NASA - National Aeronautics and Space Administration

While Pathfinder and Sojourner were digging in the martian dirt, a new satellite -- Mars Global Surveyor -- was on its way to study Mars from the sky above. "The idea for Surveyor was to put a spacecraft in polar orbit and systematically study the weather and surface of the planet," said Surveyor project scientist Arden Albee.

Image to right: An artist's depiction of Mars Global Surveyor orbiting the planet. Credit: NASA

The satellite left its Florida launch pad on Nov. 7, 1996. Zipping through space, the craft needed to shed speed in order to be gripped by the orbit-inducing gravity of Mars. To do so, Albee and Surveyor's large team of scientists and engineers used a radical and experimental technique to slow the racing satellite. "We pioneered aerobraking with Surveyor," said Albee.

During aerobraking, a spacecraft lightly skims a planet's atmosphere with each oversized orbit to create drag that gradually scrubs off speed. The entire process of aerobraking takes place over hundreds of passes and gradually reduces the size of the orbit. Aerobraking eliminates the need for extra fuel that would be used to power engines during a braking maneuver.

While ultimately successful, the early phases of Surveyor's aerobraking process proved to be a bit tricky. "When our solar panels came up against the pressure of the atmosphere, we ended up with what I'll call a sprained wing," said Albee. In response, engineers helped to lessen pressure on the ailing panel by easing the satellite into orbit at a slower pace.

Once Surveyor was circling Mars in its targeted orbit, the spacecraft began using its cameras and sensors to show us the planet's majestic features. From high in space, the craft revealed a planet with dramatic terrain. Mars was shown to be capped with smooth poles of frozen carbon dioxide, while its mid-latitudes feature dormant volcanoes, wide deserts and rocky plateaus. Perhaps most visually stunning was the discovery of ancient gullies likely carved by running water. Surveyor also detected near the equator a 300-mile-wide patch of ferric oxide, a chemical that forms in standing water.

Image to left: Mars Global Surveyor spotted this ancient river delta flowing through a giant crater. Credit: NASA

On a global scale, the satellite determined that Mars has ebbing and flowing magnetic fields too. Early in the planet's history, molten magma appears to have seeped through its floating crust. The rising magma cooled in sporadic pockets, creating fields of erratic magnetic intensity. The shape and arrangement of the fields also indicates the planet's plates of crust shift and crunch together to form riffs and mountains like those on Earth.

In 2001, Surveyor was joined in orbit by the Odyssey satellite. Odyssey was sent to analyze the composition of the planet's surface and radiation levels. Initially, the two satellites had independent missions. But before long, two new spacecraft in need of their cooperation would be on the way: the Mars Exploration Rovers.

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