Voyager 2 joins its twin in interstellar space

Voyager 2, the multi-planetary exploratory probe launched in 1977, has finally entered interstellar space, some six years after its twin, Voyager 1, did the same. Interstellar space starts where the sun's "heliosphere" ends — the big ball of radiation and plasma in which the planets bathe and by which they are protected. Both Voyagers have instruments on board that monitor all this stuff, and both have shown a major drop-off in electrical and plasma activity, suggesting they've crossed over.