Heliosphere

The Sun's magnetosphere, astrosphere, and outermost atmospheric layer — a vast bubble-like region of space inflated by solar wind plasma. In plasma physics terms, it is the cavity carved by the Sun in the surrounding interstellar-medium. It shields the Solar System from significant amounts of galactic cosmic radiation (though not uncharged gamma rays).

The heliosphere is the Sun-specific instance of an astrosphere — the general term for the wind-bubble around any star. Named by Alexander J. Dessler, who first used the word in scientific literature in 1967. Its study is the domain of heliophysics.

Structure (Outward from the Sun)

Solar wind zone — Plasma streams from the solar corona at 300–800 km/s, extending billions of kilometres beyond Neptune.

Termination shock (~75–90 AU) — The solar wind slows abruptly from supersonic to subsonic as it encounters interstellar medium pressure. Solar wind compression, heating, and magnetic field change occur here. *Voyager 1* crossed it in 2004; *Voyager 2* in 2007.

Heliosheath (80–160+ AU) — The turbulent region between the termination shock and the heliopause. Not smooth — filled with magnetic bubbles ~1 AU wide, formed by magnetic reconnection between opposite solar magnetic field sectors. *Voyager 1* detected a stagnation region at ~113 AU where solar wind velocity dropped to zero and galactic electron flux increased 100-fold.

Heliopause — The outer boundary where solar wind and interstellar medium pressures balance. The edge of the heliosphere; the boundary between solar matter and galactic matter. *Voyager 1* crossed it on 25 August 2012 (measured 40-fold plasma density increase); *Voyager 2* on 5 November 2018.

Heliotail — The heliosphere is not spherical. It resembles a comet: roughly spherical on the sunward side (~100 AU) and stretching into a long tail thousands of AUs in the direction behind the Sun's path through the galaxy.

Shape

Earlier models predicted a blunt-nosed, comet-like shape compressed by interstellar medium flow. Observations in 2009 revised this; the heliosheath appears to be filled with magnetic foam rather than a smooth subsonic flow. A bow shock was long hypothesized ahead of the heliosphere but was ruled out in 2012 — the Sun moves through the interstellar medium too slowly (~23 km/s) for a true shock to form; a gentler bow wave may exist instead.

Exploration

Five spacecraft have explored the outer heliosphere: Pioneer 10 (to 67 AU), Pioneer 11 (44 AU), Voyager 1 and 2 (ongoing, past heliopause), and New Horizons.

Voyager current status (as of April 2026): Voyager 1 and 2 are the only spacecraft ever to operate outside the heliosphere. Nearly 49 years into their mission, both continue to return science data but face severe power constraints as their radioisotope thermoelectric generators (RTGs) decay. In April 2026, NASA shut down one of Voyager 1's remaining instruments to conserve power, leaving it with two science instruments still operating. The twin Voyagers are escaping the solar system in different directions at more than 3 AU per year. Though beyond the heliopause, they remain within the Solar System's gravitational boundary — the outer edge of the Oort Cloud.