But one of the instruments on Voyager 1 was broken, and Voyager 2 was able to send back an even more detailed treasure trove of data from 11 billion miles away.

It found that the heliopause - the barrier where the solar and interstellar winds meet - was much more defined than scientists anticipated.

Voyager 2 took less than a day to cross the heliopause, whereas Voyager 1 had taken 80 days.

As it left the solar system Voyager 2 also recorded particles leaking out into interstellar space, whereas Voyager 1 had recorded elements of the interstellar wind coming the other way.

Comparison of where the two probes left the solar system helped to show the shape of the heliosphere.

Don Gurnett, astronomy professor at the University of Iowa, said: "There's almost a spherical front to this. It's like a blunt bullet."