What are the long-lasting waves detected by Voyager 1? Our first working interstellar probe — admittedly never designed for that task — is operating beyond the heliosphere, which it exited back in 2012. A paper just published in Nature Astronomy explores what’s going in interstellar space just beyond, but still affected by, the heliosphere’s passage through the Local Interstellar Medium (LISM).
We have a lot to learn out here, for even as we exit the heliosphere, the picture is complex. The so-called Local Bubble is a low-density region of hot plasma in the interstellar medium, the environment of radiation and matter — gas and dust — that exists between the stars. Within this ‘bubble’ exists the Local Interstellar Cloud (LIC), about 30 light years across, with a slightly higher hydrogen density flowing from the direction of Scorpius and Centaurus. The Sun seems to be within the LIC near its boundary with the G-cloud complex, where the Alpha Centauri stars reside.
Image: Map of the local galactic neighborhood showing the Sun located near the edge of our local interstellar cloud (LIC). Alpha-Centauri is located just over 4 light-years away in the neighboring G-cloud complex. Outside these clouds, the density may be lower than 0.001 atoms/cc. Our Sun and the LIC have a relative velocity of 26 km/sec. Credit: JPL.
But if the interstellar medium is a sparse collection of widely spaced particles and radiation, it proves to be anything but quiet. We learn this from Voyager 1’s Plasma Wave Subsystem, which involves two antennae extending 30 meters from the spacecraft (see image below). What the PWS can pick up are clues to the density of the medium that show up in the form of waves. Some are produced by the rotation of the galaxy; others by supernova explosions, with smaller effects from the Sun’s own activity.
Vibrations of the ionized gas — plasma — in the interstellar medium have been detectable since late 2012 by Voyager 1 in the form of ‘whistles’ that show up only occasionally, but offer ways to study the density of the medium. The new work in Nature Astronomy, led by Stella Koch Ocker (Cornell University), sets about finding a more consistent measure of interstellar medium density in the Voyager data.
Image: An illustration of NASA’s Voyager spacecraft showing the antennas used by the Plasma Wave Subsystem and other instruments. Credit: NASA/JPL-Caltech.
A weak signal appearing at the same time as a ‘whistle’ in the 2017 Voyager data seems to have been the key finding. Ocker describes it as “very weak but persistent plasma waves in the very local interstellar medium.” When whistles appear in the data, the tone of this plasma wave emission rises and falls with them. Adds Ocker:
“It’s virtually a single tone. And over time, we do hear it change – but the way the frequency moves around tells us how the density is changing. This is really exciting, because we are able to regularly sample the density over a very long stretch of space, the longest stretch of space that we have so far. This provides us with the most complete map of the density and the interstellar medium as seen by Voyager.”
So we have an extremely useful instrument, Voyager 1’s Plasma Wave Subsystem, continuing to return data with increasing distance from the Sun. Analyzing the data over time, we learn that the electron density around the spacecraft began rising in 2013, just after its exit from the heliosphere, and reached current levels in 2015. These levels, which persist to the end of 2020 through the dataset, show a 40-fold increase in electron density. Up next for Ocker and team is the development of a physical model of the plasma wave emission that will offer insights into its proper interpretation.
As we begin to think seriously about interstellar probes in this century, it’s striking how much we have to learn about the medium through which they will pass. Voyager 1 is helping us learn about conditions immediately outside the heliosphere. A probe sent to Alpha Centauri will need to cross the boundary between the Local Interstellar Cloud and the G-cloud, a region we have yet to penetrate. The nature of and variation within the interstellar medium will require continuing work with our admittedly sparse data.
The paper is Ocker et al., “Persistent plasma waves in interstellar space detected by Voyager 1,” Nature Astronomy 10 May 2021. Abstract / Preprint.
Could these resources be useful to you? Particularly the “top-down” version of the map?
I was curious about where the Voyagers are going. According to NASA/JPL:
Voyager 2 is escaping the solar system at a speed of about 3.1 AU per year, 48 degrees out of the ecliptic plane to the south toward the constellations of Sagittarius and Pavo. In about 40,000 years, Voyager 2 will come within about 1.7 light years of a star called Ross 248, a small star in the constellation of Andromeda…
Now, finding where these stars are at the moment proved challenging, but using the 3D 10 parsecs map and orienting it from the top so Altair is to the upper right Gliese 445 is a star labelled G 254 29, right at the 6 pc line to the Sun’s upper left. Ross 248 is also to our upper right, but at ca. 2.5 pc. This is the same orientation shown on the zoomable top down 10 parsecs map. Gliese 445 is left out of that one, however, perhaps due to its being too far out on the Z axis.
