Termination Shock

At a fluctuating distance of about 7 -8.5 billion miles from the sun, the solar wind is decreased to speeds less than the speed of sound as it reaches a barrier of plasma and dust. The solar wind is the stream of charged particles ejected by the sun and travels at supersonic speeds until it reaches this barrier. However, the intensity of the solar wind fluctuates due to the changing solar surface, and so the termination shock distance changes dramatically.
Voyager 2 will be reaching the shock (the position of which is terribly difficult to calculate and predict) sometime at the end of 2007 or beginning of 2008, and its experience will be widely observed and studied. Because of the changing distance, Voyager 2 will actually cross the shock multiple times. Voyager 1 reached the shock in 2004.
Eventually, both craft will reach another milestone, the heliopause, at which the solar wind is stopped completely and the spacecraft will be in galactic space.