In 1977, NASA launched the twin Voyager spacecraft with a bold mission: to explore the outer reaches of the Solar System and venture into interstellar space. Decades later, both Voyager 1 and Voyager 2 succeeded in crossing a mysterious boundary known as the heliopause — a region sometimes referred to as a “wall of fire”, where temperatures reached an astonishing 30,000 to 50,000 kelvin (roughly 54,000 to 90,000 degrees Fahrenheit).
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There are several ways to define where the Solar System ends. Some consider it to be where the planets stop, while others extend the boundary as far as the Oort Cloud — a vast shell of icy bodies at the very edge of the Sun’s gravitational influence. However, many scientists prefer a more dynamic frontier: the heliopause. This marks the boundary where the solar wind, a continuous stream of charged particles from the Sun, is balanced by the pressure of the interstellar medium.
As NASA explains: “The Sun sends out a constant flow of charged particles called the solar wind, which ultimately travels past all the planets to some three times the distance to Pluto before being impeded by the interstellar medium. This forms a giant bubble around the Sun and its planets, known as the heliosphere.”
Beyond the heliosphere lies the heliopause — the point at which the outward push of the solar wind meets the incoming pressure of interstellar space. The resulting standoff causes the solar wind to deflect and stream backward in a long tail, while a bow shock — much like that of a boat moving through water — forms ahead of the heliosphere as it moves through space.
On 25 August 2012, Voyager 1 became the first human-made object to pass through the heliopause, followed by Voyager 2 in 2018. Before these crossings, scientists could only speculate where this boundary might lie. The fact that the two spacecraft encountered the heliopause at different distances from the Sun helped confirm that its location shifts over time — expanding and contracting with the solar cycle.
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“Scientists expected that the edge of the heliosphere, called the heliopause, can move as the Sun’s activity changes, sort of like a lung expanding and contracting with breath,” a NASA statement noted. This variable nature explains why the Voyager probes crossed the heliopause at separate locations.
Though sometimes called a “wall”, the heliopause is not a solid surface. However, both spacecraft detected extremely high temperatures while passing through, giving rise to the dramatic moniker “wall of fire”. Fortunately, the density of particles in this region is so low that even such energetic material could not transfer enough heat to damage the spacecraft.
Nearly five decades after their launch, the Voyagers remain the only spacecraft to have ventured beyond the heliopause. From this vantage point, they continue to send back invaluable data — revealing not just the boundary itself, but also the conditions that exist in the space between the stars.
One key finding came when Voyager 2 confirmed a surprising result previously observed by Voyager 1: the magnetic field just beyond the heliopause runs parallel to the magnetic field inside the heliosphere. At first, scientists weren’t sure whether Voyager 1’s readings were unique or representative of a broader phenomenon. Voyager 2’s observations confirmed that the alignment was not coincidental.
“An observation by Voyager 2’s magnetic field instrument confirms a surprising result from Voyager 1,” NASA explained. “The magnetic field in the region just beyond the heliopause is parallel to the magnetic field inside the heliosphere.”
These discoveries mark a significant step forward in our understanding of the boundary between our Solar System and the vast unknown of interstellar space — a frontier the Voyagers continue to explore, long after they left Earth behind.
