Hang a Right at Jupiter

For space navigators, the best course to a distant object is never a straight line.

The tumbling asteroid Eros (shown here in a time sequence taken during NEAR's approach) was a challenging target for space navigators. (NASA/JHU Applied Physics Laboratory)
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Farquhar delights in spacecraft navigation, and actually seems to relish all the complicated dips and detours. His longtime colleague Dave Dunham, who works out of a small basement office elsewhere on the rural APL campus, sifts numbers through his computers to find trajectories that offer the greatest advantage in terms of “delta-V”—a measure of the total change in spacecraft velocity, which translates roughly to the number and duration of engine burns and therefore to the amount of fuel a spacecraft has to carry. A mission that requires too much delta-V is dead before it even gets to the launch pad.

Farquhar’s team, for instance, could have launched NEAR in 1998 on a direct path to Eros, but it would have required so much delta-V that the mission would have needed an expensive Atlas rocket—prohibitive for a program with a budget cap of $150 million. The extra fuel would have left less room for scientific instruments. And there was one other fatal flaw. “Mars Pathfinder would have launched first,” Dunham says, in a tone suggesting that he would have found it unthinkable to let a JPL spacecraft take credit as NASA’s first Discovery-class mission off the launch pad.

So Dunham set his computers humming and found a more roundabout trajectory that used a gravity assist from Earth to pick up some free delta-V. It called for a launch in 1996, thereby beating Mars Pathfinder into space and using a cheaper and less powerful Delta II rocket. Looking over computer printouts of the spacecraft’s course, Farquhar noticed another dividend: With a little more delta-V, NEAR could fly past a smaller asteroid called 253 Mathilde on the way to Eros. So they added one extra engine burn—in technical parlance a “trajectory correction maneuver,” or TCM—to take the closest-ever portraits of an asteroid only one day after Farquhar’s wife’s birthday in 1997. (The Eros rendezvous, by the way, took place on Valentine’s Day 2000—just right, Farquhar figured, for an asteroid named for the Greek god of love.)

TCMs are at once a space navigator’s best friend and worst enemy. They give the mission designer control over the spacecraft’s route and the power to adjust errors in its course. But if they don’t go just right—engine burns are timed down to the second—they can introduce new errors in the trajectory. And each burn presents one more opportunity for something to go wrong, whether it’s a fuel line breaking, a valve sticking, or a tank exploding. “Every time you enable the thrusters, you’re taking a chance,” says Bobby Williams, leader of the JPL team providing navigational support for the NEAR mission, whose accent identifies him as a member of the Texas Mafia.

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.

About Michael Milstein

Michael Milstein is a freelance writer who specializes in science. He lives in Portland, Oregon.

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