Number theory. Third time proves charm for prime-gap theorem.

into shock waves in the solar wind upstream of planets. Shortly after the oscillations, Voyager was in the new solar wind regime of heightened magnetic field. Everyone, including Krimigis, now agrees that this new regime is the heliosheath. Now that they are in it, researchers are eager to understand the heliosheath. They missed recording the actual passage through the shock because it occurred during one of the gaps in Voyager monitoring by the big radio telescopes of the Deep Space Network. But they will be studying the heightened turbulence within the heliosheath and how the turbulence helps deflect galactic cosmic rays. The spacecraft’s reports from the heliosheath should also help scientists understand similar shock-bounded “astrospheres” seen around other, more energetic stars. Researchers are also looking outward toward the next Voyager milestone: leaving the heliosphere entirely. Estimates of the distance to the heliopause—where solar wind ends and the interstellar medium begins— vary widely. Gurnett’s interpretation of radio signals emanating from that frontier place it anywhere from 116 AU to 177 AU. But 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 Gurnett, because 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. –RICHARD A. KERR N E W S O F T H E W E E K