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Fact file

Type

Region at the edge of the Solar System

Interesting facts about the heliosphere

• The Sun constantly blows a stream of charged particles out into space: the solar wind. It forms a gigantic bubble around the whole Solar System – the heliosphere.
• Its outer boundary, the heliopause, lies around 120 times as far from the Sun as Earth does. Beyond it, interstellar space begins.
• Voyager 1 left the bubble in 2012, Voyager 2 in 2018 – the only human-made objects ever to do so.

Formation

The solar wind races outwards at 400 to 800 km/s until it meets the thin gas between the stars. There it is slowed down and piles up.

Structure

At about 90 AU the solar wind slows abruptly (termination shock); at about 120 AU it comes to an end (heliopause). Because the Sun is moving through the Milky Way, the bubble is probably not round but has a kind of tail.

Exploration

The Voyager probes measured the boundary directly: particles from the solar wind suddenly became rarer, those from the Milky Way more common. NASA's IBEX probe maps the boundary from Earth orbit.

Importance for Earth

The heliosphere protects us like a shield from a large part of the cosmic radiation coming from the Milky Way.

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