Aerobraking has been used twice before: experimentally at Venus after the Magellan spacecraft's mission was complete in 1994, and with the Mars Global Surveyor spacecraft when it arrived at Mars in 1997. The technique uses the spacecraft's solar panels like wings, or a parachute, to slow it down and lower its orbit, and greatly reduces the need for propellant that would otherwise be needed to place the spacecraft in the desired orbit. Complicating Global Surveyor's aerobraking was a faulty hinge on one of its solar wings. Extra care had to be taken to keep the wing from tearing off the spacecraft, adding a year and a half extra to the aerobraking phase of that ultimately successful mission, which has now returned more data than all other Mars missions combined.

Mars scientists have long desired to have their instruments in a low, circular orbit that affords beneficial lighting conditions and a uniform altitude for photography and other data taking, and Odyssey's aerobraking phase is designed to deliver that. "But at Mars, especially, aerobraking comes with its own set of risks," says Whetsel, who, with his experience on Mars Global Surveyor, may be the most seasoned aerobraking specialist in the solar system. [More information on aerobraking and orbits can be found in "The Basics of Space Flight"]

"Aerobraking relies on a 'Goldilocks' approach," says Whetsel: "Not too big, not too little, not too deep, not too shallow, because if it's too deep you'll damage the spacecraft and if it's too shallow you won't get to the right orbit in time." And if that happens, to borrow from another fairy tale, the big, bad wolf will get you.

"Once you start aerobraking, you're putting the spacecraft through the atmosphere hundreds of times, and the trick is to do that deep enough to shrink the orbit down so you get to a good geometry while the lighting is still good, but not so fast that you overheat the solar arrays," Whetsel said.

That would be easy, says Whetsel, if the Martian atmosphere were just a big unvarying sphere, "but there are real things like weather and dust storms that cause the atmosphere to move up and down."

For weeks, in fact, Mars has been in the midst of a gigantic global dust storm. Scientists on the Mars Global Surveyor team are using their instruments to keep a close watch on the storm. A well-designed drag pass through the atmosphere could doom the mission if, unbeknownst to planners, a dust storm swelled the atmosphere and snagged Odyssey out of the Martian sky. So data from Global Surveyor will be crucial in guiding sibling Odyssey through its aerobraking phase.

"The team has to monitor that process on pretty much a daily basis to see if the atmosphere is growing, how is the spacecraft responding, are the temperatures ok, are we going fast enough, are we keeping the spacecraft healthy," says Whetsel. [See aerobraking interviews and animation at http://www.jpl.nasa.gov/videos/mars/odyssey.html ]