Sunday, September 5, 2021

The extraordinary region 'full of chaotic and foamy activity' explored by the Voyager mission outside the Solar System!

 After the Voyager 1 and 2 space probes made it out of the Solar System, much more was learned about the space between stars in the Universe.
 The mysterious dark vacuum of interstellar space is finally being revealed by the two spacecraft that became the first man-made objects to leave our Solar System.
 Away from the sun's protective embrace, the edge of our solar system appears to be a cold, empty, dark place.  The open space between us and the nearest stars has long been considered a frighteningly vast expanse of nothingness.
 Until recently, it was a place humanity could only spy on from afar.  Astronomers paid only passing attention, preferring to focus their telescopes on the bright masses of nearby stars, galaxies and nebulae.
 But two spacecraft built and launched in the 1970s have in recent years sent back our first glimpses of this strange region we call interstellar space.
 As the first man-made objects to leave our Solar System, they are venturing into uncharted territory billions of miles from home.  No other spacecraft has traveled that far.
 And they revealed that, beyond the limits of our Solar System, there is an invisible region of chaotic and frothy activity.
 "When you look at different parts of the electromagnetic spectrum, this area of ​​space is very different from the darkness we perceive with our eyes," says Michele Bannister, an astronomer at the University of Canterbury in Christchurch, New Zealand, who studies the outer limits of the Solar System. .
 "The magnetic fields are fighting, pushing and tying together. The image you should keep in mind is like a swimming pool under Niagara Falls."
 Rather than falling water, however, turbulence is the result of the solar wind (a steady and powerful stream of charged particles, or plasma, spreading in all directions from the Sun) as it crashes into a cocktail of gas, dust and cosmic rays that blow between star systems, known as the "interstellar medium".
 Scientists are building an image of the composition of the interstellar medium, thanks in large part to observations with radio telescopes and X-rays.
 They revealed that it is composed of extremely diffuse ionized hydrogen atoms, dust and cosmic rays interspersed with dense molecular clouds of gas believed to be the birthplace of new stars.
 But the exact nature outside our solar system has been a great mystery, mainly because the Sun, the eight planets and a distant debris disk known as the Kuiper Belt are all contained within a giant protective bubble formed by the solar wind, known as the heliosphere.
 As the Sun and its surrounding planets dart through the galaxy, this bubble spreads against the interstellar medium like an invisible shield, keeping most harmful cosmic rays and other materials away.
 But its properties also make it harder to study what lies beyond the bubble.  Even determining its size and shape is difficult from the inside.
 "It's like you're inside your house and you want to know what it's like. You have to go outside and take a look to really say it," says Elena Provornikova, a postdoctoral researcher at the Johns Hopkins University Applied Physics Laboratory.
 "The only way to get an idea is to travel away from the Sun, look back and take an image outside the heliosphere."
 This is not a simple task.  Compared to the entire Milky Way, our Solar System looks smaller than a grain of rice floating in the middle of the Pacific.
 And yet, the outer edge of the heliosphere is still so far away that it took more than 40 years for the Voyager 1 and Voyager 2 spacecraft to get there after departing Earth.
 Voyager 1, which took a more direct route through the Solar System, moved into interstellar space in 2012, before Voyager 2 joined it in 2018.
 Currently about 13 billion and 11 billion miles from Earth respectively, they are now moving farther and farther out into space beyond our Solar System, sending back more data.
 What these two ancient probes revealed about the boundary between the heliosphere and the interstellar medium provided new clues to how our Solar System formed and how life on Earth is possible.
 Far from being a sharp boundary, the edge of our Solar System is actually churning with turbulent magnetic fields, conflicting stellar windstorms, high-energy particle storms, and swirling radiation.
 The size and shape of the heliosphere's bubble changes as the Sun's output changes and as we pass through different regions of the interstellar medium.  When the solar wind rises or falls, it changes the external pressure in the bubble.
 In 2014, the Sun's activity increased, sending what amounted to a hurricane of solar wind sweeping through space.
 The explosion hit Mercury and Venus quickly at about 800 km per second.  After two days and 150 million km, it enveloped the Earth.  Fortunately, our planet's magnetic field has protected us from its powerful and harmful radiation.
 The blast passed through Mars a day later and continued through the asteroid belt towards the distant gas giants — Jupiter, Saturn, Uranus and after more than two months, Neptune, which orbits about 4.5 billion km from the sun.
 After more than six months, the wind finally reached a point more than 13 billion km from the Sun, known as the "termination shock".
 Here, the Sun's magnetic field, which drives the solar wind, becomes weak enough for the interstellar medium to push it.
 The solar wind gust emerged from the termination shock traveling at less than half its previous speed—it's the hurricane downgraded to a tropical storm.
 Then, in late 2015, it surpassed the irregular shape of the Voyager 2, which is the size of a small car.  The plasma wave was detected by Voyager's detection technologies, powered by a slowly decomposing plutonium battery.
 The probe sent data back to Earth, which even at the speed of light took 18 hours to reach us.
 Astronomers could only receive Voyager's information thanks to an enormous array of 70-meter satellite dishes and advanced technology that had not been imagined, much less invented, when the probe left Earth in 1977.
 The solar wind wave hit Voyager 2 while it was still within our Solar System.  A little over a year later, the last gasps of wind reached Voyager 1, which had crossed into interstellar space in 2012.
 The different routes taken by the two probes meant that one was about 30 degrees above the solar plane, the other the same amount below.  The explosion of the solar wind hit them in different regions at different times, which provided useful clues about the nature of the heliopause.
 The data revealed that the turbulent border is millions of kilometers thick.  It covers billions of square kilometers around the surface of the heliosphere.
