The Sun's reach: the solar wind and the edge

The Sun gives out more than light. It also blows a wind.

It is not a wind of air. It is a very thin stream of tiny electric particles, rushing away from the Sun in every direction at about 400 km every second.

The wind blows past all the planets. It is what pushes a comet’s blue tail straight back from the Sun.

Far beyond the last planet, the wind finally runs out of push. That is where the Sun’s own space ends and the space between the stars begins. The whole region inside is like a giant bubble with the Sun and planets in the middle.

Two spacecraft, Voyager 1 and Voyager 2, have flown all the way out of the bubble. They were launched in 1977 and they are still going.

Why there is a wind. The Sun’s outer atmosphere, the corona, is so hot that the Sun’s gravity cannot hold on to it. It boils away into space all the time. This is the solar wind: mostly protons and electrons, carrying some of the Sun’s magnetic field along with them.

The solar wind
Speed	About 400 km a second, with faster streams of 700 or more
Time to reach the Earth	Two to four days
How thin	A few particles in each cubic centimetre by the time it gets here: far emptier than the best vacuum made in a laboratory
What the Sun loses to it	More than a million tonnes every second

That loss sounds enormous and is not. Over its whole life so far the Sun has shed only about a hundredth of one per cent of its mass this way.

What it does to planets. The Earth has a strong magnetic field, which turns the wind aside well above the atmosphere. Mars has none to speak of. Measurements by NASA’s MAVEN spacecraft show the wind stripping gas from the top of Mars’s atmosphere today, and over billions of years this is thought to be a large part of how Mars lost the thick air it once had.

Where it stops. Space between the stars is not quite empty. It holds a very thin gas of its own, and the solar wind has to push against it. The wind thins out as it spreads, and at about 90 times the Earth’s distance from the Sun it abruptly slows. Further out, at about 120 times, it stops altogether. That boundary is the heliopause, and everything inside it is the heliosphere.

Two spacecraft have crossed it.

	Crossed the heliopause	At a distance of (Earth = 1)
Voyager 1	August 2012	121
Voyager 2	November 2018	119

Both found the same things on the far side: the Sun’s particles fell away, and the gas became denser and colder. Voyager 1 is now about 170 times as far from the Sun as the Earth is, and its radio signals take nearly a day to arrive.

What the bubble does for us. The galaxy is full of cosmic rays, fast and damaging particles from exploded stars. The heliosphere turns a large share of them away before they reach the planets.

What is not known. Only two spacecraft have sampled the boundary, at two points, so its overall shape is uncertain. It may trail a long tail like a comet, or be more rounded. NASA’s IMAP spacecraft, launched in 2025, is mapping it from near the Earth.

Not the edge of the Solar System. The heliopause is where the Sun’s wind ends. Its gravity reaches very much further. The Oort Cloud of comets is thought to stretch out hundreds of times beyond the heliopause, and the Voyagers will not be clear of it for tens of thousands of years.