The heliopause is the outer edge of the heliosphere, the bubble that the solar wind blows around the Sun. There the wind's outward push is matched by the pressure of the thin gas between the stars. Voyager 1 crossed it on 25 August 2012, about 121.6 AU from the Sun, and Voyager 2 on 5 November 2018, at about 119 AU.
Understanding Heliopause
Three boundaries on the way out
Leaving the Sun, a spacecraft passes three landmarks of the outer heliosphere. Both Voyagers have now crossed all three, on different paths: Voyager 1 climbs north of the plane of the planets and Voyager 2 heads south, so each sampled a different part of the boundary, at a different stage of the Sun's activity cycle.
| Boundary | What happens there | Voyager 1 | Voyager 2 |
|---|---|---|---|
| Termination shock | The solar wind slows abruptly from faster than sound to slower than sound | 16 December 2004, about 94 AU | 30 August to 1 September 2007, about 84 AU, crossed several times |
| Heliosheath | The slowed, heated wind piles up and is swept back towards the heliosphere's tail | Crossed from 2004 to 2012 | Crossed from 2007 to 2018 |
| Heliopause | The solar wind meets interstellar gas and can go no further | 25 August 2012, about 121.6 AU | 5 November 2018, about 119 AU |
How scientists knew Voyager 1 had crossed
Voyager 1's plasma instrument had failed in 1980, so it could not measure the surrounding gas directly. In August 2012 its particle detectors saw solar particles plunge and galactic cosmic rays climb, but the magnetic field barely changed direction, which left room for doubt. The decisive evidence came from the plasma wave instrument. When shock waves from eruptions on the Sun reached the spacecraft months later, in late 2012 and again in 2013, they set the surrounding plasma ringing at a pitch that reveals its density: about 0.08 electrons per cubic centimetre, some 40 times denser than in the outer heliosphere and close to the value expected for interstellar gas. NASA announced the crossing in September 2013 and dated it to 25 August 2012. Six years later, Voyager 2's plasma instrument, which still worked, recorded its own crossing directly: the change took less than a day, and the plasma beyond was denser and cooler than the solar wind inside, though hotter than expected at roughly 30,000 to 50,000 kelvin.
Mapping the boundary from afar
Two spacecraft give only two points on a vast surface. To see the whole boundary, NASA's IBEX spacecraft, launched in 2008, began mapping it from orbit around Earth by catching energetic neutral atoms: particles created where the solar wind meets interstellar gas, which fly straight back into the inner solar system. Its successor, IMAP, launched in September 2025, has worked since early 2026 from about 1.5 million km sunward of Earth and maps the boundary in much finer detail. Together with the Voyager measurements, these maps show a boundary that swells and shrinks with the solar cycle and is moulded by the interstellar magnetic field pressing on it. Whether the heliosphere trails a long, comet-like tail or is closer to round is still an open question.