The year was 1977. Jimmy Carter succeeded Gerald Ford as President of the United States, Rod Stewart topped the charts with "Tonight’s The Night,” and "Star Wars: A New Hope” premiered in cinemas, followed by "Close Encounters of the Third Kind.”
The public appetite for everything space-related was at its peak.
In August of that year, NASA launched its two longest-ever missions. The Voyager 2 spacecraft took to the heavens from Cape Canaveral aboard a Titan-Centaur rocket on August 20, 1977, followed two weeks later by Voyager 1 on September 5. Between them, the two hardy spacecraft unlocked some of the mysteries of our solar system’s outer planets.
Voyager 1 visited Jupiter and Saturn before angling upwards to shoot out into interstellar space, while Voyager 2 visited Jupiter, Saturn, Uranus, and Neptune, then angled downwards to exit the solar system at an entirely different point. They each carry identical copies of a Golden Record, which holds everything from recordings of Mozart to photographs of the Earth’s sun in hopes of running into alien intelligence somewhere in the galaxy.
What makes these missions special is their intergenerational nature. While the spacecraft sent data back from Jupiter less than two years after launch, Voyager 2 didn’t pass Neptune until 1989, by which time many of the project’s original scientists had retired or passed away.
It wasn’t until 2012 that Voyager 1 achieved the next great milestone – passing beyond the heliosphere into interstellar space. In November 2018, Voyager 2 achieved the same feat, and the information has been analyzed by an entirely new generation of researchers.
Here are some of the findings revealed by Voyager 1 and 2’s crossing of the heliosphere.
1. Understanding the Heliosphere
The heliosphere is a bubble that extends 11 billion miles from the sun; it is caused by the sun’s magnetic field, which is carried in all directions by the solar wind, or electrically charged particles. At its edge, it collides with the interstellar medium, described by National Geographic as the “debris from ancient stellar explosions that lurks in the space between stars.” Scientists are fascinated by the threshold between the heliosphere and the interstellar medium and are using the combined data of Voyagers 1 and 2 to understand more about the nature of the sun’s movement through the galaxy.
One of the many questions that still remain involves the shape of the heliosphere. The sun orbits the galactic center at 450,000 miles an hour, meaning the heliosphere has a leading-edge comparable to a football thrown through the air. Voyager 1 burst through the heliosphere at the leading edge in 2012, while Voyager 2 exited on the left flank – meaning scientists have data from just two exit points, which only gives researchers enough information to take a guess.
Is it a sphere or bubble held in that shape by the interstellar medium’s pressure? Or does it have a tail?
2. Exploring the Leaky Edge
The edge of the bubble (the heliopause) is not impervious to the interstellar medium. In fact, as Voyagers 1 and 2 discovered, there’s plenty of “spillage” both ways. Scientists found the strength of the interstellar magnetic field to be two to three times stronger than expected, exerting up to ten times more pressure on the heliosphere than previously thought.
While Voyager 1’s instruments that measure the temperature of plasma failed in the 1980s, Voyager 2’s plasma instruments were fully operational as it approached the heliosphere. It discovered that:
- The plasma surrounding objects approaching the heliopause heats up, slows down, and becomes denser from about 140 million miles away.
- Beyond the heliopause the interstellar medium is hotter than expected; at least 54,000° Fahrenheit (although the plasma is so thin and diffused that the Voyager craft remained extremely cold).
- The border leaks in both directions. Voyager 1 discovered that tendrils of interstellar particles have punched through the heliopause “like tree roots through rock.” On the other side, Voyager 2 revealed that low-energy particles from our heliosphere extend a hundred million miles into space beyond the heliopause.
3. Discovering Mysterious Doldrums
Back in 2012, Voyager 1 discovered a strange patch 800 million miles before the heliopause where the outward-gusting solar wind slowed to a crawl. In 2018, Voyager 2 found an “altogether different kind of layer” of solar wind that was nearly the same width as that discovered six years beforehand. There’s currently no explanation for this phenomenon, which has led researchers to call for more data.
While NASA has launched other spacecraft beyond the solar system since 1977, none have been built to transmit data from the heliosphere and beyond like Voyagers 1 and 2. Scientists are now calling for a new interstellar probe with a multigenerational mission beyond the solar wind.
This is genuine, long-term thinking: the people who fund, design, build, and launch the next probe will be long gone by the time humanity receives our next package of data from the heliosphere’s edge.