Half a century ago, a man walked on the moon, signaling to the cosmos our species’ intent to explore nearby planets. But, as any fan of Carl Sagan will note, the solar system is small and space is large and our destiny is out there among the stars. While a manned interstellar mission is a hope for the distant future, before sending a person into the unknown, we will need to what it’s like “out there.” And, thanks to a space mission launched 42 years ago, we now know a bit more about the nature of interstellar space.
In 1977, two probes were launched to explore the outer solar system. These probes, named Voyager 1 and 2, took advantage of an alignment of the planets that occurs only once every 175 years that allows a spacecraft to use the gravitational fields of Jupiter and even more distant planets to steer the craft. Voyager 1 visited Jupiter and Saturn a few years after it was launched. Voyager 2 visited both of those planets and then added Uranus and Neptune to its itinerary, with the Neptunian transit occurring in 1989.
Their planetary missions complete, these two probes continued at high speed, leaving the comforting confines of the solar system and heading towards interstellar space. Voyager 1 reported it had left the solar system in 2012 and Voyager 2 made a similar transition in 2018. This week, NASA reported the results of measurements made by Voyager 2.
People often think of the solar system as a collection of planets, but the reality is much bigger than that. Our sun protects us from the harsher environment of the Milk Way. The sun’s magnetic field deflects high energy cosmic rays from the solar system, and the solar wind, which is the flux of protons given off by the sun, pushes against interstellar gas. Together, they insulate our planetary environment. But these protections weaken with distance from the sun and eventually the interstellar environment takes over.
In 2012, when Voyager 1 reached a distance of 122 au, it crossed over from the realm of the solar system to that of interstellar space. (An astronomical unit, or au, is the distance from the Earth to the sun.) The character of the charged particle densities changed markedly, transitioning from the lower energy particles from the sun to much higher energy charged particles of the interstellar environs. Astronomers were surprised to not see a simultaneous change in the direction of the magnetic field. This was certainly not what was expected. Sadly, the plasma instrumentation in Voyager 1 was damaged back in 1980, so there was no measurement of how the hot and diffuse plasma from the sun transitioned to the cooler and denser interstellar medium.
However, the Voyager 2 plasma instrumentation is intact and functioning. As Voyager 2 moved from the solar system into interstellar space in November 2018, the mix of plasmas changed markedly, as did the flux of charged particles. This occurred at a distance of 119 au from the sun and at a different location than Voyager 1. Voyager 2 also saw a change in the magnetic fields. It is unclear whether this difference in the transition of magnetic fields is due to different amounts of magnetism at the location of the individual spacecraft or because the sun has changed over the six years. It turns out that the sun’s activity changes over time and the Voyager 1 transition occurred at a period of low solar activity and the Voyager 2 transition occurred when the solar activity was much higher.
Scientists don’t actually know the shape of the “bubble” of solar activity within the larger interstellar medium. The local environment is called the heliosphere, and it could be spherical, although most astronomers expect it to be more elliptical. This is because the interstellar medium flows over the heliosphere like a stream flowing around a rock sticking up through the water’s surface. However, both Voyager craft left the heliosphere in locations that were fairly near one another, so it is impossible to determine the shape of the heliosphere with these two measurements.
It will be a long time before any other measurements are made of the interstellar environment. Even the New Horizons probe that visited Pluto in 2015 isn’t moving fast enough to add to the data any time soon. However, scientists are still combing through both Voyager 1 and 2 data and will do so for decades.
It would be nice if the two craft would continue to make measurements for decades to come, but the reality is that they were launched over forty years ago and it is a miracle that they have lasted so long. Their batteries are dying, and it will be necessary to start turning off instruments to conserve power. In less than a decade, the two probes will go silent forever.
It will be decades before humanity ventures outside the solar system to encounter the environment of deep space. The Voyager probes will have given us a glimpse of the realities of interstellar space, but there is always more to learn. Scientists and engineers have discussed a follow on probe, but there are no firm plans for a new craft to follow in the Voyager probe’s footsteps.