The Martian North Polar Cap
If Santa Claus were a martian, he'd be in for one bumpy ride.
That's the assessment of navigators and engineers controlling the
flight of NASA's 2001 Mars Odyssey spacecraft as it presently flies four times
daily above the north polar region of Mars.
"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.
Fasten Your Seatbelt
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.
Aerobraking: It's a Drag
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
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.
"Try to Go with the Flow"
"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
-By Mary Beth Murrill