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Nasa's Voyager 2 probe 'leaves the Solar System' (bbc.co.uk)
405 points by grahamel on Dec 10, 2018 | hide | past | favorite | 100 comments

My all-time favorite graph is the plot of incident solar particles on Voyager 1 in 2012 [0][1]. That to me is the perfect image of scientific thinking. "We anticipate that the probe will enter interstellar space and see a significant drop in the number of incident solar particles." And then it happens, clear as day.

I teach high school math and science, and I have shown this graph to students every year. Many students get what this really means. They suddenly understand how far Voyager is from us, and how small and isolated our solar system really is in the universe.

Now it will be fun to show them the same graph from Voyager 2 [2][3]!

[0] https://3c1703fe8d.site.internapcdn.net/newman/gfx/news/hire...

[1] https://phys.org/news/2012-10-voyager-left-solar.html

[2] http://earth-chronicles.com/wp-content/uploads/2018/11/voyaz...

[3] http://earth-chronicles.com/space/voyager-2-is-even-closer-t...

Thanks for these graphs!

Your Voyager 1 graph [footnote 0, in the parent comment] shows two periods of particles-per-second drops to around 10 particles per second followed by a rebound and then a dramatic drop to 2 to 3 particles per second.

The Voyager 2 graph [footnote 2, in the parent comment] shows a single drop to around 17 to 18 particles per second and what looks like the beginning of a rebound. Wouldn't interstellar space be characterized by the drop to below 3 particles per second?

Is this the same issue we had with Voyager 1 where Voyager 2 is beginning the process of entering interstellar space but has not yet actually entered it? If not, why are the particle counts about an order-of-magnitude higher for Voyager 2 then they were for Voyager 1 in interstellar space?

My first reaction when I saw this - probably a reaction due to previous repeated news about Voyager 1 leaving - was "But hasn't it already left, multiple times?". This is explained somewhat in GP's footnote [3]:

In early November, the team noticed a sharp decline in the number of particles, but not to zero. This means that the spacecraft still has a long way to go until scientists can declare it free from the solar system.

Due to the fact that the power of the Sun rises and decreases, the position of the heliopause is not constant. Therefore, it is impossible to say exactly when Voyager 2 will leave the Solar System.

So your interpretation is correct, Voyager 2 is approaching the boundary, but hasn't yet definitively crossed into interstellar space. The phrase 'leaves the Solar System' is misleading, depending on your interpretation of where it ends. We can probably expect a few more confused articles announcing this in the next few months until it is certain.

[3] http://earth-chronicles.com/space/voyager-2-is-even-closer-t...

> why are the particle counts about an order-of-magnitude higher for Voyager 2 then they were for Voyager 1 in interstellar space

Better detector? Those are detected particle counts

Voyager 2 was launched a couple weeks before Voyager 1. They are pretty much the same.

Radiolab did one of my favorite episodes ever on this! Skip to minute mark 21 if anyone is in a hurry. But the whole episode is very good.


Sorry, not on-topic but how were you able to navigate the archive? I can't get to old episodes easily.

I didn’t. I remembered it was a space episode and figured radiolab only probably did a handful. So, I simply googled space and radiolab. After a few misfired links, I came upon the episode. I just remembered it was recent because they talked about the voyager 1 passing the solar winds.

This is one my favorite episode. My eyes become teary every time I listen to this.

That's really interesting to see, thank you.

It's worth noting that [2] uses a non-zero baseline value, so the actual difference between the top of the plot and the bottom isn't nearly as pronounced as the difference shown in [0].

Why isn't it a distance squared relation?

It has to do with the interaction between solar wind and interstellar plasma.

There is a relatively sharp transition— the Heliopause— from solar wind dominating to interstellar “wind” (from other stars) dominating, which occurs where their respective radiation pressures are roughly equal:


Is there a prevailing wind in interstellar space from other stars? Seems like it would be chaotic eddies near net zero. But maybe due to solar system motion or something like that there is a prevailing direction. If so would that mean we could use solar sails to continue to accellerate a spacecraft (at a much reduced rate) outside the heliosphere as long as we want to go in a specific direction arc?

