What We Know About Voyager 2's Adventures Outside the Solar System

Pablo Tucker
March 23, 2020

Researchers have looked at the data returned by the Voyager 2 spacecraft to confirm that it has entered the interstellar medium (ISM), the region of space outside the bubble-shaped border produced by solar winds. Voyager 2 also sent out signals that hint that the plasma outside the heliosphere could be compressed as it is slightly warmer, though it is unclear what is causing the compression. Voyager 2 left the solar system past year.

The two Voyager probes were launched all the way back in 1977 to study the outer Solar System. Professor Ed Stone, of the California Institute of Technology, who has been working on the mission since before its launch in 1977, said: "We didn't know how large the bubble was and we certainly didn't know that the spacecraft could live long enough to reach the edge of the bubble and enter interstellar space".

However, damage to the plasma instrument meant that complete data on the transition could not be gathered. Confirmation of its exit came a few months after the fact.

A lot of the findings confirmed what was discovered when Voyager 1 left our tiny back yard in the universe, which is useful in itself. In a series of five papers in the journal Nature Astronomy, researchers describe what they observed during and since Voyager 2's historic exit.

Comparing what Voyager 2 learned with the data available from the Voyager 1 crossing provides interesting similarities and contrasts, Stone said.

Somehow, there is balance between these two very different regions, and astronomers believe it's the density that helps even out the pressure differences.

The electromagnetic junction just outside the heliosphere was thought to be a deeper transitional place of intermingling cosmic weather, but Voyager 2's plasma wave instrument - built by University of Iowa researchers - detected sharp jumps in plasma density, much like two different fluids coming into contact with one another. An AU is the distance between Earth and the Sun.

The similarity in distance "is very unusual in the sense that one occurred at the solar minimum when solar activity is the least and the other one occurred at solar maximum", he said. Earth and the other solar system planets are nestled inside this bubble, called the heliosphere.

Voyager 1 is still up and about, and its instruments are recording that the plasma density is rising. Voyager 2's magnetometer observations confirm the Voyager 1 finding and indicate that the two fields align, according to Stone.

This higher temperature suggests either more compression of the plasma than predicted or heating by reconnection says Richardson and co-authors.

Sadly, it's extremely unlikely the Voyager probes will remain operational for that distance, or that any of us now will be alive to see them reach it. "And that remains a puzzle".

"Without the new Voyager 2 data, we wouldn't know if what we were seeing with Voyager 1 was characteristic of the entire heliosphere or specific just to the location and time when it crossed". Scientists are now aware that the magnetic field just past the heliopause is parallel to the one inside the heliosphere.

It was the second human-made probe known to have sailed beyond the heliosphere - the expansive region made of plasma and magnetic fields generated by the Sun. "So, it's a puzzle".

"The two Voyagers will outlast Earth", Kurth said. "In other words, material from the solar bubble was leaking outside upstream into the galaxy...and that was very different than what happened with Voyager 1 where hardly any material was leaking out" but some galactic material was leaking in, he explained.

Mr Kurth added: "That says that these two points on the surface are nearly at the same distance".

"It's kind of like looking at an elephant with a microscope", Kurth says. Scientists are studying the discrepancies in the data in hopes of gaining a better understanding of the boundary between our solar system and the wider galaxy.

Though NASA continues to monitor, communicate with, and collect data from both Voyager probes, converting this data into useful scientific insights is largely the responsibility of scientists based at different institutions throughout the US.

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