Scientists Shocked Voyager 2 Just Crossed the Edge of Our Solar System

[00:00:00]There's a moment in every great journey where the traveler crosses a threshold that cannot be uncrossed. A point where what lies behind and what lies ahead become fundamentally different environments and the act of passing through that boundary changes the nature of the journey itself. For Voyager 2, that moment arrived on November 5th, 2018 at a distance of approximately 11 billion miles from Earth. On one side of that invisible line, the sun, its wind, its magnetic field, its charged particles, its protective influence. On the other side, something that no scientific instrument had ever directly measured from the inside before, the actual interstellar medium, the plasma between the stars, the ancient environment that exists in the space our sun does not control. Voyager 2 crossed that line. And what it found on the other side was so different from what the models predicted that five separate teams of scientists felt compelled to publish five separate research papers on the exact same day, one year after the crossing, to explain what they had seen.

[00:00:54]This is not a story about a mystery that no one can explain. This is a story about a boundary that every physicist studying the outer solar system had been theorizing about for decades that we thought we understood and that a single spacecraft proved we'd gotten significantly wrong. And the implications of what we got wrong reach back across 11 billion miles of space to right here to the shield that makes life on Earth possible and to questions about how stable that shield actually is over longtime scales. I am your name and this is your channel. Subscribe now if you want more stories like this. We go this deep into the real science every week.

[00:01:27]Now, let us start from the beginning.

[00:01:28]Because to understand what Voyager 2 found at the edge of our solar system, you first need to understand what the edge of our solar system actually is and why getting there took 41 years. Most people think of the solar system as ending somewhere around Pluto. This is a reasonable assumption based on how the solar system is usually depicted in textbooks and documentary graphics.

[00:01:47]Pluto was the outermost known planet for decades, and pictures of the solar system tend to end around there. But Pluto is not the edge of the solar system in any physically sense. It orbits within a region that is still thoroughly dominated by the sun's influence. The actual boundary of the solar system, the point where the sun's influence genuinely ends and something else begins, is a different kind of structure altogether. And it is vastly further away. The sun does not just provide light and heat. It continuously releases a stream of charged particles called the solar wind. Flowing outward in every direction at speeds of several hundred km/s. The solar wind pushes against the surrounding space and inflates an enormous bubble around our entire planetary system. The bubble is called the heliosphere. Inside it, the sun's magnetic field dominates. Inside [music] it, the solar wind is the primary particle environment. Inside it, everything from Mercury to Pluto and far beyond exists within a continuous stream of material that originated in our star.

[00:02:42]The outer edge of this bubble is called the helopause. It is the point where the solar wind finally runs out of energy and gets stopped by the pressure of the interstellar medium pushing back from the other side. Beyond the helopause, there is no more solar wind. The magnetic field environment is the galactic magnetic field, not the suns.

[00:02:58]The particles present are not solar particles. The environment is fundamentally physically different from anything inside the heliosphere.

[00:03:05]Crossing the helopause means leaving in a genuine physical sense the bubble our star built and entering the galaxy itself. Voyager 2 took 41 years to reach that boundary. It launched in August 1977, swung past Jupiter in 1979, past Saturn in 1981, past Uranus in 1986, past Neptune in 1989, and then kept going. For the next three decades, it traveled outward, gradually slowing due to the sun's gravity, but still moving fast enough over 40,000 kmh at certain points to keep making progress toward the outer edges of the heliosphere. And on November 5th, 2018, its instruments detected the unmistakable signature of having crossed the helopause. The solar wind plasma reading dropped to essentially zero. Galactic cosmic rays spiked sharply. The magnetic field environment changed character. In less than 1 day of travel time, Voyager 2 had passed from one physical environment into a completely different one. It became only the second spacecraft in history to reach interstellar space.

