Where does interstellar space begin? That wasn’t a question that anyone had to ask for millennia, as the geocentric and then heliocentric models of the universe had no place for an “interstellar” anything. But once our knowledge of the universe expanded far beyond the planets and the fixed firmament of stars into one where our Sun was only one of the billions of stars, the heliopause, as the boundary of our solar system is known, suddenly became very relevant.
While the heliopause is very likely not something any of us are going to physically visit one day, it is an important marker for astronomy, much like the boundaries of a city on the map of a country. But just like the boundaries of a city, where that line lies isn’t always so clear. And while cities have the benefit of using rivers, valleys, and other geographical features as a convenient demarcation (as well as GPS coordinates), how do you do that in the emptiness of space?
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What is heliopause?
The heliopause is the point where the influence of our solar system balances against that of the interstellar medium along the edge of the heliosphere, which is a bubble created by the clouds of interstellar gas that surrounds the solar system. It is a theoretical boundary where the strength of the Sun's solar wind is not strong enough to counter the stellar winds of the surrounding stars.
"The heliosphere is the bubble this solar wind blows out into the local interstellar medium," said Richard Marsden, a Ulysses project scientist at the European Space Agency's technical center (ESTEC) in the Netherlands who works to study the heliosphere. "It defines the volume of space over which our Sun's influence predominates."
This heliosphere extends far beyond the orbit of Pluto, as much as three times the distance, in fact, before it comes into conflict with the interstellar medium. Just as the Earth's magnetic field protects the Earth from the ravages of the solar wind, the heliosphere protects the solar system from the interstellar (or galactic) wind.
“The effect the heliosphere has on cosmic rays allows for human exploration missions with longer duration," said Arik Posner, a heliophysicist at NASA Headquarters in Washington, D.C. "In a way, it allows humans to reach Mars. The challenge for us is to better understand the interaction of cosmic rays with the heliosphere and its boundaries.”
Marsden agrees. "Without the heliosphere," he said, "life would certainly have evolved differently - and maybe not at all."
As we approach the edge of the heliosphere, the solar winds and magnetic field of the Sun encounter the countervailing forces of the interstellar wind. There are a couple of key parts of this process at play. First, the influence of the interstellar medium is apparent as soon as we reach the Termination Shock. This is the beginning of the end of the heliosphere, where the solar wind slows to subsonic speeds and heats up because of countervailing pressure from interstellar winds.
The highly charged particles of the solar wind are compressed together and form what we call the heliosheath; a region where the solar winds and the interstellar winds are interacting, but where the influence of the solar winds is still stronger than that of the interstellar winds.
The difference in the influence between the two gradually shrinks the further away from the sun you get, and the heliopause is the point where the interstellar wind starts to overpower the solar wind.
This doesn’t produce a particularly clear boundary, however. While the sun might look static from our perspective, you can’t forget that it is in motion too, orbiting around the galactic core at a speed of about 240 kilometers a second. As it does, it more or less carves a path through surrounding interstellar gas the way a ship sails through the ocean.
Since the interstellar medium itself isn’t static, there are differences in pressure that can warp the outline of the heliopause, just as the waves of the ocean can push back against the displaced water ahead of a ship traveling at speed.
In a similar way, heliopause is characterized by a redirection of the solar winds, which are initially pushing through the interstellar medium in a more or less outward direction from the Sun. At the heliopause, however, these solar winds can no longer move forward, since the interstellar winds are stronger. This forces the solar winds to turn back towards, but around, the Sun, which produces some rather interesting astrophysical phenomena.
Why is the heliopause shaped like that?
If we were to describe the heliopause as a boundary, the closest analogy would be a windsock, constantly extended and flowing out with the prevailing wind, and it's shaped that way for pretty much the same reason as far as physics is concerned. Another close analog is a comet flying towards the Sun with its sublimating gases forming the coma around the comet's nucleus.
In the case of the heliopause, its shape is a product of the interaction of the solar wind and the countervailing pressure of interstellar gas and magnetic fields originating from outside the heliosphere. If these forces are stronger than the outward push of the solar wind, then the heliopause bends backward, just like a windsock does when it meets a gust of wind here on Earth, or like how the gases blow off a comet flow around the central nucleus to trail behind it.
"The shape of the heliosphere is not symmetrical around the Sun," according to the European Space Agency. "The motion [of the Sun] through the [local interstellar medium] compresses [the heliosphere] at the front, and drags it out into a tail at the back, very much like a planetary magnetosphere."
In addition to the force of the interstellar wind on the heliosphere, another major factor helps to define the shape of the heliopause, namely the solar wind.
"The distance of the heliopause from the Sun changes as the heliosphere breathes in and out on the timescale of the solar cycle," Marsden explains.
This is partially due to the polarity of the Sun's magnetic field flipping during the solar cycle, which gives something of a three-dimensional sine waveform to the solar wind, which in turn shapes how the heliosphere and heliopause form as it pushes up against the interstellar wind.
How far away is the heliopause?
Since the shape of the heliopause isn't static, how far away it depends largely on which direction you are facing relative to the movement of the Sun through the galaxy. If you were looking "straight-ahead" as the Sun pushes through the interstellar medium, it's estimated that the heliopause begins at around 123 astronomical units (AU), with one AU being the distance between the Earth and the Sun, roughly 93 million miles or 150 million kilometers.
This puts the leading edge of the heliopause far beyond the orbit of Pluto (which has a mean distance from the Sun of about 35 AU) and even farther than Eris, the most distant dwarf planet ever discovered, with an aphelion of about 97.5 AU.
That distance is constantly in flux, however, and scientists are still working to map the extent, shape, and distances to various points along the heliopause, which is something much easier said than done. How far the heliopause extends in other directions is not exactly an easy question to answer.
