- 3336 - VOYAGER SPACECRAFT - still making new discoveries? - When the 40 year old Voyager 1 and Voyager 2 spacecraft entered interstellar space in 2012 and 2018, respectively, scientists celebrated. These spacecraft had already traveled 120 times the distance from the Earth to the sun to reach the boundary of the “heliosphere“, the bubble encompassing our solar system that's affected by the solar wind.
------- 3336 - VOYAGER SPACECRAFT - still making new discoveries?
- The Voyagers discovered the edge of the heliosphere bubble but left scientists with many questions about how our Sun interacts with the local interstellar medium. The twin Voyagers' instruments provide limited data, leaving critical gaps in our understanding of this region.
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- NASA and its partners are now planning for the next spacecraft, currently called the Interstellar Probe, to travel much deeper into interstellar space, 1,000 astronomical units (AU) from the sun, with the hope of learning more about how our home heliosphere formed and how it evolves.
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- The Interstellar Probe will go to the unknown local interstellar space, where humanity has never reached before. For the first time, we will take a picture of our vast heliosphere from the outside to see what our solar system home looks like
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- The mission plans to take "images" of our heliosphere using energetic neutral atoms, and perhaps even "observe extragalactic background light from the early times of our galaxy formation -- something that can't be seen from Earth.
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- The heliosphere is important because it shields our solar system from high-energy galactic cosmic rays. The sun is traveling around in our galaxy, going through different regions in interstellar space.
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- The sun is currently in what is called the Local Interstellar Cloud, but recent research suggests the sun may be moving toward the edge of the cloud, after which it would enter the next region of interstellar space -- which we know nothing about.
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- Such a change may make our heliosphere grow bigger or smaller or change the amount of galactic cosmic rays that get in and contribute to the background radiation level at Earth.
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- As Voyagers 1 and 2 continue their epic journeys through interstellar space, they're resolving the true shape of the heliosphere.
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- The two Voyager spacecraft keep expanding our horizons. Having flown past the giant planets in the late 1970s and 1980s, Voyagers 1 and 2 are now well beyond all their planetary targets, with Voyager 1 more than five times farther out than Neptune and Voyager 2 not far behind.
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- During the past decade, both spacecraft reached a new realm, entering the interstellar medium, that tenuous material that fills the vast space between the stars.
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- The interstellar magnetic field has surprised researchers with both its strength and its direction, and the new data have even fed a controversy over the geometry and activity of the heliosphere, the Sun’s magnetic domain.
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- Is the heliosphere the shape of a comet, as has long been assumed, or is it instead more spherical? And does it expand and contract when sunspots wax and wane, or is it more stable?
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- Voyager 2 left Earth in 1977, followed by Voyager 1. They weren’t the first spacecraft to reach the nearest of the giant planet, that honor went to Pioneers 10 and 11. But the Voyagers were more sophisticated than the Pioneers and made many startling discoveries.
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- Voyager 1 took a shorter route than Voyager 2 and arrived at Jupiter first, in 1979, finding that the planet’s colorful moon Io sported erupting volcanoes. In 1980, Voyager 1 sped past Saturn, spying intricate details in the planet’s rings and discovering the first nitrogen atmosphere beyond Earth, around the moon Titan.
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- Voyager 2 took the more scenic route, visiting Jupiter in 1979 and Saturn in 1981, then ventured past Uranus in 1986 and Neptune in 1989. Voyager 2 provided outstanding views of the green and blue planets and spotted geysers on Neptune’s large moon Triton.
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- The spacecraft then headed for interstellar space. As astronomers have defined it, the interstellar medium begins where the solar wind, the outflow of charged particles from the Sun ends.
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- This ionized gas, or plasma, presses against the cooler, denser interstellar plasma flowing around it like a pebble obstructing water in a stream. The Sun-carved cavity is called the heliosphere and its edge the heliopause, just as the top of Earth’s troposphere is called the “tropopause“.