 The heliosphere is also unexpectedly large, suggesting that the interstellar medium in this part of the galaxy is less dense than people thought.
 The Sun cuts a path through interstellar space like a boat moving on water, creating a "bow wave" and extending a wake behind it, possibly with a tail (or tails) in comet-like shapes.
 Both Voyagers exited through the "nose" of the heliosphere and therefore did not provide information about the tail.
 "The estimate based on Voyagers is that the heliopause is about one astronomical unit thick (93 million miles, which is the average distance between the Earth and the Sun)," says Provornikova.
 "It's not really a surface. It's a region with complex processes. And we don't know what's going on there."
 The car-sized Voyager spacecraft was launched in 1977 and is now transmitting data from interstellar space - NASA
 Not only do solar and interstellar winds create a turbulent tug of war at the border, but the particles seem to shift charge and momentum.  As a result, a portion of the interstellar medium becomes converted to solar wind, increasing the outward momentum of the bubble.
 And while a solar wind wave can provide interesting data, it appears to have a surprisingly small effect on the bubble's overall size and shape.
 It seems that what happens outside the heliosphere is much more important than what happens inside.  The solar wind can increase or decrease over time without appearing to drastically affect the bubble.  But if that bubble moves into a region of the galaxy with denser or less dense interstellar wind, it will shrink or increase.
 However, many questions remain unanswered, including those about how typical our solar wind protective bubble might be.
 Provornikova says that understanding more about our own heliosphere can help us answer whether we are alone in the universe.
 "What we study in our own system will tell us about the conditions for the development of life in other star systems," she says.
 This is mainly because, by keeping the interstellar medium under control, the solar wind also prevents a bombardment of radiation and high-energy deadly particles (such as cosmic rays) from deep space.
 Cosmic rays are protons and atomic nuclei flowing through space at almost the speed of light.  They can be generated when stars explode, when galaxies collapse into black holes and other cataclysmic cosmic events.
 The region outside our Solar System is dense with a constant shower of these high-speed subatomic particles, which would be powerful enough to cause deadly radiation poisoning on a less protected planet.
 "Voyager definitely said that 90% of this radiation is filtered by the sun," says Jamie Rankin, a heliophysics researcher at Princeton University and the first person to write a doctoral thesis based on the Voyagers' interstellar data.
 "If we didn't have the solar wind protecting us, I don't know if we'd be alive."
 The Sun produces a constant barrage of high-energy particles known as the solar wind, which can rise and fall with the activity of our star - NASA
 The Sun produces a constant barrage of high-energy particles known as the solar wind, which can rise and fall with the activity of our star.
 Image: NASA
 Three additional NASA probes will soon join Voyagers in interstellar space, although two have already run out of power and stopped returning data.  These little pinpricks on the giant frontier will only provide limited information on their own.  Fortunately, a more comprehensive observation can be made closer to home.
 NASA's International Boundary Explorer (Ibex), a tiny satellite that has orbited the Earth since 2008, detects particles called "neutral energy atoms" that cross the interstellar boundary.  Ibex creates three-dimensional maps of the interactions that take place around the edge of the heliosphere.
 "You can think of Ibex maps as a kind of 'Doppler radar' and Voyagers as ground weather stations," says Rankin.
 She used data from Voyagers, Ibex and other sources to analyze smaller bursts of solar wind and is currently working on an article based on the much larger explosion that started in 2014. So far, evidence shows that the heliosphere was shrinking when Voyager 1 overtook the border, but was expanding again when Voyager 2 crossed it.
 "It's a pretty dynamic boundary," she says.
 "It's amazing that this discovery was captured on Ibex's 3D maps, which allowed us to track local Voyager responses at the same time."
 Ibex revealed how dynamic the limit can be.
 In its first year, it detected a giant strand of energetic atoms snaking across a boundary that changed over time, with features appearing and disappearing as quickly as six months.  The tape is a region in the nose of the heliosphere where particles from the solar wind bounce off the galactic magnetic field and are reflected back to the Solar System.
 But there is a surprise in the Voyager story.  Although they have left the heliosphere, they are still within reach of many of our Sun's other influences.
 Sunlight, for example, would be visible to the naked eye from other stars.
 Our star's gravity also extends well beyond the heliosphere, holding in place a sparse and distant sphere of ice, dust, and space debris known as the Oort Cloud.
 Oort objects still orbit the Sun, although they float a lot in interstellar space.
 Although some comets have orbits that extend to the Oort cloud, a region of 300 to 1,500 billion km is generally considered too far away for us to send our own probes.
 These distant objects have barely changed since the beginning of the Solar System and may hold the keys to everything from how planets form to the likelihood of life in our universe.  And with each wave of new data, new mysteries and questions also arise.
 Provornikova says there may be a hydrogen cap covering part or all of the heliosphere, the effects of which have yet to be decoded.
 Furthermore, the heliosphere appears to be turning into an interstellar cloud of particles and dust left over from ancient cosmic events whose effects on the boundary (and those of us living within it) were not anticipated.
 "This can change the dimensions of the heliosphere, it can change its shape," explains Provornikova.  "It can have different temperatures, different magnetic fields, different ionizations and all these different parameters. It's very exciting because it's an area of ​​so much discovery, and we know very little about this interaction between our star and the local galaxy."
 Whatever happens, two car-sized varieties of metal screwed into small parabolic dishes — the intrepid Voyager probes — will be the vanguard of our Solar System, revealing more and more about this strange and uncharted territory as we advance through space .

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