Voyager 1 Crosses Into Interstellar Space: How We Knew

You cannot see a border in space. There is no sign, no line, no change in the view. Voyager 1 crossed the edge of the Sun’s influence in 2012, and humans learned about it by what the spacecraft heard.

• Voyager 1 crossed the heliopause on August 25, 2012, at roughly 122 AU, about 11 billion miles (18 billion km) from the Sun ^[1].
• Its plasma science instrument had failed in 1980, so confirmation came nearly a year later from the plasma wave subsystem ^[1].
• Voyager 2 crossed on November 5, 2018, confirmed by both particle and plasma instruments ^[1].
• The Voyagers are the only human-made objects to reach interstellar space ^[1].

The border

NASA defines the heliopause as the outer edge of the heliosphere, a protective bubble created by the Sun’s magnetic field and the outward flow of the solar wind ^[1]. Voyager 1 crossed it on August 25, 2012, at about 122 astronomical units from the Sun, roughly 11 billion miles or 18 billion kilometers ^[1].

A broken instrument and a patient wait

The crossing was not obvious in real time. Voyager 1’s plasma science instrument had malfunctioned in 1980 ^[1]. Nearly a year after the crossing, its plasma wave subsystem detected oscillations in the surrounding plasma, which showed that the spacecraft was moving into an increasingly dense region, a clear sign that it had entered interstellar space ^[1]. Picture a swimmer who knows they have left the shallows because the water has grown thicker around their hands.

Voyager 2’s cleaner crossing

Voyager 2 crossed on November 5, 2018, on a different trajectory and at a slower pace. Its crossing was confirmed by data from both its particle instruments and its plasma science instrument ^[1].

Speeds and milestones

NASA’s fast-facts page puts the crossing in a longer timeline. Voyager 1 became the most distant human-made object in space on February 17, 1998, entered interstellar space on August 25, 2012, and travels at about 3.6 astronomical units a year, while Voyager 2 reached interstellar space on November 5, 2018 at about 3.3 astronomical units a year ^[2]. At 3.6 astronomical units a year, Voyager 1 covers about 0.0099 astronomical units, or roughly 1.5 million kilometers, each day (my arithmetic) ^[2].

The same page reminds us what the spacecraft did before leaving: they explored all the giant planets, 48 moons and the systems of rings and magnetic fields around those planets, with Voyager 2 still the only spacecraft to have visited Uranus and Neptune ^[2]. The interstellar crossing is a late chapter of a mission that began in 1977 and is still returning data.

What the Voyagers still do

NASA calls the twin Voyagers the only human-made objects to reach interstellar space. They continue to measure magnetic fields, particles and plasma waves beyond the heliosphere ^[1]. The same two spacecraft carry the Golden Record.

• Voyager 1: August 25, 2012.
• Voyager 2: November 5, 2018.
• Method: plasma density and oscillations, not a visible boundary.

Questions readers ask

When did Voyager 1 enter interstellar space?

On August 25, 2012, at about 122 AU from the Sun, according to NASA ^[1].

How did scientists confirm the crossing?

Voyager 1’s plasma wave subsystem detected oscillations showing denser plasma, nearly a year after the crossing ^[1].