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Voyager 2 Spacecraft Enters Interstellar Space
After a journey of more than four decades, Voyager 2 has passed beyond the sun’s influence
By Jonathan O'Callaghan on December 10, 2018

This illustration shows the position of NASA’s Voyager 1 and Voyager 2 probes beyond the heliosphere, a region of space dominated by our sun that extends well past the orbit of Pluto. Credit: JPL-Caltech and NASA

NASA’s Voyager 2 spacecraft has crossed into interstellar space, agency officials announced today.

The milestone makes Voyager 2 humanity’s second operating spacecraft in history to go interstellar after the Voyager 1 spacecraft did in August 2012. “One kind of feels like a lucky fluke,” says Justin Kasper, a scientist involved in the Voyager missions from the University of Michigan in Ann Arbor. “Two feels like we’re becoming a society that’s capable of exploring interstellar space.”

Voyager 2’s new interstellar status is based on data from its Plasma Science Experiment (PLS), which logged a decrease in particles around the spacecraft that had been ejected from our sun. The PLS measurements of this “solar wind” plummeted to zero on November 5—the official date of Voyager 2’s departure. Now the mission team is confident the spacecraft has joined its predecessor in passing beyond a key boundary called the heliopause and into interstellar space. “November 5 was the day that the galactic cosmic-ray intensity abruptly increased, and that same date was when the heliospheric particle intensity dropped significantly,” says Voyager project scientist Ed Stone. “That same day the magnetic field increased, and that’s also the point at which the plasma [instrument] quit measuring the solar wind. So that’s the correlation we were looking at [to confirm interstellar space].”

The heliopause is the region at which the solar wind’s outward expansion is countered by the influence of incoming interstellar particles. It is considered to be one of the limits of the sun’s influence on surrounding space—although the two spacecraft are still said to be inside the solar system, they are now in a region of space dominated more by the Milky Way Galaxy than our sun. Prior to this, Voyager 2 crossed a region known as the termination shock back in 2007, where the speed of the solar wind dropped dramatically as it began to encounter interstellar particles and radiation. The entire region of the sun’s bubble of influence in the galaxy is known as the heliosphere, whereas the region Voyager 2 has just traversed is called the heliosheath, which lies between the termination shock and the heliopause.

NASA launched the twin Voyager spacecraft in 1977 on a mission to explore the outer planets. After both studying Jupiter and Saturn—with Voyager 2 taking a detour to also visit Uranus and Neptune—the two probes continued their journeys toward the edge of the solar system. In August 2012 Voyager 1 became the first human-made vehicle in history to reach the heliopause and enter interstellar space. But both missions have raised questions about our solar system’s true boundary. Originally some scientists speculated our star’s winds would peter out in the vicinity of Mars, but the Voyager spacecraft have gradually pushed this boundary far beyond. The solar system’s actual limit remains contentious, however, with some researchers defining it not by solar winds but rather by the most distant objects thought to be held in thrall by our sun’s gravity—comets in the Oort Cloud up to about two light-years away. Regardless of where one believes the sun’s sphere of influence ends, both Voyagers are poised to greatly extend it—each has a famous Golden Record onboard in the event any other spacefaring species happens across them on their lonely, eons-long sojourns between the stars.

Voyager 2 has traveled about 120 astronomical units—one AU is the Earth–sun distance—which equates to just over 18 billion kilometers, a distance light itself takes more than 16 hours to traverse. Although far out, this is about one AU closer in than Voyager 1’s earlier exit from the solar system. The most obvious explanation for this disparity is that our solar system’s heliosphere is not perfectly spherical—instead it is oddly shaped and asymmetric, perhaps due to the influence of the Milky Way’s magnetic field. “You can think of the galactic magnetic field as an array of bungee cords,” says Eric Christian from NASA Goddard Space Flight Center, a scientist on the Voyager team. “The solar system is this soccer ball you’re pushing through these bungee cords,” flexing and distorting its shape as it moves.

Another possible explanation is the sun’s fluctuating activity, measured via outbursts such as solar flares and powerful explosions called coronal mass ejections. These events can affect the heliosphere much like gas pumped into a balloon, causing it to grow—conversely, when their numbers decrease the heliosphere can shrink, changing the location of the heliopause. It is possible that in coming years, as the sun reaches the peak of its roughly 11-year activity cycle, its outbursts could push the heliopause farther out again, perhaps even beyond Voyager 2. “There’s a chance [the probe could enter interstellar space twice],” Kasper says. “It’s all going to depend on how long the solar minimum lasts.” In fact, something like this happened when Voyager 2 crossed the termination shock in 2007. Fluctuating solar activity made the boundary oscillate, so the spacecraft ended up crossing the termination shock several times.

