Fade to Black
Now and then, the faintest whisper returns from NASA's distant space probes.
- By J. Kelly Beatty
- Air & Space magazine, July 2001
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.
Today both craft continue to race outward at nearly a million miles per day, a speed that will remain constant. With its trajectory pitched well north of the planets’ orbital plane, Voyager 1 will pass near a star in the constellation Ursa Minor about 40,000 years from now. By then, Voyager 2, taking a more southerly route, will be cruising past Ross 248 in Andromeda en route to a distant rendezvous with dazzling Sirius in the year 296,036.
Meanwhile, somewhere not far ahead of them lies the boundary marking the limit of the sun’s electromagnetic influence, a kind of Holy Grail long sought by space physicists. The first evidence of the approaching frontier should be a region called the termination shock, where the solar wind becomes contorted and redirected as it slows to subsonic speed. Bowed but not broken, the wind should limp outward until it can no longer make any headway against the tenuous interstellar ether. That will mark the heliopause, the end of the solar line, beyond which lies true interstellar space. “Our best estimate is that the distance to the termination shock is 80 or 90 astronomical units [eight or nine billion miles], and Voyager 1 will reach 80 AU in three years,” says Edward Stone, Voyager’s project scientist. The transition region might lie considerably farther out, but that seems unlikely. During the last six months of 1992, both Voyagers recorded a 10-trillion-watt burst of low-frequency radio noise triangulated to be no more than about 100 AU from the sun. Project scientists believe that this was a hail from the heliopause, created when a fast-moving solar wind shock front hit the interstellar wall, causing redirected electrons to groan in protest.
When and if the termination shock is reached—and conceivably that crossing could start any day now—the five experiments still working on each Voyager spacecraft will know it. Cosmic ray energies will jump, magnetic field lines will rear-end one another, and the solar wind plasma will shriek and sizzle with wave activity. “It’ll be quite an exciting time,” Stone says.
The Pioneer team, on the other hand, may need to be a little more patient. Even though Jupiter’s gravity gave Pioneer 10 an 82,000-mph boot out of the solar system, the craft is racing toward the constellation Taurus while the sun is headed in the opposite direction, toward Hercules. So if the solar wind bubble is shaped like a teardrop, as most physicists believe, the spacecraft is unlikely to break out before its power fails. But Van Allen, whose cosmic ray detector is the sole Pioneer 10 instrument still switched on, takes a skeptical view, arguing that the heliosphere is, in fact, nearly spherical. “What we’re looking for is the absence of fluctuations caused by the sun,” he explains.
Of course, all that conjecture becomes moot if no one ever hears again from the “Gallant Lady,” as O’Brien’s TRW team once christened Pioneer 10. The spacecraft is now 7.1 billion miles from Earth, requiring a round-trip communication time of 21.3 hours. Ric Campo and Paul Travis, members of Lasher’s now-disbanded mission team, have been tending to Pioneer 10’s needs on a voluntary basis for years. Now they’re hoping for another chance to slip back into their old control consoles and pull in just a little more of its data.
Campo and Travis attempted to tweak the spacecraft’s orientation last July. Although Pioneer 10 relayed some data to Earth a month later, it never confirmed that the command was received or executed. The probe’s silence could have been the result of a transmitter failure or a drop in voltage from its plutonium-powered RTGs. But Lasher suspects that the craft was simply pointing at the wrong spot in Earth’s orbit. Last March, NASA’s Deep Space Network tracking stations in California, Australia, and Spain began to listen for the craft’s eight-watt signal, and two-way communication was attempted in April. On the 28th, the Madrid station achieved contact with Pioneer 10.
So could the 29-year-old spacecraft become the first to send a signal from the heliosphere? Unfortunately, Pioneer is last in the queue for Deep Space Network tracking passes. Officially, the project ended on April 1, 1997—a few weeks after a “celebration” of Pioneer 10’s silver anniversary at the Smithsonian’s National Air and Space Museum. “It was a funeral service,” Van Allen snips, “and I gave a eulogy.” But he also worked the hallways, protesting the cutoff to NASA officials. They responded with a reprieve, agreeing to let engineers use the spacecraft’s weakening signal to test a new tracking scheme based on chaos theory.
IMP 8 doesn’t have to compete for time on the Deep Space Network, ironically, because its telemetry system is obsolete. For more than 28 years, this oldster has been continuously transmitting six kilobits of data per second—it has no tape recorder—at the long-abandoned VHF frequency of 137.98 megahertz. The signal is gathered by a trio of Yagi-style receivers (think of rooftop TV antennas on steroids) in Virginia, Belgium, and Australia that are dedicated to the IMP 8. “One of the most challenging aspects of my job,” says project scientist Joseph H. King at NASA’s Goddard Space Flight Center in Maryland, “is cobbling together the VHF ground network.” Once at Goddard, the IMP data is routed to science teams around the country, arriving as an encrypted jumble of timing code, spacecraft positions, and instrument readouts that takes a lot of massaging by decades-old software to be useful.
