Hang a Right at Jupiter
For space navigators, the best course to a distant object is never a straight line.
- By Michael Milstein
- Air & Space magazine, January 2001
NASA/JHU Applied Physics Laboratory
(Page 4 of 6)
The NEAR team was rudely reminded of this risk in December 1998, when NEAR fired its thrusters to enter orbit around Eros. For reasons that still aren’t clear, the spacecraft went into a tumble. In what Farquhar now benignly calls an “unscheduled fuel dump,” it started spewing propellant, then shut itself off. For a tense day, the team feared the craft had been lost. In fact, it had reverted to a backup “safe mode,” aiming its solar panels at the sun to recharge its batteries, which had taken it out of contact with Earth. Controllers finally reestablished contact, but the mishap threw NEAR far off course.
Fortunately, Dunham, well-prepared navigator that he is, had devised a plan to use in the unlikely event the maneuver failed. It took another year to loop back to Eros, and a little more delta-V, but the spacecraft finally made it, getting as close as three miles from Eros this October.
“You should always have a contingency plan and a generous fuel supply,” Dunham says with a satisfied grin.
Extra fuel wouldn’t have been much help to the hapless engineers in charge of JPL’s Mars Climate Orbiter, whose story serves as the great cautionary tale of modern space navigation.
Launched in December 1998 to study the Martian atmosphere and relay signals from the Mars Polar Lander, which followed it, the Mars orbiter had an idiosyncrasy that flustered navigators: Unlike the Mars Global Surveyer that preceded it, the craft had solar panels that stuck out to one side. The lopsided design created a kind of sail that caught the solar wind, torquing the spacecraft around. Controllers had to counteract this force every day using onboard reaction wheels—spinning flywheels that could absorb the unwanted momentum. But the flywheels could store up only so much energy before they too had to be “unloaded” by a thruster firing in the opposite direction. And heavy use of the reaction wheels required the spacecraft to fire its thrusters 10 times more often than the navigators had expected.
Every time the thrusters fired, the navigators calculated the spacecraft’s change in trajectory. But because of a procedural mixup that began with a parts subcontractor, the calculations used English units instead of metric. The firings were actually more than four times stronger than they should have been, pushing the spacecraft slowly and steadily off course. “Even very small thrusts over time can really add up,” explains JPL’s Watkins.
Wandering off course should not in itself have spelled disaster. But in this case, the other essential ingredient of space navigation—precise tracking—also broke down. Navigators typically keep track of a spacecraft just as Farquhar’s team follows NEAR: by watching the Doppler shift in its signal. But this method only measures the distance in one direction: along the line of sight between Earth and the spacecraft. It does a poor job measuring the spacecraft’s motion out of that line, which unfortunately was the direction of the Mars orbiter’s error.
Had the spacecraft been carrying a camera during its final approach, it would have been obvious from the photographs of the Martian moons that the craft wasn’t where it was supposed to be. But photography wasn’t one of the mission’s scientific objectives, and cameras were left behind as an unnecessary luxury. Cost considerations also had led NASA to allow a supplementary tracking system known as VLBI (Very Long Baseline Interferometry) to fall into disuse. If the navigators had had the giant VLBI antennas on opposite sides of Earth, they could have used triangulation to fix the spacecraft’s position in three-dimensional space. But only the line-of-sight Doppler tracking was available, so mission control didn’t know the craft was off course until it was effectively too late.