When Voyager 2 crossed the heliopause on November 5, 2018, it did so 119 astronomical units from the sun. Voyager 1 had already crossed in August 2012 at 121 AU. One AU is about 93 million miles, or 149.6 million kilometers. Voyager 1 became the first human-made object to enter interstellar space; Voyager 2 became the second.
Voyager 2 lifted off on August 20, 1977; Voyager 1 followed 16 days later. Voyager 1 flew past Jupiter and Saturn. Voyager 2 took a longer route, visiting Jupiter and Saturn before becoming the only spacecraft to have visited Uranus and Neptune up close.
What the crossings found
Researchers reported the Voyager 2 results a year later in Nature Astronomy, and the boundary proved stranger than expected. Stamatios Krimigis of Johns Hopkins University called the heliosphere “very leaky.” Voyager 1 had already seen interstellar material streaming into the bubble; Voyager 2 found solar material flowing the other way. Edward Stone of Caltech said the two observations confirmed the heliopause leaks in two very different places, though no one yet knows why.
Donald Gurnett of the University of Iowa said the boundary itself is sharp. Voyager 2’s plasma wave instrument measured densities comparable to Voyager 1’s. Solar plasma is about 1 million °C, while interstellar plasma is 10,000 °C, so the density jumps by a factor of 20 to 50 across the border. “That’s a characteristic of fluids, which oftentimes form very sharp boundaries,” Gurnett said.
Krimigis was struck that both spacecraft crossed at nearly the same distance, 121 AU and 119 AU, despite different solar activity. Models had predicted a wider gap. Leonard Burlaga of NASA’s Goddard Space Flight Center found that the magnetic field direction barely changed at the crossing, which also surprised astronomers. The heliopause Voyager 2 saw was thinner and simpler than the one Voyager 1 traversed.
The new data also showed coronal mass ejections, shock waves of plasma from the sun, move past the heliopause and reduce cosmic rays beyond the bubble. Gurnett likened that to shock waves from supernovae stirring the interstellar medium, though at far greater intensity. Stone said every star likely has similar features, and what scientists learn about the heliosphere will help them understand the astrospheres of other stars.
Power and the long quiet
Stone recalled that “when the two Voyagers were launched, the space age was only 20 years old. It was hard to know at that time that anything could last 40 years.” The Voyagers run on radioisotope thermoelectric generators powered by decaying plutonium-238. “We know that somehow, in another five years or so, we may not have enough power to have any scientific instruments on any longer,” Stone said in 2019. In January 2026, the Planetary Society reported the twin probes were still sending data home nearly 50 years after launch.
There are currently no plans for a successor to the Voyager program. New Horizons, the only other spacecraft headed to such distances, will run out of power at 90 AU.
Sources
- Five things we’ve learned since Voyager 2 left the solar system | MIT Technology Review
- Voyager and the heliopause: Exploring where… | The Planetary Society
