From NASA Goddard Space Flight Center: “New Evidence Our Neighborhood in Space Is Stuffed With Hydrogen”

From NASA Goddard Space Flight Center

Oct. 30, 2020

Miles Hatfield
miles.s.hatfield@nasa.gov
NASA’s Goddard Space Flight Center, Greenbelt, Md.

Only the two Voyager spacecraft have ever been there, and it took than more than 30 years of supersonic travel.

NASA/Voyager 1.

NASA/Voyager 2.

Heliosphere-heliopause showing positions of Voyager spacecraft. Credit: NASA.

It lies well past the orbit of Pluto, through the rocky Kuiper belt, and on for four times that distance. This realm, marked only by an invisible magnetic boundary, is where Sun-dominated space ends: the closest reaches of interstellar space.

In this stellar no-man’s land, particles and light shed by our galaxy’s 100 billion stars jostle with ancient remnants of the big bang. This mixture, the stuff between the stars, is known as the interstellar medium. Its contents record our solar system’s distant past and may foretell hints of its future.

Measurements from NASA’s New Horizons spacecraft are revising our estimates of one key property of the interstellar medium: how thick it is.

NASA/New Horizons

Findings published today in The Astrophysical Journal share new observations that the local interstellar medium contains approximately 40% more hydrogen atoms than some prior studies suggested. The results unify a number of otherwise disparate measurements and shed new light on our neighborhood in space.

Slogging through interstellar fog

Just as Earth moves around the Sun, so our entire solar system hurtles through the Milky Way, at speeds exceeding 50,000 miles per hour. As we cruise through a fog of interstellar particles, we’re shielded by the magnetic bubble around our Sun known as the heliosphere.

NASA Heliosphere.

Many interstellar gases flow around this bubble, but not all.

Our heliosphere repels charged particles, which are guided by magnetic fields. But more than half of local interstellar gases are neutral, meaning they have a balanced number of protons and electrons. As we plow into them the interstellar neutrals seep right through, adding bulk to the solar wind.

“It’s like you’re running through a heavy mist, picking up water,” said Eric Christian, space physicist at NASA’s Goddard Space Flight Center in Greenbelt, MD. “As you run, you’re getting your clothes all soggy and it’s slowing you down.”

Soon after those interstellar atoms drift into our heliosphere, they are zapped by sunlight and slammed by solar wind particles. Many lose their electrons in the tumult, becoming positively-charged “pickup ions.” This new population of particles, though changed, carry with them secrets of the fog beyond.

“We don’t have direct observations of interstellar atoms from New Horizons, but we can observe these pickup ions,” said Pawel Swaczyna, postdoctoral researcher at Princeton University and lead author of the study. “They are stripped of an electron, but we know they came to us as neutrals atoms from outside the heliosphere.”

NASA’s New Horizons spacecraft, launched in January 2006, is the one best suited to measure them. Now five years past its rendezvous with Pluto, where it captured the first up-close images of the dwarf planet, today it ventures through the Kuiper belt at the edge of our solar system where pickup ions are the freshest. The spacecraft’s Solar Wind Around Pluto, or SWAP instrument, can detect these pickup ions, distinguishing them from the normal solar wind by their much higher energy.

The amount of pickup ions New Horizons detects reveals the thickness of the fog we’re passing through. Just as a jogger gets wetter running through thicker fog, the more pickup ions New Horizons observes, the denser the interstellar fog must be outside.

Diverging measurements

Swaczyna used SWAP’s measurements to derive the density of neutral hydrogen at the termination shock, where the solar wind butts up against the interstellar medium and abruptly slows down. After months of careful checks and tests, the number they found was 0.127 particles per cubic centimeter, or about 120 hydrogen atoms in a space the size of a quart of milk.

This result confirmed a 2001 study [COSPAR Colloquia Series] which used Voyager 2 – about 4 billion miles away – to measure how much the solar wind had slowed by the time it arrived at the spacecraft. The slowdown, largely due to intervening interstellar medium particles, suggested a matching interstellar hydrogen density, about 120 hydrogen atoms in a quart-sized space.

But newer studies converged around a different number. Scientists using data from NASA’s Ulysses mission, from a distance slightly closer to the Sun than Jupiter, measured pickup ions and estimated a density of about 85 hydrogen atoms in a quart of space.

NASA/ESA Ulysses

A few years later, a different study combining Ulysses and Voyager data found a similar result.

“You know, if you discover that something different than previous work, the natural tendency is to start looking for your errors,” said Swaczyna.

But after a little digging, the new number began to look like the right one. The New Horizons measurements fit better with observations based on faraway stars. The Ulysses measurements, on the other hand, had a shortcoming: they were made much closer to the Sun, where pickup ions are rarer and measurements more uncertain.

“The inner heliosphere pickup ions observations go through billions of miles of filtering,” Christian said. “Being most of the way out there, where New Horizons is, makes a huge difference.”

As for the combined Ulysses/Voyager results, Swaczyna noticed that one of the numbers in the calculation was outdated, 35% lower than the current consensus value. Recalculating with the currently accepted value gave them an approximate match with the New Horizons measurements and the 2001 study.

“This confirmation of our old, almost forgotten result comes as a surprise,” said Arik Posner, author of the 2001 study at NASA Headquarters in Washington, D.C. “We thought our rather simple methodology for measuring the slowdown of the solar wind had been overcome by more sophisticated studies conducted since, but not so.”

A new lay of the land

Going from 85 atoms in a quart of milk to 120 may not seem like much. Yet in a model-based science like heliophysics, a tweak to one number affects every other.

The new estimate may help explain one of the biggest mysteries in heliophysics of the last few years. Not long after NASA’s Interstellar Boundary Explorer or IBEX mission returned its first complete dataset, scientists noticed a strange stripe of energetic particles coming from the forward edge of our heliosphere. They called it the “IBEX ribbon.”

