Voyagers’ Dive out of the Solar System
Earlier this week, we got news that 1 year after Voyager 2 left out solar system there are published papers discussing what was found. These 5 papers each detail the findings of one of Voyager 2’s 5 still functional instruments. These instruments were chosen to continue receiving power and heat because of their ability to study magnetic fields, plasma, and the high energy particles that are diagnostic of the different parts of the outer solar system and the galaxy just beyond.
Voyager 2, as its name implies, is the second Voyager mission to accomplish a variety of tasks. Launched in August 1977, it was the second mission to visit either Jupiter or Saturn, and now it becomes the second Voyager to exit the solar system. Oddly, due to the nature of orbital mechanics, it actually launched roughly 2 weeks before Voyager 1. As essentially the younger sibling, its job has been to confirm what Voyager 1 has seen, and when it comes to exiting the solar system, we had to wait 6 long years for it to find its way past the protection of the Sun.
Trying to identify the edge of the Solar System is complicated by our Sun’s changing nature. We define the edge as the place where the Sun’s wind and magnetic field are no longer forming a barrier. A barrier from what… well that was what we were hoping to learn from the Voyager missions. The catch is, our Sun’s sphere of influence changes in side with the Sun’s activity levels. A more active Sun can push out farther and influence a larger volume, while the less active Sun lets the coldness of space get a little bit closer.
While Voyager 1 and 2 both made trips to Jupiter and Saturn, their paths diverged at the ringed-planet. At Saturn, Voyager 1 made a series of maneuvers designed to give a clear view of cloudy Titan. This took it under the pole of Saturn on a trajectory out of the plain of the planets. For this mission, exploration ended and the run toward the edge of the Solar System began.
Voyager 2 would continue on to both Uranus and Neptune, and then begin its delayed mission out of our Solar System.
Throughout the 2000s, there was press release after press release about how this Voyager or that had entered or not entered the outer regions of our Solar System that make up the boundary between our Solar System and interstellar space. Specifically, the space craft have each passed at least once through the Heliosheath, where the Solar wind is bent in a sphere as it interacts with interstellar wind, and the Heliopause where the Sun stops effectively blocking charged particles particles. It is the crossing of the Heliopause the marks a mission’s final exit, and for Voyager 1 that exit came in 2012 and was marked by an increase in plasma, increase in cosmic rays, and unexpected magnetic field. Since this was essentially a fly by, with measurements being taken as Voyager 1 passed through this region, scientists would have to wait until Voyager 2 caught up to see if it would detect the same thing.
And, last year when Voyager 2 made its way out of the Solar System, it replicated Voyager 1’s results to a great degree.
In this week’s issue of the journal Nature Astronomy, 5 separate stories highlight the science of 5 separate instruments. I know I should call them journal articles, but when your science describes that adventures of a spacecraft leaving our Solar System, the word story just makes more sense. These articles collectively highlight how at a distance of about 119 AU — or 119 times the distance from the Earth to the Sun, Voyager 2 found itself in a region of space that had a strictly different temperature, plasma density, and incidence of energetic particles, but the magnetic field of our Solar System merged smoothly into surrounding space. The transition from in our Solar System to outside of it took roughly a day, and in that time, the mission saw the low energy ions from the sun drop in number while the high energy cosmic rays from external sources increased in number. At the same time, the plasma density radically increased to approximately what was seen by Voyager 1, which happened to be more plasma — and also hotter plasma — than had been expected.
Humankind has now sent four spacecraft out of our Solar System, and while Pioneer 10 and 11 went silent in 2003 and 1995, both Voyager 1 and 2 continue to function and send us back data as we leave them behind in the galaxy. These missions are early 1970s technology, kind of like me, and like the Generation X they belong to, they are continuing to just get their job done while sometimes getting lost between the history making Mercury-Gemini-and Apollo missions, and the social media stars of the modern day. We don’t know how much longer these missions will work. Remarkable, they seem to still be capable of course corrections and so much more, if someone will just notice them and ask.
(Spacecraft) Children of the 70s, we see you out there doing your science. We see all 5 new journal articles of your science and it is awesome, if a bit dry and particle-filled.
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Want to read more about these discoveries? Here are the original science papers in Nature Astronomy:
- “Cosmic Ray Measurements from Voyager 2 as It Crossed into Interstellar Space,” Edward C. Stone et al., 2019 Nov. 4, Nature Astronomy .
- “Voyager 2 Plasma Observations of the Heliopause and Interstellar Medium,” John D. Richardson, et al., 2019 Nov. 4, Nature Astronomy.
- “Magnetic Field and Particle Measurements Made by Voyager 2 at and Near the Heliopause,” L. F. Burlaga et al., 2019 Nov. 4, Nature Astronomy [https://doi.org/10.1038/s41550-019-0920-y].
- “Energetic Charged Particle Measurements from Voyager 2 at the Heliopause and Beyond,” Stamatios M. Krimigis et al., 2019 Nov. 4, Nature Astronomy.
- “Plasma Densities Near and Beyond the Heliopause from the Voyager 1 and 2 Plasma Wave Instruments,” D. A. Gurnett & W. S. Kurth, 2019 Nov. 4, Nature Astronomy.
