Fade to Black

Now and then, the faintest whisper returns from NASA’s distant space probes.

Air & Space Magazine

Dickens might have called it A Tale of Two Terminals. For Karolen Paularena, it is the best of times. The morning brings a new batch of ones and zeros, beamed to Earth from the depths of space and zapped overnight to her Sun workstation. Paularena and her colleagues at MIT’s Space Plasma Laboratory in Cambridge, Massachusetts, are studying the solar wind, the sun’s supersonic exhalation of protons, electrons, and magnetic energy, and key to that effort are the speed, direction, and intensity measurements they get from a NASA probe that almost no one has ever heard of: the Interplanetary Monitoring Platform 8.

For Lawrence Lasher, a continent away at NASA’s Ames Research Center in Mountain View, California, things are not so rosy. It may not be the worst of times, but he’s had little to cheer about lately. Lasher serves as project scientist for the only two active spacecraft under Ames’ control: Pioneer 6, which NASA no longer listens to even though it is still functioning, and Pioneer 10, which has been heard from only once since last August. Although outwardly optimistic, Lasher is hedging his predictions about when—or whether—the control-room computers waiting to communicate with the spacecraft will be used again.

The Ames and MIT teams share the distinction of working with craft that left Earth improbably long ago: IMP 8 in 1973, Pioneer 10 in 1972, and Pioneer 6—incredibly—in 1965. “We’ve got graduate students coming in to work with a spacecraft launched before they were born,” Paularena observes. Given NASA’s recent run of bad luck in getting to relatively nearby Mars, the endurance of these Space Age elders seems all the more remarkable.

The engineers who designed and built them aren’t really surprised. B.J. O’Brien joined the Pioneer development team at Space Technology Laboratories (later incorporated into TRW) in 1964 and took over as project manager in 1967. “As the program name implied, we knew we’d be breaking new ground,” O’Brien recalls. “To us, reliability meant simplicity.” All critical subsystems, such as the radio transmitter and power supplies, utilized designs that had already flown in space, and each had a backup. Because very-large-scale integrated circuits hadn’t yet appeared, the Pioneers used smaller boards (and, for the early models, discrete transistors) that were more tolerant of faults and radiation damage. Instead of asking banks of thrusters to maintain rock-steady orientation in space, the Ames-STL team stabilized their craft by spinning them. Finally, these birds had no brains—they transmitted their data continuously and executed changes only when commanded to by ground controllers.

It was a bullet-proof design philosophy that paid off handsomely. When Pioneer 6 headed off into solar orbit 36 years ago, project scientists hoped to glean six months of readings from its magnetometer, plasma sensors, and cosmic ray detectors. If the craft lasted that long, the STL team would earn a sizable performance bonus. “Obviously,” O’Brien wryly observes, “we didn’t have to worry.” NASA stopped tracking Pioneer 6 in 1997, though last December 16 a receiving station in the Mojave Desert locked onto its radio beacon—a carrier “tone” that included no data—for two hours to mark the craft’s 35th anniversary. Lasher thinks it can continue indefinitely. And for all anyone knows, its sibling Pioneers, 7 and 8, remain in good shape too; when last contacted in the mid-1990s, they were still phoning home.

Emboldened by the project’s initial success, in 1967 the agency approved a plan dreamed up by renowned space physicist James Van Allen and other members of the agency’s Lunar and Planetary Missions Board. A pair of Pioneers, each bristling with 11 experiments, would trek to Jupiter and dash through its surrounding radiation belts.

Puttering around in solar orbit was one thing, but plunging headlong through lethal doses of high-speed electrons was another altogether. The design by O’Brien’s team used radiation-hardened electronics and shielded critical components wherever possible. “They were pretty rugged spacecraft,” says Van Allen. And because weak sunlight at Jupiter’s distance would have required enormous solar cells, these long-haul craft carried their own juice: plutonium-fueled powerplants called radioisotope thermoelectric generators, or RTGs.

Pioneer 10 rocketed away on March 2, 1972, and reached Jupiter 21 months later after threading the uncharted asteroid belt without incident. The target point was just 81,000 miles from the giant planet’s colorful cloud tops, and as Jupiter loomed larger several of the radiation detectors topped out. “The counts just kept going up up up,” Van Allen recalls. Back in California, anxiety peaked as the craft slipped behind the planet and out of radio contact. “Those were 15 of the longest minutes in my life,” says O’Brien. Telemetry later showed that a few transistors had failed and exposed optics had darkened, but there were no serious malfunctions. The triumphant flyby earned the project team an Emmy (for its real-time broadcast of Jovian cloudscapes) and O’Brien a bottle of gin (a side bet with one of the scientists).

Pioneer 11 proved up to the task as well, sweeping past Jupiter in 1974 and Saturn in 1979. No one really knew what kind of environment lay beyond Jupiter, but the Pioneers might find out. Nor could anyone predict exactly how long the spacecraft would last, though in theory their RTGs would keep electricity flowing for a dozen years or more. Pioneer 11 was the first to go: Its transmissions ceased in November 1995, 22-and-a-half years after launch, when it apparently lost track of the sun.

Following the trail blazed by their predecessors, Voyagers 1 and 2 ricocheted their way across the outer solar system, culminating with Voyager 2 making a final flyby of Neptune in August 1989, five days past the 22nd anniversary of its launch. The big, beefy Voyagers were outfitted for the long haul: more RTG power, stronger transmitters, a bigger radio dish, sophisticated experiments, and a modest degree of computational intelligence.

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