Voyager 1 and 2: Frontiers of the Solar System

“Honestly, some of the values ​​were just guesswork,” says Gurnett. An early indication saw the heliopause as close as Jupiter. In 1993, Gurnett calculated a distance about 25 times further from 116 to 177 AU. These figures, he recalls, were not well received by the teaching staff. By 1993, Voyager 1 had already passed 50 AU. “If the heliopause were to be 120 AU, that meant we still had 70 AU ahead of us.” So, at a rate of about 3.5 AU per year, it would take the probes two decades to exit the heliosphere.

This raised troubling questions: Would Voyagers last that long? How is the mission funded? This was also extended in anticipation of finding the heliopause at about 50 AU. But none of the expected signs of an interstellar transit could be detected here. For example, the corresponding Voyager probe should have registered a strong increase in galactic cosmic rays from supernovas and other high-energy processes in surrounding space. The heliosphere’s magnetic field deflects most low-energy cosmic rays before reaching the inner solar system. “It shields us from at least 75 percent of what’s going on out there,” Stone says. The Voyager team also expected the prevailing magnetic field to change direction. The interstellar fields are thought to come from nearby stars and huge clouds of ionized gas, and are likely to have a different orientation from the heliosphere’s magnetic field. Again, there was no change in the 50 AU range.

»With such a long mission, those involved are gradually becoming a family«Linda Spilker, planetary scientist

Gurnett’s estimates turned out to be prophetic — in fact, one of the probes didn’t reach heliopause until two decades later. By then, they had just managed to secure further funding as the team dwindled from hundreds to a few dozen. Most of them are still in action and feel all the more connected. “With such a long mission, everyone involved gradually becomes a family,” says Spilker. “We became parents around the same time. We went on vacation together. We are now working across generations and some of the younger team members weren’t even born when the mission started.”

When Voyager 1 finally crossed heliopause in August 2012, some data was startling. “We delayed the announcement because we couldn’t agree on whether interstellar space had actually been reached,” Cummings recalled. “Talks went on for about a year.” Voyager 1 had indeed detected the expected jump in plasma density. According to the plasma wave detector developed by Gurnett, it has increased up to 80 times. However, there was no evidence of a change in the orientation of the surrounding magnetic field. “It was a shock,” Cummings says. “And it still bothers me.”

Second border crossing with new puzzles

Voyager 2 eventually reached the border as well. When the time came in November 2018, their instruments also recorded no anomaly in the magnetic field. Adding another mystery, the probe encountered the heliopause at a distance of 120 AU — the same distance as its twin six years earlier. The close agreement did not fit the theoretical models that the heliosphere should expand and contract in accordance with the sun’s 11-year activity cycle. During this period, the solar wind decreases occasionally. Voyager 2 arrived at the heliopause when the impact should have been quite severe and the border regions suitably further away. “Nobody expected that,” crime fiction says. “The theory turned out to be insufficient in light of the measurements.”

Now that real data is available, models of the interactions between the heliosphere and the interstellar environment are becoming increasingly complex. Gary Zank, an astrophysicist at the University of Alabama in Huntsville, explains the picture that now prevails: Our sun first came out of a hot, ionized region of the Milky Way, then entered a region that was only partially ionized. The hot region was probably formed as a result of supernovae. One or more nearby ancient stars exploded at the end of their lives, tearing the electrons from the surrounding atoms with the emitted energy. According to Zank, the adjacent areas can be imagined as “a kind of sea surf, with turbulent water and crashing waves”. We are in such a turbulent region, he explains. “The magnetic fields are warped and not as smooth as theorists would like.” However, the magnitude of the turbulence can vary depending on the type of observation. The Voyager data shows only small field fluctuations on a large scale, but many small-scale fluctuations around the heliopause. They are caused by the influence of the heliosphere on the interstellar medium. Eventually, the spacecraft would have to leave these troubled zones and eventually encounter the undisturbed interstellar magnetic field.

