Heliosphere boundary≈119-121 AU

The heliopause

The moving boundary where the Sun’s outward wind loses its push against the surrounding interstellar medium.

Crossed by Voyager 1 in 2012 and Voyager 2 in 2018 · not a solid wall · not the edge of the Sun’s gravity

Typical crossing
About 119-121 AU from the Sun
Inside
The heliosphere: a bubble filled with solar-wind plasma
Outside
The local interstellar medium
Voyager 1
Crossed in August 2012 at about 121 AU
Voyager 2
Crossed in November 2018 at about 119 AU
Important
Both spacecraft remain within the Sun’s gravitational influence
Light-yellow solar-wind particles stream outward from the Sun through the termination shock and heliosheath. A faint curved boundary separates the solar-wind bubble from the local interstellar medium, with two Voyager crossing markers. LOCAL INTERSTELLAR MEDIUM SOLAR WIND → SUN TERMINATION SHOCK HELIOSHEATH HELIOPAUSE VOYAGER 1 · 2 CROSSINGS
Schematic and not to scale · the real boundary moves and is not spherical
  • Solar wind and heliosheath
  • Heliopause boundary
  • Interstellar medium

A bubble inflated by the Sun

The Sun continuously releases a stream of charged particles called the solar wind. Near the Sun it dominates the surrounding plasma, creating the heliosphere: a vast, asymmetric bubble carried outward through the Milky Way.

At the termination shock, the solar wind abruptly slows. Beyond it lies the heliosheath, where solar plasma piles up and flows around the bubble. The heliopause is the outer surface where solar-wind pressure no longer overwhelms the interstellar medium.

Two crossings, two different distances

Voyager 1 · 2012

Crossed the heliopause at about 121 AU. Its plasma, cosmic-ray and magnetic-field instruments recorded a clear change in the surrounding environment.

Voyager 2 · 2018

Crossed at about 119 AU while still operating more of its plasma instrument. It supplied a second, independent boundary measurement.

A moving boundary

The different distances are expected: solar activity, the interstellar magnetic field and the direction of travel all change the pressure balance.

Interstellar space is not the Solar System’s edge

The heliopause marks the end of the Sun’s wind-dominated bubble, not the end of its gravity. Voyager 1 and Voyager 2 remain bound to the Sun and are still climbing out through a much larger gravitational domain.

The Oort Cloud is inferred to begin thousands of astronomical units farther out, with an outer population extending toward 100,000 AU. The heliopause and Oort Cloud therefore answer different questions: one is a pressure boundary, while the other is a distant reservoir of orbiting bodies.

Why the heliosphere is not a sphere

The Sun moves through an interstellar medium that is itself magnetised and uneven. The outward solar wind can be stronger in some directions, while the pressure and flow of interstellar material vary in others.

That produces a distorted, changing boundary rather than a fixed circle. Even the familiar teardrop shapes in diagrams are simplified: recent work shows a more complex interaction than a comet-like bubble moving through an obstacle.

Worth knowing

  • The heliopause sits roughly four times farther from the Sun than Neptune’s orbit.

  • Voyager 2 crossed the boundary only about six years after Voyager 1 but encountered a measurably different plasma environment.

  • The heliopause does not mark where the Sun’s gravity ends.

Where to go next

Interactive view

Return to the Solar System

Open the remote with the heliopause boundary selected on the diagram.

 Far beyond

The Oort Cloud

Travel outward to the inferred reservoir beyond both Voyagers.

 Inner frontier

The Kuiper Belt

Compare the heliopause with the icy region that begins just beyond Neptune.

Sources: NASA heliosphere facts · NASA Voyager · NASA Oort Cloud overview