This is the current position of these stars of course, they will encounter the Voyagers when they are much closer – indeed, they will both be among the closest stars to the Sun within the next 100k years, which is a puzzling coincidence.
So am I correct in interpreting this as indicating that the Voyagers aren’t moving much of anywhere in the direction of the Alpha Centauri system? Voy1 is explicitly stated to be moving in the same direction as the Sun, i.e., towards Altair, according to the small map shown above. And Voy2 is doing the same, but down out of the ecliptic and more in the direction of the Galactic center?
Neither seems to be sampling the medium of what lies in the direction of our nearest neighbors, right?
That’s right, Lucy. I was only using the Alpha Centauri situation because we’d like to send a probe there, and to point out that learning more about the interstellar medium would be necessary for such a mission. But I didn’t mean to imply that the Voyager data were related to the route to the Centauri stars.
Actually it seems Voy 2 is heading roughly in the direction of A Cen: Hubble Provides Interstellar Road Map for Voyagers’ Galactic Trek | NASA
This shows 4 guidestars Hubble is using to chart what’s along the probes’ trajectories. For Voy 2 only one of them shows up on the 10 pc 3D map,, GJ 780, which turns out to be Delta Pavonis. Thus Voy 2’s path seems to be quite close to the A Cen system, at least in the X+Y dimensions. I read once that a probe sent their way would need no small amount of extra delta V to get where it’s going, that a probe to somewhere else further away but not so far out of the ecliptic might actually be an easier feat to accomplish. No doubt this has been discussed here.
If only we had a longer lifetime for the Voyagers, whose days of active data-return are rapidly dwindling!
I’m not entirely clear what we have learned from this. Voyager 1 is measuring a thin transect in a particular direction. We already know that the ISM is turbulent just beyond teh edge of the heliosphere. We had an approximate estimate of the density of the ISM, but now the narrow bandwidth signal around 3 kHz allows us to estimate the ISM density with finer granularity along that transect.
But if we are planning a hypothetical starship voyage in the direction of Alpha-Cen, what new information does this buy us? It seems to me that if this was a sea voyage, we would want detailed 2D, even 3D maps of the ocean, but our prevoyage survey only supplied some feature of the ocean along a line in one direction and not the one we thought our destination lay.
Given the interest in this story, I feel I must be missing something. Can someone explain what that is?
As explorers of new worlds, we are in pretty much the same position as our ancestors five centuries ago when they fitted out the ships to round the horn of Africa or to cross to the New World.
As insufficient and fragmentary as they may be, our charts and sailing directions are much more complete and detailed than theirs were. The Polynesian seafarers sailed across the Pacific with even less preparation and resources than we, or the sailors of the Age of Discovery, ever had available to them.
I’m a navigator myself, I can sail a ship to any port on the planet I wish to visit using only a compass, chart, sextant and almanac. But my own pride at having mastered these skills is tempered by the realization that our exploring ancestors set off with much less than that. Their ignorance of what lay before them was no obstacle to their going; on the contrary, it was the reason they had to.
I fear that there may be biological, natural or technological barriers to interstellar voyaging that we do not know. But partial, or even complete, ignorance of what lies on our journeys or at their ends should not stop us from going. And if our tools and preparations prove to be inadequate, then so be it. Improvising on insufficient or incorrect information is what we do best.
In the case of the Polynesian navigators, the skills bordered on the supernatural. They could estimate water speed from the pattern and movement of waves; wind speed from tactile sensations; boat speed from the appearance of the wake, and the islands beyond the horizon from the alteration of lighting in clouds near the horizon by reflected light from those islands.
The skills were derived from acutely tuned physical senses. The data extracted from interstellar space is not directly accessible to human senses. Managing the computerized crunching of data will be within the purview of humans (within hailing distance) but that won’t work if the communication lag is more than minutes. Hopefully AI will have enough of both lizard brain and monkey brain hardwired or programmed into it.
Talking about “alteration of lighting”… When I was a boot petty officer in the Navy, our frigate was having a difficult time finding Midway Island. We had to locate it to top off on bunker oil. This was in the pre-GPS era, and we had had a stretch of bad weather–no celestial fixes for a few days, and only one sun line of position that morning. We were approaching the island along the LORAN ‘blind spot’ and the land was too low (and below the horizon) to show up on radar. We only had a vague idea where we were.