NASA/IBEX

“The IBEX ribbon was a big surprise – this structure at the edge of our solar system a billion miles wide, 10 billion miles long, that no one knew was there,” Christian said. “But even as we developed the models for why it was there, all of the models were showing that it shouldn’t be as bright as it is.”

The ribbon remains one of IBEX’s biggest discoveries. It refers to a vast, diagonal swath of energetic neutrals, painted across the front of the heliosphere. Credits: NASA/IBEX.

“The 40% higher interstellar density observed in this study is absolutely critical” said David McComas, professor of astrophysical sciences at Princeton University, principal investigator for NASA’s IBEX mission and coauthor of the study. “Not only does this show that our Sun is embedded in a much denser part of interstellar space, it also may explain a significant error in our simulation results compared to the actual observations from IBEX.”

Most of all, though, the result gives an improved picture of our local stellar neighborhood.

“It’s the first time we’ve had instruments observe pickup ions this far away, and our picture of the local interstellar medium is matching those from other astronomical observations,” said Swaczyna. “It’s a good sign.”

See the full article here.

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NASA’s Goddard Space Flight Center is home to the nation’s largest organization of combined scientists, engineers and technologists that build spacecraft, instruments and new technology to study the Earth, the sun, our solar system, and the universe.

Named for American rocketry pioneer Dr. Robert H. Goddard, the center was established in 1959 as NASA’s first space flight complex. Goddard and its several facilities are critical in carrying out NASA’s missions of space exploration and scientific discovery.

NASA/Goddard Campus

From AAS NOVA: “New Discoveries from Old Spacecraft”

From AAS NOVA

7 October 2020
Susanna Kohler

This illustration shows NASA’s Voyager 1 and Voyager 2 probes outside of the heliosphere, the protective bubble created by the Sun around our solar system. Credit: NASA/JPL-Caltech.

In 1977, two space probes were launched from Earth, flung out toward the farthest reaches of our solar system. Now, 43 years later, Voyager 1 and Voyager 2 are journeying through interstellar space — and they’re still providing new insights.

NASA/Voyager 1.

NASA/Voyager 2.

Voyaging to the Outer Edge

The original mission of the Voyager spacecraft was to study the giant planets in our outer solar system. But across 43 years and three mission extensions, these little probes have gone on to do so much more — most recently crossing out of the heliosphere and providing our first up-close look at interstellar space.

NASA Heliosphere.

What’s the heliosphere? As the solar wind streams from the Sun, it carries magnetic fields outward, inflating a bubble around the solar system that separates us from the surrounding interstellar medium (ISM). As the Sun orbits through the galaxy, the heliosphere is compressed on one side and elongated on the other, forming a blunt “nose” and a streaming “tail”.

Into the Unknown

When Voyagers 1 and 2 were launched, they were sent in slightly different directions — so they’re now exploring two different regions of the interface between the heliosphere and the interstellar medium. In 2012, Voyager 1 crossed the boundary of the heliosphere on one side of the nose, at a distance of ~122 au from the Sun. Voyager 2 followed suit in 2018, crossing the other side of the nose at a distance of ~119 au.

Now, both spacecraft are traveling through the very local ISM beyond the heliosphere. But despite their distance (the one-way light travel time to Voyager 1 is ~21 hours!), the probes are still reporting back data — including from the Plasma Wave Science (PWS) instrument on each craft, which uses the long, V-shaped pair of antennae to measure oscillations in the surrounding plasma. From these oscillations, we can infer the electron density of the ISM that the Voyager spacecraft are traveling through.

Denser and Denser

In a new publication, University of Iowa scientists William Kurth and Donald Gurnett report the latest PWS measurement from Voyager 2, which indicates that the electron density of the ISM is currently increasing as the probe travels away from the Sun. This discovery is neatly consistent with the data from Voyager 1, which has also been reporting an increasing radial density gradient since crossing the boundary of the heliosphere and entering interstellar space.

Electron density vs. radial distance from the Sun, as measured by the Voyager 1 (black) and Voyager 2 (red) spacecraft. The radial density gradient in the ISM can be seen in the data from both probes at distances above ~120 au. Credit: Kurth & Gurnett 2020.

Voyagers 1 and 2 have trajectories that differ by 67° in latitude and 43° in longitude — so with the new Voyager 2 data published by Kurth and Gurnett, we now have confirmation that the radial density gradient first measured by Voyager 1 is a large-scale feature around the heliospheric nose.

Still More to Learn

What’s causing the gradient? Two theories have been put forward:

the interaction of the solar wind with the very local ISM creates a pile-up region outside of the heliosphere, or
draping of magnetic field lines over the outer boundary of the heliosphere depletes the plasma just inside the heliosphere.

We’ll potentially be able to differentiate between these two models once we have density measurements from even farther out in the ISM — so we’ll have to see if the Voyager probes last long enough to provide them!

Citation

“Observations of a Radial Density Gradient in the Very Local Interstellar Medium by Voyager 2,” W. S. Kurth and D. A. Gurnett 2020 ApJL 900 L1.
https://iopscience.iop.org/article/10.3847/2041-8213/abae58

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From Horizon The EU Research and Innovation Magazine: “Five things we’re going to learn from Europe’s Solar Orbiter mission”

From Horizon The EU Research and Innovation Magazine

ESA/NASA Solar Orbiter depiction

07 February 2020
Jonathan O’Callaghan

At 23.03 (local time) on Sunday 9 February, Europe’s newest mission to study the sun is set to lift off from Cape Canaveral in Florida, US. Called Solar Orbiter, this European Space Agency (ESA) mission will travel to within the orbit of planet Mercury to study the sun like never before, returning stunning new images of its surface.

