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 3 of 6)
Over the years, ephemerides—and therefore space navigation—have become more precise as more observations are entered into the database and new instruments are built that can measure celestial positions ever more accurately. But space navigators still accept unpredictability as a part of doing business. Because of the new precision, subtle forces of gravity and solar wind come more into play when calculating routes through the solar system. Navigators try to anticipate every tug on a spacecraft but it’s a constant struggle.
Part of what makes Farquhar so good at his business is that he was among the first to appreciate that in the new era of cost-constrained space missions, you had to substitute imagination and cleverness (free) for rocket fuel (expensive). Instead of countering gravity’s pushes and pulls, he found new ways to use them. In the 1980s he earned kudos in the space community by re-routing a little-known spacecraft called the International Sun-Earth Explorer 3—using gravity swings past the Earth-moon system—to fly through the tail of a comet called Giacobini-Zinner months before the world’s space agencies managed their own much more expensive missions to Comet Halley.
Now Farquhar is back to his old tricks, using some very delicate maneuvers to close in on Eros. Although the asteroid’s gravitational pull is so weak that a person standing on the surface could easily lift a car, it’s just enough to hold the 1,800-pound spacecraft in orbit. It’s slow going, though: If NEAR were to orbit much faster than its current few miles per hour, it would break free and fly off into space.
Sitting in the conference room, which doubles as mission control, Farquhar and his team track NEAR’s position by watching the radio signals coming back to Earth. If the signals are Doppler-shifted—like the change in pitch of an ambulance siren as it gets nearer—controllers know that gravity is accelerating the spacecraft in ways they hadn’t expected. Careful tracking is essential when dealing with an enigma like Eros: before NEAR’s arrival, nobody knew the asteroid’s exact shape or dimensions. Lacking a good map going into the encounter, engineers had to rely on tracking data—and first-time pictures coming back from NEAR’s cameras—to nail down the spacecraft’s location before committing it to its next move.
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.