As an automobile ages, it usually requires more trips to a mechanic. Spacecraft are designed for a primary mission; when all is going well, the mission is often extended to obtain more data for other research objectives. But spacecraft are also subject to the ravages of time and wear.
The Mars Global Surveyor is now in its second extension following a very successful primary mission and is providing scientists with valuable information about how the red planet changes over time. Since there are no service garages in space, mission planners have to perform any needed repairs from the ground, including finding ways to save fuel.
Fuel Savings Important
To make mission extensions possible, it is important to economize on the fuel that keeps the spacecraft in its proper alignment in space. The spacecraft's power comes from its solar panels, but it needs fuel to continually adjust its position in space so it can keep itself steady to make observations and turn to communicate with Earth. The spacecraft is currently in a position 16 degrees off-nadir, looking slightly backwards. (Nadir is the point directly below the spacecraft.) Scientists call this position Relay 16. This saves fuel because in this orientation the spacecraft is more aerodynamic, thus reducing drag that occurs when any vehicle travels through air or space. The more fuel Surveyor saves, the longer the mission can be extended to continue science activities.
The off-nadir position also aids scientists in their studies, since it enables them to obtain three-dimensional images of some areas already imaged.
Problem-Solvers "R" Us
"We've been so successful because the operations team has always found a way to overcome any problem the mission has had over all the years it's been in operation," said Mars Global Surveyor Mission Manager Gene Brower. "The high-gain antenna fix is a prime example."
Shortly after mapping began in March 1999, the high-gain antenna experienced an obstruction when commanded to point at directions less than 41.5 degrees azimuth. (Altitude and azimuth form an astronomical coordinate system for defining position.) In consequence, during times of the year when the Earth reaches this location as seen from Mars, a special operation known as a "beta supplement" is required. Between March 2002 and September 2003, the high-gain antenna must be flipped over against its boom to drive it in the opposite direction. The antenna must then flip back as it crosses the equator of Mars each orbit in this configuration. All of this movement combines to "disturb" the motion of the spacecraft slightly, increasing momentum buildup in the reaction wheels that control attitude. Excess momentum is removed by using the thrusters to apply torque. This technique consumes fuel too, but that use is partially offset by the off-nadir configuration of the spacecraft.
"That ingenious fix solved the problem," Brower said.
Do Spacecraft Age?
"I do get concerned as Surveyor ages," said Mars Global Surveyor Project Manager Tom Thorpe. "The spacecraft has gone into contingency mode four times during the past six months, but each time we were able to restore communications."
The contingency mode situation is the result of confusion with the spacecraft's star tracker. The star tracker fixes on a star pattern to help the spacecraft orient itself in space. Several times, the tracker fixed on a wrong star field after the spacecraft turned back from off-nadir targeting. The onboard software wasn't originally programmed to account for the Relay 16 attitude and the star tracker became confused. This caused the spacecraft to turn the high-gain antenna away from Earth. A built-in script placed Surveyor in contingency mode, as it was programmed to do. Ground controllers restored communications with the spacecraft and are now updating the software code. "This fix should keep us out of star sensor-induced contingency mode," said Thorpe.
"We're confident in the Surveyor's abilities and, all in all, we have a healthy spacecraft," he added.
The Global Surveyor has studied the entire martian surface, atmosphere, and interior, and has returned more data about the red planet than all other Mars missions combined. Observations by the spacecraft are expanding our understanding of the martian climate and may indicate the climate is changing significantly even today. Among key science findings, Surveyor has taken pictures of gullies and debris flow features that suggest there may be current sources of liquid water, similar to an aquifer, at or near the surface of the planet.