The heliosphere's a bubble in space "blown" into the interstellar medium (the hydrogen and helium gas that permeates the galaxy) by the solar wind. Although electrically neutral atoms from interstellar space can penetrate this bubble, virtually all of the material in the heliosphere emanates from the Sun itself. For the first ten billion kilometres of its radius, the solar wind travels at over a million kilometres per hour. As it begins to collide with the interstellar medium, it slows down be… (More on Heliosphere)