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

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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.

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