Voyager 2 reaches the heliosheath

Like its sister craft Voyager 1 several years before it, Voyager 2 has now flown far enough out into the solar system to cross the termination shock, where the supersonic wind of solar particles—the wind that blows a bubble in space around our solar system—is abruptly slowed by pressure from the interstellar medium. Beyond that thin shock, the heliosheath extends to the farthest reaches of the Sun’s magnetic influence, which ultimately ends at the yet-unseen heliopause (see the schematic). As of 1 September 2007, Voyager 1 was 104 astronomical units from the Sun (1 AU is the mean distance between the Sun and Earth), traveling at 3.6 AU per year, while Voyager 2 was 84 AU away and moving at a more sedate 3.3 AU per year. Although Voyager 1 reached the termination shock first, its plasma-measuring instrument stopped working before it arrived there. Voyager 2’s working instrument found the temperature beyond the shock surprisingly lower than predicted, which could indicate that energy is being transferred to so-called pickup ions and shock-accelerated cosmic rays. In addition, the data from the spacecrafts’ two different locations confirm that the termination shock is not spherical. (Results reported by several researchers at the December 2007 meeting of the American Geophysical Union in San Francisco.)