A plucky little spacecraft may have passed into a strange and unexplored region near the edge of the solar system. If so, then for the first time a humanmade object has reached the verge of interstellar space.
The sun creates a kind of bubble that shields the solar system from interstellar space. A torrent of particles flows from the sun, and this solar wind gushes in all directions at supersonic speeds. Far beyond the planets, the solar wind slows to subsonic speeds, and that transition creates a shock wave that loosely resembles the one created by the supersonic airplane Concorde. Past that "termination shock," the solar wind tapers and eventually melds with the interstellar medium at the edge of the solar system, which is called the heliopause. Scientists are hoping to observe NASA's far-flung Voyager 1 and Voyager 2 spacecraft pass into the little-understood region between transition shock and heliopause. Both were launched in 1977, and the Voyager 1 probe has sped more than 13 billion kilometers from Earth--more than 85 times the distance to the sun. Voyager 2 lags 2.7 billion kilometers behind.
In the summer of 2002, researchers saw evidence that Voyager had crossed the termination shock. A detector on the spacecraft registered a 100-fold increase in low-energy particles, Stamatios (Tom) Krimigis of Johns Hopkins University's Applied Physics Laboratory in Laurel, Maryland, and colleagues report in the 6 November issue of Nature. Equal numbers of the particles were heading toward and away from the sun. That's what researchers would expect to see if Voyager 1 had passed into the region where the solar wind blows more gently. "We're getting into the transition to interstellar space," Krimigis says.
But Frank McDonald of the University of Maryland, College Park, and colleagues report, also in Nature, that higher energy particles increased less than they should have if Voyager 1 had crossed the termination shock. "Have we passed it?" McDonald says, "or are we just in the vicinity?"
The directions of the low-energy particles reflect the speed of the solar wind more directly than the rate of high-energy particles, says Len Fisk of the University of Michigan, Ann Arbor, who suspects that Voyager 1 did cross the termination shock. Ironically, the spacecraft carries a "plasma detector" designed specifically to measure the speed of the solar wind, but it conked out several years ago. The plasma detector on Voyager 2 still works, so researchers should know for sure whether that probe has passed the termination shock when it cruises through similar territory 4 years hence.
NASA's Voyager site