Equipped with instruments and cameras, the decade-long mission is set to provide scientists with key information in their ongoing solar research. We spoke to three solar physicists about what the mission might teach us and the five unanswered questions about the sun it might finally help us solve.

1. When solar eruptions are heading our way

Solar Orbiter will reach a minimum distance of 0.28% of the Earth-sun distance throughout the course of its mission, which could last the rest of the 2020s. No other mission will have come closer to the sun, save for NASA’s ongoing Parker Solar Probe mission, which will reach just 0.04 times the Earth-sun distance.

NASA Parker Solar Probe Plus named to honor Pioneering Physicist Eugene Parker

Dr Emilia Kilpua from the University of Helsinki in Finland is the coordinator of a project called SolMAG, which is studying eruptions of plasma from the sun known as coronal mass ejections (CMEs).

Coronal mass ejections – NASA-Goddard Space Flight Center-SDO

NASA/SDO

She says this proximity, and a suite of cameras that Parker Solar Probe lacks, will give Solar Orbiter the chance to gather data that is significantly better than any spacecraft before it, helping us monitor CMEs.

‘One of the great things about Solar Orbiter is that it will cover a lot of different distances, so we can really capture these coronal mass ejections when they are evolving from the sun to Earth,’ she said. CMEs can cause space weather events on Earth, interfering with our satellites, so this could give us a better early-warning system for when they are heading our way.

2. Why the sun blows a supersonic wind

One of the major unanswered questions about the sun concerns its outer atmosphere, known as its corona. ‘It’s heated to (more than) a million degrees, and we currently don’t know why it’s so hot,’ said Dr Alexis Rouillard from the Institute for Research in Astrophysics and Planetology in Toulouse, France, the coordinator of a project studying solar wind called SLOW_SOURCE. ‘It’s (more than) 200 times the temperature of the surface of the sun.’

ESA China Double Star mission continuous interaction between particles in the solar wind and Earth’s magnetic shield 2003-2007

ESA China Smile solar wind and Earth’s magnetic shield – the magnetosphere spacecraft depiction

Magnetosphere of Earth, original bitmap from NASA. SVG rendering by Aaron Kaase

A consequence of this hot corona is that the sun’s atmosphere cannot be contained by its gravity, so it has a constant wind of particles blowing out into space, known as solar wind.

This artist’s rendering shows a solar storm hitting Mars and stripping ions from the planet’s upper atmosphere. NASA/GSFC

This wind blows at more than 250km per second, up to speeds of 800km per second, and we currently do not know how that wind is pushed outwards to supersonic speeds.

Dr Rouillard is hoping to study the slower solar wind using Solar Orbiter, which may help us explain how stars like the sun create supersonic winds. “By getting closer to the sun we collect more (pristine) particles, he said. “Solar Orbiter will provide unprecedented measurements of the solar wind composition. (And) we will be able to develop models for how the wind (is pushed out) into space.”

3. What its poles look like

During the course of its mission, Solar Orbiter will make repeated encounters with the planet Venus. Each time it does, the angle of the spacecraft’s orbit will be slightly raised until it rises above the planets. If the mission is extended as hoped to 2030, it will reach an inclination of 33 degrees – giving us our first ever views of the sun’s poles.

Aside from being fascinating, there will be some important science that can be done here. By measuring the sun’s magnetic fields at the poles, scientists hope to get a better understanding of how and why the sun goes through 11-year cycles of activity, culminating in a flip of its magnetic poles. They are set to flip again in the mid-2020s.

‘By understanding how the magnetic fields are distributed and evolve in these polar regions, we gain a new insight on the cycles that the sun is going through,’ said Dr Rouillard. ‘Every 11 years, the sun goes from a minimum activity state to a maximum activity state. By measuring from high latitudes, it will provide us with new insights on the cyclic evolution of (the sun’s) magnetic fields.’

4. Why it has polar ‘crowns’

Occasionally the sun erupts huge arm-like loops of material from its surface, which are known as prominences. They extend from its surface into the corona, but their formation is not quite understood. Solar Orbiter, however, will give us our most detailed look at them yet.

‘We’re going to have very intricate views of some of these active regions and their associated prominences,’ says Professor Rony Keppens from KU Leuven in Belgium, coordinator of a project called PROMINENT which is studying solar prominences. ‘It’s going to be possible to have more than several images per second. That means some of the dynamics that had not been seen before now are going to be visualised for the first time.’

Some of the sun’s largest prominences come from near its poles, so by raising its inclination Solar Orbiter will give us a unique look at these phenomena. ‘They’re called polar crown prominences, because they are like crowns on the head of the sun,’ said Prof. Keppens. ‘They encircle the polar regions and they live for very long, weeks or months on end. The fact that Solar Orbiter is going to have first-hand views of the polar regions is going to be exciting, especially for studies of prominences.’

5. How it controls the solar system

By studying the sun with Solar Orbiter, scientists hope to better understand how its eruptions travel out into the solar system, creating a bubble of activity around the sun in our galaxy known as the heliosphere.

NASA Heliosphere

This can of course create space weather here on Earth, so studying it is important for our own planet.

‘One of the ideas we have is to take measurements of the solar magnetic field in active regions in the equatorial belt of the sun,’ said Professor Keppens. ‘We’re going to extrapolate that data into the corona, and then use simulations to try and mimic how some of these eruptions happen and progress out into the heliosphere.’

Thus, Solar Orbiter will not just give us a better understanding of the sun itself, but also how it affects planets like Earth too. Alongside the first-ever images of the poles and the closest-ever images of its surface, Solar Orbiter will give us an unprecedented understanding of how the star we call home really works.

The research in this article is funded by the European Research Council. Sharing encouraged.

