The eyes of the entire world may be focused on Mars. Last week, the Ingenuity helicopter flew, and the Perseverance Rover made oxygen. Voyager 1, the oldest and farthest human-made space probe, still performs science.
It is well into its fourth decade and has not come within a few hundred kilometres of any planet since 1980, when it passed Saturn. Even though it is drifting further away from a dimming star, the probe still sends information back to Earth, as scientists reported in The Astrophysical Journal.
Voyager has been sailing at 11 miles per second for decades. It travels another 3.5 AU (the distance between Earth’s sun and Earth) each year. It is now sending messages home as it prepares to depart from this solar system.
There are many ways to view the “edge” of the solar system. One way is to consider heliopause, a region that borders the interstellar medium. This is the boundary where the solar wind, the soup of charged particles constantly thrown off by the sun, is too weak to stop the interstellar medium, which is the plasma, dust and radiation that fills the bulk of space.
Bill Kurth is an Iowa astrophysicist who worked with Voyager 1 from the beginning. Some scientists believed the heliopause was within 10 to 5 AU of the orbits around Jupiter and Saturn.
The heliopause is only 120 AU away in reality. This is partly because Voyager 1 reached heliopause in August 2012, three and a quarter decades after it departed Earth. This puts the probe in interstellar orbit.
Space is full of interstellar material, but you won’t see much. A cube of Earth’s air contains over a trillion molecules, compared to a cube of the interstellar medium with the densest parts. Voyager 1 is travelling through a sparsely populated region. It’s also quiet for the most part.
Voyager 1 continues to record more information about the plasma and dust here every few years. Every so often, however, it discovers something. Voyager 1 experienced a shock in 2012 and again in 2014.
Kurth claims that Voyager 1 experienced a magnetic spike and a burst in energetic electrons, which caused an intense oscillating electric field.
These shocks, which are the farthest effects of the sun and can be seen radiating outwards beyond the heliopause, are the most extreme.
Voyager 1 saw a second increase in magnetic field strength in 2020, but intense electrical oscillations did not accompany it. Scientists believe it is a pressure front, a subtler disturbance that moves into interstellar space. Voyager 1 had previously seen something similar in 2017.
Scientists have discovered a way to monitor the density of Voyager 1 from more than 13 billion miles away. Jon Richardson is an astrophysicist from MIT, and this latest discovery shows that Voyager 1 still surprises scientists. He says that usually, the probe would have to be subject to a shock in surrounding plasma to measure its density.
Richardson claims that Voyager 1 still feels the sun’s tendrils billions of kilometres beyond the heliopause. Richardson says the sun’s influence is still strong, even far from the heliosphere.
Voyager 1 remains within the sun’s gravitational reach. Scientists predict that Voyager 1 will enter the Oort cloud’s inner edge in 300 years. This shroud of comets stretches several light-years from Earth.
Although we haven’t witnessed evidence of the Oort cloud yet, Voyager 1 is unlikely to be the one that will. The probe is literally on borrowed time. The half-life of Plutonium-238 (the radioisotope powering the probe’s generator) is approximately 88 years.
Voyager 1 is now experiencing fuel shortages. Scientists have already had to decide which probe parts should be kept functional. Likely, the search will only be able to power an instrument by the middle of 2020s.
Scientists like Kurth still hope to extend the probe’s life to 2027, which will mark its 50th anniversary since its launch. Kurth claims that this milestone is something Voyager 1’s engineers could not have imagined.