Voyager 1 enters interstellar space
On 25 August 2012 Voyager 1 crossed the outer edge of the solar wind's domain, 121 AU from the Sun. It was the first time a human-made object entered the space between the stars.
Available views: Orbit ✓ · Sky — (not visible in the sky)
The story
The Sun blows a wind of charged particles, the solar wind, out in every direction. The huge bubble this wind fills is called the heliosphere, and its outermost boundary is the heliopause. On 25 August 2012 the solar-wind particles around Voyager 1 suddenly dropped away, while cosmic rays, high-energy particles arriving from outside the solar system, rose sharply. Voyager 1 had crossed the boundary. It was 121 AU from the Sun, meaning 121 times the distance between Earth and the Sun.
A firm conclusion took more than a year. The plasma detector that measured the density of the surrounding gas directly had stopped working in 1980. Then, in the spring of 2013, a shock from an outburst on the Sun reached the probe and set the gas around it vibrating. Such gas vibrates at a pitch that depends on its density. Voyager's radio instrument picked up the vibration, and its pitch matched the density expected in interstellar space. The broken instrument had been worked around with a different one.
The phrase "left the solar system" needs care, though. What Voyager 1 left is the region the solar wind reaches. The Sun's gravity reaches much farther, and the Oort cloud, the distant home of comets, lies far beyond. Pressure waves from outbursts on the Sun also kept reaching Voyager 1 after the crossing. So the heliopause is less a sharp line than a zone where the Sun's influence fades.
Voyager 1 is still moving away at about 17 km per second, or roughly 1.5 million km a day. Its power is slowly shrinking and its science instruments are being switched off one by one, but the remaining ones keep measuring interstellar space. The 3-D interstellar view marks its present position outside the heliopause.
Mission goals
That day the position of the heliopause was measured directly for the first time. Before then it could only be estimated by calculation, and the predictions varied widely. In 2018 Voyager 2 crossed the same boundary in a different direction at 119 AU. That two probes met the boundary at similar distances in different directions became an important clue to the shape of the heliosphere.
This is the only way to measure interstellar space from the inside. From Earth, astronomers can only infer the nature of the gas between the stars from the small amounts of starlight it absorbs. Voyager 1 measures the gas's density, its magnetic field and the strength of cosmic rays from right inside it. Thanks to these data, much more is known about the interstellar gas the solar system is passing through now.
How the conclusion was reached also matters. Because the direction of the magnetic field barely changed at the crossing, some scientists argued that Voyager 1 had not crossed yet. The debate settled only after the density measurement of spring 2013 and NASA's formal announcement on 12 September 2013. It is an example of how a scientific conclusion becomes firm as evidence builds up.
Mission chronology and background
There were signs before the crossing. At the end of 2004, at about 94 AU, Voyager 1 passed the termination shock, the boundary where the solar wind suddenly slows from faster than sound to slower than sound. For nearly eight years after that it flew through this region of slowed solar wind, and from 2010 the outward speed of the solar wind around it was seen to fall to almost zero.
The first measurements beyond the boundary were not what was expected. The density of the interstellar gas was dozens of times higher than just inside. This means the solar system sits in a fairly empty bubble that the solar wind has pushed into the interstellar gas. Yet the magnetic field only grew stronger while keeping almost the same direction. That is why the verdict was delayed. Voyager 2, which crossed in 2018, still had a working plasma detector and confirmed its crossing at once.
Voyager 1's electricity comes from nuclear power sources carried since its launch in 1977, and their output shrinks every year. NASA is switching instruments off one at a time in the hope of keeping contact into the 2030s. After the signal ends, the probe will keep flying. NASA calculates that it will enter the inner Oort cloud in about 300 years and pass the star AC+79 3888 in Camelopardalis at 1.6 light-years in about 40,000 years.
Other milestones of this mission
- Voyager 2 launch — the Grand Tour
- Voyager 1 launch
- Voyager 2 at Neptune
- Voyager 1 passes one light-day from Earth(planned)
Where to see it — sky and simulator
Voyager 1 lies in the direction of Ophiuchus, about 172 AU from the Sun as of September 2026. It is far too distant and small to be seen with any telescope. Even light takes about a day to cover that distance one way, so sending a signal to Voyager 1 and receiving a reply takes nearly two days.
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. Play the custom journey "Voyager 1 round-trip signal" and the long trip of a light-speed signal from Earth to the probe and back is reproduced.
Follow Voyager 1 in the orbit view and move the time to 25 August 2012 to reproduce its position at the crossing (121 AU). Speed up the playback and it is shown moving away by about 3.6 AU every year since.
See this for yourself — replay the moment in the orbit view
FAQ
Has it left the solar system?
It has left the heliosphere, the region the solar wind reaches. The Oort cloud, still held by the Sun's gravity, lies much farther out and will take tens of thousands of years to cross.
Why did confirmation take a year?
The instrument that measured gas density directly had failed in 1980. In 2013 a shock from a solar outburst set the surrounding gas vibrating, and that vibration revealed the density of interstellar space.
Where was the boundary?
At 121 AU from the Sun, on 25 August 2012. Voyager 2 crossed in a different direction at 119 AU in 2018.
How long will it keep transmitting?
Its power is shrinking and instruments are being switched off one by one. NASA aims to keep in contact into the 2030s.
Related
- Voyager 1 launch
- Voyager 2 launch — the Grand Tour
- The Pale Blue Dot
- Voyager 1 — object index
- Voyager 2 — object index
Times and figures are approximate — verify with official sources when planning travel. Simulation uses J2000 mean elements.