The
Sun emits a steady flow of
electrically charged particles called the
solar wind. As the cloud expands into space at
supersonic speeds, it creates the
heliosphere. This expanding cloud of particles will eventually reach the electrically charged particles and
magnetic field in the
interstellar gas. At some distance from the Sun, the solar wind can no longer push back against the interstellar gas and it slows abruptly from supersonic to
subsonic speeds. This region is known as the termination shock, where termination refers to the termination of supersonic speed, not the termination of the solar wind, which occurs at the
heliopause.
There is currently insufficient data regarding the termination shock to locate it exactly but predictions place it between 62 and 90 astronomical units from the Sun, with most estimates converging on about 82 AU. For comparison, Pluto's average distance from the Sun is 39.53 AU. As the speed of the solar wind varies, the boundary will move back and forth. The velocity of the wind peaked in 1996, which was thought to coincide with the solar minimum and this pushed the termination shock away. As the wind speed decreased, it was brought closer again.
Currently, the Voyager spacecraft are the only man made objects near this boundary to make any measurements and they are expected to reach it in 2003, unless the solar wind speeds up again and they will have to wait a few more years to catch up.