Yes, the apparent direction of the interstellar wind is due to the motion of our solar system through a 30-light-year-wide cloud of gas [1].

The specific dynamics of the cloud (e.g., its turbulence) are still largely unknown, though the fact that the direction of intersellar wind has shifted by several degrees over a few decades hints that it’s not totally calm.

I’m not sure about solar sails outside the heliosphere... that’s an interesting question. I’m not an expert, just a guy who likes reading about this stuff.

[1]: https://en.wikipedia.org/wiki/Local_Interstellar_Cloud

Sorry but the chart shows nothing. At this distance from the sun the gradient would be quite smoothed out so you just need to pick energy level (they choose 0.5MeV) and saw there drop below in October, if they choose 0.6 the drop would be sooner, if they choose 0.4 the drop would be later. I am aware about bow shock and termination shock but this number is just randomly picked it doesn't mean anything.

What I'm trying to say is that if you are trying to show that voyager's surrounding environment suddenly changed, this is the worst possible chart to use. If the gradient was perfectly linear, you would still see sudden drop at some point, depending on which random threshold you choose.

Why would there be a sudden drop in that graph even if the actual boundary environment had a linear gradient of particle density as you move through it?

I was assuming the "0.5 MeV/nuc ions" just meant they are counting all nucleons they encounter with at least that much (kinetic?) energy. I don't see how your interpretation works with that, but I could very easily be missing something.

My naive interpretation is that the graph does a great job of demonstrating the environmental change.

It's possible the particle energies are very homogeneous i.e before the drop they were all just above 0.5MeV with very low std deviation and after the drop they were all just below 0.5MeV. You cannot tell from the graph what the true drop off is because you cannot tell how their energies were distributed.

It's kind of disingenuous to present this as an impressive prediction: "the data are changing in ways that the team didn't expect". https://phys.org/news/2012-10-voyager-left-solar.html

Also, looking at the voyager-1 data I would expect that voyager-2 is not yet out. The rate for voyager-2 only dropped to ~17 particles/sec which is more similar to the "false starts" seen in the voyager-1 data. From the article I got the impression these dips are only seen right at the heliopause.

Just FYI: Your last 2 links [2] and [3] are marked as high risk due to the presence of "Mal/HTMLGen-A" by Sophos web protection.

If you haven't watched it yet, "The Farthest: Voyager in Space" is an incredibly great documentary recently released by PBS


Thanks. I just Googled this, expecting it to be hard to get hold of in the UK, but was delighted to discover it was broadcast on BBC Four last night and is therefore available on iPlayer for the next 30 days. Looking forward to that.

That sequence of shots of Saturn’s moons and the music....gets me every time.

When I'm stressed out, I watch this documentary. It made me feel so good about America.

Cannot watch from Germany. :(

It's probably still available in your region as a $2 rental from one of the other streaming services, and would be well worth it.


Netflix? Can't even browse the catalog without signing up first. iTunes? No results PBS? Not available due to rights restrictions.

Pirate Bay? Hmm...

Ahem, at the cost of burning a bit more karma, iTunes still returns no cookie, perhaps it’s because I’m on the NL Store? Might be, eh... USA != rest of World. Netflix? I still haven’t figured out how to bypass the signup hero but I might be daft... maybe it’s a regional setting? Who knows!? PBS? Oh well... it’s their right to restrict access to their legitimate property, I guess I’ll have to wait for it to enter public domain, or hope it capriciously enters the catalog of one of the content services I do pay subscription for...

"We're sorry, but this video is not available in your region due to rights restrictions."

So it seems @eecc was correct about at least the "PBS? Not available due to rights restrictions." part.

Voyager 2 has been "leaving the Solar System" regularly for quite a while now. Example from 2007: https://www.nature.com/news/2007/071210/full/news.2007.365.h...

I guess the actual thresholds crossed are different but at this point who cares, unless you actually know those thresholds. It's out there and getting farther out. Yay.

If you can't describe an achievement without sensationalizing the hell out of it for the Nth time, maybe don't describe it.

