The heliosphere

At a glance

Key points

Its outer edge, the heliopause, lies about 120 AU out, where interstellar gas pushes back.

• Voyager 1 crossed the heliopause at about 122 AU in 2012
• Voyager 2 crossed in November 2018
• Voyager 1 is projected to be one light-day from Earth on 18 November 2026

Three boundaries

At the termination shock the supersonic solar wind abruptly slows. Beyond it lies the heliosheath, a turbulent zone of compressed wind. The heliopause marks where solar plasma ends and interstellar plasma begins.

Not the edge of the Solar System

The Sun's gravity still dominates far beyond the heliopause. The icy Oort cloud starts at a few thousand AU and may reach 100,000 AU, so the Voyagers are inside the Solar System's reach for tens of thousands of years.

What the Voyagers found at the edge

Voyager 1 crossed near the bubble's leading face and Voyager 2 near its flank, at different points in the solar cycle. Beyond the heliopause the plasma was denser and colder, as expected, but the magnetic field kept almost the same direction as inside, which surprised researchers. Voyager 2 also found the flank leakier, with some particles escaping outward. Both probes still sit in a disturbed transition zone rather than pristine interstellar space.

Mapping it from Earth orbit

Only two spacecraft have sampled the boundary directly, so the rest is mapped remotely. When solar-wind ions grab electrons from interstellar atoms they become fast neutral atoms that fly in straight lines, ignoring magnetic fields. IBEX caught these from Earth orbit and in 2009 revealed an unexpected bright ribbon across the sky, apparently shaped by the interstellar magnetic field. NASA's IMAP, working at the L1 point since early 2026, maps the same atoms in much finer detail.

What we still don't know

The overall shape: some models trail a long comet-like tail, others a compact, rounder bubble. Only two direct crossings exist.

Missions

1. 1977–

   Voyager 1

   NASAActive

   Jupiter 1979, Saturn 1980; interstellar space since 25 Aug 2012.

   Mission page (opens in a new tab)

2. 1977–

   Voyager 2

   NASAActive

   The only spacecraft to visit Uranus (1986) and Neptune (1989); interstellar since 5 Nov 2018.

   Mission page (opens in a new tab)

3. 2006–

   New Horizons

   NASAActive

   Pluto flyby 14 Jul 2015; Arrokoth 1 Jan 2019; now studying the outer heliosphere.

   Mission page (opens in a new tab)

Related entries

• Voyager 1First spacecraft in interstellar space
• Voyager 2The only visitor to Uranus and Neptune
• Oort cloudThe Solar System's distant icy shell
• The SunG2V main-sequence star · 4.6 billion years old

Sources

1. NASA JPL: Voyager 2 illuminates boundary of interstellar space (opens in a new tab)
2. NASA Goddard SVS: IMAP arrives at L1 (opens in a new tab)
3. NASA Science: Voyager interstellar mission (opens in a new tab)
4. NASA Science: Voyager 1: what is a light-day? (opens in a new tab)
5. NASA Science: Oort cloud facts (opens in a new tab)
6. Gurnett et al. 2013, Science 341:1489 (opens in a new tab)
7. McComas et al. 2009, Science 326:959 (opens in a new tab)
8. NASA Science: IMAP reaches its destination (opens in a new tab)

Last reviewed 10 Oct 2026 against these sources. Built from public NASA, ESA and JPL data; not affiliated with or endorsed by any agency.