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From astrobites: “Where the Solar System Ends”

From astrobites

Feb 6, 2020
Briley Lewis

Title: Voyager 2 plasma observations of the heliopause and interstellar medium
Authors: John D. Richardson, John W. Belcher, Paula Garcia-Galindo, Leonard F. Burlaga
First Author’s Institution: Kavli Institute for Astrophysics and Space Research; Massachusetts Institute of Technology

Status: Published in Nature Astronomy [closed access]

Where does the solar system actually end? We could say it’s where the Sun’s gravity stops being strong enough to hold onto things. This would make it the edge of the Oort Cloud, the loosely bound sphere of rocky and icy bits left over from the solar system’s formation, extending almost 3 light-years from the Sun.

Oort Cloud, The layout of the solar system, including the Oort Cloud, on a logarithmic scale. Credit: NASA, Universe Today

Or, we could say it’s where the energetic particles from the Sun (the solar wind) stop flowing away from us, blocked by the pressure of all the other gas that’s between stars, the interstellar medium

Today, we’ll focus on the latter: the Heliopause, the boundary where the solar wind meets the interstellar medium (ISM), which marks the edge of the Heliosphere, the bubble of gas surrounding the Sun.

Heliopause https://galnet.fandom.com/wikiHeliopause

Both the solar wind and the ISM are made of plasma, the 4th state of matter. In a plasma, some of the electrons have been stripped off the atoms, leaving charged particles (ions) to move around. There are a few different parts of the heliosphere, and the Voyager missions, launched in the 1970s, have traveled through all of them.

NASA/Voyager 1

NASA/Voyager 2

After traveling far beyond the planets, the two Voyager missions first encountered a region where the solar wind slows down below the speed of sound, known as a termination shock. Their next big milestone would be the Heliopause (see Figure 1 for illustration). This is an important and unique part of our solar system to understand; it is where our solar system and our star interact with the surrounding galaxy. There’s a lot we can learn here about how forces and magnetic fields in the plasma of the interstellar medium confine and influence the solar wind. The heliopause is even important for understanding how life comes about in solar systems – it’s what protects us from dangerous cosmic rays and other high energy radiation that could be disastrous for life.

Figure 1: Illustration (not to scale) showing the planets and the different features of the Heliosphere. (Image from Encyclopedia Britannica)

Voyager 1 crossed the heliopause first in 2012, at 121.7 Astronomical Units, (AU) meaning Voyager 1 was 121.7 times further from the Sun than the Earth is. Voyager 2 finally reached this milestone in late 2018, at 119 AU, passing through a slightly different flow of the solar wind than Voyager 1 did, as shown in Figure 2. Although Voyager 1 gave us the first information on the heliopause, it passed through a weird spot, where the solar wind seemed to be flowing more slowly and in ways we wouldn’t expect. It also didn’t get to take all its measurements, since its plasma instrument was broken. Since 2012, astronomers have been waiting for Voyager 2 to reach this milestone, so that they can take new measurements and understand another perspective of the heliopause, including the speed and direction of the plasma’s flow, its temperature, and its density in that region.

Figure 2: Diagram of the Voyager 1 and 2 trajectories, illustrating the different paths they took towards the outer reaches of the solar system. (Image from NASA/JPL)

So, what did Voyager 2 see out there? As it approached the heliopause, it entered a “boundary layer” – a region where the density and magnetic field increase as the solar wind encounters the ISM. Voyager 1 also traveled through this layer, and observed something unusual: the flow of the solar wind was stagnated, traveling much more slowly than expected. Voyager 2 saw very different velocities of the solar wind near the boundary, and although we’re not sure why these two observations were so different, the authors think they might be due to instabilities in the boundary layer; the heliosphere isn’t a perfect bubble, instead its edges might have swirls and uneven patches. It took the spacecraft 8 days to cross this boundary region, but the actual heliopause is so sharply defined that it only took 1 day to cross (as shown in Figure 3)!

Figure 3: Data from Voyager 2’s plasma instruments, showing the steep drop in current once the spacecraft reached the heliopause. The rise in current before the heliopause happens as V2 crosses the boundary layer (Figure 1 in the paper).

After the heliopause, Voyager 2 was officially out in the “very local interstellar medium” (VLISM). The VLISM isn’t perfectly smooth either; Voyager 2 observed variations in the speed, flow direction, density, and temperature of the plasma out there, and found that the further it gets from the heliopause, the more dense the VLISM gets. This makes sense, since it’s cooler out there (around 7500 K, a bit hotter than the Sun’s surface) than it is closer to the heliopause, where gas gets compressed as the solar wind presses into the ISM, and the plasma is observed to be much hotter – around 30,000 K! This is actually hotter than expected, suggesting that the plasma is getting more compressed or heated in other ways. Voyager 2 also passed through an interesting region where the current in the plasma spiked up (illustrated in the data in Figure 4); the authors think this is a shock, a sudden change in pressure and density.

Figure 4: Measurements of the current in the VLISM (very local interstellar medium) flow as measured by Voyager 2. The heliopause is marked “HP” and the spike at Day 418 shows when V2 may have passed through a shock. (Figure 5 in the paper)

Having direct measurements of all this plasma and matter beyond the heliopause is important, because it’s literally the stuff that is between ALL the stars! Most of the universe (besides dark matter, of course) is made of plasma, and after more than 30 years of traveling and waiting, now we have the chance to directly observe it.

Originally created as a mission to study Jupiter and Saturn up close, the Voyager probes ended up flying by Jupiter, Saturn, Uranus, Neptune, and 48 of their moons. Now, they’re continuing past the planets, past the heliopause, and into interstellar space. Both Voyagers are now out there observing the VLISM – and we can look forward to getting info on all this stuff between stars as long as the spacecraft stay alive and communicating with Earth.

See the full article here .

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Astrobites is a daily astrophysical literature journal written by graduate students in astronomy. Our goal is to present one interesting paper per day in a brief format that is accessible to undergraduate students in the physical sciences who are interested in active research.
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From Eos: “Sampling the Space Between the Stars”

From AGU

From Eos

19 August 2019
Mark Zastrow

Data from the Cassini and Voyager spacecraft reveal new information about the Sun’s magnetic bubble.

