The science goals of the Mars Polar Lander could be severely compromised if another satellite link is not established to replace the lost Mars Climate Orbiter, project scientists say.
The best-case scenario is that the Mars Global Surveyor will be able to perform the services originally assigned to the Climate Orbiter, but the surveyor is already busy with its own task of mapping the planet. It is not clear how the Global Surveyor's current mission would be affected by a role change, or whether such a change is feasible.
Teams at NASA's Jet Propulsion Laboratory are fully investigating the possibility, though, said David Paige, leader of the Polar Lander's integrated instrument payload that includes most of the science instruments aboard the Lander.
Initial indications from JPL, though, are that the Mars Global Surveyor will be available as a transmission relay for data from the Lander, said Paige, a professor of planetary sciences at the University of California at Los Angeles.
"We'll go forward assuming we are [going to use the Global Surveyor as a relay]," he said.
Even if the Mars Global Surveyor is used, it only fills half the gap, however.
The craft can only be used to relay data to Earth from the Polar Lander, not to the Lander from Earth, Paige said. It is also likely that it will not be ready until several days after the Dec. 3 touchdown.
Science and engineering teams are now scrambling to rewrite codes and command sequences for the mission, he said. Paige is meeting this week with several members of the mission's science-instrument teams to plan necessary changes.
"You make a change like that and it affects everything," he said.
Pending more analysis and a final decision though, the Lander still may be left in the lurch, having to rely only on its own 2-foot antenna dish to transmit data and receive commands all the way to and from Earth.
This would significantly cut the amount of data that can be returned, because sending information all the way to Earth requires 15 times more power than is needed to transmit to an orbiting satellite.
The direct-to-Earth link was only meant to serve as a backup for brief periods when the Climate Orbiter relay was down, said Hop Bailey, project manager for the Thermal and Evolved Gas Analyzer, an instrument built to vaporize soil samples and spectrally analyze the gas.
"Now we're posed with a question: You don't have a relay. It turns all of the sequence planning on its head because sequences have all been built up and tested," said Bailey, a physicist at the University of Arizona's Lunar and Planetary Laboratory.
"Now those are all trash because we don't have the resources, the energy in the batteries, or through the solar panels, to support the same transmission of data to Earth," he said.
According to Bailey and Paige, the power requirements of direct-to-Earth transmission will cut into the amount of time the science instruments will be able to operate.
As a result, the activities of all the instruments will have to be scaled back, Bailey said.
For example, the thermal soil analyzer that Bailey works with has 8 small oven chambers, each of which can cook a small soil sample. After the Lander's robotic arm digs a sample from the ground and drops it into one of the chambers, the instrument heats it and uses a laser to get a spectrum of the gas that is released at high temperatures.
The original plan called for one sample to be collected, cooked and analyzed in a single Martian day. If less power will be available for science, the Thermal Analyzer's heavy power draw will make it necessary to spread the process over two days, effectively doubling the time that other instruments will not be fully operational, Bailey said.
Furthermore, direct-to-Earth transmission uses so much juice that the spacecraft will not be able to return all its scientific data.
"Instead of sending down every spectrum, or every fifth, or every tenth spectrum down, we're looking at only sending every twentieth down," Bailey said.
"And it's just the nature of this business that the thing you decide not to send down will be the one thing that you really need to understand what happened. Everything is just kind of up in the air at the moment."
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