September 1997 Volume 6 Number 3
First picture of Mars from MGS camera, obtained July 2, 1997. Courtesy Malin Space Science Systems
Ready for MOI-- Mars Global Surveyor and ASU's Thermal Emission Spectrometer
--August 21, 1997 Status--by Greg Mehall, TES Systems Engineer, Arizona State University
Long, Cold Journey-- To Mars!
Since my last TES News update many exciting events have taken place on the Mars Global Surveyor(MGS) Project, including our first TES observation of Mars! The "cruise" phase of the mission is winding down quickly as we approach the MGS Mars Orbit Insertion (MOI) activities that are scheduled to take place on the evening of September 11, 1997. Since the November 7, 1996, launch, MGS has covered nearly 500 million miles on a semi-elliptical trek between the planets. It's been a long, cold journey to the Red Planet, but we're almost there. I hope that by the time many of you read this, we are celebrating a successful entry into Mars orbit!
Cruise Action-- May 1997
MGS's cruise activities in the past few months have included a checkout and testing of each of the sub-systems aboard the spacecraft, as well as the final preparations for MOI and aerobraking. During testing in early May, the spacecraft automatically entered into "Safe Mode" after it lost inertial reference during a spacecraft gyroscope calibration test. This test involved commanding the spacecraft to rotate in various directions in order to check the performance of the Inertial Measurement Unit's (IMU) gyroscopes. The MGS on-board computers are programmed to keep track of the orientation of the spacecraft in space. They do this by calculating the expected positions of the Sun and various stars, and then MGS periodically measures their positions using the Sun Sensor Assemblies and the Celestial Sensor Assembly.
The spacecraft first entered into "Contingency Mode" when the direction to the Sun, as measured by the Sun sensors, disagreed with the predicted direction to the Sun by about 5 degrees. Entry into "Safe Mode" occurred about five hours later when the MGS computers encountered a software glitch. This event posed no danger to the spacecraft because the MGS computers responded exactly as programmed. After correcting the pointing and software issues, MGS was commanded out of "Safe Mode" in mid-May and has operated flawlessly since. Even though these events created a lot of extra work, they actually ended up being an excellent "Operational Readiness Test" for how to recover the spacecraft from "Safe Mode" if it ever happens again.
Gyroscopes (yes, like those toys...!) Point the Spacecraft
An unrelated problem with one of the MGS gyroscopes occurred in mid-May when the flight computer powered down the IMU's gyroscope #2. This power-down occurred automatically when the electrical current used by the gyroscope exceeded a pre-determined limit. The functions of gyroscope #2 were automatically switched to gyroscopes #1 and #3. As long as these back-up gyroscopes continue to operate normally, there should be no impact on the ability of MGS to point at targets in space and at Mars.
Mars-- Ready or Not, Here We Come!
Despite the little glitches that occurred back in May, the spacecraft and science experiments are all in excellent condition and are ready to begin exploring Mars in September. On August 25th, the final cruise sequence was set to perform the fourth Trajectory Correction Maneuver (TCM-4; see Figure 1). This final TCM is designed to place the spacecraft's flight path about 188 miles above the Martian surface in order to go into orbit with the MOI maneuver. The actual MOI events are called the T1 Sequence. T1 includes the pressurization of the propulsion system (rocket engines) three days before MOI.
MOI Events-- What to Expect on September 11th
To go into orbit, the spacecraft's main engine is pointed in the direction it is travelling and fires for about 25 minutes to slow the spacecraft down. In other words, this rocket firing acts as a break. MGS needs to be slowed, or it will zip right past the planet.
The main engine firing will begin around 6:15 p.m. Arizona time (MST) on Thursday, September 11. This burn is designed to place the MGS in a highly elliptical, 45 hour orbit about Mars (See Figures 1 and 2). Mars Global Surveyor should be in orbit by about 6:45 p.m. Arizona time (MST) on September 11, 1997!
