Voyager scientists announced today that their craft has entered a new realm, one long hypothesized but never observed, that marks the doorstep to true interstellar space. The territory is called the heliosheath; it lies beyond the supersonic solar wind that bathes all the planets and contains solar wind that's slowed to subsonic speeds. The news comes just two months after NASA managers decided that the Voyager 1 spacecraft--28 years and 14 billion kilometers out from Earth--might have outlived its usefulness as the spacecraft glides into the void far beyond the farthest planets (Science, 11 March, p. 1541).
Researchers are eager to understand the heliosheath after years of debate about where it is, much less what it does. It and its inner boundary are thought to both shield the solar system from galactic cosmic rays and generate their own cosmic rays that reach as far as Earth. The heliosheath is also Voyager's last stop before sampling the vanishingly thin stuff between the stars.
At the Joint Assembly of the American Geophysical Union in New Orleans, Louisiana, Leonard Burlaga of NASA's Goddard Space Flight Center in Greenbelt, Maryland, and the rest of the Voyager magnetometer team reported finding a key marker of the heliosheath. Last December, at a distance of 94 times the sun-Earth distance (94 astronomical units, AU), Voyager 1 recorded a sustained, three-fold increase in the intensity of the feeble magnetic field dragged along by the charged particles of the solar wind. Only a slowing of the solar wind and its resulting compression could do that.
Researchers are also looking outward toward the next Voyager milestone: leaving the heliosphere entirely. Estimates of the distance to the heliopause--where the solar wind ends and the interstellar medium begins--vary widely. Voyager 1 will run short of power from its radioisotope thermal generator as early as 2020 and go silent about 147 AU out.
Now, knowing where the termination shock is, researchers are suggesting 125 AU as a best estimate of the distance to the heliopause. "That's a comforting number," says Donald Gurnett of the University of Iowa in Iowa City, since it would get Voyager 1 there around 2014. Perhaps NASA managers will be equally comforted and remove Voyager 1 and its lagging companion Voyager 2 from the list of space physics missions to be considered this fall for termination.