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- As the spacecraft sped ever outward, estimates of the distance to the heliopause kept going up. As a result, no one knew when or where Voyager would enter interstellar space. In July 1992, both Voyagers began detecting strong radio waves at frequencies between 2 and 3 kilohertz.
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- Astronomers attributed these radio waves to six big flares that had erupted on the Sun more than a year earlier. The researchers said that plasma ejected during the flares had eventually hit the heliopause, causing electrons there to oscillate and emit the radio waves.
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- The oscillation frequency is proportional to the square root of the plasma’s electron density, and the observed frequency of the radio waves implied a density matching that expected for the interstellar medium. Although so tenuous it would pass for a perfect vacuum on Earth, the local interstellar medium is much denser than the outer heliosphere.
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- Knowing the approximate speed of the outbound solar material and the length of time it took to hit the boundary revealed the heliopause’s distance from the Sun: between 116 and 177 astronomical units, where 1 astronomical unit is the mean distance between the Sun and the Earth .
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- By comparison, Neptune is only 30 astronomical units from the Sun, and on average Pluto is about 40 astronomical units from the Sun. Voyager 1 shot through the heliopause on August 25, 2012 at 121.6 astronomical units, about four times Neptune’s distance and right in line with predictions two decades earlier.
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- Voyager 1 saw some indications that it had crossed the heliopause. High-energy particles from the Sun vanished, a likely sign that the rest of the solar wind had been left behind as well. Also, cosmic rays from beyond the solar system, which the heliosphere partially blocks, intensified after Voyager’s passage.
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- Voyager 1’s plasma instrument had stopped working and so could not record the jump in particle density when the spacecraft broke from the heliosphere into interstellar space. The magnetic field beyond the heliosphere was expected to point in a different direction and failed to do so. It is still not clear why the magnetic field outside the heliosphere aligns with that inside.
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- Solar storms had erupted earlier in 2012, and the next year they shocked the plasma that Voyager 1 was speeding through, causing electrons there to oscillate and give off radio waves that the spacecraft detected. The frequency of those radio waves indicated that Voyager had indeed entered a much denser domain .
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- Voyager 1 thus became the first spacecraft ever to reach the interstellar medium. Roughly a trillion icy bodies revolve around the Sun far beyond the orbits of Neptune and Pluto; every now and then one of them plunges toward the Sun and we see a new comet in the sky.
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- The farthest of these distant icy objects are probably 1 to 2 light-years, or 63,000 to 126,000 astronomical units, away. Someone in the center of the continental United States who walks three miles west has gotten closer to the Pacific Ocean, relatively speaking, than Voyager has to the solar system’s edge.
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- On November 5, 2018, Voyager 2 also crossed the heliopause. This time the passage was not controversial. The spacecraft’s plasma instrument was working and detected the leap in particle density as protons, electrons, and other charged particles struck the instrument .
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- It also recorded the temperature: between 30,000 and 50,000 Kelvin, much hotter than the local interstellar medium, probably because the plasma gets compressed as it hits the heliosphere.
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- Like Voyager 1, the spacecraft saw the solar wind vanish and cosmic rays from outside the solar system increase, but the magnetic field again failed to change direction, indicating that the result six years earlier was no fluke.
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- The crossing distance was 119.0 astronomical units. That is almost exactly the same distance where the spacecraft’s twin had crossed, a surprise because the solar cycle was then in a different state.
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- Over an 11-year period, as sunspots wax and wane, the solar wind strengthens and weakens, pushing harder and then less so on the heliopause, with a time delay of about 2.5 years.
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- So the heliosphere should expand and contract. Because the pressure from the solar wind was less in 2012 than in 2018, the heliosphere should have been considerably smaller during Voyager 1’s passage than Voyager 2’s. Instead the nearly equal distances mean the heliopause must be sturdier than had been thought.