[00:04:03]Voyager 1 had done it first, crossing the helopause in August 2012 from a different direction. But here is where the story gets genuinely important and why the Voyager 2 crossing produced five simultaneous research papers while Voyager 1's crossing 6 years earlier had left major questions unanswered. When Voyager 1 crossed the helopause in 2012, it was doing so with a damaged scientific toolkit. Its plasma science instrument, the device designed specifically to measure the properties of the plasma it was moving through had stopped working back in 1980. By the time Voyager 1 reached the helopause, that instrument had been dead for 32 years. Scientists could study the crossing using its magnetometer, its cosmic ray detectors, and its other instruments, and they learned a great deal from those readings. But the most direct window into what was happening to the plasma itself on both sides of the boundary was not available. Researchers had to infer plasma conditions from secondary measurements, which met some of the most fundamental questions about the Helopause crossing remained uncertain. Voyager 2's plasma science instrument was still working. This was, by the standards of a 41-year-old spacecraft operating in the radiation environment of the outer solar system, almost miraculous. The instrument had been built in the 1970s, had been operating nearly without interruption for over four decades. And when Voyager 2 arrived at the Helopause in November 2018, it was ready. For the first time in history, scientists had a spacecraft crossing the helopause with a working plasma instrument capable of directly measuring the temperature, density, and speed of plasma on both sides of the boundary in real time as the crossing happened. What it measured changed the picture in ways that five teams of researchers spent a year analyzing before they published. The first surprise was the helopause itself. For decades, theoretical models of the heliosphere had predicted that the boundary between the solar wind and the interstellar medium would be a broad, gradual transition zone. The solar wind was expected to slow down and thin out over a significant distance before giving way to interstellar plasma. The two environments were expected to blend together over an extended region, like two rivers meeting and slowly merging rather than colliding at a hard edge.

[00:06:06]What Voyager 2's plasma instrument recorded was something dramatically different. The crossing happened in less than one day. On November 4th, 2018, Voyager 2 was in the helio sheath, the outermost layer of the heliosphere. On November 5th, it was in the interstellar medium. The transition between the two environments was not a gradual fade. It was at the scale of the instrument's measurements essentially a sharp line, a boundary so thin and so abrupt that it crossed within a single day of travel time where the plasma on one side belonged unmistakably to the solar environment and the plasma on the other side belonged unmistakably to the interstellar environment. The research paper describing the plasma instrument findings published in Nature Astronomy in November 2019 put the width of the actual helopause transition layer at less than 0.06 astronomical units. One astronomical unit is the distance from the Earth to the Sun, roughly 93 million miles, less than 0.06 of that distance for the transition. At the scale of space physics, where distances are typically measured in units of millions and billions of miles, this is extraordinarily thin. The models had not predicted a boundary this sharp. The second surprise came from the temperature of the interstellar plasma on the other side. Theoretical models had predicted that the interstellar medium near the helopause would be cold.

[00:07:22]The temperature expected was significantly lower than what the plasma instrument actually measured. The interstellar plasma outside the helopause was hotter than the model said it should be. Not dramatically hotter in absolute terms, but measurably significantly hotter in the specific sense of not matching what decades of theoretical work had predicted. The same paper noted that the very local interstellar medium, the specific region of interstellar space immediately surrounding our heliosphere, is variable near the helopause, meaning its properties are not uniform, but change as you move through it. The expected quiet, cold, uniform void was neither as cold nor as uniform as the models had described. The third finding came from the magnetometer data published in a separate nature astronomy paper analyzing Voyager 2's magnetic field readings through the crossing in the region of the helioath just inside the helopause. The magnetic instrument detected what researchers as a magnetic barrier. This is a zone where the magnetic field intensity increases sharply above the typical helio sheath values forming a layer of elevated magnetic pressure between the bulk of the heliosphere and the helopause itself. This barrier had been partially suggested by Voyager 1's magnetometer data during its 2012 crossing. But Voyager 2's crossing confirmed it more clearly and added detail about how it interacts with cosmic rays entering the heliosphere from the interstellar side.

[00:08:39]Cosmic rays are among the most energetic particles in the universe produced by supernova explosions and other violent events across the galaxy. They stream through interstellar space at nearly the speed of light. The heliosphere partially shields the inner solar system from them, reducing the intensity of cosmic radiation reaching the planets.

[00:08:56]The specific mechanism of that shielding turns out to involve the magnetic barrier that Voyager 2 detected. The magnetic pressure of the barrier impedes the inward flow of cosmic rays, deflecting or slowing them before they can penetrate deeper into the heliosphere. This is a real measurable effect with real consequences for the radiation environment of the inner solar system, including Earth. and the details of how it actually works were not correctly described by any model built before Voyager 2 crossed through it. The fourth finding was one that required comparing what Voyager 2 measured with what Voyager 1 had measured 6 years earlier, and it produced what may be the most conceptually significant result of the entire crossing. The two probes crossed the helopause at different locations, heading in different directions relative to the solar systems orientation in the galaxy. Voyager 1 crossed in the northern solar hemisphere heading in the direction the sun is moving through the galaxy roughly toward the nose of the heliosphere where the solar wind pushes hardest against the interstellar medium. Voyager 2 crossed in the southern solar hemisphere heading more toward the flank a different part of the boundary. If the heliosphere were a symmetric sphere with the same properties in every direction, both crossings should have found essentially similar conditions at the helop. They did not. Voyager 2 encountered a thinner boundary than Voyager 1 had encountered with stronger interstellar magnetic fields and different plasma conditions.

