The point where the solar wind becomes slower than the speed of sound is called the termination shock; the solar wind continues to slow as it passes through the heliosheath leading to a boundary called the heliopause, where the interstellar medium and solar wind pressures balance. Voyager 1, the man-made object most distant from Earth, crossed the termination shock in 2004 and the heliopause in 2012, putting it into interstellar space.