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- This could be attributable to the unexpectedly strong magnetic field in the interstellar medium. Voyager 1 put it at 5 microgauss, about twice the predicted value, and Voyager 2 found an even stronger interstellar magnetic field, around 7 microgauss. The pressure of the strong interstellar field acts like a straitjacket, suppressing most of the expansion and contraction of the heliosphere.
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- The magnetic field is so strong it also changes the shape of the heliosphere. The standard view is that the heliosphere had the shape of a comet, with a nose and a long tail. The nose faces the direction the solar system is moving through the interstellar medium, while the tail trails in the opposite direction. But because the interstellar magnetic field is so strong, the magnetic pressure, which goes as the square of the field’s strength, squeezes the heliosphere and makes it round.
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- Asteronomers had earlier used the Cassini spacecraft, then orbiting Saturn, to reach the same conclusion . Cassini detected energetic neutral atoms believed to come from near the heliopause. These varied in sync with the sunspot cycle and did so in all directions at about the same time, suggesting that the heliopause is equidistant in all directions, in other words, that the heliosphere is round.
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- Whatever the heliosphere’s exact shape, Voyagers 1 and 2 continue to dart away from it. By year’s end, 2021, Voyager 1 will be 155 astronomical units from the Sun. The spacecraft’s signals, traveling at the speed of light, will take more than 21 hours to get to us. Voyager 2 will be 129 astronomical units out and in 2023 will overtake the silent Pioneer 10 to become the second farthest spacecraft of all. The two Voyagers are dashing away in different directions and are even farther from each other than either is from Earth.
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- They continue to transmit data about interstellar space. Both can measure the electron density, because they detect the radio waves generated when solar eruptions cause electrons in the interstellar medium to oscillate.
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- These eruptions should become more frequent as the sunspot cycle peaks around 2025. The current measurements indicate that the interstellar density has increased further from the value it had outside the heliopause, but no one knows what will happen next.
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- Voyager 2, which has a working plasma instrument, will keep tabs on the interstellar temperature. This temperature will likely fall, because astronomers have measured the local interstellar medium’s temperature to be only 7,500 Kelvin; the high temperature just beyond the heliopause probably results as the plasma there gets compressed and heated.
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- Both spacecraft will monitor the interstellar magnetic field. If the magnetic field gets compressed and strengthened near the heliopause, then the field should eventually weaken at greater distances. And Voyager could even see the magnetic field change direction, just as researchers had expected at the heliopause.
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- The ultimate goal is to sample the unperturbed interstellar medium, space so distant that the heliosphere barely affects it. The interstellar medium is of interest to astronomers because it is the spawning ground of stars as well as the place where they deposit newly minted chemical elements, which future generations of stars and planets inherit.
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- Astronomers must peer through interstellar gas and dust and correct for their effects to study stars and galaxies. Although researchers use ground-based and Earth-orbiting telescopes to observe the interstellar medium from afar, the two spacecraft yield unique data on its density, temperature, and magnetic field by actually being inside it.
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- No one knows how much longer the two spacecraft will survive. They have to stay warm so that the fuel they need to keep their antennas pointed toward Earth doesn’t freeze. Voyager’s heat comes from plutonium, but as the radioactive element decays, it provides less and less power every year.
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- Surprisingly, Voyager 1 is the warmer spacecraft, even though it’s farther from the Sun. After NASA launched Voyager 2, engineers noticed it was colder than expected, so before launching Voyager 1, they added more insulation to keep it warm. That extra warmth may well prolong the spacecraft’s life.
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- And because Voyager 1’s plasma instrument has failed, that instrument doesn’t consume any energy, leaving more for heating the spacecraft. As a result, the Voyager that’s exploring a more distant region of space may continue to return data even after its mate falls silent.
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- If we make it to 2027, then it would be fifty years since Voyager was launched.
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- November 9, 2021 VOYAGER - making new discoveries? 3336
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