Martian North Polar Cap on September 12, 1998

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.

Talk About the Weather

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.