NASA/ESA/ASI Cassini-Huygens Spacecraft

NASA/Voyager 1

NASA/Voyager 2

The basic shape and properties of the heliosphere, the protective magnetic bubble created by the solar wind, shown in this schematic are based on measurements of heliosheath proton distributions from Voyager 1 and 2 (illustrated in the diagram) and of energetic neutral atoms by Cassini. The location of the inner edge of the heliosheath, called the termination shock, is roughly 10 astronomical units (AU; 1 AU is equivalent to the mean Sun-Earth distance of about 150 million kilometers) farther from the Sun where Voyager 1 crossed it compared with Voyager 2, but the location of the outer edge, the heliopause, is about the same distance at along both Voyager trajectories. Red arrows represent the interstellar plasma flow deflected around the heliosphere bubble. Credit: K. Dialynas, S. M. Krimigis, D. G. Mitchell, R. B. Decker and E. C. Roelof

Charged particles that spew into space as part of the solar wind create a protective magnetic bubble tens of billions of kilometers wide around the solar system. This bubble, called the heliosphere, plows through the harsh cosmic radiation of interstellar space.

Understanding the physics at the bubble’s edge, called the heliosheath, is not easy. The boundary is in constant flux and pushes out against the broader interstellar magnetic field that permeates our corner of the Milky Way. Only two spacecraft—Voyager 1 and 2, originally launched by NASA in 1977—have ever traversed the frontiers of our local bubble.

Now Dialynas et al. [Geophysical Research Letters] have combined Voyager data with observations from NASA’s Cassini mission, which orbited Saturn from 2004 to 2017, to gain more insight into this region of space. The researchers recognized that the missions, although launched 20 years apart, had collected complementary data. Voyager 1 and 2 had instruments that measured energetic ions as the craft crossed the heliosheath and exited the solar system. Cassini, meanwhile, was able to remotely observe energetic neutral atoms (ENAs) arriving in all directions from the heliosheath.

These two phenomena are related: ENAs come from the heliosheath, where fast solar wind protons collide with neutral hydrogen atoms from interstellar space and “steal” an electron from the interlopers. The Voyager probes took in situ measurements of the parent heliosheath proton distributions as they passed through this region. Meanwhile, the protons with newly added electrons become ENAs and zip off in all directions.

The synergy among the spacecrafts’ observations allowed the researchers to use Voyager data from the heliosheath to ground truth and calibrate ENA data from Cassini, which was more sensitive to lower energetic particles than Voyager was. Together, the spacecraft extended data on the intensity of both ENAs and ions to include a broader range of energies, which gave the team a window into the physics in the heliosheath as the solar wind and interstellar medium press against each other.

The researchers found that in the energy range considered in their study (>5 kiloelectron volts), lower-energy ions with energies between about 5 and 24 kiloelectron volts played the largest role in maintaining the pressure balance inside the heliosheath. This allowed the team to calculate the strength of the magnetic field and the density of neutral hydrogen atoms in interstellar space—about 0.5 nanotesla and 0.12 per cubic centimeter, respectively.

On the basis of calculations from Voyager 2 data, the researchers predict that the heliopause, the outer boundary of the heliosheath, is located roughly 18 billion kilometers from the Sun, or 119 times the distance from the Sun to the Earth—right where Voyager 2 found it in November 2018.

Furthermore, the finding that the lower-energy ions dominate the pressure balance in the heliosheath means that space physicists will have to rethink their assumptions about the energy distribution of such particles in the heliosheath.

See the full article here .

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From Asgardia via Medium: “Which Spacecraft Will Reach Interstellar Space Next?”

From Asgardia

via

Medium

NASA’s Voyager 2spacecraft reached interstellar space in December 2018, following in the footsteps of its sister, Voyager 1. Currently, only five spacecraft have been launched that can make such a grand exit, including the Voyagers. The other three are Pioneers 10 and 11, and New Horizons. Which one will make a great escape next?

NASA/Voyager 2

NASA/Voyager 1

NASA Pioneer 10

NASA Pioneer 11

NASA/New Horizons spacecraft

Reaching interstellar space is a milestone that is thought of as leaving the solar system by a specific definition. In 1990, the New York Times reported that Pioneer left the solar system when it flew past Neptune’s orbit. But that’s not what Voyager 2’s scientists used as their definition. Instead, the more recent measurements said the crossing of the sun’s heliopause, the theoretical boundary to its heliosphere, is the determining factor for entering interstellar space.

Heliopause https://galnet.fandom.com/wikiHeliopause

The heliosphere is a bubble of charged particles that are created by and flows past the sun. It is used by scientists to mark where interstellar space starts.

NASA Heliosphere

However, the heliosphere is tricky, and there are many changes such as the sun’s 22-year solar cycle, the shrinking and growing with the solar wind, and stretching out behind the sun in the star’s direction of travel. It’s not something that can be measured easily from Earth. Thus, NASA’s Interstellar Boundary Explorer (IBEX) mission is trying to define the edges of the bubble remotely.

Observations from the Voyager probes’ indicate that they’ve pierced this bubble. However, since researchers think the Oort Cloud also surrounds the sun, an area of icy bodies that is estimated to stretch from 1,000 to 100,000 astronomical units — far beyond the heliopause — the Voyager probes cannot be considered entirely outside the solar system. (One astronomical unit, or AU, is the distance between the Earth and the sun — 93 million miles, or 150 million kilometres).

Oort cloud Image by TypePad, http://goo.gl/NWlQz6

Oort Cloud, The layout of the solar system, including the Oort Cloud, on a logarithmic scale. Credit: NASA, Universe Today

When Voyager 1 and 2 crossed the heliopause, their still-working particle instruments unveiled the historical events. The heliosphere functions as a shield, keeping out many of the higher-energy particles created by the cosmic rays generated by other stars.

