The collision occurs at the "termination shock", which is roughly 80–100 AU from the Sun upwind of the interstellar medium and roughly 200 AU from the Sun downwind. Here the wind slows dramatically, condenses and becomes more turbulent, forming a great oval structure known as the "heliosheath". This structure is thought to look and behave very much like a comet's tail, extending outward for a further 40 AU on the upwind side but tailing many times that distance downwind; evidence from "Cassini" and Interstellar Boundary Explorer spacecraft has suggested that it is forced into a bubble shape by the constraining action of the interstellar magnetic field.