Pioneer 10, launched in 1972 and expected to complete its mission the next year, is still sending messages from far beyond Pluto, confounding even its makers
American Heritage of Invention & Technology, Winter 2001
The dark emptiness beyond Pluto, the farthest planet in our solar system, defies human comprehension. The sun is little more than a bright star in the trackless void, the Earth lost in its feeble glare.
Yet even out here, there's a little piece of Earth. Almost seven billion miles from home, faithfully calling back to a planet which has almost forgotten it exists, is humanity's oldest functioning spacecraft: Pioneer 10. After trailblazing the path into the outer solar system as the first space probe to Jupiter, Pioneer 10 confounds its makers by functioning long past its expected life span, returning useful data nearly thirty years after its launch. But back on Earth, only a few people are still listening.
One is Dr. Larry Lasher, Pioneer's current project manager. Although NASA headquarters officially ended the project in 1997, Lasher and a small team of diehards managed to wangle the facilities to maintain Pioneer 10's link with Earth. "No one seems to want to let it go, even if it requires coming in after hours or on weekends," says Lasher. "Some will come in the middle of the night because that was the time commands had to be sent up."
The people involved in space exploration tend to be a dedicated bunch--they have to be, to overcome the technical, financial and physical obstacles that come with the work. But volunteering your weekends and late nights for a project that even your bosses think of as extinct is decidedly above and beyond the call. How does this humble spacecraft, built with 1960s technology, weighing less than six hundred pounds and smaller than a compact car, inspire such devotion?
Maybe it's because the Pioneer project proves that sometimes,
simpler really is better. The story of Pioneer 10 remains an
impressive example of what can be accomplished by a team of
skilled scientists and engineers working on a limited budget while
literally opening up brand-new scientific frontiers.
Although proposals for a deep space mission to the outer planets had been bouncing around NASA circles since the early 1960's, it wasn't until early 1969 that official approval for a Jupiter mission was granted, amidst the heyday of the Apollo moon missions. The sterling performance of NASA's Ames Research Center in Mountain View, California, on an earlier series of Pioneer solar probes convinced headquarters to give them the Jupiter project. Pioneer 10 would investigate the interplanetary medium, assess the dangers of the asteroid belt between Mars and Jupiter, and make the first fly-by of Jupiter. An identical sister craft, Pioneer 11, would follow a year later.
Time was crucial. The best time to send a spacecraft to
Jupiter occurs about every thirteen months, when the relative
positions of Earth and Jupiter permit a launch at minimal energy.
Allowing a reasonable period for the design and construction of
the spacecraft and the selection of scientific experiments,
Pioneer's first launch window opened in late February to early
March of 1972. To avoid the time-consuming process of competitive
bidding for a prime contractor of the spacecraft, project manager
Charles F. Hall pushed for a "sole-source" selection of TRW, Inc.,
the builder of the previous Pioneers. As Hall assigned TRW to
complete a design study, NASA review committees began ruthlessly
winnowing down over 150 initial experiment proposals to the final
eleven chosen for Pioneer. The group of principal investigators
(PIs) was a roll call of the world's most distinguished planetary
astronomers and physicists, including the University of Chicago's
John Simpson and Van Allen Belt discoverer James Van Allen.
Despite Pioneer's ambitious objectives, Charlie Hall insisted on simplicity and reliability in every aspect of the spacecraft and its mission. To avoid the expense and complexity of a three- axis stabilization thruster system, the Pioneer craft would be spin-stabilized: once released on its Jupiter trajectory by its launch booster, it would be set spinning around its axis at about five revolutions per minute, like a cosmic top. Trajectory specialist Daze Lozier observes, "The spinning spacecraft idea sure makes things simple. If we lost communication, we always knew it was going to be pointed in the same direction." Three pairs of small hydrazine jets, evenly spaced around the rim of the nine-foot main antenna, would control Pioneer's spin rate, velocity, and orientation.
Hall also made sure that wherever possible, only component designs previously tested in spaceflight were used. Bernard J. O'Brien, who headed the Pioneer effort at TRW, points out that the Pioneer 10 spacecraft design was essentially a refinement and evolution of TRW's earlier models. "We used a lot of the same design concepts, which in turn helped the cost and schedule. I think Charlie was smart in doing that." Redundancy was built into vital systems such as the radio transmitters and receivers (two of each), with Pioneer designed to switch automatically to backup systems should a primary fail. "Charlie insisted that wherever there was an automatic system, we'd be able to bypass it," says mission analysis chief Jack Dyer.
