The elusive edge is known as the heliopause boundary, where the pressure of the solar wind becomes equal to that of the interstellar wind of cosmic rays. It¿s thought to be immediately preceded by the termination shock, where the solar wind is abruptly slowed from super- to sub-sonic flow. Pioneer¿s Geiger tube telescope should be able to locate the boundary by measuring an increase in cosmic-ray counts.
But, after a history filled with dramatic firsts, Pioneer 10 will have to settle for being a runner-up in this particular marathon. The first to encounter interstellar space will most likely be the more recently launched Voyager 1 spacecraft, itself an accomplished long-distance runner.
Voyage to Jupiter and Beyond
Also celebrating a milestone birthday this year, turning 25 on September 5, 2002, Voyager 1 overtook Pioneer in February 1998, to become the farthest man-made artifact. Like its pioneering cousin, Voyager 1 defied the odds, surviving for more than six times its original four-year mission length.
Voyager 1 and Voyager 2 set out in 1977, to Jupiter and beyond. The timing took advantage of a rare arrangement of the planets, occurring once every 175 years, which allowed a bargain tour of Jupiter, Saturn, Uranus and Neptune. To save weight on fuel, the twin spacecraft both employed the "slingshot" method, using the gravity of each planet passed to bend the flight path and increase velocity. Following in the footsteps of Pioneer 10, Voyager 1 traveled to Jupiter and Saturn before embarking on its interstellar mission, while Voyager 2 took the full four-planet tour. The two Voyagers found the first signs of volcanic activity outside Earth, on Jupiter¿s moon Io, and discovered that the atmosphere of Saturn consists almost entirely of hydrogen and helium.
The Voyagers¿ longevity can be attributed in part to last-minute design changes made following feedback from the Pioneer 10 and 11 missions. (The mission of Pioneer 11, which also flew by Jupiter, ended after its last transmission was received on September 30, 1995.) "The Pioneers discovered that Jupiter¿s radiation belt was much more intense than anticipated, and the Voyagers, while more sophisticated than the Pioneers, were also more vulnerable," Massey explains. "Optical glasses and electronic components that would withstand the intense radiation had to be found, and instruments and other space subsystems had to be modified."
Credit must also be given to the original design team¿s foresight when preparing the spacecraft for potential mission extensions. Each craft houses on-board backup systems that can be activated from the ground if needed. In April 2002, the team had to call on one of those reserve systems when Voyager 1¿s position-sensing capability was in jeopardy, setting a new record for most-distant spacecraft maintenance in the process. "We were switching to a system that had not been used for 20 years, and, while we felt confident, we were not positive that it would work," Massey says. During a temporary changeover, on-board computers became confused about the spacecraft¿s location, mistaking the sun for Earth, almost leading to loss of communications with the ground. The team had to instruct Voyager 1 to try to keep itself steady with gyroscopes during the final switch because it couldn¿t rely on its computer sun-position sensors. "There was only about a 15-minute interval to analyze the data after the switch and decide whether to continue with the permanent switch," Massey says. But the team had faith that the transition had been successful, and today Voyager 1¿s backup is functioning as smoothly as the original did 25 years ago.
Now both Voyagers are also heading out to the heliopause. With more sophisticated instrumentation than Pioneer, they may be used to study interstellar fields, particles and waves, in regions unaffected by the solar wind. To make sense of the data received, however, it¿s important for scientists to have a clear understanding of how galactic cosmic rays are modulated by the solar wind. And who better to look to for advice than IMP-8, a spacecraft with almost 30 years of experience collecting data on long-scale solar processes?