Voyager 2 is also entering interstellar space in a completely different region from its sister ship. Whereas the latter traveled out of the heliosphere’s northern hemisphere (the planet-filled ecliptic plane is the equator), Voyager 2 is heading out of the southern hemisphere. Here, the galactic magnetic field is thought to be weaker, which may also affect the shape of the heliosheath. Having outbound spacecraft from both heliospheric hemispheres opens up fascinating opportunities for science. Both spacecraft have a working magnetometer, to measure the local magnetic field, and two particle detectors—one for solar particles and another for incoming cosmic rays. Only Voyager 2, however, continues to have a functioning plasma instrument, which could tell us much more about this unexplored region, including the temperature, density and velocity of any electrically charged material flowing around the spacecraft.

The Voyager team will continue to take measurements as both spacecraft travel away from the sun into a region called the outer heliosheath, although neither probe has much time remaining to make observations. “We’re probably only going to have four to five years left of data,” Christian says. But they may later be joined in this region by other spacecraft in the coming years. Although NASA’s Pioneer 10 and 11 spacecraft launched before the Voyagers paved the way on similar journeys beyond the Asteroid Belt to the outer solar system, eventually leading into interstellar space, they are no longer communicating with Earth. But NASA’s New Horizons spacecraft, which flew by Pluto and is about to conduct the most distant-ever rendezvous in the solar system, could continue operating into interstellar space, giving us a third functioning interstellar probe in coming decades.

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Odp: [Scientific American] Voyager 2 Spacecraft Enters Interstellar Space
« Odpowiedź #1 dnia: Grudzień 13, 2018, 19:30 »
Voyager 2 enters interstellar space
by Jeff Foust — December 10, 2018 [SpaceNews]

Both of NASA's twin Voyager spacecraft have crossed the heliopause and entered interstellar space, with scientists hoping the spacecraft will keep operating for up to a decade. Credit: NASA

WASHINGTON — As NASA’s Voyager 2 spacecraft enters interstellar space, project officials have high hopes that it and its twin spacecraft will continue to operate for as long as a decade.

NASA announced Dec. 10 that Voyager 2 passed what is known as the heliopause, the boundary between where the solar wind from the sun dominates and where the interstellar medium is dominant. That crossing, around Nov. 5, was detected by Voyager 2 in the form of a sharp drop in solar wind particles and corresponding increase in galactic cosmic rays detected by the spacecraft’s instruments.

The discovery, announced at the Fall Meeting of the American Geophysical Union here, comes six years after Voyager 1 also crossed the heliopause into interstellar space.

“This is a very exciting time again in Voyager’s 41-year journey,” said Ed Stone, Voyager project scientist at Caltech and former director of the Jet Propulsion Laboratory, during a press conference announcing the discovery.

The signature of the heliopause crossing from Voyager 2 is not quite the same as Voyager 1, Stone noted. “That’s what makes it interesting,” he said. “We’re in a different place — one is in the northern hemisphere and the other is in the southern hemisphere — and it’s a different time in the solar cycle. We would have been amazed if it looked the same.”

With both Voyager 1 and Voyager 2 beyond the heliopause, scientists are eager to combine those measurements with those from spacecraft and instruments within the sun’s heliosphere. “I’ve been studying galactic cosmic rays for many years from within our heliosphere,” said Georgia Denolfo, a space scientist at NASA’s Goddard Space Flight Center, at the press conference. “So it’s especially exciting to be able to think that we will be having a mission in the very same space that I have been studying, and many others, from afar.”

“I often get asked, ‘Is this it for Voyager?’” said Nicky Fox, director of the heliophysics division at NASA Headquarters. “Absolutely not. This is really for me the beginning of a new era of heliophysics science.”

Project officials are hopeful that this new era can last for several years, despite the age of the two Voyager spacecraft. Both were launched within weeks of each other in 1977 on missions to fly by Jupiter and Saturn. Voyager 2 also made flybys of Uranus and Neptune.

“Both spacecraft are very healthy, if you consider them senior citizens,” said Suzanne Dodd, Voyager project manager at JPL. “They are operating just fine.”