Such flexibility was crucial because unlike later probes, Pioneer lacked an on-board computer. "It was a real-time spacecraft," says Dave Lozier. "Everything you wanted to do was commanded from Earth." Microprocessors and sophisticated integrated circuits were still only engineering pipe dreams, and a full-blown on-board computer would be much too heavy and power- hungry. Lozier explains that Pioneer "had a five-command storage feature so if we had a power problem and had to shut instruments off in an emergency, it would just cycle through these five commands. Anything we wanted to do, fire the jets, turn an instrument on and off, we had to load a register and execute it."
But because the spacecraft lacked a "brain" and thus wasn't self-sufficient, the issue of communications was absolutely vital. With the light-speed-dictated time lag between message transmission and reception ever increasing as Pioneer sped away from Earth, spacecraft maneuvers and possible problems had to be anticipated well in advance. And over the immense distances to the outer solar system, Pioneer's 8-watt signals would be a mere whisper, lacking enough energy to power the dimmest night light and pushing the extremely sensitive 70-meter antenna dishes of NASA's worldwide Deep Space Network to their limits.
Designing a spacecraft is always a series of tradeoffs between what you want to do and what you can do given the unavoidable limitations of budget, time, and plain old physics. Electrical power is always precious and must be carefully rationed among spacecraft systems. Weight--or more precisely, mass--is always a critical concern in spaceflight, because the more mass, the more energy required to send it someplace. More energy means a larger, more expensive launch vehicle. The Pioneer craft had to be small and light enough for an Atlas-Centaur rocket to lift it into space and put it on the proper trajectory at the speed necessary to make its Jupiter rendezvous. At only 550 pounds, Pioneer was a mere featherweight compared to the massive Voyager and Galileo probes which would follow.
Pioneer's final form evolved in an intense back-and-forth between Charlie Hall and TRW engineer Herb Lassen, who authored TRW's initial proposal. It wasn't an easy process. "The conceptual design was by Herb Lassen," Hall recalled in a 1999 interview. "He started out laying out various designs and presenting them, and he knew I wasn't liking 'em. So then I'd present something I'd been thinking about, and I knew he didn't like them. I'd say, keep trying, it'll come. Finally one day he calls up and says, 'Get down here fast! I got it!' So I go down there and he presented the Pioneer 10 and 11 that were actually built. It was obvious that it was just a breakthrough."
Lassen conceived an elegant and ingenious design. "He had thrown all constraints out the window," Hall said. "And some of the constraints were artificial, because people assumed something that was not necessarily true." One assumption concerned the fundamental nature of the spin stabilization concept. "Most people in designing a spacecraft have the spin axis go through the center of the antenna so it won't wobble," Hall explained. A wobbling antenna dish could lead to a slight fluctuation in signal power back at Earth. "Herb threw that out and said it doesn't matter. We're talking about 8 centimeters out of about 60 billion kilometers."
Some big questions still remained, including how to power the craft. All previous probes had traveled within the orbit of Mars, close enough to the Sun that solar panels could provide adequate electrical power. But out at Jupiter, 485 million miles from the sun, light is only 1/27th as bright as at Earth. Pioneer would need very large and delicate solar panels to do its job. Some previous spacecraft had used radioisotope thermoelectric generators: devices which generate electricity from thermocouples using the heat produced by small capsules of decaying plutonium- 238. But early RTGs were considered unreliable for long-term use, especially for a deep space mission such as Pioneer. "At first they looked like they were going to be one big pain in the ass," Hall admitted frankly. "They weren't developed far enough long yet."
With all their limitations, solar panels seemed the only answer--until a representative of the Atomic Energy Commission informed Hall that the AEC and the Teledyne corporation had developed a new RTG model, the SNAP-19, which promised a much longer service life. "He was really anxious to get a spacecraft to put a unit on, and ours was the only one at that time of an interplanetary nature," Hall said. "He said we'll build the prototypes free, and all you have to do is pay for the flight units. Well, this was a big bonus. Saving us fifteen million bucks--I couldn't turn that down!" Four SNAP-19s would provide about 155 watts of power. Although the RTG output would slowly decrease over time because of the deterioration of the thermocouple junctions, there would still be plenty of power available at Jupiter.