Magnetosphere of Earth, original bitmap from NASA. SVG rendering by Aaron Kaase

By tracking both the low-energy particles found inside the solar system and the high-energy particles from outside of it, the instruments could reveal a sudden surge of cosmic rays alerting scientists that the spacecraft had left the solar system.

The ever-changing nature of the heliosphere makes it impossible to tell when Pioneer 10 and 11 will enter interstellar space. It’s even possible that one of them may have already.

As per NASA’s e-book Beyond Earth: A Chronicle of Deep Space Exploration, from Nov. 5, 2017, Pioneer 10 was approximately 118.824 AUs from Earth, farther than any craft besides Voyager 1. H(?), Although Pioneer 11 and the Voyager twins were all heading in the direction of the sun’s apparent travel, Pioneer 10 is headed toward the trailing side. 2017 research showed that the tail of the heliosphere is around 220 AU from the sun. Since Pioneer 10 travels about 2.5 AU/year, it will take Pioneer until roughly 2057–40 years — to reach the changing boundary.

Pioneer 11 was thought to be approximately 97.6 AUs from Earth as of Nov. 5, 2017, according to the same e-book. Unlike its twin, the spacecraft is travelling in about the same direction as the Voyagers. Voyager 2 crossed into interstellar medium at approximately 120 AUs. Since Pioneer 11 is moving at 2.3 AU/year, it should reach interstellar space in about eight years, around 2027 — assuming the boundary doesn’t change, which it probably will.

On Jan. 1, 2019, New Horizons made its most recent flyby of a solar system object, and it was launched much later than the other four. During this flyby, New Horizons was 43 AU from the sun. The mission’s principal investigator, Alan Stern, told Space.com that the spacecraft was travelling approximately 3.1 AU each year, or 31 AU in ten years. In another two decades, the spacecraft has a good chance of reaching interstellar space. If New Horizons crossed at Voyager 2’s same border (it won’t, but just consider as a baseline), it would make the trip in just under 24 years, in 2043. But it’s possible the ISM line will move inward, allowing it to cross sooner.

Although there won’t be a direct confirmation of crossing the heliopause with the Pioneer spacecraft, it’s possible that New Horizons will still be working, and will give us a detailed study of interstellar space. The particle detectors that it holds are much more potent than the ones on Voyager, Stern said. Moreover, New Horizons holds a dust detector that would offer insight into the area beyond the heliosphere.

However, whether or not they will still be functioning remains to be seen. As per Stern, power is the limiting factor. New Horizons runs off of decaying plutonium dioxide. Presently, the spacecraft has enough power to work until the late 2030s, said Stern, and it is currently in good working order.

If in the unlikely event that the ever-changing heliosphere remains static Pioneer 11 will be the next to cross the heliopause in 2027, followed by New Horizons in 2043. Pioneer 10, the first of the five spacecraft to launch, will be the last to leave the heliosphere, in 2057. Once again, this assumes the extremely unrealistic chance that the heliopause remaining static for the next four decades.

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From JPL-Caltech: “NASA’s Voyager 2 Probe Enters Interstellar Space”

From JPL-Caltech

Dec. 10, 2018

Dwayne Brown
Headquarters, Washington
202-358-1726 / 301-286-6284
dwayne.c.brown@nasa.gov

Karen Fox
Headquarters, Washington
301-286-6284
karen.c.fox@nasa.gov

Calla Cofield
Jet Propulsion Laboratory, Pasadena, Calif.
626-808-2469
calla.e.cofield@jpl.nasa.gov

This illustration shows the position of NASA’s Voyager 1 and Voyager 2 probes, outside of the heliosphere, a protective bubble created by the Sun that extends well past the orbit of Pluto.

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

NASA/Voyager 2

Members of NASA’s Voyager team will discuss the findings at a news conference at 11 a.m. EST (8 a.m. PST) today at the meeting of the American Geophysical Union (AGU) in Washington. The news conference will stream live on the agency’s website.

Comparing data from different instruments aboard the trailblazing spacecraft, mission scientists determined the probe crossed the outer edge of the heliosphere on Nov. 5. This boundary, called the heliopause, is where the tenuous, hot solar wind meets the cold, dense interstellar medium. Its twin, Voyager 1, crossed this boundary in 2012, but Voyager 2 carries a working instrument that will provide first-of-its-kind observations of the nature of this gateway into interstellar space.

NASA/Voyager 1

Voyager 2 now is slightly more than 11 billion miles (18 billion kilometers) from Earth. Mission operators still can communicate with Voyager 2 as it enters this new phase of its journey, but information – moving at the speed of light – takes about 16.5 hours to travel from the spacecraft to Earth. By comparison, light traveling from the Sun takes about eight minutes to reach Earth.

Artist’s concept of Voyager 2 with 9 facts listed around it. Image Credit: NASA

The most compelling evidence of Voyager 2’s exit from the heliosphere came from its onboard Plasma Science Experiment (PLS), an instrument that stopped working on Voyager 1 in 1980, long before that probe crossed the heliopause. Until recently, the space surrounding Voyager 2 was filled predominantly with plasma flowing out from our Sun. This outflow, called the solar wind, creates a bubble – the heliosphere – that envelopes the planets in our solar system. The PLS uses the electrical current of the plasma to detect the speed, density, temperature, pressure and flux of the solar wind. The PLS aboard Voyager 2 observed a steep decline in the speed of the solar wind particles on Nov. 5. Since that date, the plasma instrument has observed no solar wind flow in the environment around Voyager 2, which makes mission scientists confident the probe has left the heliosphere.

Animated gif showing the plasma data. Image Credit: NASA/JPL-Caltech

“Working on Voyager makes me feel like an explorer, because everything we’re seeing is new,” said 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.”