To date, only two instruments have passed the heliopause, Voyager 1 and Voyager 2—in 2012 and 2018, respectively—and are currently the only human-made objects to reach interstellar space. Their data was essential to establishing the heliopause's existence in the first place. But trying to measure a three-dimensional structure in space isn't possible from just two instruments.
“Trying to figure out the entire heliosphere from two points, Voyager 1 and 2, is like trying to determine the weather in the entire Pacific Ocean using two weather stations,” said Eric Christian, a lead heliosphere research scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. Fortunately, we don't have to rely on the data from the Voyager missions alone.
We've made considerable progress in mapping the heliopause thanks to NASA's Interstellar Boundary Explorer (IBEX) mission (launched in 2008) which collects particles known as energetic neutral atoms (ENAs). These are high-energy particles produced by the complex interaction of the solar wind and the interstellar wind along the heliopause. Since these ENAs originate from the solar wind itself, IBEX measures the outgoing solar wind and records the incoming ENAs to produce a kind of solar pulse that charts the distance to the heliopause in various directions.
“Every time you collect one of those ENAs, you know what direction it came from,” said David McComas, the IBEX principal investigator at Southwest Research Institute (SwRI) in San Antonio, Texas. “By collecting a lot of those individual atoms, you're able to make this inside-out image of our heliosphere.”
“Just as bats send out sonar pulses in every direction and use the return signal to create a mental map of their surroundings," Dan Reisenfeld, a scientist at Los Alamos National Laboratory and the lead author of a paper published in Astrophysical Journal in 2021 that presented the first detailed 3D map of the heliopause, explained, "We used the Sun’s solar wind, which goes out in all directions, to create a map of the heliosphere."
IPEX was also instrumental in probing the much more ephemeral heliotail. Analogous to the wake left by a ship speeding through the water, the heliotail extends much farther "behind" the heliosphere, possibly for more than 350 AU, but it's nearly impossible to get an accurate measurement. Thanks to IBEX though, we at least know what the heliotail looks like: a four-leaf clover.
"Many models have suggested the heliotail might look like this or like that, but we have had no observations," said McComas, who was the lead author of the 2013 paper announcing the discovery of the heliotail's shape. "We always drew pictures where the tail of the solar system just trailed off the page, since we couldn't even speculate about what it really looked like."
Where the heliotail ends is anybody's guess; after all, where does the wake of a speedboat truly end and the waves of the ocean begin? At some indistinguishable point behind us, the heliotail and the dwindling heliopause around it simply blend into the surrounding interstellar gas around us.
How did astronomers discover it?
The heliopause was the natural outgrowth of the discovery of the heliosphere at the dawn of the space age in the 1950s. Once satellite data provided evidence of the heliosphere, physical models of the heliosphere and interstellar medium interaction quickly showed that the heliopause had to be out there somewhere.
Early probes into deep space started providing evidence for the heliopause, most significantly with the Pioneer 10 and Pioneer 11 missions. Launching in 1972, the Pioneer probes continued to measure solar wind activity for decades out to a distance of about 67 AU, with the final coherent broadcast back to Earth from Pioneer 10 occurring in January 2003.
By then, however, we had even stronger evidence of the heliopause from data sent back by the Voyager 1 and Voyager 2 probes in 1993. The two probes were in a unique position in deep space to detect strong, low-frequency radio waves produced by the violent interaction between intense solar winds produced in May and June 1992 and the interstellar medium, providing the first direct evidence of a definable boundary with interstellar space.
"These radio emissions are probably the most powerful radio source in our solar system," Dr. Don Gurnett, the principal investigator of the Voyager plasma wave subsystem that detected the radio emissions, said in 1993 after the announcement of the discovery. "We've estimated the total power radiated by the signals to be more than 10 trillion watts. However, these radio signals are at such low frequencies, only 2 to 3 kilohertz, that they can't be detected from Earth."
"We've seen the frequency of these radio emissions rise over time," Gurnett added. "Our assumption that this is the heliopause is based on the fact that there is no other known structure out there that could be causing these signals."
It wasn't until 2012, however, that we got the most solid evidence for the heliopause when Voyager 1 detected a sudden drop in solar wind particles and a corresponding spike in galactic cosmic-ray particles, indicating that it had crossed the boundary into interstellar space. While pretty conclusive in its own right, final confirmation came in 2018 when Voyager 2, which is on a very different trajectory from Voyager 1, detected the same sudden drop in solar wind particles and spike in galactic-ray particles, showing that the phenomenon wasn't local to Voyager 1.
Here be space dragons: what exists beyond the heliopause?
While it's obvious that what lies beyond the heliopause is interstellar space, there's still more mystery—and controversy—around the heliopause and what lies beyond.
For decades, it's been theorized that a "Bow Shock" exists just beyond the heliopause, where the weaker solar wind particles and magnetic field of the heliosphere disturb—but do not overpower—the interstellar medium ahead of it. The idea of a more gentle "Bow Wave" has gained traction more recently as some argue that the solar system is not moving fast enough through the interstellar medium to generate a "shock".
Then there is the matter of the IBEX Ribbon, a band of intense ENAs along the heliosphere that is significantly "brighter" than the surrounding ENAs. For now, no one has been able to explain what is causing the IBEX Ribbon or what implications it might have for our models of the heliopause.
“Our Sun is a star-like billions of other stars in the universe," said Justyna Sokol, a research scientist at SwRI. "Some of those stars also have astrospheres, like the heliosphere, but this is the only astrosphere we are actually inside of and can study closely. We need to start from our neighborhood to learn so much more about the rest of the universe.”