The spacecraft, powered by radioisotope thermoelectric generators (RTGs), are gradually losing power as their plutonium power sources decay. Dodd said the power produced by each spacecraft’s RTG is declining by about four watts per year, and the heat they generate that keeps the spacecraft warm is also declining.

Dodd said they’re closely watching the temperatures on Voyager 2, which are approaching the freezing point for its hydrazine fuel. “So we have to make a lot of trades, particularly with Voyager 2, between power and thermal,” she said.

As the power levels continue to decline, project officials will have to turn off some of the instruments on each spacecraft. “The difficult decisions are going to be made by Dr. Stone and the science team on which instruments to turn off first,” she said. “Those decisions will be made with getting the most science value back.”

Dodd said it’s possible for each spacecraft to continue operating for up to 10 more years. “My own personal goal would be to get these spacecraft to last 50 years” from their 1977 launch, she said. “If we get out to 2027, that will be a 50-year mission. I think that would be fantastic.”

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Odp: [Scientific American] Voyager 2 Spacecraft Enters Interstellar Space
« Odpowiedź #2 dnia: Grudzień 13, 2018, 19:31 »
Voyager 2 Has Just Entered Interstellar Space, NASA Confirms
By Tom Hale 10 DEC 2018 []

Going, going, gone. After a momentous 41-year journey, Voyager 2 has fulfilled its fate to leave the heliosphere and become the second interstellar spacecraft after Voyager 1, NASA has just confirmed.

The Voyager 2 probe was launched by NASA on August 20, 1977, 16 days before its twin Voyager 1, on a mission to explore Jupiter and Saturn. After this success, Voyager 2’s mission to explore the Solar System continued and it went on to become the first spacecraft to explore the ice giants, Uranus and Neptune. Ever since completing this part of its mission in 1989, the pair of galactic wanderers has been pointed towards deep space.

"For the second time in history, a human-made object has reached the space between the stars," NASA said in a statement on Monday. "NASA’s Voyager 2 probe now has exited the heliosphere – the protective bubble of particles and magnetic fields created by the Sun."

This image shows the crescents of Neptune and Triton, acquired by Voyager 2 just three days after its closest approach to Neptune in 1989. Voyager 2/ARC/NASA

It’s worth noting that the spacecraft has not totally “left the Solar System”. Here, we are noting that Voyager 2 has entered interstellar space by passing the heliosheath, the outermost layer of the Sun’s “magnetic bubble” beyond which the velocity of solar winds dramatically drop.

In Voyager 1's case, it proved difficult to definitively confirm it had passed the heliosheath and entered interstellar space, taking almost a year to verify. However, it’s been noted that the Voyager 2 spacecraft was picking up on an ever-decreasing quantity of heliospheric particles since November this year, fueling rumors that its grand exit was imminent. Throughout early December, a further decline was noted.

Graph showing the decreasing heliospheric particles being registered by the Voyager 2 spacecraft on December 9, 2018. NASA/JPL

It may be goodbye for now, but there is still work to be done. Unlike its twin, the Voyager 2 spacecraft is still armed with its Plasma Science (PLS) instrument, meaning it should be able to beam back some invaluable information on the nature of the heliosheath and the interstellar medium outside our Solar System.

“Working on Voyager makes me feel like an explorer, because everything we’re seeing is new,” added John Richardson, principal investigator for the PLS instrument and a principal research scientist at the Massachusetts Institute of Technology in Cambridge. “Even though Voyager 1 crossed the heliopause in 2012, it did so at a different place and a different time, and without the PLS data. So we’re still seeing things that no one has seen before.”

The Voyager spacecraft are also known for their Golden Records, two phonograph records that contain sounds and images of life on Earth, designed to act as a time capsule for intelligent extraterrestrial life or future humans.

Farewell, Voyager 2, it's been a pleasure.

Source: Voyager 2 Has Just Entered Interstellar Space, NASA Confirms

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Odp: [Scientific American] Voyager 2 Spacecraft Enters Interstellar Space
« Odpowiedź #3 dnia: Grudzień 13, 2018, 19:32 »
Voyager 2 spacecraft enters interstellar space
It’s the second probe ever to exit the heliosphere
BY LISA GROSSMAN 12:57PM, DECEMBER 10, 2018 [Sciencenews]

OVER AND OUT  The Voyager 2 spacecraft has crossed into interstellar space, six years after its twin probe, Voyager 1. The two spacecraft are providing an unprecedented look at the edge of the sun’s influence (lighter blue bubble in this illustration).