In addition to the plasma data, Voyager’s science team members have seen evidence from three other onboard instruments – the cosmic ray subsystem, the low energy charged particle instrument and the magnetometer – that is consistent with the conclusion that Voyager 2 has crossed the heliopause. Voyager’s team members are eager to continue to study the data from these other onboard instruments to get a clearer picture of the environment through which Voyager 2 is traveling.

“There is still a lot to learn about the region of interstellar space immediately beyond the heliopause,” said Ed Stone, Voyager project scientist based at Caltech in Pasadena, California.

Together, the two Voyagers provide a detailed glimpse of how our heliosphere interacts with the constant interstellar wind flowing from beyond. Their observations complement data from NASA’s Interstellar Boundary Explorer (IBEX), a mission that is remotely sensing that boundary. NASA also is preparing an additional mission – the upcoming Interstellar Mapping and Acceleration Probe (IMAP), due to launch in 2024 – to capitalize on the Voyagers’ observations.

“Voyager has a very special place for us in our heliophysics fleet,” said Nicola Fox, director of the Heliophysics Division at NASA Headquarters. “Our studies start at the Sun and extend out to everything the solar wind touches. To have the Voyagers sending back information about the edge of the Sun’s influence gives us an unprecedented glimpse of truly uncharted territory.”

While the probes have left the heliosphere, Voyager 1 and Voyager 2 have not yet left the solar system, and won’t be leaving anytime soon. The boundary of the solar system is considered to be beyond the outer edge of the Oort Cloud, a collection of small objects that are still under the influence of the Sun’s gravity.

Oort Cloud NASA

The width of the Oort Cloud is not known precisely, but it is estimated to begin at about 1,000 astronomical units (AU) from the Sun and to extend to about 100,000 AU. One AU is the distance from the Sun to Earth. It will take about 300 years for Voyager 2 to reach the inner edge of the Oort Cloud and possibly 30,000 years to fly beyond it.

The Voyager probes are powered using heat from the decay of radioactive material, contained in a device called a radioisotope thermal generator (RTG). The power output of the RTGs diminishes by about four watts per year, which means that various parts of the Voyagers, including the cameras on both spacecraft, have been turned off over time to manage power.

“I think we’re all happy and relieved that the Voyager probes have both operated long enough to make it past this milestone,” said Suzanne Dodd, Voyager project manager at NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, California. “This is what we’ve all been waiting for. Now we’re looking forward to what we’ll be able to learn from having both probes outside the heliopause.”

Voyager 2 launched in 1977, 16 days before Voyager 1, and both have traveled well beyond their original destinations. The spacecraft were built to last five years and conduct close-up studies of Jupiter and Saturn. However, as the mission continued, additional flybys of the two outermost giant planets, Uranus and Neptune, proved possible. As the spacecraft flew across the solar system, remote-control reprogramming was used to endow the Voyagers with greater capabilities than they possessed when they left Earth. Their two-planet mission became a four-planet mission. Their five-year lifespans have stretched to 41 years, making Voyager 2 NASA’s longest running mission.

The Voyager story has impacted not only generations of current and future scientists and engineers, but also Earth’s culture, including film, art and music. Each spacecraft carries a Golden Record of Earth sounds, pictures and messages.

NASA Voyager Golden Record

Since the spacecraft could last billions of years, these circular time capsules could one day be the only traces of human civilization.

Voyager’s mission controllers communicate with the probes using NASA’s Deep Space Network (DSN), a global system for communicating with interplanetary spacecraft. The DSN consists of three clusters of antennas in Goldstone, California; Madrid, Spain; and Canberra, Australia.

NASA Deep Space Network dish, Goldstone, CA, USA

NASA Canberra, AU, Deep Space Network

NASA Deep Space Network Madrid Spain

The Voyager Interstellar Mission is a part of NASA’s Heliophysics System Observatory, sponsored by the Heliophysics Division of NASA’s Science Mission Directorate in Washington. JPL built and operates the twin Voyager spacecraft. NASA’s DSN, managed by JPL, is an international network of antennas that supports interplanetary spacecraft missions and radio and radar astronomy observations for the exploration of the solar system and the universe. The network also supports selected Earth-orbiting missions. The Commonwealth Scientific and Industrial Research Organisation, Australia’s national science agency, operates both the Canberra Deep Space Communication Complex, part of the DSN, and the Parkes Observatory, which NASA has been using to downlink data from Voyager 2 since Nov. 8.

For more information about the Voyager mission, visit:

https://www.nasa.gov/voyager

More information about NASA’s Heliophysics missions is available online at:

https://www.nasa.gov/sunearth

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Jet Propulsion Laboratory (JPL) is a federally funded research and development center and NASA field center located in the San Gabriel Valley area of Los Angeles County, California, United States. Although the facility has a Pasadena postal address, it is actually headquartered in the city of La Cañada Flintridge, on the northwest border of Pasadena. JPL is managed by the nearby California Institute of Technology (Caltech) for the National Aeronautics and Space Administration. The Laboratory’s primary function is the construction and operation of robotic planetary spacecraft, though it also conducts Earth-orbit and astronomy missions. It is also responsible for operating NASA’s Deep Space Network.

From Goddard: “NASA’s Cassini, Voyager Missions Suggest New Picture of Sun’s Interaction with Galaxy”

NASA Goddard Space Flight Center

April 24, 2017
Sarah Frazier
sarah.frazier@nasa.gov
NASA’s Goddard Space Flight Center, Greenbelt, Md.

New data from NASA’s Cassini mission, combined with measurements from the two Voyager spacecraft and NASA’s Interstellar Boundary Explorer, or IBEX, suggests that our sun and planets are surrounded by a giant, rounded system of magnetic field from the sun — calling into question the alternate view of the solar magnetic fields trailing behind the sun in the shape of a long comet tail.

