When NASA launched Voyager 2 in 1977, the spacecraft was designed to explore the outer planets of our Solar System. More than four decades later, it continues its journey, now travelling through interstellar space. In November 2018, Voyager 2 became only the second human-made object to cross the boundary of the heliosphere, the vast bubble of particles and magnetic fields created by the Sun. What it has found since then has raised more questions than it has answered.