Hacker News Reader

Comment by japhyr | Hacker News Reader

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...

View on HN

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?

> 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.

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...

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.

https://www.wnycstudios.org/story/sun-dont-shine

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:

https://en.wikipedia.org/wiki/Heliosphere#Additional_heliosp...

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.

Reply on news.ycombinator.com