In some sense the Voyager spacecraft have been leaving the solar system since launch. :) As the Nature article points out, Voyager 2 crossed the termination shock in 2007, entering the heliosheath. The recent announcement is about Voyager 2 exiting the heliosphere and entering interstellar space. Here's a nice graphic that shows the various regions - https://www.nasa.gov/sites/default/files/thumbnails/image/pi...

Someone has been counting: https://m.xkcd.com/1189/

I was wondering if I had deja vue over the probe leaving the solar system.

I came here to say this. I was sure I had read it before.

Voyager 2 is leaving the atmosphere!

Have we been able to launch faster spacecraft since? I'm thinking whether there could potentially be a newer interstellar probe that could overtake the Voyagers 1 and 2 on the way out of the Solar system.

Not really, no. These probes don't really accelerate by burning fuel; instead they pick up velocity by doing a bunch of gravity assists. The more of these you do, the faster you go, but sometimes you have to coast around the sun for years to align yourself with the next gravity assist, and this gets worse and worse until you hit solar escape velocity. Using this method, your final interstellar velocity will just depend on how many objects you can slingshot around on your final 'kick' out of the solar system. (So, in other words, you can get in the same ballpark as Voyager, but not orders of magnitude faster.)

Here's a video of what it takes "simply" to get out near Jupiter, nevermind out of the solar system. https://www.youtube.com/watch?v=ktrtvCvZb28

The Voyager spacecraft took advantage of a special planetary alignment that won't happen again for over a hundred years.


Having said that, if by 'spacecraft' you mean something weighing a few grams, the Starshot program might be able to obliterate the Voyager records. https://en.wikipedia.org/wiki/Breakthrough_Starshot

What if you use an ion drive or other propulsion that uses less fuel, and run it for years or decades? Yes, it generates little thrust, but won't it add up over long timescales?

That'll certainly help a bit--you'll pick up several km/s of delta-v--but even this will still be pretty irrelevant in terms of interstellar distances.

> a laser with a gigawatt of power (approximately the output of a large nuclear plant) would provide only a few newtons of thrust.

wow... that's not very efficient...

For anyone who wants to play with doing slingshots around the solar system, there's always Kerbal. Or alternatively there's Spaceflight Simulator https://play.google.com/store/apps/details?id=com.StefMorojn... for which https://xkcd.com/1356/ is true for me.

Only five probes were launched with the ability to leave de Solar System, but the Voyagers are still the fastest ones...


Depends on what you mean. There have been recent craft (New Horizons and the Parker Solar Probe) that were launched to a higher velocity than they Voyagers. However, the Voyagers used gravity assists from the gas giants to achieve a high final velocity. It seems unlikely that we'll be able to overtake them unless/until we step up to a more powerful form of spacecraft propulsion.

> Voyager 1 departed Earth on 5 September 1977, a few days after its sister spacecraft, Voyager 2.

Well that’s unusual isn’t it? Perhaps there was a temporary issue/delay with #1? I didn’t see the explanation in the article.

IIRC Voyager 1 traveled faster than Voyager 2, so it quickly passed Voyager 2. They named it Voyager 1 because it was going to arrive on target sooner.

From Wikipedia: "Voyager 2 is a space probe launched by NASA on August 20, 1977, to study the outer planets. Part of the Voyager program, it was launched 16 days before its twin, Voyager 1, on a trajectory that took longer to reach Jupiter and Saturn but enabled further encounters with Uranus and Neptune.[4]"

> They named it Voyager 1 because it was going to arrive on target sooner.

Do you know this or are you speculating?

I watched the documentary The Farthest (really good, highly recommend https://www.imdb.com/title/tt6223974/) on Netflix a few weeks ago. I might be misremembering; feel free to fact check me.

Thanks. This bit from Wikipedia doesn't explicitly confirm, but it makes your answer very like:

> Two trajectories were selected. One was designated JST: its mission would take it to Jupiter, Saturn, and Titan, with the probe's trajectory designed to optimize the Titan flyby. The second was designated JSX: it would be launched on a trajectory that would preserve the option of a Grand Tour, while serving as backup for the first probe. It would arrive after JST, and if JST were successful, it could continue with the Grand Tour. If JST was unsuccessful, JSX could be diverted to perform the Titan flyby itself, which would eliminate the possibility of a Grand Tour.

