HAMPTON, VIRGINIA -- NASA's Mars Odyssey has encountered a strange, unexpected phenomenon as it slips over the red planet's north polar region. An intense polar vortex has been detected, causing Mars' atmosphere to be less dense than predicted for that area.

Likened to a jet stream on Earth, the baffling high-latitude, planet-circling vortex is being carefully eyed by scientists.

Odyssey has begun weeks of delicate aerobraking maneuvers, repeatedly skimming through the Martian atmosphere to slow down and arrive at a correct orbit to start full-time science operations early next year.

Here at NASA's Langley Research Center, some 18 specialists are divided up into teams that provide aerobraking support for navigation officials at the Jet Propulsion Laboratory (JPL) in Pasadena, California. JPL is the lead center for day-to-day operations of Odyssey.

In performing its aerobraking duties, Mars Odyssey swings through a different region of atmosphere than that experienced by Mars Global Surveyor (MGS). Now in Mars orbit, MGS made use of the fuel-saving aerobraking technique starting in 1997.

"The polar vortex is kind of a different animal," contrasted to conditions at Mars' southern regions, said Gerald Keating, a Langley-based senior research staff scientist from George Washington University in Washington, D.C. "It's an intense jet stream for the north polar region," he said.

Data gleaned by Odyssey shows a colder region over Mars' north pole, said Michelle Munk, flight mechanics engineer for the Langley Odyssey team.

"The polar vortex has given us some excitement in terms of seeing a different density profile [of the atmosphere] than our computer models predicted at the outset," Munk said.

Trying to predict what impact the vortex might play in aerobraking Odyssey is underway, said Alicia Dwyer, staff research scientist for the Institute for Computer Applications in Science and Engineering, a contractor at Langley working on the Mars mission. "That's become a very big part of what we're doing," she said.

"There's a large drop in density at that northern latitude. It wasn't something we foresaw. We didn't realize it existed," Dwyer said.

A step by step dance plan for gingerly putting Odyssey through its aerobraking paces has been scripted.

Following Mars arrival on October 23, the spacecraft kicked off a "walk-in" phase. In this period of time, engineers calibrated the probe's overall performance as ground controllers started validating the processes and computer models necessary to start aerobraking and establishing the density of Mars' atmosphere.

With the walk-in phase behind them, Odyssey has begun taking dips into the thin Martian atmosphere to adjust the shape of its orbit around Mars, said Mary Kae Lockwood, team leader for planetary exploration in Langley's vehicle analysis branch.

"We're right were we want to be," Lockwood told SPACE.com. "We're taking small steps, closer and closer into the atmosphere. It's designed to be conservative, moving toward the corridor Odyssey needs to be in for main phase aerobraking," she said.

Odyssey's close-up swing, called periapsis, has been lowered to 67 miles (108 kilometers) above the planet's surface. At the far end of the elliptical orbit, known as apoapsis, the spacecraft speeds outward, some 10,530 miles (16,960 kilometers) away from the red planet.

After the upcoming aerobraking steps are completed, Odyssey should reach a circular orbit of 249 miles (400 kilometers) above Mars. It will be circling Mars once in just under two hours, passing over the same part of the planet at roughly the same local time each day.

Data gathered daily at a Langley Odyssey operations center is being fed to a JPL atmospheric advisory group. As Odyssey makes consecutive low altitude sweeps over Mars, devices on the craft sense the density of Martian atmosphere being encountered.

"We track the aerodynamics of Odyssey, to understand the atmospheric density. Also, we make sure the spacecraft is behaving as we think it should as it flies through the atmosphere. And that's all been excellentright on," Lockwood explained.

With each pass of the probe through the upper layers of Mars' atmosphere, Langley teams watch for heat loads and stresses on Odyssey. This information helps guide a decision on diving deeper into the atmosphere or backing away a bit on subsequent swingbys, Lockwood said.

An integrated team from JPL, Langley, George Washington University, and Lockheed Martin Astronautics in Denver, Colorado, the firm that built Odyssey, work together to support decision making.

"There's an art to the decision, but it's a group consensus," Lockwood said.

Odyssey's solar array serves as a long distance thermometer.

Both the face and back of the segmented panel are outfitted with a set of heat sensing sensors, said John Dec, a Langley thermal analyst. The sensors, called thermocouples, show heat loads on the solar array as it whisks through the Martian atmosphere. Accelerometers are also in use to gauge forces on Odyssey, he said.

"There's a lot of margin built into the solar panel," said Joe Gasbarre, an assistant who works with Dec on profiling the thermal loads on Odyssey.

"We do predictions for each orbit pass, and after each pass we do reconstructions using actual data collected. That gives us the heating profile that is correlated against the earlier predictions," Dec added. By archiving the information, computer models of how Odyssey is handling each atmospheric dousing can be updated and fine-tuned, he said.

Serving as a weather station, the Mars Global Surveyor is helping to finesse Odyssey into an accurate orbit. MGS is monitoring the planet's atmosphere, particularly dust storm conditions.

Keating, the George Washington University scientist, said measuring the aerodynamic effects on Odyssey is akin to putting your hand out the window of a moving car and feeling the force.

"We've been feeling our way down," Keating said. "It's all pretty exciting stuff. Every day, there's new data and we get another look. We have to keep moving on."

Flight mechanics, atmosphere, thermal, and aerodynamic teams at Langley each correlate data relayed from the Mars orbiting satellite, said Munk.

"Every week we look back over the data for the past week. We see how well our computer models predicted. So we kind of do a score card every week, and adjust our models accordingly," Munk said. "It's very exciting to get some density. Now we've got real data, and that's a lot better," she added.

Real data about Mars' atmosphere is always welcome news to all concerned.

"Things are going very well in aerobraking. I think we are on track, so far, to get into mapping orbit in mid-January as planned," said JPL's Stephen Saunders, 2001 Mars Odyssey project scientist, in a separate interview.

But Saunders adds a caveat. "The Mars atmosphere, however, has seldom acted as predicted and is quite variable. We have gotten close to a condition for 'immediate action', i.e. raise periapsis, at least once," he told SPACE.com.

No doubt, the weeks ahead will become more intense as the $297 million Mars Odyssey mission settles in to churn out scientific data.