The solar wind blows a bubble about 120 AU across in the gas between the stars
The Sun sends out a wind of charged particles at about 400 km per second. Far out, it runs into the thin gas between the stars, the interstellar medium, which flows past the solar system at about 26 km per second. The wind slows abruptly to below the speed of sound at the termination shock, about 85 to 95 AU from the Sun, then piles up in a turbulent layer, the heliosheath, until it meets the interstellar gas at the heliopause, about 120 AU out on the upwind side. The heliosphere shields the planets from most of the galaxy's cosmic rays.
Voyager 1 crossed into interstellar space in 2012 and is now one light-day from Earth
- Voyager 1 crossed the termination shock in December 2004 at 94 AU and the heliopause in August 2012 at 121.6 AU, the first spacecraft in interstellar space. On November 18, 2026, it becomes the first human-made object one light-day from Earth, 25.9 billion km away; radio signals take 24 hours each way.
- Voyager 2 crossed the termination shock in 2007 at about 84 AU, several times as the boundary moved, and the heliopause in November 2018 at 119 AU. It is now about 143 AU away.
Both are running out of power. Voyager 1 is down to two working instruments, its magnetometer and plasma wave subsystem; Voyager 2 has three. A power-saving reconfiguration in July 2026 bought Voyager 2's instruments at least another year, and engineers expect the spacecraft to fall silent in the early 2030s.
New Horizons should reach the termination shock between 2029 and 2040, with better instruments than the Voyagers
NASA's New Horizons, about 66 AU out after flying past Pluto and Arrokoth, is the only working spacecraft in the outer heliosphere. Its solar wind and energetic particle instruments measure pickup ions, interstellar atoms ionized by sunlight, which the Voyagers could not. Because the heliosphere swells and shrinks with the solar cycle, models put the crossing anywhere from 2029 to 2040, and it may cross more than once. It has power to keep working until about 2050.
IMAP, launched in 2025, maps the boundary from near Earth
NASA's Interstellar Boundary Explorer (IBEX), launched in 2008, mapped the boundary from Earth orbit by catching energetic neutral atoms, particles that start at the edge as ions, steal an electron and fly straight back. It found an unexpected bright ribbon across the sky, shaped by the interstellar magnetic field. The Interstellar Mapping and Acceleration Probe (IMAP) launched on September 24, 2025, reached its post at the Sun-Earth L1 point, 1.5 million km sunward of Earth, in January 2026 and began science on February 1, 2026. Its three neutral-atom cameras produced the first complete all-sky maps of the boundary in 2026, with far sharper detail than IBEX.
Whether the heliosphere has a long tail like a comet or is rounder is still argued
For decades the heliosphere was drawn like a comet, with a short nose upwind and a long tail. Some models and Cassini and IBEX measurements suggest a rounder or croissant-shaped bubble whose tail splits in two. Only spacecraft that cross the boundary in other directions, or sharper maps from IMAP, can settle it.
China's Shensuo probes and NASA's Interstellar Probe would explore the boundary in new directions
China's Shensuo program, first called Interstellar Express, plans two probes, one toward the nose of the heliosphere and one toward its tail, with flybys of Jupiter and Neptune on the way; papers published in 2025 and 2026 suggest the mission is nearing formal approval. NASA's Interstellar Probe concept would launch in the 2030s and travel more than twice as fast as the Voyagers, reaching beyond 350 AU in 50 years. The Oort Cloud · The Kuiper Belt · Solar system missions