Heliopause

The outer boundary of the heliosphere — the surface where the Sun's solar wind is stopped by the interstellar-medium and the two pressures come into equilibrium. Beyond the heliopause lies true interstellar space; matter on the solar side originates from the Sun, matter on the other side from the rest of the galaxy.

The heliopause is the outermost layer of the heliosphere's edge structure, preceded outward by the termination shock and the heliosheath.

Crossing Signatures

Three simultaneous signals mark a crossing of the heliopause:

• Sharp drop in temperature of solar wind charged particles
• Change in direction of the magnetic field
• Sudden increase in galactic cosmic rays

Voyager Crossings

Voyager 1 crossed the heliopause on 25 August 2012 at ~121 AU from the Sun — confirmed when proton flux from the Sun dropped from 25 particles/second to ~2 particles/second, and cosmic rays spiked 9% in a single month. NASA announced the crossing in September 2013. Unexpectedly, the galactic magnetic field at the crossing was found to be aligned with the solar magnetic field.

Voyager 2 crossed on 5 November 2018 at ~119 AU. Its Plasma Science Experiment recorded a sharp decline in solar wind particle speed; cosmic rays, low-energy charged particles, and magnetic field instruments all registered the transition simultaneously. Unlike Voyager 1, Voyager 2 did not detect interstellar flux tubes while crossing the heliosheath.

The twin Voyagers are the only human-made objects to have entered interstellar space — though they remain within the Solar System's gravitational boundary, which extends to the outer edge of the Oort Cloud.

Note on Distance

Voyager 1 and 2 crossed the heliopause at slightly different distances (121 AU vs 119 AU), implying the heliopause is not a perfect sphere — consistent with the heliosphere's asymmetric, comet-like shape.