The Guardian reports:

Twelve billion miles from Earth, there is an elusive boundary that marks the edge of the sun’s realm and the start of interstellar space. Voyager 2, the longest-running space mission, has finally beamed back a faint signal from the other side of that frontier, 42 years after its launch.

The Nasa craft is the second ever to travel beyond the heliosphere, the bubble of supersonic charged particles streaming outwards from the sun. Despite setting off a month ahead of its twin, Voyager 1, it crossed the threshold into interstellar space more than six years behind, after taking the scenic route across the solar system and providing what remain the only close-up images of Uranus and Neptune.

Now Voyager 2 has sent back the most detailed look yet at the edge of our solar system – despite Nasa scientists having no idea at the outset that it would survive to see this landmark.

“We didn’t know how large the bubble was and we certainly didn’t know that the spacecraft could live long enough to reach the edge of the bubble and enter interstellar space,” said Prof Ed Stone, of the California Institute of Technology, who has been working on the mission since before its launch in 1977.

The heliosphere can be thought of as a cosmic weather front: a distinct boundary where charged particles rushing outwards from the sun at supersonic speed meet a cooler, interstellar wind blowing in from supernovae that exploded millions of years ago. It was once thought that the solar wind faded away gradually with distance, but Voyager 1 confirmed there was a boundary, defined by a sudden drop in temperature and an increase in the density of charged particles, known as plasma. [Continue reading…]