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Aerobraking team will help guide Odyssey into Mars orbit

When NASA's 2001 Mars Odyssey spacecraft arrives at the red planet Oct. 23, things may get a little tense for New Mexico State University astronomer Jim Murphy and other scientists in the project's Aerobraking Atmospheric Advisory Group.



New Mexico State University astronomer Jim Murphy is a member of the Aerobraking Atmospheric Advisory Group for NASA's Mars Odyssey project. (NMSU photo)

The group's recommendations will play an important role in maneuvering the spacecraft into a circular orbit around Mars so it can achieve its scientific goals in preparation for an eventual manned mission to the planet.

Aerobraking is the technique that will move Odyssey from its initial elliptical orbit into a circular one, using frictional drag as the craft flies through the upper part of the atmosphere, about 100 kilometers above the planet's surface.

"It's a bit like dragging your feet on the ground to slow down when you're riding a bicycle," said Murphy, whose research focuses on Mars' atmosphere and weather.

Putting a spacecraft directly into a circular orbit would require a large amount of fuel and add tremendously to the weight of the launch and the cost of the mission, he explained.

"An alternative is to get yourself initially into an elliptical orbit and slow the spacecraft down a little bit each time it goes through the atmosphere" on the near part of its orbit, he said.

The atmospheric advisory group provides recommendations on whether or not to use the onboard rockets to nudge the spacecraft deeper into the atmosphere or to move it to a higher point in the atmosphere as it passes through.

"You don't want to go too deeply into the atmosphere and burn the spacecraft," he said. On the other hand, the craft needs to be deep enough within the atmosphere to experience enough friction to slow down the required amount to achieve a circular orbit within the window of time available for the mission.

Those critical decisions will be based partly on the expertise of Murphy and others on the aerobraking advisory team.

"Our role is to take as much of the data and theoretical understanding we have of the atmosphere on Mars and attempt to forecast, a certain number of orbits out, what the spacecraft might encounter," Murphy said. His contributions to the effort include computer modeling of the Martian atmosphere.

Like weather on Earth, Martian weather can be difficult to predict. To complicate matters, Odyssey will arrive during the planet's prime dust storm season, Murphy said. A global dust storm that developed in June is subsiding now, he said, but the historical record shows that two storms can occur during one Mars year.

The prospect of another storm is a big concern to the advisory group, he said. "We would have to deal with its rapid intensification in real time, and also have to deal with its ongoing effects while still being constrained with the idea that aerobraking needs to accomplish its work in a specified period of time to achieve the orbit that is desired for the mission."

Odyssey's scientific objectives are to characterize the climate and geology of Mars, look for evidence of water, and study the planet's radiation environment. The details it provides will contribute to a better understanding of the planet and help prepare for human exploration.

An instrument called THEMIS (Thermal Emission Imaging System) will determine the distribution of minerals on the planet's surface, particularly minerals that can form only in the presence of water. Using infrared images, Murphy said, this high-tech camera should be able to map any thermal hot spots, which could be useful in identifying future landing sites.

Odyssey's Gamma Ray Spectrometer will provide information on the distribution of chemical elements, including hydrogen, which will help detect the presence of water in the soil.

An instrument called MARIE (Mars Radiation Environment Experiment) will investigate the radiation levels on Mars, which could present hazards to human explorers, he said.

Much of the data Odyssey sends back to Earth will be useful to Murphy as he refines his models of the Martian atmosphere. But being directly involved in a NASA mission adds another dimension to his work.

"Most of the time in theoretical work, you have no time constraints," he said, while a mission like Odyssey involves quick decisions and changing conditions.

"It's a bit nerve-wracking at times," said Murphy, who has taken part in previous Mars missions. "We do have some direct impact on the health of the craft."