Voyager 2 has entered interstellar space. The spacecraft slipped out of the huge bubble of particles that encircles the solar system on November 5, becoming the second ever human-made craft to cross the heliosphere, or the boundary between the sun and the stars.

Coming in second place is no mean achievement. Voyager 1 became the first spacecraft to exit the solar system in 2012. But that craft’s plasma instrument stopped working in 1980, leaving scientists without a direct view of the solar wind, hot charged particles constantly streaming from the sun (SN Online: 9/12/13). Voyager 2’s plasma sensors are still working, providing unprecedented views of the space between stars.

“We’ve been waiting with bated breath for the last couple of months for us to be able to see this,” NASA solar physicist Nicola Fox said at a Dec. 10 news conference at the American Geophysical Union meeting in Washington, D.C.

NASA launched the twin Voyager spacecraft in 1977 on a grand tour of the solar system’s planets (SN: 8/19/17, p. 26). After that initial tour was over, both spacecraft continued travelling through the bubble of plasma that originates at the sun.

“When Voyager was launched, we didn’t know how large the bubble was, how long it would take to get [to its edge] and whether the spacecraft could last long enough to get there,” said Voyager project scientist Edward Stone of Caltech.

For most of Voyager 2’s journey, the spacecraft’s Plasma Science Experiment measured the speed, density, temperature, pressure and other properties of the solar wind. But on November 5, the experiment saw a sharp drop in the speed and the number of solar wind particles that hit the detector each second. At the same time, another detector started picking up more high-energy particles called cosmic rays that originate elsewhere in the galaxy.

Waning wind

Voyager 2 saw a steep drop in solar wind, or heliospheric, particles on November 5, and recorded an increase in galactic cosmic ray particles, which come from the rest of the galaxy. Those signs make scientists think the spacecraft has left the sun’s protective bubble.

Using particles to track Voyager 2’s journey

Those measurements suggest that Voyager 2 has reached the region where the solar wind slams into the colder, denser population of particles that fill the space between stars. Voyager 2 is now a little more than 18 billion kilometers from the sun.

Intriguingly, Voyager 2’s measurements of cosmic rays and magnetic fields — which Voyager 1 could still make when it crossed the boundary — did not exactly match up with Voyager 1’s observations.

“That’s what makes it interesting,” Stone said. The variations are probably from the fact that the two spacecraft exited the heliosphere in different places, and that the sun is at a different part of its 11-year activity cycle than it was in 2012. “We would have been amazed if they had looked the same.”

The Voyagers probably have between five and 10 years left to continue exploring interstellar space, said Voyager project manager Suzanne Dodd from NASA’s Jet Propulsion Laboratory in Pasadena, Calif.

“Both spacecraft are very healthy if you consider them senior citizens,” Dodd said. The biggest concern is how much power they have left and how cold they are — Voyager 2 is currently about 3.6° Celsius, close to the freezing point of its hydrazine fuel. In the near future, the team will have to turn off some of the spacecraft’s instruments to keep the craft operating and sending data back to Earth.

“We do have difficult decisions ahead,” Dodd said. She added that her personal goal is to see the spacecraft last until 2027, for a total of 50 years in space. “That would be fantastic.”

Source: Voyager 2 spacecraft enters interstellar space

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Odp: [Scientific American] Voyager 2 Spacecraft Enters Interstellar Space
« Odpowiedź #4 dnia: Lipiec 13, 2019, 05:17 »
To Keep Boldly Going: NASA's New Plan for the Voyager Mission
By Paul Scott Anderson, on July 9th, 2019 [AS]

Artist’s concept of one of NASA’s Voyager spacecraft, including the location of the cosmic ray subsystem (CRS) instrument. Image Credit: NASA/JPL-Caltech

With all of the current and recent planetary missions throughout the Solar System, it may be easy to forget sometimes that there are still some older spacecraft that have been traveling for decades now. Two of these – Voyagers 1 and 2 – have even left the Solar System entirely, and now engineers need to figure out how to keep them active and functioning with much lower power levels and degrading thrusters.

“It’s incredible that Voyagers’ instruments have proved so hardy,” said Voyager Project Manager Suzanne Dodd, at NASA’s Jet Propulsion Laboratory in Pasadena, California. “We’re proud they’ve withstood the test of time. The long lifetimes of the spacecraft mean we’re dealing with scenarios we never thought we’d encounter. We will continue to explore every option we have in order to keep the Voyagers doing the best science possible.”

So how do you maintain these aging probes so they can continue to return science data, after nearly 42 years? Mission engineers have come up with a new plan.