> The two spacecraft that launched retained the same mission concept. Voyager 1's course was optimized for the Titan flyby and Voyager 2 for the Grand Tour. Voyager 2 would reach Saturn nine months after Voyager 1, giving plenty of time to decide if it should proceed with the Grand Tour. Additionally, by launching Voyager 2 first, Voyager 1's launch could be re-targeted to perform the Grand Tour if Voyager 2 were lost in a launch failure. An option to skip Voyager 1's Titan flyby and proceed from Saturn to Pluto was identified, though Titan was still considered the more interesting target, especially after images from Pioneer 11 indicated a very substantial atmosphere.

> With Voyager 1's mission complete, Voyager 2 was cleared for an extended mission to Uranus and Neptune, fulfilling the goal of a Grand Tour as proposed in 1964.


I don't have a source but I remember this being a known fact.


Haha, I'm rewatching the series now. Just finished season 2 last night.

This is not reddit guys. Please refrain and skedaddle.

They were built in parallel and probably didn't get firm launch dates until long after they had names, associated teams, etc.

How do they manage to keep sending data back? Is there a really really directional antenna that points at the Earth?

This guy goes into great depth at an EE and signals level on how the Deep Space Network does this.


Yes. Directional antenna with EXTREMELY low bitrates so that the bits have a long time to accumulate any noise out of the signal. Specifically its at 160 bits per second at a transmit frequency of 8 GHz.

160 bits per second is actually pretty high. As comparison the German radio time signal used by parts of Europe sends out 1 bit per second: https://en.wikipedia.org/wiki/DCF77

Yeah, at the time the probes were launched, 160bps would have been a fairly respectable transmit rate between two computers connected by a cable, located in the same city!


Direction + a large enough receiver on Earth. Oh, and lots of error correction.

And we have giant antennas that listen.

Pretty much. The big dish on the voyagers is the comms antenna, not a science instrument.

How much of a lag is there in the data transmission? I guess that should correspond to how many light years it is away? How does the probe know which direction to send the data? I have so many questions! I gotta watch that documentary and listen to the podcast people are suggesting here!

The current one way transmission times are 20 hours and 5 minutes to Voyager 1 and 16 hours and 38 minutes to Voyager 2: https://voyager.jpl.nasa.gov/mission/status/

The probes use a star tracker to determine their orientation (they apparently look for the sun and one other star). I assume they keep track of their expected position in the solar system, and this is fine-tuned with updates from the ground station. If you have position and orientation, you can compute where the Earth should be at the current time and point the antenna in that direction.

Also, at their distance, they don't really have to be that close in aiming. Their beam is plenty wide by the time it reaches Earth.

All with 1970s technology, simply amazing stuff.

"Light years" is a bit optimistic. Voyager 1 is about 20 light-hours away (144 times further from the sun than we are).


you know, it's pretty damn incredible that we've sent little tiny space probes out of our solar system and they can still talk to us. sometimes i like imagining this tiny ball of circuits floating in space talking to its home planet and it just boggles my mind.

Quote from the article:

"the US space agency says that Voyager 2 has a working instrument aboard that will provide "first-of-its-kind observations of the nature of this gateway into interstellar space"."

Does anyone know what Voyager 2 is carrying that Voyager 1 wasn't that might give interesting data in this part of space?

It's the "Plasma Science Experiment" I believe. Voyager 1 did have one, but it broke along the way. This is based on https://www.jpl.nasa.gov/news/news.php?feature=7301

Thanks very much for the info!

They should send these out every decade or so. Especially if they could get to where they can relay data from probes further out. Technology advances so quickly, after all.

Yeah, as well as visit Titan and Europa more seriously...Why "we" don't do such things that are silly inexpensive relatively, but burn millions on reality shows is something I'll never learn to accept...

Be satisfied that hundreds of years from now, our descendants will look back at how today’s society allocates and deploys its capital and shake their heads asking “WTF were they thinking?”

Or they scavange the ruins of it ...

Pure science missions must compete for funding against efforts that inspire and capture the imagination of the people.