[00:10:13]Both probes found the helopaw at roughly the same distance from the sun, approximately 120 astronomical units, which was itself somewhat surprising given that the crossings happened under different conditions of solar activity.

[00:10:25]But the character of the boundary itself differed between the two locations. The heliosphere is not a perfect sphere. It has a shape and a symmetry determined by the direction the sun is moving through the galaxy and the way the interstellar medium flows around it. Now the two Voyager crossings at different points on this asymmetric surface measured different things because the surface itself is different in different places.

[00:10:44]This asymmetry matters for one particular reason that researchers have begun exploring since the crossing data was published. If the heliosphere is asymmetric, if it is compressed on one side and extended on the other, then the thickness and strength of the protective barrier provides against cosmic radiation from outside. also varies depending on direction. The inner solar system is not equally shielded in every direction. The amount of cosmic radiation reaching Earth from different parts of the sky may be subtly influenced by the shape of the heliosphere. A more compressed heliosphere might provide somewhat less shielding to the inner solar system.

[00:11:16]over geological time scales. If the sun passes through regions of the galaxy where the interstellar medium is denser or more magnetically active than the current local environment, the heliosphere could be compressed significantly enough to expose the inner solar system to higher cosmic ray intensities than we currently experience. The Voyager data does not prove that this has happened or will happen on any specific timeline, but it establishes that the interstellar medium exerts more force on the heliosphere than the models accounted for, and that changes the baseline calculation for how stable and how consistent the heliosphere's protective properties actually are. All five of these findings, the sharp boundary, the hotter interstellar plasma, the magnetic barrier, the asymmetric heliosphere shape, and the stronger interstellar magnetic field and pressure were delivered to the scientific community simultaneously on November 4th, 2019, exactly one year after the crossing in five papers published together in Nature Astronomy. The simultaneous publication was deliberate. The findings from the five different instrument teams on Voyager 2 were interconnected. Each one adding context to the others and publishing them together allowed the broader scientific community to see the full picture at once rather than assembling it peacemeal over months or years. The reaction within the astrophysics and helopysics communities was not panic, but it was significant recalibration. Patrick Conn, a helopysics program scientist at NASA headquarters, described Voyager 2 as the first platform to actually experience the interstellar medium from the inside, calling it quite literally a pathfinder.

[00:12:46]The word pathfinder is carefully chosen, not just because it sounds dramatic, but because it is accurate. There was no path before. There were models built from observations taken from far inside the heliosphere and theoretical reasoning about what should be on the other side. And those models, for all the sophistication that went into them, missed the sharpness of the boundary, the temperature of the plasma, the intensity of the magnetic field, and the asymmetry of the heliosphere itself. A pathfinder is something that goes somewhere first and comes back with information that changes the map.

[00:13:16]Voyager 2 is not coming back, but the information it sent home changed every map that mattered. Now consider what this means for the two instruments Voyager 2 still has operating in in interstellar space right now more than 7 years after the crossing. The heliosphere boundary was not the end of the scientific story. It was the beginning of a new chapter. The interstellar medium that Voyager 2 entered on November 5th, 2018 is not a uniform quiet background. It has structure. It has turbulence. The plasma in it varies in density and temperature as the spacecraft moves through it.

[00:13:49]Compression waves from solar events, eruptions on the sun powerful enough to send shock waves all the way out past the helopause and into interstellar space have been detected by Voyager 2's instruments from outside the heliosphere, demonstrating that the two environments are connected in ways that go beyond the static boundary picture the pre-crossing models described. The heliosphere breathes, in a sense, responding to solar activity and to the interstellar medium pressing against it.

