"If he were flying above the North Pole of Mars, my advice to Santa would be 'Hang tight onto your reins,'" said Odyssey navigator John C. Smith. "You could be in for a rough ride."

In the midst of aerobraking maneuvers that are lowering the spacecraft's orbit around Mars, the Odyssey team has discovered an unexpected and somewhat unpredictable north polar atmospheric disturbance that is making the job a real adventure, Smith said.

Called the "polar vortex," this cold, low density region forms each winter in the atmosphere above the planet's latitudes 70 degrees north and higher. The region between the polar vortex and the rest of the atmosphere is called the 'transition zone.' In this zone, strong winds swirl around the pole and the zone itself weaves in and out in the typical fashion of a terrestrial jet stream. It is an area where sometimes surprising shifts in the atmospheric density can become fasten-your-seatbelt territory for Odyssey.

"When we're in the transition zone, the atmosphere is very unpredictable," said Smith.

Scientists and engineers have long known that Mars' atmosphere "breathes" -- moving up and down, growing or decreasing in density with the effects of dust storms, winds and other influences. But scientists and navigators are just getting to know up-close the peripatetic polar vortex and its shifty transition zone.

Launched April 7, the Odyssey spacecraft entered orbit around Mars Oct. 24. Over a three-month period that ends in January, Odyssey's orbit is being lowered and circularized through aerobraking, carefully designed passes through the top of the atmosphere that slow the spacecraft through the effects of atmospheric drag.

Mars weather reports come from Odyssey's older sibling, Mars Global Surveyor, which has been in orbit around Mars for four years.

So where, exactly, is the polar vortex? "That's a good question," said Smith. "We ask that every day.

"We have daily meetings to decide - 'do we need to lower ourselves farther down to get more drag out of the atmosphere, or do we need to raise ourselves higher in the atmosphere to keep from getting overheated?'" said Smith.

Every day at 11:30 a.m. Pacific time, scientists from the Surveyor and Odyssey teams hold a telephone conference to talk about the weather - on Mars - and how it might impact the next 24 hours' worth of Odyssey's aerobraking maneuvers. They depend upon Mars Global Surveyor and Odyssey data to assess martian atmospheric behavior, with a special watch for the large martian dust storms which can increase in a big way the air density at aerobraking altitudes of about 100 kilometers, or about 60 miles.

Then at 1 p.m. each day, this atmospheric advisory group joins the project navigation and spacecraft teams to work out the next aerobraking maneuvers.

In planning each aerobraking pass through the atmosphere, the team walks a fine line between getting the desired amount of drag out of the pass without subjecting the spacecraft to overheating from unforeseen pockets of dense air.

"Basically, we use the Mars weather report on what the atmosphere is going to be like and make sure we fly at an altitude to keep the spacecraft safe while getting sufficient drag," said Smith.

"We monitor it closely and try to go with the flow on the variability," Smith said. "We have to take into account how much the prediction could be wrong and use that as margin. When the atmosphere is as highly variable as it is in this region, it means we have to be a little more cautious."

With concerns about atmospheric changes that any pilot could relate to, "it might be said this is as close as we've ever come to flying on another planet," said Dr. Richard Zurek of JPL, who co-chairs the atmospheric advisory group to the project.

But, says Smith, "It's a little different from flying. We don't have the option of flying around a disturbance. We have to fly right through it. We can raise our altitude or lower it, but we can't avoid going through that region."

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