Interstellar space is the space between the stars. For a spacecraft leaving the Sun it begins at the heliopause, about 120 AU out, where the solar wind gives way to thin interstellar gas, dust, cosmic rays and magnetic fields. Voyager 1 entered it in 2012 and Voyager 2 in 2018, the only spacecraft to report from it.
Understanding Interstellar Space
What the Voyagers found on the other side
Crossing into interstellar space changed the readings on both Voyagers within days. Particles from the Sun all but disappeared, while galactic cosmic rays, which the heliosphere partly screens from the inner solar system, jumped and stayed high. The surrounding plasma became much denser and, as Voyager 2's plasma instrument showed in 2018, cooler than the solar wind inside, though hotter than expected at roughly 30,000 to 50,000 kelvin. One surprise was the magnetic field: just beyond the boundary it runs almost parallel to the field inside, rather than swinging to a new direction, probably because the interstellar field drapes itself around the heliosphere. Since then Voyager 1 has also picked up a faint, steady hum of plasma waves, which lets scientists keep track of how the density of the gas changes as it travels outwards.
Interstellar space is not the end of the solar system
Two different edges are easy to confuse. The heliopause is the edge of the Sun's wind and magnetic field, and crossing it is what scientists mean by reaching interstellar space. The Sun's gravity, though, dominates far beyond it, out through the Oort cloud, a vast, thinly scattered swarm of icy bodies that are the source of many long-period comets. Its inner edge is uncertain, with estimates from about 1,000 to 5,000 AU, and it may stretch to 100,000 AU. In the gravitational sense the Voyagers are still in the solar system, and will be for tens of thousands of years.
| Landmark | Distance | Voyager 1 |
|---|---|---|
| Termination shock | About 84 to 94 AU from the Sun | Crossed in December 2004 |
| Heliopause, where interstellar space begins | About 120 AU from the Sun | Crossed on 25 August 2012 |
| One light-day | About 173 AU from Earth | From 18 November 2026 |
| Inner edge of the Oort cloud | Perhaps 1,000 to 5,000 AU from the Sun | In about 300 years |
| Outer edge of the Oort cloud | Up to about 100,000 AU from the Sun | In perhaps 30,000 years |
| Proxima Centauri, the nearest star | About 268,000 AU (4.24 light-years) | About 75,000 years at its speed, though it is not heading that way |
The neighbourhood the Sun is moving through
The heliosphere is a small bubble inside a much larger interstellar landscape. The Sun and its planets are passing through a patch of warm, thin gas known as the Local Interstellar Cloud, which sits inside a far larger, hotter and emptier cavity called the Local Bubble, blown out by exploding stars over roughly the last 14 million years. As the Sun moves, interstellar gas streams past it like a breeze, and the shape of the heliosphere is set by that flow and by the interstellar magnetic field. Some of the breeze's neutral atoms slip right through the heliosphere, and missions watching from near Earth, such as NASA's IMAP, sample them: a taste of interstellar space without travelling there.