Navigating the Unknown
9 Nov 2006
A junk-food buffet lined the long table in the conference room where the small and celebrated team of Mars Reconnaissance Orbiter navigators has made their home over the last year or more. Getting an interview with the team became nearly as difficult as securing time with a celebrity, but in the end, the navigators all sat down for a group dialogue about their experience.
Tung-Han You, an energetic leader, was quick to praise the team's hard work and dedication to a daunting task. He modestly described himself as the "firefighter" of the team - there to deal with emergencies that, hopefully, are stamped out while still only small brush fires. He is clearly a lot more than that.
During aerobraking, the navigators were responsible for ensuring the spacecraft's safety while nudging it every "go-round" into an increasingly tighter orbit. Each time the orbiter circled, the team directed the spacecraft to skim the martian atmosphere and use that friction to slow it down and put it into a close, low orbit around Mars.
Neil Mottinger, a friendly and jovial veteran navigator, equates the erratic schedules of the aerobraking phase with those of his Viking days in the 1970s. "In hindsight, it's almost like a dream," he laughed. "Sometimes outdoor webcams were my only clue that morning was coming."
What keeps the team holed up on the eighth floor of a mission operations building at JPL is the martian atmosphere.
"We are here 24/7 due to its volatility," said navigator Earl Higa. "During aerobraking, we tried to predict the atmosphere, and it was jumping all over the place. The thicker the atmosphere, the more drag you're going to get and, if you dip too deeply, you'll burn up."
Avoiding that dire fate, the team achieved their goal of putting Mars Reconnaissance Orbiter in the lowest orbit of any spacecraft at Mars.
With a Little Help From Their Friends
When Mars Reconnaissance Orbiter first arrived at Mars, an approximately 35-hour orbit allowed navigators time to assess what atmospheric challenges their spacecraft would face. However, as the orbits became smaller and smaller (down to about two hours at the end of aerobraking), the team had to work at a frenzied pace to ensure that no harm came to the orbiter.
Assistance came from across the country, as Jill Prince, Scott Striepe, and their team at NASA's Langley Research Center in Virginia used software (developed with teams at NASA's Johnson Space Center in Texas) to make atmospheric prediction models. The Langley team also provided valuable contributions of trajectory, aerodynamic, and thermal analyses supporting navigation team decisions.
"With shorter orbits, we had less time to do analysis and make adjustments, so the intensity level was higher," said Prince. "We used the best atmospheric models available, but the orbiter experienced a denser atmosphere than expected. So, we had to make several critical adjustments."
Managing Martian Traffic
As more spacecraft are sent to Mars, navigators must also perform frequent trajectory analyses so that orbiters don't cross paths. In "navigation speak," this is called COLA, or collision avoidance. "COLA was never done in any previous aerobraking mission," said orbit determination lead Stuart Demcak. "To accomplish it, we needed to build the whole process from ground zero." Mars Reconnaissance Orbiter's chief navigation analyst Dolan Highsmith pointed out that the chances of impacting are very small, but monitoring is necessary. That vital task went to chief traffic coordinator Stacia Long who said the intensity of the aerobraking work periods made her feel like she'd been exiled to a remote island.
"The COLA issues began with Mars Odyssey, which had to avoid Mars Global Surveyor," said navigator Eric Graat. "When the European Space Agency's Mars Express orbiter came into the mix, it became more complicated. With Mars Reconnaissance Orbiter, it was even more complex due to the unstable aerobraking orbits. When you have a stable orbit, you can calculate these things, but when you are in aerobraking, you are constantly adjusting."
No Rest For the Weary
The successful end of aerobraking hardly signaled a lighter workload for the navigation team.
"When we finished aerobraking, we were in an orbit that wasn't anything like our final science orbit," said chief navigation designer Allen Halsell. "We had to make some sizable changes in the shape and trajectory of the orbit." The second of these orbit adjustments was the largest mission maneuver, excluding Mars Orbit Insertion (MOI) in March, 2006. The team's maneuver analyst Ram Bhat redesigned the post-aerobraking maneuver sequence, saving significant fuel, potentially enough to extend the science mission a few more years!
As scientists waited eagerly for their instruments to be turned on, they furiously planned where they wanted to image and collect data. Navigators needed to plan for the spacecraft to be where it was promised. The spacecraft's low orbit is new territory for them.
As Highsmith points out, the navigators must predict tricky atmospheric dynamics to direct the spacecraft to specific targets chosen by scientists. Brilliant images from the HiRISE (High Resolution Imaging Science Experiment) and CTX (Context Imager) cameras have already shown how successful the navigators have been; it's reason enough to justify their reclusive behavior and their celebrity status.