, the instruments are swept across the planet over 15 minutes. This allows the TES to collect a single swath across the planet. The ANS observations make up the remainder of the orbit and account for the majority of the TES data. In this mode, the spacecraft's high gain antenna is pointed directly at the Earth to allow communications. While the antenna is pointed at the Earth, the spacecraft performs a complete roll, or spin (with the high gain antenna at the spin axis) every 100 minutes. By continually adjusting the TES pointing mirror during this roll, we are able to build up TES images of the disk of Mars. These observations allow us to monitor the southern hemisphere of the planet on a more global scale (like in the picture located HERE).
What TES Is Seeing
The close-up TES data collected during the drag and roll-out passes give us the most detail of the planet and the atmosphere ( CLICK HERE for examples of TES Spectra of Mars). These data have given us the best measurements of the martian geology. On each pass, the TES can observe the abundance of minerals, such as pyroxene (SEE PYROXENE FIGURE), as the spacecraft passes over the surface. As these data are analyzed, we plan to build up mineral maps of entire surface of Mars. The drag pass data also allow us to make detailed measurements of the martian atmosphere; we can monitor the pressure, temperature, density and winds aloft in the atmosphere just like meteorologists (weather-persons) do on Earth.
TES Spies Huge Dust Storm
The MGS project is especially interested in the TES atmospheric data since it provides valuable insight into the density of the atmosphere. This helps the flight team to plan subsequent aerobraking activities while minimizing the risk to the spacecraft. This is especially important because large dust storms, sometimes global, are known to occur at Mars during this time period.
On orbits 41-60, a large storm did form in the southern hemisphere! We were able to use TES to monitor the size and intensity of this storm. The global snap-shots of the southern hemisphere allow us to monitor the position of the storm every 100 minutes.
Other MGS Findings So Far
The Mars Orbiter Camera (MOC) on MGS has provided stunning images of the surface of Mars (see Valles Marineris Figure and Nirgal Vallis Figure). These images have shown unique details that were not detected by the Viking orbiters in the 1970's. However, because it doesn't have a pointing mirror, MOC can only image the planet for the short 20 minute "roll-out" period following each aerobraking drag pass and during the "peri-scan" orbit passes that occur when we are not aerobraking. The Mars Orbiter Laser Altimeter (MOLA) has obtained valuable topographic profiles across the martian landscape on each of the "peri-scan" passes that were done in September and October. The Magnetometer and Electron Reflectometer (MAG/ER) have also collected interesting data, early results have found magnetic anomalies at various locations on the surface, but there is no sign of a global magnetic field.
Check Our Web Page for the Latest News!
The TES Web site (http://tes.la.asu.edu/) contains some of the latest TES surface and atmospheric results from Mars. The TES team plans to submit its first scientific paper about their results to the journal, Science, by the end of January 1998. In the next TES News, I will update you on the latest discoveries made by the TES at Mars. 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.