On 25 August last year, Voyager 1 left our solar system and entered interstellar space. The exploration probe launched by Nasa in 1977 is about 125 AU from the sun, or more than 11 billion miles away (one AU is the distance between the sun and Earth - 93 million miles). How do we know? Because of the change in plasma readings ascertained by the Voyager team. Plasma - ionised gas - is the most important marker that distinguishes whether Voyager 1 is inside the solar bubble, known as the heliosphere, which is inflated by plasma that streams outward from the sun, or in interstellar space and surrounded by material ejected by the explosion of nearby giant stars millions of years ago.
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