Or maybe the picture is all wrong. Some researchers, such as Lennard Fisk of the University of Michigan, believe the Voyager probes have not yet passed through the heliosphere. “There’s no reason that the magnetic fields in the heliosphere and in the interstellar medium have exactly the same orientation,” Fisk says. He has been working on a new model of the heliosphere with colleague George Gloeckler, a longtime member of the Voyager mission team. It shifts the edge of the heliosphere out another 40 AU.

However, most experts find the measured dramatic increase in galactic cosmic rays and plasma density convincing enough. “Given that, it’s very difficult to argue that the probes aren’t actually in interstellar space,” Cummings says. »On the other hand, it is not the case that everything fits together perfectly. That’s why we need an interstellar probe.”

McNutt has been advocating a new mission for decades. His group at Johns Hopkins University has detailed plans for an interstellar probe in a detailed report. It could launch in the 2030s and reach the heliosphere within 15 years, 20 years faster than Voyager 1. Unlike the Voyager mission, the interstellar probe would be specifically designed to study the outer zones of the heliosphere. The U.S. Science Academy umbrella has yet to decide whether the mission should be one of NASA’s priorities for the next decade.

“It’s like trying to describe a goldfish bowl from the fish’s point of view.”Ralph McNutt, Johns Hopkins University Applied Physics Laboratory

An interstellar probe could answer a fundamental question about the heliosphere: What does its structure look like from the outside? “We just don’t know,” admits McNutt. “It’s like trying to describe a goldfish bowl from the fish’s point of view. We have to somehow be able to see everything from the outside.” According to some models, interstellar matter flows gently along the heliosphere at about 200 kilometers per second, like water on the bow of a ship. This should result in a shape with a long tail like a comet. However, a computer model developed by a team led by Boston University astronomer Merav Opher predicts more turbulent dynamics, giving the heliosphere more of a croissant shape. “You can discuss that at scientific conferences,” McNutt says of the situation, “but you have to take measurements to see what’s really going on.”

Sustainable technology with relentlessly diminishing power

The Voyager probes run on 50-year-old hardware. “Software practically does not exist,” Krimigis says. “There are no microprocessors on board – they weren’t even available then!” Designers couldn’t rely on thousands of lines of code to operate it. Krimigis believes the technology lasts so long because almost everything was wired. »Today’s engineers have no idea how to do that. I don’t know if it would even be possible to rebuild such a simple spacecraft. The Voyager mission is the last of its kind.” Cummings underlines how hard it is for everyone to say goodbye: “We’ve reached heliopause and we’ve accomplished something amazing with it.”

Five instruments are still working on Voyager 2 and four on Voyager 1. All are powered by a component that converts heat from the radioactive decay of plutonium into electricity. The power decreases by about four watts per year. In 2019, NASA had to turn off the heating for the cosmic ray detector, which was crucial in determining when the heliopause was crossed. Everyone expected the device to fail. “The temperature dropped by 60 or 70 degrees Celsius, well outside the tested operating limits,” says Spilker, “and the instrument continued to function. It was great.”

The last survivors will likely be a magnetometer and plasma detector. They are located in the hull of the spaceship, where they are kept at operating temperature by the heat released there. The other instruments are mounted on a boom. “If you turn off the fire,” Dodd says, “they get very, very cold.” So how long will the Voyagers last? Spilker hopes: »If all goes well, we may be able to extend the missions into the 2030s. It just depends on the power supply. That is the limiting factor.«

The voyages of the Voyager probes continue even after the end of the mission. They will float through the Milky Way more or less intact, even long after our sun has disappeared. Should they ever be discovered by an alien civilization, they will each deliver a final message on a metal disk. Images and sounds are encoded in their grooves, intended to give an impression of the world from which they originate. In addition to the chirping of crickets and the sound of falling rain, a recording of Bach’s Second Brandenburg Concerto is heard. Also included is a statement from Jimmy Carter, who was president of the United States at the time the missile was launched. “We are sending this message out into the cosmos,” it reads. “One day, after we solve our problems, we want to join a community of galactic civilizations. This record represents our hope, determination and goodwill in the midst of an immense, awe-inspiring universe.«

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