The two Voyager spacecraft have left the solar system NASA/JPL-Caltech

Voyager 2 has sent back its first data from interstellar space. The spacecraft, launched in 1977 to study the outer planets of our solar system, passed Neptune in 1989 and then hurtled onwards to the edge of our solar system. It was declared to have exited the solar system in 2018, and has now sent back some of its first measurements from beyond.

The official edge of the solar system is called the heliopause. This is where particles blown out by the sun in the solar wind give way to the interstellar medium that permeates the entire galaxy. Voyager 2 is only the second spacecraft to have crossed the heliopause, after Voyager 1 left the solar system in 2012.

Now that researchers have analysed data from Voyager 2’s crossing, they have spotted a few differences between its measurements of the heliopause and the surrounding region and those from its predecessor. One is that Voyager 2’s crossing seemed to be smoother due to a thinner heliopause on its path.


The probe also sent measurements from just beyond the heliopause. “Material from the solar bubble was leaking out into the galaxy at distances up to…170 million kilometres, and that was very different than what happened with Voyager 1, where barely any material was leaking out,” said Stamatios Krimigis at Johns Hopkins University in Maryland during a press conference. In fact, Voyager 1 actually saw material leaking into the galaxy from the interstellar medium.

One scientific instrument on Voyager 1 that measured the surrounding plasma – a form of matter in which a gas loses its electrons – was broken by the time the craft passed the heliopause, so Voyager 2 was able to look at some things that Voyager 1 could not. That included a layer inside the heliopause where the plasma seemed to pile up and get very dense, as well as a layer between the heliopause and interstellar space where the plasma from the two areas was mixed.

The heliopause remains largely mysterious despite the information from the Voyager missions: we don’t know its exact shape or structure, partially because both spacecraft left the solar system travelling in approximately the same direction. “Here’s an entire bubble that we’ve only crossed at two points,” said Krimigis. “Two examples are not enough.”

The spacecraft could still send back more data. Both are still functioning and taking measurements in interstellar space, but they will probably run out of power in the next five years or so. No further missions to interstellar space are currently planned.

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Journal reference: Nature Astronomy, DOI: 10.1038/s41550-019-0918-5