NASA/Voyager 1

NASA/ESA/ASI Cassini Spacecraft

NASA/IBEX

The sun releases a constant outflow of magnetic solar material — called the solar wind — that fills the inner solar system, reaching far past the orbit of Neptune. This solar wind creates a bubble, some 23 billion miles across, called the heliosphere. Our entire solar system, including the heliosphere, moves through interstellar space. The prevalent picture of the heliosphere was one of comet-shaped structure, with a rounded head and an extended tail. But new data covering an entire 11-year solar activity cycle show that may not be the case: the heliosphere may be rounded on both ends, making its shape almost spherical. A paper on these results was published in Nature Astronomy on April 24, 2017.

“Instead of a prolonged, comet-like tail, this rough bubble-shape of the heliosphere is due to the strong interstellar magnetic field — much stronger than what was anticipated in the past — combined with the fact that the ratio between particle pressure and magnetic pressure inside the heliosheath is high,” said Kostas Dialynas, a space scientist at the Academy of Athens in Greece and lead author on the study.

New data from NASA’s Cassini, Voyager and Interstellar Boundary Explorer missions show that the heliosphere — the bubble of the sun’s magnetic influence that surrounds the inner solar system — may be much more compact and rounded than previously thought. The image on the left shows a compact model of the heliosphere, supported by this latest data, while the image on the right shows an alternate model with an extended tail. The main difference is the new model’s lack of a trailing, comet-like tail on one side of the heliosphere. This tail is shown in the old model in light blue.
Credits: Dialynas, et al. (left); NASA (right)

An instrument on Cassini, which has been exploring the Saturn system over a decade, has given scientists crucial new clues about the shape of the heliosphere’s trailing end, often called the heliotail. When charged particles from the inner solar system reach the boundary of the heliosphere, they sometimes undergo a series of charge exchanges with neutral gas atoms from the interstellar medium, dropping and regaining electrons as they travel through this vast boundary region. Some of these particles are pinged back in toward the inner solar system as fast-moving neutral atoms, which can be measured by Cassini.

“The Cassini instrument was designed to image the ions that are trapped in the magnetosphere of Saturn,” said Tom Krimigis, an instrument lead on NASA’s Voyager and Cassini missions based at Johns Hopkins University’s Applied Physics Laboratory in Laurel, Maryland, and an author on the study. “We never thought that we would see what we’re seeing and be able to image the boundaries of the heliosphere.”

Many other stars show tails that trail behind them like a comet’s tail, supporting the idea that our solar system has one too. However, new evidence from NASA’s Cassini, Voyager and Interstellar Boundary Explorer missions suggest that the trailing end of our solar system may not be stretched out in a long tail. From top left and going counter clockwise, the stars shown are LLOrionis, BZ Cam and Mira. Credits: NASA/HST/R.Casalegno/GALEX

Because these particles move at a small fraction of the speed of light, their journeys from the sun to the edge of the heliosphere and back again take years. So when the number of particles coming from the sun changes — usually as a result of its 11-year activity cycle — it takes years before that’s reflected in the amount of neutral atoms shooting back into the solar system.

Cassini’s new measurements of these neutral atoms revealed something unexpected — the particles coming from the tail of the heliosphere reflect the changes in the solar cycle almost exactly as fast as those coming from the nose of the heliosphere.

“If the heliosphere’s ‘tail’ is stretched out like a comet, we’d expect that the patterns of the solar cycle would show up much later in the measured neutral atoms,” said Krimigis.

The heliosphere is the bubble-like region of space dominated by the Sun, which extends far beyond the orbit of Pluto. Plasma “blown” out from the Sun, known as the solar wind, creates and maintains this bubble against the outside pressure of the interstellar medium, the hydrogen and helium gas that permeates the Milky Way Galaxy. The solar wind flows outward from the Sun until encountering the termination shock, where motion slows abruptly. The Voyager spacecraft have actively explored the outer reaches of the heliosphere, passing through the shock and entering the heliosheath, a transitional region which is in turn bounded by the outermost edge of the heliosphere, called the heliopause. The overall shape of the heliosphere is controlled by the interstellar medium through which it is traveling, as well as the Sun, and is not perfectly spherical.[1] The limited data available and unexplored nature[2] of these structures have resulted in many theories.
Wikipedia.

But because patterns from solar activity show just as quickly in tail particles as those from the nose, that implies the tail is about the same distance from us as the nose. This means that long, comet-like tail that scientists envisioned may not exist at all — instead, the heliosphere may be nearly round and symmetrical.

A rounded heliosphere could come from a combination of factors. Data from Voyager 1 show that the interstellar magnetic field beyond the heliosphere is stronger than scientists previously thought, meaning it could interact with the solar wind at the edges of the heliosphere and compact the heliosphere’s tail.

The structure of the heliosphere plays a big role in how particles from interstellar space — called cosmic rays — reach the inner solar system, where Earth and the other planets are.

“This data that Voyager 1 and 2, Cassini and IBEX provide to the scientific community is a windfall for studying the far reaches of the solar wind,” said Arik Posner, Voyager and IBEX program scientist at NASA Headquarters in Washington, D.C., who was not involved with this study.

“As we continue to gather data from the edges of the heliosphere, this data will help us better understand the interstellar boundary that helps shield the Earth environment from harmful cosmic rays.”

See the full article here.

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NASA’s Goddard Space Flight Center is home to the nation’s largest organization of combined scientists, engineers and technologists that build spacecraft, instruments and new technology to study the Earth, the sun, our solar system, and the universe.

Named for American rocketry pioneer Dr. Robert H. Goddard, the center was established in 1959 as NASA’s first space flight complex. Goddard and its several facilities are critical in carrying out NASA’s missions of space exploration and scientific discovery.

NASA/Goddard Campus