Voyager 1 and 2 in Interstellar Space: What They Found Beyond the Heliosphere
Launched in 1977, Voyager 1 and Voyager 2 are the most distant human-made objects in existence. Both have crossed into interstellar space and are still transmitting data. What they found at the edge of the solar system has surprised scientists.
In the summer of 1977, NASA launched two spacecraft within two weeks of each other on trajectories that would carry them past Jupiter, Saturn, and beyond — a Grand Tour of the outer solar system made possible by a rare alignment of the outer planets that occurs only once every 175 years. Voyager 1 and Voyager 2 flew past Jupiter in 1979, Saturn in 1980 and 1981, and Voyager 2 alone continued to Uranus (1986) and Neptune (1989). Then they kept going. In 2012, Voyager 1 became the first human-made object to cross into interstellar space — the region beyond the Sun's influence. In 2018, Voyager 2 followed. As of 2026, both spacecraft are still operating, transmitting data over distances that take more than 20 hours to traverse at the speed of light, providing measurements of an environment no other instrument has ever sampled directly.
What happened
The heliosphere is the vast bubble of space dominated by the Sun's influence — filled with the solar wind (charged particles streaming from the Sun) and a magnetic field that extends outward from the Sun in a spiral pattern. It is roughly spherical but compressed on the sunward side by the interstellar medium pushing in. At the boundary — the heliopause — the solar wind's pressure equals the pressure of the interstellar medium, and the Sun's magnetic influence ends.
Voyager 1 crossed the heliopause in August 2012 at a distance of 121 AU from the Sun (121 times the Earth-Sun distance). Scientists knew the crossing had occurred because the intensity of energetic particles from the Sun dropped suddenly and the intensity of galactic cosmic rays (which the heliosphere partially shields against) jumped sharply. The local magnetic field direction also changed, indicating Voyager had moved from field lines rooted in the Sun to field lines rooted in the interstellar medium.
What surprised scientists was the magnetic field orientation. Voyager 1 found that the interstellar magnetic field is nearly aligned with the heliopause boundary — suggesting the heliosphere has compressed the local interstellar field in ways not predicted by models. The pressure at the heliopause was also different from pre-mission predictions: the heliosphere extends farther than models suggested on one side and is more compressed on another.
Voyager 2 crossed the heliopause in November 2018 at 119 AU in a different direction (south of the ecliptic plane), providing the first two-point measurement of the heliopause boundary. Its instruments, including a working plasma instrument that Voyager 1 lost in 1980, provided a more complete characterization. It found that the heliopause is very thin — just a few AU thick — and that there is a sharp transition in plasma density and temperature between the heliosphere and the interstellar medium. The plasma density outside the heliopause is about 40 times higher than inside.
Both spacecraft continue to transmit data. Voyager 1 is now more than 163 AU from the Sun (as of mid-2026), moving at about 17 km/s. Voyager 2 is about 136 AU away. They are powered by radioisotope thermoelectric generators (RTGs) fueled by plutonium-238, which decay and provide less power each year. Both spacecraft have been progressively shutting down non-essential systems as power decreases; it is estimated that Voyager 1 will run out of sufficient power to operate any science instruments sometime in the late 2020s.
Why it matters
The Voyager spacecraft are the only direct probes of the interstellar medium beyond the Sun's domain. Every other measurement of the interstellar medium is remote — done from within the heliosphere by observing how distant light is modified as it traverses the ISM, or by studying cosmic rays and neutral particles that enter the heliosphere. Voyager 1 and 2 are in the ISM itself, measuring its plasma, magnetic field, and particle populations directly.
The measurements have already produced surprises that have revised models of the heliosphere's structure and the properties of the local interstellar medium. The thickness and sharpness of the heliopause were unexpected. The alignment of the interstellar magnetic field with the heliospheric boundary requires explanation. The particle fluxes and composition outside the heliopause differ from predictions in multiple ways.
For future interstellar missions — a generation ship, an Interstellar Probe, or Breakthrough Starshot's laser-pushed lightsail — Voyager's measurements provide the only direct characterization of the environment such vehicles would travel through. Understanding the structure of the heliosphere and the interstellar medium at the scale of thousands to tens of thousands of AU is essential background knowledge for planning any real interstellar mission.
- The only direct measurements of the interstellar medium beyond the heliopause, providing ground truth for models of the heliosphere and the local galactic environment.
- The dual Voyager measurements from different directions (north and south of the ecliptic) allow the first two-point characterization of the heliopause, revealing asymmetries that single-point measurements could not detect.
- Voyager's 48+ years of continuous operation is a remarkable engineering achievement, demonstrating that spacecraft can function and provide science for decades beyond their design lifetimes.
- Both spacecraft are approaching the end of their operational lives as RTG power decreases; science return will decline through the late 2020s and essentially cease by the early 2030s.
- Voyager 1's loss of its plasma instrument in 1980 means its interstellar plasma measurements are indirect (inferred from wave phenomena) rather than direct, limiting the precision of some measurements.
- With only two measurement points, the three-dimensional structure of the heliopause is extremely poorly constrained — a comprehensive heliosphere characterization would require a constellation of Interstellar Probe-class spacecraft in multiple directions.
How to think about it
The Voyager mission represents one of the greatest engineering and scientific achievements of the 20th century, and one that continues to deliver in the 21st. The spacecraft were designed for a 5-year mission to Jupiter and Saturn. They are now 48 years old, operating in an environment no instrument was designed to reach, and still returning data about the boundary between our solar system and the galaxy.
The most important thing Voyager has taught us about the heliopause is that our pre-existing models were substantially wrong. The interstellar boundary is sharper, more complex, and in important ways different from what theoretical models predicted. This is the normal pattern of science — new measurements refine and sometimes overturn existing models — but the Voyager data are particularly valuable because they represent the only direct measurement from a regime that is otherwise entirely inaccessible.
When Voyager 1 was launched in 1977, it was not known whether interstellar space could even be reached in a human lifetime with existing propulsion technology. It was not known whether the instruments would survive, whether the spacecraft would be navigable, whether the power systems would last. Every assumption was validated. The spacecraft built in the early 1970s with 1970s technology are now humanity's ambassadors to the galaxy — and they will continue to drift outward long after their power fails, carrying a golden record with sounds and images of Earth for any civilization that might, in some unimaginably distant future, encounter them.
FAQ
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