“Interstellar medium” sounds like a psychic with super-long-distance capabilities, but it’s actually a region of space just beyond the heliosphere — the bubble in the solar system that holds the eight planets (plus poor downgraded Pluto). Created by the solar wind, the bubble is what protects the planetary system, including us, from galactic radiation. But what happens at the edge of the bubble?
A year ago, NASA’s Voyager 2 spacecraft made headlines when it escaped the heliosphere and entered the interstellar medium. This month the craft made headlines again as NASA published a trove of intriguing data that shed light on the stuff filling out the farthest regions of the solar system, including plasmas and particles. National Geographic characterized the findings as “even weirder than expected.”
Voyager 2 is only the second craft to have made the trip into interstellar space, which it entered in November 2018: In August 2012, its identical sibling, Voyager 1, crossed a similar threshold. Among many other things, scientists were curious about the outermost edge of the heliosphere, called the heliopause. This is the cosmic shore where the electrically charged solar wind stops. “In a historical sense, the old idea that the solar wind will just be gradually whittled away as you go further into interstellar space is simply not true,” said University of Iowa physicist Don Gurnett, corresponding author on one of the five new studies. (All were published in Nature Astronomy.) “We show with Voyager 2 — and previously with Voyager 1 — that there’s a distinct boundary out there. It’s just astonishing how fluids, including plasmas, form boundaries.” The team said in a news release that the “marked increase in plasma density is evidence of Voyager 2 journeying from the hot, lower-density plasma characteristic of the solar wind to the cool, higher-density plasma of interstellar space.” Voyager 2 data, though, also showed that the boundary is leaky in both directions, and it also measured the temperature on the other side of the heliopause.
According to NASA, Voyager 2 left the heliosphere equipped with five operating instruments: two for studying plasma in addition to a magnetic field sensor and two devices to detect energetic particles. “Taken together,” the agency concluded in a release, “the findings help paint a picture of this cosmic shoreline, where the environment created by our Sun ends and the vast ocean of interstellar space begins.”
That’s pretty good work for a 42-year-old machine that wasn’t expected to make it nearly this far. (How far? Where Voyager 2 encountered it, the heliopause is some 11 billion miles away.) The primary mission of both probes, launched in 1977, was to get a good look at Jupiter and Saturn.
“NASA considered everything past the Saturn encounter a bonus,” said Howard Butler, who ran GE’s Aerospace Electronic Systems Department and managed the development of the command computers for Voyager 2. What’s kept the probe’s instruments and other onboard systems going is a GE creation known as the radioisotope thermoelectric generator. The company also helped design the computer command subsystem, the flight control processor and other technology for both Voyager probes, according to a NASA “backgrounder” (PDF).
If everything else is gravy, NASA plans to keep passing the gravy boat — the agency expects sensors on both Voyager craft to keep working for about another five years. They probably won’t still be functioning when they reach the actual edge of the solar system, though. The farthest part of our neighborhood is a zone called the Oort cloud, and Voyager 1, at least, has a ways to go until it gets there: some 300 years. Then it’ll just be another 30,000 years till the other side.
In the meantime, similar GE technology is busy at work on other spacecraft, including NASA’s New Horizons. Earlier this year, the RTG-powered probe took photos of comet Arrokoth (née Ultima Thule) — the most distant object ever visited by a human craft.