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

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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.

 “It should have been a slam dunk,” says Steven Synnott, a spacecraft imaging expert at JPL. “It should have been, but it wasn’t.”

The loss of the Mars orbiter was a crushing blow for JPL’s navigation section. As missions had grown more precise and ambitious, navigation had always kept up. Until now. “You have the impression that navigation is a floundering science,” sighs Standish, the planet-tracker. “It’s not. It’s a precise science, but you’ve got to have the right numbers.”

You also have to be honest about what you don’t know. When a team of engineers at JPL was plotting a proposed mission to Neptune several years ago, they wanted to know the planet’s exact distance from Earth so they could calculate how long the trip would take and, consequently, how much fuel they would need. Standish gave them the distance plus or minus 400 kilometers, or about 250 miles.

“They wanted it more accurate than that,” he recalls. “I said, ‘No—400 kilometers is the best I can do.’ You’ve got to plan your strategy on the fact that the distance to Neptune is not going to be known much better than that. Otherwise you’re fooling yourself.”

In the wake of the Mars Climate Orbiter loss, JPL assigned a kind of navigational SWAT team to make sure that the Mars Polar Lander did not go awry as well. Among other fixes, the team drew on a newly opened NASA checkbook to resurrect the VLBI tracking system, which helped navigators keep the lander on course for its target. In the end, they got the directions right. But an unrelated problem with the software that controlled the lander’s descent through the atmosphere led to an embarrassing crash on Mars.

Even if these kinds of disasters can be avoided, the future of space navigation may lie not in better calculations on the ground but in teaching spacecraft how to find their own way. Auto-navigation will be particularly important for small, distant targets whose coordinates aren’t well known from ground observations. Deep Space 1, a pioneering spacecraft launched in 1998 to test new spacecraft technologies and (as a bonus) encounter two asteroids, carried such a system. It was supposed to pick out its targets against a background of stars stored in onboard memory, then fly past them without any direction from the ground or independent tracking. NASA hailed the $152 million project as a success because it proved other technologies, including an ion drive engine. But that wasn’t the whole story: One asteroid proved too dim for the camera to recognize, a failure that scotched the auto-navigation experiment. To home in on your target, it helps to be able to see it.

The navigators in charge of NEAR intend to make sure they don’t lose track of their whereabouts at Eros. In a back room at JPL, astronomer William Owen uses the spacecraft’s detailed photographs of the asteroid to create computerized maps of the nearly 1,200 craters that pock Eros’ surface. Navigators will use the craters as road markers to identify where the orbiting spacecraft is and what part of the asteroid it’s looking at. Automating the system is a worthy goal for the future, but for now it would demand too much computer power, says Owen. So he maps the craters by hand. “The eyeball is still a wonderful computer,” he says.

Back at NEAR’s mission control in Maryland, Farquhar’s attention is focused on the numbers on the screen that tell him the spacecraft has begun firing its thrusters to lower its orbit. For a space navigator, this is the critical moment. Finally the numbers begin climbing, registering the signal’s gradual Doppler shift, which means the craft is dropping closer to the surface.

About Michael Milstein

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

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