|Computer screenshot of aerobraking-related data
It's 7:30 a.m., and Brian Kennedy's about to clock out, having
successfully driven his spacecraft around Mars four times in the
last eight hours.
"I just delivered four orbits," he says matter-of-factly.
"That's a record for me."
Kennedy is one of 11 navigators at JPL who are responsible for the
'round-the-clock guidance of NASA's Mars Odyssey spacecraft through the
mission's aerobraking phase - repeatedly flying Odyssey through the
top of the martian atmosphere, using friction to lower and circularize
the spacecraft's orbit.
As Odyssey gets closer to achieving the orbital geometry required for
the mission, the end of the delicate, three-month-long aerobraking phase
is nearing an end. And Kennedy and his colleagues are looking forward
to getting off the graveyard shift: "I'm not going to miss it," he
says, having polished off a thermos of coffee and a one-liter bottle of
soda to help stay awake, now looking forward to a four-hour nap
before eating dinner at noon.
Legacy of Aerobraking Expertise
With the successful completion of the aerobraking effort, the Odyssey
navigation team is leaving a legacy of well-honed interdisciplinary tools and
techniques certain to be used on future missions using aerobraking.
Aerobraking represents a huge cost savings to space exploration.
Instead of using a large amount of propellant to deliver a spacecraft into
its final orbit around Mars, aerobraking uses well-controlled "drag
passes" through the upper atmosphere to slowly shrink and adjust
the altitude of the spacecraft. By not having to carry hundreds of extra
kilograms of propellant, the spacecraft is lighter. And a less massive
spacecraft can use a smaller, less expensive launch rocket.
How Low Can You Go?
|Simulated aerobraking passes
The hardest part is determining where to send the spacecraft on each
pass. Too low an altitude, and the denser atmosphere could cause parts
of the spacecraft to overheat. Too high, and there may not be enough
atmosphere there to provide the desired resistance for effective
aerobraking. Adding drama to the process, the martian atmosphere is
constantly changing in altitude and density. The design of each
aerobraking pass is based upon daily weather assessments provided
by Odyssey's scientific atmospheric advisory group, which watches Mars
weather through Odyssey and another NASA spacecraft - Mars Global
Kennedy and his colleagues recall the early days of aerobraking shortly
after Odyssey first entered orbit around Mars Oct. 23, 2001. "It was
very calm. There would be several hours between drag passes, and we
would accumulate several hours of data and deliver it to the operations
team at Lockheed Martin in Denver," he recalls. But as the orbit
tightened around Mars and the spacecraft has rounded the planet more
frequently, the pace has quickened. As time progresses, there has been
less time to assemble the results of each drag pass before the next one
starts. "It's been getting more interesting," says Kennedy.
Ahead of Schedule Despite Some Challenges
|Printout of aerobraking-related data
Odyssey's lead navigator, Bob Mase, says the aerobraking effort has
"generally gone better than expected.
"The atmosphere did not behave exactly as we predicted, but we
were able to compensate for the weather patterns that we experienced.
We are actually finishing ahead of schedule, which is better than we
Challenges along the way included occasional telecommunications
outages between Odyssey and NASA' Deep Space Network, when bad
weather or other glitches cut off the two-way radio link with the
spacecraft. And despite daily reports, Mars' weather created some
hurdles, too, says Mase.
"We were surprised by the polar vortex that we observed in
the atmosphere near the north pole. The atmosphere was very
turbulent in that region, but very quiet near the pole. So although we
had to slow down to get through the edge of the vortex, we were able to
make up time while we were in the quiet polar region."
But no matter what problems were thrown at the team, he says,
"we were always able to work around them."
Odyssey navigator Darren Baird had volunteered for the graveyard
shift because of the lonely but challenging learning opportunity it offered.
He'll miss the frequent 5 a.m. visits by Odyssey project manager Matt
Landano, who'd pop in for early morning updates on the mission's
progress. Little perks also made the graveyard shift more easily
manageable. "As stupid as this may sound, you're guaranteed a
great parking spot" at the sprawling JPL campus, Baird noted.
Nonetheless, he looks forward to a return to the sunlit working world so
he can resume acquaintances with his girlfriend and family and return to
a life of normal sleep cycles.
For navigator Peter Antreasian, "the hardest part is going home
exhausted at 9 a.m., wanting to sleep, but getting a wide-awake
three-year-old jumping on your stomach instead." He described the
aerobraking phase as stressful, but well worth the sacrifice. The successful
results Odyssey's aerobraking, he says, "have led us to a standard
for aerobraking that's going to be pretty commonplace on missions
in the future."