Our navigator, a salty old QM1 took me to one side and pointed out the color shading on the underside of the cumulus clouds on the horizon. I hadn’t noticed it, but after he mentioned it it became obvious the bottoms of the clouds were a subtle shade of blue, reflecting the deep blue sea beneath them. But one of them was just a hint of the palest green.
“Midway’s a coral atoll, with a big, shallow, sandy lagoon. The sea is green there…”
From Wikipedia:
“In cool, dense regions of the ISM, matter is primarily in molecular form, and reaches number densities of 10? molecules per cm³ (1 million molecules per cm³)”.
“Compare this with a number density of roughly 10¹? molecules per cm³ for air at sea level, and 10¹? molecules per cm³ (10 billion molecules per cm³) for a laboratory high-vacuum chamber.”
“By mass, 99% of the ISM is gas in any form, and 1% is dust.”
Over the course of light years of travel, it must add up to some significance, and is therefore worth knowing about.
By human standards, it’s nothing at all. Wikipedia goes into an abstruse, almost arcane description of how waves propagate through that “nothing at all”, and not having that much math & physics (wasn’t required for my specialization at my time & place), I have to take them at their word. But this, like a few other items based on physics, seem like gaps in the world-view.
Given that these “clouds” are what everyone else would call a hard vacuum, I’ve always thought they should have a Symposium on the Local Interstellar Medium titled “Much Ado About Nothing”
This discovery reveals two things, one of practical use, and the other a double mystery.
That tone sits right at the (electron) Langmuir plasma frequency, which depends on the number of electrons per cubic centimeter. Plasma waves won’t propagate at frequencies below that; what’s strange here is that there is a wave phenomena confined to that frequency. Why this is of practical use is that it’s continuous, so they don’t have to wait for occasional events (“whistlers”) to determine the plasma frequency, and thus the density, but can do so continuously. (It’s not really too surprising that that density varies up and down some.) Why this is doubly mysterious is that it indicates that something is continually putting wave power into the ISM (and we don’t know what that is) and that that power is either confined or trapped at the plasma frequency (and we don’t know why that is).
This, from the companion paper
https://iopscience.iop.org/article/10.3847/1538-4357/abeb6a
struck my eye:
“In reality, since it now requires an array of four Deep Space Stations (DSS) (one 70 m and three 34 m antennas) at one of the Deep Space Network (DSN) complexes to achieve a usable signal-to-noise ratio to successfully transmit the wideband data, the ability to schedule the DSN for the playback is difficult and does not always occur before older data is overwritten on board.”
The Voyagers are literally beginning to fade into the dark; it takes basically all of the antenna area present at a DSN complex to fully receive their transmissions.
Where this may give us interesting results is if Voyager spacecraft come near enough to any object with a magnetic field. Rogue planets, brown dwarfs or any such object would cause large scale effects on the plasma waves from much further distances then the magnetosphere in the solar system. The suns solar wind encapsules earth’s and the gas giants but in interstellar space the magnetosphere expand to huge scales. The wake from such objects may be hundreds of times larger and would be observed as changes in the plasma waves, like solitons that form from the wake of the bow of a ship.
The correct link from Wikipedia: Interstellar medium
A bit further than Voyager’s current position (but still in the local cloud) we reach the Inner Oort Cloud. And I just noticed that Mike Brown and Konstantin Batygin have finished a new paper, still trying to pin down their Planet Nine, in light of the updated census of KBOs, and posted about (on the slowest-updated blog in the world) here: http://findplanetnine.blogspot.com/2021/04/the-inner-oort-cloud-connection.html In case you would like to write anything about Planet Nine one day Paul!
Yes, Planet Nine is always of interest. We’ll be talking about new work in this area soon.
A testament to ingenuity of Voyager space probes who continue to bring us amazing discoveries decade after decade.
Readers might be interested in proposal by Chinese space scientists to launch two probes studying Jupiter, Neptune, heliosphere and interstellar medium. The proposed launch date seems ambitious(2024) and they would, if launched, reach 100 AU in 2049.
https://spacenews.com/china-to-launch-a-pair-of-spacecraft-towards-the-edge-of-the-solar-system/
It’s a good sign that exploration of interstellar space is now being considered by other space agencies besides NASA, increasing opportunities for discoveries and research.
