Voyagers at the Termination Shock
Data from Voyager 1 and 2 was used to define our solar system's final frontier, a vast region at the edge of our solar system where the solar wind runs up against the thin gas between the stars.
Voyager 2 crossed the heliosheath boundary, called the solar wind termination shock, about 10 billion miles away from Voyager 1 and almost a billion miles closer to the sun. It confirmed that our solar system is "squashed" or "dented" -- that the bubble carved into interstellar space by the solar wind is not perfectly round.
Even though Voyager 2 is the second spacecraft to cross the shock, it is scientifically exciting for a couple of reasons. The Voyager 2 spacecraft has a working Plasma Science instrument that can directly measure the velocity, density and temperature of the solar wind. This instrument is no longer working on Voyager 1 and estimates of the solar wind speed had to be made indirectly. Secondly, Voyager 1 may have had only a single shock crossing and it happened during a data gap. But Voyager 2 had at least five shock crossings over a couple of days (the shock "sloshes" back and forth like surf on a beach, allowing multiple crossings) and three of them are clearly in the data. They show us an unusual shock.
In a normal shock wave, fast-moving material slows down and forms a denser, hotter region as it encounters an obstacle. However, Voyager 2 found a much lower temperature beyond the shock than was predicted. This probably indicates that the energy is being transferred to cosmic ray particles that were accelerated to high speeds at the shock.
Last Update: 27 Jun 2011 (AMB)