Voyager 1 exited the solar wind at a distance of similar to 85 AU from the Sun

The outer limit of the Solar System is often considered to be at the distance from the Sun where the solar wind changes from supersonic to subsonic flow(1). Theory predicts that a termination shock marks this boundary, with locations ranging(2) from a few to over 100 AU (1 AU approximate to 1.5 x 10(8) km, the distance from Earth to the Sun). `Pick-up ions' that originate(3,4) as interstellar neutral atoms should be accelerated to tens of MeV at the termination shock, generating anomalous cosmic rays(5-7). Here we report a large increase in the intensity of energetic particles in the outer heliosphere, as measured by an instrument on the Voyager 1 spacecraft. We argue that the spacecraft exited the supersonic solar wind and passed into the subsonic region ( possibly beyond the termination shock) on about 1 August 2002 at a distance of similar to85 AU (heliolatitude similar to34degrees N), then re-entered the supersonic solar wind about 200 days later at, 87 AU from the Sun. We show that the composition of the ions accelerated at the putative termination shock is that of anomalous cosmic rays and of interstellar pick-up ions.