This involves making some difficult choices.

There is only so much power available, especially after such a long time in deep space, and it becomes harder to maintain both the science instruments and the heaters. So engineers have had to choose what continues to receive power and what gets turned off now.

Diagram of the various instruments and other components of the two Voyager spacecraft (both are identical). Image Credit: NASA/JPL-Caltech

One option is to turn off heaters for instruments that can get by now without them.

On Voyager 2, the decision was made to turn off the heater for the cosmic ray subsystem (CRS) instrument, despite it still returning data even though temperatures had dropped to minus 74 degrees Fahrenheit (minus 59 degrees Celsius). That is lower than the temperatures at which the instrument was tested more than 42 years ago, down to minus 49 degrees Fahrenheit, or minus 45 degrees Celsius, a testament to just how well the CRS was built. This decision was based on the fact that the CRS can only take data in certain fixed directions, so a bit more limited in its use.

The CRS was valuable last November, proving that Voyager 2 had finally exited the Solar System’s heliosphere, the protective bubble created by a constant “wind” of ionized particles from the Sun. Voyager 2 is now considered to be in interstellar space.

Even with the CRS heater now turned off, Voyager 2 still has five functioning instruments: the CRS, two instruments to study plasma (a gas with ionized atoms) and a magnetometer (to measure magnetic fields).

Voyager 1 reached interstellar space back in August 2012, and is still collecting data with its own CRS. Other instruments are still functioning as well, including a plasma instrument, the magnetometer and the low-energy charged particle instrument.

Illustration showing the locations of both spacecraft as of December 2018, when Voyager 2 reached interstellar space. Voyager 1 crossed into interstellar space in 2012. Image Credit: NASA/JPL-Caltech.

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Voyager 2 Enters Interstellar Space. Video Credit: NASA/JPL

Both Voyager 1 and Voyager 2 are powered by three radioisotope thermoelectric generators (RTGs), which produce heat via the natural decay of plutonium-238 radioisotopes. The heat is then converted into electrical power. But they don’t last forever, and the heat energy produced gradually declines, with 4 less watts of electrical power per year. As of now, 40% less power is being generated than at launch 42 years ago. Because of this, additional heaters may need to be turned off over the next few years. That is not an easy decision to make, since temperatures on both spacecraft need to be carefully controlled to prevent freezing of working (and needed) instruments. If the fuel lines froze, that could affect the thrusters, and the spacecraft might not be able to keep its antenna oriented toward Earth.

Heat isn’t the only problem however.

The thrusters on both spacecraft have started to degrade. In 2017, engineers noticed that some thrusters on Voyager 1 needed to give off more “puffs” or tiny pulses in order to maintain orientation of the spacecraft and keep the antenna pointed at Earth. To fix this, engineers switched to another set of thrusters that hadn’t been used in 37 years. It’s a good thing those additional thrusters were there and still functioning.

With both spacecraft now nearly 42 years into their missions, since their launch in 1977, it is not surprising that more problems are showing themselves now as instruments and other hardware age. But the mission engineers have come up with clever ways to keep them going, hopefully for at least several more years. They are now literally in unchartered territory, sending back new data about the boundary between our Solar System and interstellar space.

Illustration of the paths through the outer Solar System, including the Kuiper Belt, of Voyager 1 and Voyager 2, as well as Pioneer 10, Pioneer 11 and New Horizons. Image Credit: NASA/Johns Hopkins University Applied Physics Laboratory

There is also NASA’s Interstellar Boundary Explorer (IBEX), a mission that explores that boundary too, but by remote sensing. NASA is also preparing the Interstellar Mapping and Acceleration Probe (IMAP), scheduled to launch in 2024, to further build on the Voyagers’ observations.

“Both Voyager probes are exploring regions never before visited, so every day is a day of discovery,” said Voyager Project Scientist Ed Stone, at Caltech. “Voyager is going to keep surprising us with new insights about deep space.”

Before leaving the Solar System, the Voyagers conducted a Grand Tour of the outer Solar System, sending back close-up images and data of Jupiter, Saturn, Uranus and Neptune, helping to revolutionize our understanding of these distant worlds.

Even after both probes eventually die, they will continue to silently navigate the space between our Solar System and the nearby stars, but they are not programmed to fly past any particular stars. Instead they will just drift alone through deep space, having accomplished their incredible missions of exploring the Solar System and beyond.

You can read more about the Voyager mission at the JPL website.


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