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 2 of 6)
That’s what happened to the Mars Climate Orbiter, which vanished in September 1999 as it prepared to enter Mars orbit because of a now-infamous confusion between metric and English units—and more to the point, because it wasn’t where the navigators thought it was. The incident forced JPL space navigators into an unfamiliar and uncomfortable spotlight. It also proved—the hard way—that accurate navigation is every bit as vital to space exploration as raw rocket power.
In the early days of space travel, accuracy was relative. Mission planners were more than happy in the mid-1970s to put a Viking spacecraft within 25 to 30 miles of its planned orbit around Mars, recalls Myles Standish Jr., who left Yale’s astronomy department for JPL in 1972 to work on the Viking project. Tall and debonair, Standish, a distant relation to the Mayflower captain of the same name, is one of the few top JPL navigators who doesn’t belong to the “Texas Mafia,” a contingent of University of Texas astronomy and engineering graduates who seem to dominate the laboratory’s navigation and mission design section.
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.