Concern over the deployment mechanism for the high-gain communication antenna on the Mars Global Surveyor spacecraft has caused NASA managers to consider postponing the antenna's deployment in order to maximize the probability of mission success.
The project team is studying a postponement of up to nine months in the antenna deployment, which currently is scheduled to take place in March 1999. The spacecraft, now in orbit around Mars, uses the undeployed high-gain antenna to communicate with Earth, but the entire spacecraft must be turned to point the antenna toward Earth during each communication session.
"We have not made any decisions yet, but we want to take a conservative approach in order to protect the mission as fully as possible," said Glenn E. Cunningham, Mars Global Surveyor project manager at NASA's Jet Propulsion Laboratory (JPL), Pasadena, CA. "A delay in the antenna deployment would reduce the flow of imagery and science data somewhat, but we have some ideas about how to compensate for that."
Launched in November 1996 and in Mars orbit since September 1997, Mars Global Surveyor carries a dish-shaped high-gain antenna that is to be deployed on a 6.6-foot-long (two-meter) boom for the global mapping portion of the mission. The antenna is stowed during launch and the early orbital phase at Mars so that it is not contaminated by the exhaust plume from the spacecraft's main engine. The mission plan calls for the antenna boom to be deployed following the final use of the main engine next spring, at the completion of the spacecraft's orbit-shaping aerobraking activity.
During deployment, the boom is pushed outward by a powerful spring. A damper mechanism cushions the force of the spring and limits the speed of the deployment, somewhat like an automobile shock absorber or the piston-like automatic closer on a screen door. In recent months, however, engineers have become aware of problems with similar damper devices on deployable structures such as solar panels on other spacecraft.
New data suggest that, in the vacuum of space, air bubbles may develop in the viscous fluid inside the damper. This may allow the boom to move through a considerable range of motion at a high speed before any cushioning effect begins to occur.
"To the best of our knowledge, we could deploy the antenna boom without any adverse effect," said Cunningham. "However, the forces that the damper and boom would be subjected to as a result of the bubble formation are close enough to the maximum force that they are designed to withstand that we want to take a cautious approach in evaluating the deployment." In a worst-case scenario, damage resulting from damper failure could render the spacecraft unable to communicate with Earth.
"The advantage of deploying the high-gain antenna is that we can then use its gimbals to point the antenna at Earth to send data at the same time science instruments are pointed at Mars acquiring science data," said Cunningham. "Until we deploy the antenna, we must store data on the spacecraft's onboard recorder and then turn the entire spacecraft periodically to transmit data to Earth." A similar approach was used on NASA's Magellan spacecraft, which orbited Venus from 1990 to 1994.
The project team is considering postponing the antenna deployment until after the landing of another spacecraft, the Mars Polar Lander, which will reach Mars in December 1999. Mars Polar Lander carries an experiment called the Deep Space 2 microprobes, which will penetrate the soil of Mars in search of subsurface water. Deep Space 2 relies on Global Surveyor as its only possible communication link with Earth. If the high-gain antenna remains undeployed when Mars Global Surveyor begins its prime mapping mission next March, Cunningham said that small gaps would exist in coverage of the Martian surface by the spacecraft's camera and other instruments, due to the periods when the spacecraft is turned to communicate with Earth. Those gaps could be filled in later in the orbital mission.
The project team is not yet certain how a postponed deployment would affect the total amount of data returned by the spacecraft. An initial estimate for the first 30 days of the global mapping mission found that it could return approximately 40 percent of the data that could be sent with a fully articulated antenna. However, the data return rate could be improved by strategies such as using larger ground antennas on Earth so that the spacecraft could transmit data more quickly, Cunningham noted. A final decision on the antenna deployment will not be made until a review scheduled for Feb. 3, 1999, before the spacecraft's prime mapping mission begins the following month. Mars Global Surveyor is managed for NASA's Office of Space Science by JPL, a division of the California Institute of Technology. The spacecraft was built by Lockheed Martin Astronautics, Denver, CO.