[00:14:14]And Voyager 2 has been measuring that breathing from the outside since late 2018. Each year that passes brings the spacecraft closer to the point where its power supply can no longer sustain its instruments. Each year, the team at JPL makes harder decisions about which instruments to keep running and which to power down. The Plasma Science instrument that made the Helopause crossing measurement so historically complete is still operating, but it is operating on a spacecraft that loses roughly 4 watts of power per year and cannot last indefinitely. The window for direct plasma measurements from the southern interstellar medium is finite.

[00:14:47]When it closes, no replacement instrument is currently on a trajectory to fill it, and no mission to do so has been funded, designed, or approved. What Voyager 2 crossed on November 5th, 2018 was the edge of everything our sun built, everything that exists within its protective influence, everything our civilization has ever called home in any physical sense. And on the other side of that edge, it found a universe that was hotter than expected, more magnetically active than predicted, pressing harder against our solar system than any model had calculated, and bounded by a line thinner and sharper than anyone had anticipated. The textbook picture of the heliosphere, the smooth symmetric sphere gradually fading into quiet interstellar needed to be redrawn. Five teams of scientists published five papers on the same day to begin that redrawing. Ah, the spacecraft is in a very real sense drawing a map. Not a map of geographic features the way a surface probe draws a map of a planet, but a map of conditions, a record of what the interstellar medium near our solar system is like as a function of distance from the helopause, building point by point as the spacecraft moves deeper into it. No such map existed before November 5th, 2018. No telescope from Earth can substitute for it, and it can only be drawn as long as the spacecraft continues to operate. The 2023 communication blackout, which lasted 14 days before the Camber station shout successfully restored contact, interrupted that map building for two weeks. 14 days of data points that will never be recovered. In the full context of a mission lasting decades, 14 missing data points is a small gap. But every gap in the record is a section of the map that cannot be filled in later because the spacecraft only passes through each point in space once.

[00:16:22]Whatever conditions existed in that 14-day window in that specific stretch of interstellar space at that specific distance from the helopause was recorded by instruments that were not talking to anyone on Earth for 2 weeks. That data is gone. The larger gap when it eventually comes will be permanent in total. When Voyager 2 runs out of power and goes silent, the map stops where it stopped. Whatever lies further out in the very local interstellar medium, whatever changes in plasma density or magnetic field strength or cosmic ray intensity appear at greater distances from the helopause will be unknown until something else reaches those distances and nothing else is currently on its way there. There's one more aspect of what Voyager 2 found at the Helopause crossing that deserves to be here because it shifts the story from physics back into something that is harder to define but equally real. Both Voyager 1 and Voyager 2 arrived at the helopause at approximately the same distance from the sun, roughly 120 astronomical units, despite having crossed at different times and under different conditions of solar activity. Scientists found this surprising. The size of the heliosphere was expected to vary significantly with the solar cycle. The roughly 11-year rhythm of increasing and decreasing solar activity that drives sunspot counts and solar flare frequency. At solar maximum, the sun produces more solar wind, which should push the helops further out. At solar minimum, less solar wind should allow it to move inward. Voyager 1 crossed during relatively low solar activity. Voyager 2 crossed during relatively high solar activity. The expectation was that they would find the helopause at meaningfully different distances. They did not. This suggests that the interstellar medium's pressure is stable enough over the relevant time scales to keep the helopause at roughly the same location regardless of where the sun is in its activity cycle. The external environment, in other words, is more dominant in setting the heliosphere's size than the internal solar wind alone.

[00:18:06]The galaxy presses back and it presses with enough consistency to keep the boundary nearly fixed. There is something in that fact in the image of our sun doing its best to maintain its bubble against the constant external pressure of the galaxy pressing in from every direction and the galaxy holding the boundary nearly steady regardless of what the sun does internally. It is a relationship between our star and its galactic environment that our models had described incorrectly and that a 41-year-old spacecraft still functioning on hardware built before home computers existed measured directly for the first time on a November day in 2018. That spacecraft is further from Earth today than it was then. Its power supply is smaller. Its instrument count is lower, but its remaining instruments are still collecting data from inside the interstellar medium, still building the map that begins at the helopause crossing and extends outward into space that no model has correctly described from first principles. And no mission besides Voyager 2 itself has ever measured directly from within. Subscribe now if you want to keep following the story. Drop a comment telling us which discovery from this video surprised you most. and share this with someone who should know that the most important scientific data about the edge of our solar system is being collected right now by a machine built before most of the people watching this were born.

[00:19:14]Thanks for watching.