Voyager 1 enters interstellar space

Crossing the heliopause, a human-made object entered the space between the stars for the first time.

The moment of Voyager 1 enters interstellar space: https://nowsky.org/e/voyager1-interstellar — opens at the right place and time, replayable, no login.

Available views: Orbit ✓ · Sky — (not visible in the sky)

The story

On 25 August 2012 the solar-wind particles around Voyager 1 suddenly vanished and galactic cosmic rays surged. It had crossed the heliopause, the outermost boundary of the heliosphere — the first human-made object to enter the space between the stars, at 121 AU from the Sun.

Confirmation took another year. The plasma detector had failed in 1980, so density could not be measured directly. When a chance solar outburst in 2013 set the surrounding plasma ringing, the plasma-wave instrument caught the frequency, and it matched the density of interstellar space — a measurement made by working around a broken instrument on a 35-year-old spacecraft.

"Leaving the solar system" needs care. Voyager has left the domain of the solar wind, but the Sun's gravitational realm — out past the Oort cloud — lies tens of thousands of years ahead. The finding that the Sun's magnetic influence still reaches intermittently into interstellar space showed the heliopause is not a knife-edge boundary.

Voyager 1 still recedes 1.5 million km a day, sampling the interstellar plasma. Its power will run out toward the end of the 2020s, but the golden record will last billions of years. The 3-D interstellar view shows its current position beyond the heliopause.

Why it matters

The position of the heliopause was measured for the first time that day. Theory had placed it between 90 and 150 AU, and 121 AU became the anchor for heliosphere models. When Voyager 2 crossed at 119 AU in a different direction in 2018, the heliosphere was shown to be roughly spherical.

Direct measurement of interstellar plasma is impossible any other way. Ground-based work infers the interstellar medium only from absorption in starlight; Voyager measures density, magnetic field and cosmic-ray intensity from inside it, giving the first account of the "local interstellar cloud" the solar system is passing through.

The two-year argument over the crossing date — the counter-claim that the unchanged magnetic-field direction meant it had not left — was a public example of how science reaches a conclusion. The community remained divided until NASA's formal announcement in September 2013.

How to watch

Voyager 1 lies toward Ophiuchus at about 172 AU as of September 2026, beyond any telescope. Switch on the solar-system structure chip in the 3-D interstellar view and the heliopause (about 120 AU) is drawn together with the Voyager 1 marker beyond it.

Follow Voyager 1 in the orbit view and move the time to 25 August 2012 to reproduce its position at the crossing (121 AU); at high speed the 1.5 million km a day it has covered since becomes tangible.

The custom journey "Voyager 1 round-trip signal" in the interstellar view shows that even at light speed a message to the probe and back takes 47 hours.

See this for yourself — replay the moment in the orbit view

Background

The precursor was the termination shock crossing of 2004, where the solar wind drops from supersonic to subsonic (94 AU). Voyager 1 then flew eight years through the heliosheath, and from 2010 the outward speed of the solar wind was seen falling to zero.

Voyager 2 crossed the heliopause on 5 November 2018 at 119 AU. Its plasma detector still worked, so the crossing was confirmed at once, and combining the two probes' data gave the first picture of the boundary's thickness and structure.

When power runs out toward the end of the 2020s Voyager will fly on in silence. It enters the inner Oort cloud in about 300 years, leaves its outer edge only after 30,000 years, and passes the star AC+79 3888 in Camelopardalis at 1.6 light-years in about 40,000 years.

The first measurements of interstellar space were not what was expected. Plasma density beyond the heliopause was forty times higher than inside the heliosphere — meaning we live inside an empty bubble the solar wind has blown into the interstellar medium — while the magnetic field grew stronger without changing direction. That unchanged direction was what delayed the crossing verdict by two years, and Voyager 2’s data in 2018 established the interpretation that the heliospheric and interstellar fields join smoothly at the boundary.

FAQ

Has it left the solar system?

It has left the heliosphere, the domain of the solar wind, but the Sun's gravitational realm — the Oort cloud — lies tens of thousands of years ahead.

Why did confirmation take a year?

The plasma detector was dead; density was confirmed only when a 2013 solar outburst set the plasma oscillating at the interstellar frequency.

Where was the boundary?

At 121 AU from the Sun, on 25 August 2012; Voyager 2 crossed at 119 AU in 2018.

How long will it keep transmitting?

Power runs out toward the end of the 2020s; minimal contact into the early 2030s is the goal.

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Times and figures are approximate — verify with official sources when planning travel. Simulation uses J2000 mean elements.