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
(Page 2 of 4)
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.