Nearly nine years after leaving the solar system, and decades beyond its original mission, Voyager 1 is still gathering valuable data, providing plasma readings to continuously sample the density of the interstellar medium.
Scientists at Cornell University have used data from the spacecraft, launched in 1977, to uncover a weak signal that details interstellar plasma density over about 10 au (astronomical unit, roughly the distance from Earth to the Sun) with an average sampling distance of 0.03 au, according to a paper in Nature Astronomy.
Voyager 1, whose original mission was supposed to finish in 1980, crossed the heliopause in 2012, making it the first human-made object to do so. This gave researchers an opportunity to directly measure activity outside the solar system, or at least as much as the spacecraft's ageing arsenal of instruments would allow.
Interstellar plasma is a dilute gas of neutral and ionized particles. Using a repurposed Plasma Wave Subsystem, designed to record data on the magnetospheres of planets like Jupiter and Saturn, Voyager 1 was able to measure the medium, but only in the presence of solar events, lead author Stella Ocker told The Register.
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"Previously, to measure the density you had to wait for a so-called plasma oscillation event, a finite burst of plasma activity that's triggered by solar activity like coronal mass ejections which send shock waves out into the interstellar medium," she said. "Now, we can measure the plasma density in the absence of these plasma oscillation events, allowing us to trace underlying properties of the plasma and see how the plasma is responding to these shock waves from the Sun. This work gives us the most complete map of the plasma along Voyager 1's trajectory to date."
The original plan for the Voyager project was only to fly by gas giants Jupiter and Saturn following some NASA descoping. Voyager 1 and sister probe Voyager 2 passed Jupiter in 1979 with Voyager 1 performing a flyby of Saturn and its moon, Titan, in 1980. Voyager 2 made its Saturn flyby in 1981.
Having seen sense, NASA extended the mission, and Voyager 2 took in Uranus in 1986 and Neptune in 1989. Voyager 1 headed off out of the Solar System on a course that could allow the probe to turn its imaging platform back toward Earth and capture a "family portrait" of the Solar System, and the Blue Dot image made famous by Carl Sagan in 1990.
Yet a new generation of researchers is finding value in Voyager's data.
Born nearly two decades after the spacecrafts launched, Ocker has a keen interest in studying the interstellar medium as a graduate researcher at Cornell's Department of Astronomy.
"I never even thought using Voyager data would be a possibility until my second year of graduate school (in 2019), when my advisor pointed out an opportunity to work on the Voyager team through the NASA Outer Heliosphere Guest Investigator Program," she said. "When we saw the opportunity to work with Voyager, we jumped at it. It's been absolutely thrilling to work with a spacecraft that has such an incredible legacy."
But, now 14 billion miles from Earth, the probe will not continue to send data indefinitely. It is not expected to have the power remaining to operate a single scientific instrument beyond 2025.
Still, Ocker and the team hope to make the most use of the data while they can.
"I'm very hopeful that we can continue to measure this new signal with Voyager 1, but I don't know how much longer the spacecraft will be able to collect the high-resolution data that we need to detect this signal."
The information could provide insight into where extremely narrowband, weak plasma waves come from and how far away Voyager 1 needs to go to see the "quiescent" interstellar medium, beyond significant activity related to the Sun, Ocker said. ®