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)
Air & Space Magazine

(Continued from page 1)

In those days, when Mars was being reconnoitered for the first time, the scientific goals and the aiming requirements weren’t as exacting as they are now. Today’s follow-up missions demand high-resolution photography and pinpoint targeting. And, says Michael Watkins, chief of the navigation section, “We’re doing it faster, and we’re trying to do it with smaller spacecraft and less fuel.”

Navigating in space is not fundamentally different from taking a road trip on Earth. First you need a map. Then you plot your course, decide what kind of vehicle to take, and calculate how much time and fuel you’ll need. Once en route, you compare your actual progress to this plan and adjust as necessary. All the while, you have to prepare for contingencies: What if we miss this turn? What if we use more fuel than we’d planned to?

Given that everything in space is moving constantly, the spacecraft navigator faces a last, even more devilish problem: What if, upon arrival, the destination turns out not to be where we thought it was?

Squirreled away in his office at JPL, Standish works at keeping such surprises to a minimum. His computer-generated ephemerides—which list the past, present, and future positions of all nine planets, the moon, and the sun—amount to a combination map and train schedule for meeting up with any large body in the solar system. Since planets move in predictable patterns, pinning down their past locations helps Standish plot their future positions. His ephemerides extend back as far as 3000 B.C. and forward to A.D. 3000. They’re calculated from an eclectic mix of sources—everything from telescope observations by Galileo to records of eclipses in ancient Babylon.

More is at stake than just a spacecraft missing its target. Mission planners also want to make sure that some long-forgotten probe sailing through the void a thousand years from now won’t crash into a distant planet and accidentally contaminate it with terrestrial microorganisms. “When it comes to navigation,” Standish says, “you are always trying to think of things that can happen that you wouldn’t normally think of.”

With Standish watching the planets, others at JPL track the solar system’s 67 known moons as well as its thousands of asteroids and comets. All of these objects push and pull on each other in subtle and hard-to-predict ways, and these changes throw the objects off their paths and necessitate constant recalculation of their orbits. Comets prove even more difficult to track, because the action of sunlight burns off dust and gas, which produces a rocket-like thrust powerful enough to drive the comet off course. “They just don’t behave themselves,” laments Donald Yeomans, Standish’s colleague at JPL and the man charged with mapping the travels of comets through the heavens.

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.

About Michael Milstein

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

Read more from this author

Comment on this Story

comments powered by Disqus