A sentence hear the end of the article says: “A probe sent to Alpha Centauri will need to cross the boundary between the Local Interstellar Cloud and the B-cloud, a region we have yet to penetrate. ” Should “B-cloud” really be “G-cloud”?
Good catch! My typo, and I’ll correct it now.
A reminder of an earlier Centauri Dreams article on aiming the Voyager probes at certain stars to increase their chances of being found one day:
https://centauri-dreams.org/2015/12/24/voyager-to-a-star-2/
We only have until 2030, if we are lucky, to perform these maneuvers. We must not forget that they are our ambassadors into the wider Milky Way galaxy.
https://phys.org/news/2021-05-milky-unusual-astronomers.html
MAY 24, 2021
Milky Way not unusual, astronomers find
by ARC Centre of Excellence for All Sky Astrophysics in 3D (ASTRO 3D)
The first detailed cross-section of a galaxy broadly similar to the Milky Way, published today, reveals that our galaxy evolved gradually, instead of being the result of a violent mash-up. The finding throws the origin story of our home into doubt.
The galaxy, dubbed UGC 10738, turns out to have distinct ‘thick’ and ‘thin’ discs similar to those of the Milky Way. This suggests, contrary to previous theories, that such structures are not the result of a rare long-ago collision with a smaller galaxy. They appear to be the product of more peaceful change.
And that is a game-changer. It means that our spiral galaxy home isn’t the product of a freak accident. Instead, it is typical.
The finding was made by a team led by Nicholas Scott and Jesse van de Sande, from Australia’s ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D) and the University of Sydney.
“Our observations indicate that the Milky Way’s thin and thick discs didn’t come about because of a gigantic mash-up, but a sort-of ‘default’ path of galaxy formation and evolution,” said Dr. Scott.
“From these results we think galaxies with the Milky Way’s particular structures and properties could be described as the ‘normal’ ones.”
This conclusion—published in The Astrophysical Journal Letters—has two profound implications.
“It was thought that the Milky Way’s thin and thick discs formed after a rare violent merger, and so probably wouldn’t be found in other spiral galaxies,” said Dr. Scott.
“Our research shows that’s probably wrong, and it evolved ‘naturally’ without catastrophic interventions. This means Milky Way-type galaxies are probably very common.
“It also means we can use existing very detailed observations of the Milky Way as tools to better analyze much more distant galaxies which, for obvious reasons, we can’t see as well.”
The research shows that UGC 10738, like the Milky Way, has a thick disc consisting mainly of ancient stars—identified by their low ratio of iron to hydrogen and helium. Its thin disc stars are more recent and contain more metal.
(The sun is a thin-disc star and comprises about 1.5% elements heavier than helium. Thick disc stars have three to 10 times less.)
Although such discs have been previously observed in other galaxies, it was impossible to tell whether they hosted the same type of star distribution—and therefore similar origins.Scott, van de Sande and colleagues solved this problem by using the European Southern Observatory’s Very Large Telescope in Chile to observe UGC 10738, situated 320 million light years away.
The galaxy is angled “edge on,” so looking at it offered effectively a cross-section of its structure.
“Using an instrument called the multi-unit spectroscopic explorer, or MUSE, we were able to assess the metal ratios of the stars in its thick and thin discs,” explained Dr. van de Sande.
“They were pretty much the same as those in the Milky Way—ancient stars in the thick disc, younger stars in the thin one. We’re looking at some other galaxies to make sure, but that’s pretty strong evidence that the two galaxies evolved in the same way.”
Dr. Scott said UGC 10738’s edge-on orientation meant it was simple to see which type of stars were in each disc.
“It’s a bit like telling apart short people from tall people,” he said. “It you try to do it from overhead it’s impossible, but it if you look from the side it’s relatively easy.”
Co-author Professor Ken Freeman from the Australian National University said, “This is an important step forward in understanding how disk galaxies assembled long ago. We know a lot about how the Milky Way formed, but there was always the worry that the Milky Way is not a typical spiral galaxy. Now we can see that the Milky Way’s formation is fairly typical of how other disk galaxies were assembled.”
ASTRO 3D director, Professor Lisa Kewley, added: “This work shows how the Milky Way fits into the much bigger puzzle of how spiral galaxies formed across 13 billion years of cosmic time.”
Foiled again…
https://www.businessinsider.com/voyager-kitchen-aluminum-wrap-radiation-short-circuit-2017-9