While the RTGs solved Pioneer's power problem, their residual radiation threatened to interfere with some of Pioneer's particles and fields instruments, forcing other modifications. Placing the RTGs away from the craft at the end of 10-foot booms helped, but not enough. Hall recalled that "some of the scientists had to add more shielding," which also meant more weight. But Hall had anticipated this. "Fortunately, I had about fifty or sixty pounds of contingency that I could dole out." For problems like these, Hall said, "you had three choices: don't do anything, spend a lot of money, or add a little weight in the right places." Adding the small amount of extra weight was the best choice.
Charlie Hall's keep-it-simple credo led to some lively debates among Pioneer's designers, scientists and project personnel. "There were massive negotiations on things like power and weight," says James Van Allen, "hashed out in a series of group meetings in which we traded back and forth one thing for another." Scientists and engineers presented "laundry lists" of various problems and concerns, with solutions then worked out and mutually agreed upon. Once decisions were made, Hall vigorously resisted pressures to continually modify and perfect design ideas. Jack Dyer says, "Charlie viewed it as a contract: now let's do that, and not get distracted by trying to make it better, because we're going to do it within budget when Jupiter's ready at the right place."
With the spacecraft design established, experiments selected, and launch window confirmed, construction of Pioneer 10 began at TRW, its subcontractors, and the PIs' various institutions. "We didn't have an awful lot of time," O'Brien admits. But his company had quite an incentive. "There was a million-dollar penalty if we were not ready to launch in February-March '72," he recalls, adding, "And I'd have lost my job!"
O'Brien kept his job. On March 2, 1972 at 8:49 PM Eastern time, Pioneer 10 launched from Cape Kennedy, snugly within its launch window. Less than half an hour later, it set course for Jupiter, traveling faster than any previous humanmade object at over 32,000 miles per hour. Over the next few weeks, flight controllers calibrated the science instruments and tweaked the spacecraft's trajectory. Pioneer 10 soon scored its first scientific triumph by proving that the zodiacal light, a faint glow of sunlight reflected from dust particles along the plane of the zodiac, was an interplanetary rather than an Earth-generated phenomenon.
More amazing discoveries would follow, but not before Pioneer 10 faced its greatest hazards. The first hurdle came after Mars: the asteroid belt, the unavoidable threshold to the outer solar system. (Flying above the belt is possible, but only at the cost of enormous launch energies, out of the question for planetary missions.) In the vastness of the belt, the chances Pioneer would pass anywhere near a large asteroid were minuscule. But how many smaller rocks drifted among the big ones? At Pioneer 10's great velocity, a particle the size of a grain of sand could pierce the hull with more energy than a high-powered rifle bullet and destroy vital systems. And even a rock no bigger than a baseball would completely take out the spacecraft. Even a later failure of the craft at Jupiter would be easier to take, because at least in that case some valuable data would have been obtained. But as Hall explained, "If we had a loss of the spacecraft while we're in the asteroid belt, nothing else can happen." As Pioneer entered the danger zone, plans were on NASA's drawing boards for Voyager and other deep-space missions--but all depended on Pioneer 10's safe passage. The transit would last about seven months, with the possibility of disaster looming every day.
Pioneer 10 didn't simply cruise along waiting passively for catastrophe during those seven months, however. Its scientific instruments busily probed the interplanetary environment, recording new data on cosmic rays, magnetic fields, and the solar wind. Meanwhile, Pioneer's meteoroid detectors counted impacting asteroid particles. Much to everyone's surprise and relief, far fewer hits were registered than expected. The ship was going to make it. The date with Jupiter was still on.
And so was Pioneer's next challenge: penetration of
Jupiter's lethal radiation belts. Jupiter is an enormously
powerful source of radio signals, produced by whirling charged
particles trapped in an intense magnetic field. No one knew just
how strong the radiation might be, or how deeply the zone could be
penetrated by a spacecraft without its electronics being fried
into slag. John Simpson puts it succinctly: "There was no
assurance, since we were the first spacecraft in those radiation
regions, that we would survive to come out." As in the asteroid
belt, Pioneer 10 would again play the brave scout sent through no-
man's-land to draw fire for those to follow.
(C) 2000-2001 Mark Wolverton. All rights reserved.
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