Mars Global Surveyor Quick-Guide
First Science Observations From Mars Orbit Since 1989
After the spacecraft completes its first orbit and all of its systems are checked out, the science instruments will be powered on and pointed at Mars during the next closest approach. This will be the spacecraft's second orbit, and should occur September 13-15 (Arizona time). The last time Mars was seen from orbit was March 1989, when the U.S.S.R.'s Phobos 2 obtained some modest amounts of data before the spacecraft failed. The U.S. hasn't had a functioning orbiter going around Mars since Viking 1 completed its work in 1980.
Aerobraking-- Friction is the Key to Slowing Down
Following the "Science Orbit," the spacecraft will begin the aerobraking phase of the mission. Aerobraking is the process by which MGS will use the atmosphere to help slow it down. On each orbit around Mars, there is a closest-approach point called the "periapsis". In the case of MGS during aerobraking, this periapsis part of the orbit occurs slightly inside the upper atmosphere of Mars. As MGS passes through the upper atmosphere, the friction of the spacecraft pushing against the molecules of gas will help slow it down. The aerobraking phase starts with several rocket burns on successive orbits that will shrink the orbit so that MGS begins to pass through the upper Martian atmosphere during each periapsis. Figure 3 shows how the aerobraking will change the orbits of MGS as it goes around Mars during its first 56 days.
TES Watching for Dust Storms during Aerobrake Phase
Just as MGS reaches Mars in September, martian "dust storm season" will be starting. It will be the beginning of autumn in the northern hemispere on Mars. Experience with Mars over the past few centuries has shown that the planet can start having some pretty vigorous dust storms starting in northern autumn and on through the winter. Dust storm season won't end until about May 1998. Sometimes, as in 1971 and 1977, these dust storms can become global-- they can obscure the whole planet! If large regional or global dust storms occur during the aerobraking phase of the mission, they will change the temperature and pressure of the upper atmosphere-- and thus would impact the manner in which MGS conducts its aerobraking.
To monitor for dust storm activity, our TES instrument will stay turned on from Orbit 2 onward through most of the aerobraking activities. The current plan has the TES on through at least early December. During this period TES will collect large volumes of data of both the surface and atmosphere of Mars.
Aerobraking is currently scheduled to run through mid-January when the MGS orbit is reduced to a 2-hour period. For the next two months after that, the orbit is tweaked into the final mapping orbit. The 687-day mapping mission will start in mid-March 1998.
TES turned on in August-- Twice!
The TES was turned on for 4 days during the first week of August. During this period we tested all of the systems inside TES to ensure that it is still operating correctly after its long hibernation enroute to Mars. The results from this test? TES is in good shape and operating perfectly. The second part of this test was to check out several modifications to the on-board TES software in an attempt to improve the capabilities of the instrument. This is very similar to getting an upgrade for the operating system on your personal computer. All of the flight software modifications were successfully loaded into the TES computer.
First Light-- TES Sees Mars on August 19, 1997
The TES was also powered on August 19-21 to look at the Red Planet for the first time. During this test, the spacecraft was maneuvered to point TES and the camera, MOC, at Mars. On August 19, MGS was about 3.5 million miles from Mars and closing in at about 150,000 miles per day. If you could stand at this distance, Mars would appear 5 times larger than Venus appears from the Earth. This test was made up of eight separate images of Mars spaced at 45 degrees of longitude around the planet. Since the TES is not capable of generating images like the MOC, we attempted to simply measure the thermal infrared spectrum of the entire planet, like we did with Earth last November (see March 1997 TES News).
We were happy to see that the data from the first image showed a weak, but noticeable spectrum of Mars! What we could see in this spectrum is an absorption of infrared light caused by the presence of carbon dioxide gas (CO_2). The TES had received its first photons (light) from the Red Planet! Meanwhile, the MOC team was taking their eight Mars images to generate a movie of the rotating Mars which will be released to the public during MOI.
When the TES is powered on September 13th, we will be designing infrared mapping strategies, generating TES commands, sending TES commands from our ASU lab to the spacecraft, and monitoring the TES data as it comes in to ASU. We also plan to generate up-to-date Mars surface and atmosphere observations that can be viewed by the public on our Web site at http://tes.asu.edu/asu_tes/. I will update you on the post-MOI activities and the first TES data collected at Mars 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.