But space physicists aren’t complaining. The 10th and last of the Interplanetary Monitoring Platform series, IMP 8 circles Earth in an unusually high orbit that extends about halfway to the moon. In its heyday the spacecraft served as something of a sentinel, warning of stormy conditions in the solar wind. Seven of its 11 experiments still work, and their data remains a staple for hundreds of space physicists studying the sun and Earth’s magnetosphere despite the advent of state-of-the-art solar watchdogs like the Advanced Composition Explorer and Wind spacecraft. “It’s so much a part of the culture,” observes MIT’s Paularena. “We accept and use its data without really thinking about it.” For example, on July 14, 2000, the sun uncorked an eruption so powerful that the solar wind’s shock wave disabled some sensors aboard Wind. But IMP 8 took it in stride, sending back readings on the titanic shock that had the MIT team fist-pumping in exultation.
Interest in IMP 8 data soared during the mid-1990s, when there was a hiatus in solar wind coverage by other spacecraft. But the craft’s steeply inclined orbit kept it hidden from the ground stations in Virginia and Belgium for five days out of each 12-day circuit. King had been running the program on a shoestring for years, and his options were limited. But then came a chance conversation with a fellow runner at Goddard. “How’s that old spacecraft doing?” asked Michael Comberiate, who had built some of IMP 8’s electronics early in his career. It turned out that Comberiate would be returning to Antarctica in a few months to service some NASA hardware, and a plan was hatched.
Working with a local Ham wizard named Michael Staal, Comberiate fabricated eight 30-foot VHF antennas, shipped them to McMurdo Station, and mounted them atop a 65-foot-high tower. Throw in some kluged electronics and a desktop computer, and—voilà—IMP 8 data started streaming into Goddard from the South Pole. “Antarctica is a tough place to do anything like that,” Comberiate says, and after two harsh winters he disassembled the antennas and moved them to Australia.
Time is slowly catching up to IMP 8, and its data isn’t as prized as it once was. Last year’s loss of its magnetometer, whose magnetic field readings provide a context for other data sets, didn’t help. “Right now it’s like someone who is red-green colorblind,” Paularena explains. “You can still see the world, but you’re missing something.” The continuing value of IMP 8’s data will be tested later this year, when King will defend his program before a senior review board. Despite its loss of compass, there are good reasons to keep listening to what IMP 8 has to say—if for no other reason than to extend its unbroken 28-year run of solar wind data. “Sometimes I think NASA hasn’t taken sufficient pride in its long-term spacecraft,” King says.
In fact, in a world where a good VCR might last eight or 10 years, NASA’s endurance records seem nothing short of astounding. Budgets permitting, IMP 8 could continue sending its solar weather reports for years to come. The twin Voyagers could prove equally durable (the Voyager Interstellar Mission, as it’s now called, has a timeline that runs at least through 2016). No other nation’s spacefaring efforts come close to these milestones. Giotto, launched in 1985 by the European Space Agency, was tracked after plunging through two comets (Halley and Grigg-Skjellerup) until September 1992. Sakigake, another Halley watcher, remained in contact with Japanese controllers for a decade.
But the Methuselah prize may ultimately go to a spacecraft that will spend the next decade in electronic hibernation. Today NASA calls it the International Cometary Explorer, or ICE, but when launched in 1978 it was christened the International Sun-Earth Explorer 3. Under the direction of trajectory master Robert Farquhar, of the Johns Hopkins University Applied Physics Laboratory, ISEE 3 spent five years flitting here and there around the Earth-moon system. For a while it hovered near the L1 Lagrangian point, a million miles in the sun’s direction; then it crisscrossed Earth’s magnetosphere and lingered downstream for months at a time. Recast and renamed as a comet chaser, ICE dashed off to intercept the ion tail of Comet Giacobini-Zinner in September 1985.
The consummate trajectory junkie, Farquhar is never bereft of clever uses for spacecraft (see “Hang a Right at Jupiter,” Dec. 2000/Jan. 2001). He plans to revive ICE in 2010, direct it to within 125,000 miles of Earth four years later, and have it pay a return visit to Comet G-Z in 2018. He’s planned the comet encounter for September 19—a month past ICE’s 40th anniversary in space. Mark your calendars.