Getting humans into Mars to study Mars geology is priority. Once humans have been in Mars few times and the public interest vanishes–as it did during moon missions–there may be more money for telescopes and probes and cheaper robotic exploration.

53 million households in the US (over 580 million globally) watched the Apollo 11. The numbers dropped dramatically after the first mission. Apollo 13 was dramatic exception. People get bored easily and lose intrest.


If I would have a say, the priority should be the Overwhelmingly Large Telescope (OLT). US and other countries should have joined with Europeans to make it happen. The cost was 'just' 1.5 billion. Europeans opted for Extremely Large Telescope (ELT) instead and US is dong thirty meter telsecope. OLT would have been really something.

The problem isn't that the probes get too far out; the problem is that they run out of power. The Voyager probes were launched in the 70's and their RTG's won't provide enough power to keep them running for much longer.

just to be clear, it hasn't "left the solar system", it left the heliosphere. They said it still needs to leave the OOrt cloud before it has officially "left the solar system".

but, it's cool that it will continue transmitting until about 2027!

The big news is that Voyager 2 should be experiencing more interstellar medium than atmosphere from our star at this point. It will still be a while before it could be said to truly "leave the Solar System" if you consider gravitational influence to be a factor. There are objects still orbiting our star at more than a light year away for example (Oort cloud).

Do we actually know where they end up? Is it possible to make a reliable calculation on their trajectory in let's say 50.000 years? Do they get locked in a next solar system or will they continuesly be catapulted between systems? They are not solar powered right? So they won't be powered up again when reaching a next star (assuming all electronics are still alive)

Neither probe is headed towards any particular star. In roughly 40,000 years each will pass a small nearby star (Gliese 445 and Ross 248 for V1 and V2 respectively) at a distance of over 1ly, far too distant for the stars to have much effect on the trajectory of the probes. Barring any chance encounters, the spacecraft will be drifting in the interstellar space for a very, very long time.

What always amazes me is the vast difference between the visual and non-visual effect of the sun. From that distance, the sun is just another glowing speck in the distance, and yet its gravity and radiation is still felt. Like if you had a speaker the size of a pinhead that you could hear from a mile away. (Or probably a lot more).

Well, the probes definitely "feel" the visible radiation of the sun as well, for some definition of "feel". At that distance the sun is still much brighter than any other star, and it would be obvious in photos if they still had use of their cameras.

Even brighter than you might think. Based on the reasoning in this Quora answer [0], which looks reasonable at first glance, it's 10-20 times brighter than a full moon on Earth. If you were out there sitting on Voyager 2, you could read a book by sunlight.


> Voyager 1 will not approach another star for nearly 40,000 years That's a lonely ride...

Anyone happen to know if the Voyager's energy is such to exceed escape velocity wrt sun?

Yep, it and four other spacecraft. https://wikipedia.org/wiki/Interstellar_probe

Relevant xkcd: https://xkcd.com/1189/

Of course, that comic's about all the times and ways in which Voyager 1 has left this solar system. We've got decades of hearing about Voyager 2 ahead of us!

There actually isn't anything strange about it leaving solar system multiple times when the heliopause recedes towards Sol and then expands forward again enveloping the craft.

Well it's also that we have multiple ways of defining the border of the solar system.

Sure, you can say that the Solar system ends where it stops being gravitationally dominant.

But think about this, we say we left Earth for space when we actually leave Earth's atmosphere. When you cross heliopause you are actually leaving "atmosphere" of solar system. Within it it is dominated by the particles from solar wind. And the change is very abrupt and well defined (but only in the direction of the motion of the Solar system, ie. where Voyagers are) in contrast to how Earth's atmosphere just gradually thins out and we have to get into some sort of agreement when it actually ends.

yeah they can keep writing these articles til we're all dead.


I had the same initial reaction. This time it's Voyager 2, not Voyager 1. And the term 'leaves the solar system' is moderately misleading. See my initial response:


Too soon to make a "one small step" joke?

This is great and everything but is there really a need to get this status report every year. If so, shouldn't it rather read: still leaving the Solar System?

You're thinking Voyager 1, which left the solar system in 2012. This is reporting on Voyager 2.

Erm, well that's possible. Anyway I still think less would have been more:


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