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My favorite graph of all time is the one that demonstrate Voyager 1 had left the solar system. I was a high school math and science teacher at the time, and I s
by japhyr 3y ago
My favorite graph of all time is the one that demonstrate Voyager 1 had left the solar system. I was a high school math and science teacher at the time, and I spent the whole day sharing this graph with students. It was so much fun watching everyone's faces and seeing the moment they realized what it really meant.
https://phys.org/news/2012-10-voyager-left-solar.html https://phys.org/news/2012-10-voyager-left-solar.html
- alain94040 3y agoWhy isn't it linear?
- Macha 3y agoBest guess: You get particles orbiting the sun, until you pass a point where the sun's gravity is too weak to hold them, and from that point you basically only see things who's escape trajectory intersects with yours
- pdonis 3y ago> You get particles orbiting the sun The particles hitting Voyager aren't orbiting the Sun; they're from the Sun, the solar wind. The heliopause is the point where they are stopped by the interstellar medium. That point is what Voyager passed as shown in the graph.
- lazide 3y agoThey are still, indeed, orbiting the sun. In the same way the earths atmosphere is orbiting the earth.
- cshimmin 3y agoNo, they are traveling at velocities far exceeding the gravitational escape velocity of the sun. There is no meaningful sense in which they are orbiting.
- lazide 3y agoExcept they aren’t, which is why they are there and there is a heliopause instead of them being in interstellar space and there not being a heliopause. If they had greater than escape velocity, they’d be escaping and we’d not see the graph we see.
- grey-area 3y agoOrbit means going very fast around something in a circular motion. These particles are heading directly streaming out from the sun, not going round it. As I understand it it’s where these particles reach equilibrium with the stellar medium. The sun is like a comet at a large enough scale, with a long tail of particles as it moves through the galaxy. https://en.m.wikipedia.org/wiki/Heliosphere https://en.m.wikipedia.org/wiki/Heliosphere
- pdonis 3y agoThe earth's atmosphere is not orbiting the Earth. It is in hydrostatic equilibrium in the earth's gravitational field. Big difference.
- lazide 3y agoand yet, atmospheric particles that have a mean free path that doesn’t intersect with other atmospheric particles, or when they do the average velocity delta and direction reach escape velocity, escape and are lost. You might want to rethink that. It’s a useful model in bulk in the lower atmosphere, but it’s far from true in the upper atmosphere.
- pdonis 3y ago> You might want to rethink that. You might want to rethink your claim. First, while the upper atmosphere is much less dense than the lower, and the fluid approximation becomes less and less useful as you gain altitude, that still doesn't mean that "a bunch of particles in free-fall orbits" becomes a useful model. The average thermal velocity of a molecule in the upper atmosphere is still well short of orbital velocity at that altitude. Some molecules acquire sufficient velocity to escape, sure, but that doesn't mean the others are in orbit. Second, the Earth's atmosphere is not a good analogy for what is happening at the heliopause anyway.
- mattgrice 3y ago[flagged]
- cwalv 3y agoI enjoyed reading their guess. Since he started the comment with 'I guess', it seems harmless enough. You're comment, otoh, is pretty rude.
- monocasa 3y agoThere's a relatively hard boundary at the heliopause.
- idontwantthis 3y agoBut why are there hard boundaries before that bounced back?
- jacquesm 3y agoGravity. The suns gravity field is in theory infinite but there is a pretty precise boundary where it stops to have an immediate effect on the things around it and orbits around the sun are no longer possible.
- pdonis 3y ago> The suns gravity field is in theory infinite but there is a pretty precise boundary where it stops to have an immediate effect on the things around it and orbits around the sun are no longer a thing. This is not correct. The particles are not in orbit about the Sun, they're coming from the Sun--they're the solar wind. The heliopause is where the solar wind particles are stopped by the pressure of the surrounding interstellar medium. When Voyager passed that point (the heliopause), the number of particles hitting it dropped drastically.
- deleted 3y ago[deleted]
- fnordpiglet 3y agoCorrect: The heliopause is the theoretical boundary where the Sun's solar wind is stopped by the interstellar medium; where the solar wind's strength is no longer great enough to push back the stellar winds of the surrounding stars. This is the boundary where the interstellar medium and solar wind pressures balance. The crossing of the heliopause should be signaled by a sharp drop in the temperature of solar wind-charged particles,[30] a change in the direction of the magnetic field, and an increase in the number of galactic cosmic rays.[34] https://en.m.wikipedia.org/wiki/Heliosphere#Heliopause https://en.m.wikipedia.org/wiki/Heliosphere#Heliopause
- Aperocky 3y agoIt could be a 3D porous boundary. A solar eruption may impose 10~ AU of continued heliosphere at this distance.
- japhyr 3y agoAssuming you're talking about the overall sharp drop-off and not the bounce-backs, this was my favorite way to explain it to students: We live near the ocean, and we have a rocky shoreline. We have a couple coves nearby. One cove is about the a half-mile across, but the opening to the larger bay nearby is just a couple hundred feet. On most days, the cove is really calm and the bay has roughly two foot waves. So, you can go out to the cove, pick up the biggest rock you can lift, and heave it into the water. You'll make a giant splash that amazes young kids, and then you can watch the ripples fan out over the bay. But you also see those ripples stop as soon as they reach the bay, where the larger waves absorb the smaller ripples from the rock. The rock represents the sun, the ripples represent solar wind, and the waves on the bay represents interstellar space. I believe that's a reasonable way of explaining it; if I was wrong after all this time I'd love to know it.
- deleted 3y ago[deleted]
- RheingoldRiver 3y agoThat makes sense as far as explaining another situation where you would see a similar pattern, but it doesn't really explain why. What's the equivalent of the land surrounding the cove here? The sun's gravity well? But that's a gradual drop-off, are we looking at the distance where the gravitational pull on particles is canceled out by some other force?
- ikiris 3y agoHydraulic Jump on interstellar scale. Heliopause. The heliopause is the theoretical boundary where the Sun's solar wind is stopped by the interstellar medium; where the solar wind's strength is no longer great enough to push back the stellar winds of the surrounding stars. This is the boundary where the interstellar medium and solar wind pressures balance.
- ryanjshaw 3y agoBut again, why is it a sudden cut-off and not a gradual one?
- zeven7 3y agoI didn’t understand it either but the pictures and graphs on this Wikipedia entry actually helped make a lot more sense of it to me, especially the analogy of the water faucet https://en.m.wikipedia.org/wiki/Heliosphere https://en.m.wikipedia.org/wiki/Heliosphere
- kqr 3y agoThat was a good page. At first I thought "But what is out there, outside the solar system?" under the assumption that there was nothing there. And there is nothing there, but nothing in galactic terms: the entire galaxy is there! And apparently it behaves like a gas, despite its low density. So although solar wind sounds hardcore, at that distance its pressure about matches that of the nothingness that makes up most of the galaxy. Interesting!
- anon_cow1111 3y agoWow yes, that was also my main question and it looks like the answer everyone is agreeing on is "interstellar wind pushing back against the solar wind" BUT- The graph says 2-3 particles/sec hitting the detector, which in sub-atomic terms is like 2 drops of water in an ocean's worth of volume. How much meaningful particle interaction is happening when everything is so close to a true vaccuum? Is this another one of those weird quantum-field-theory things? (Asking as a layman not a physicist obviously)
- thriftwy 3y agoIt is certainly not vacuum. Vacuum is when a gas particle is more likely to hit a wall (or other solid object) than other gas particle. It is absolutely not so on the edge of solar system, where low density is compensated by the vastness of space.
- taylorius 3y agoAccording to Wikipedia, the abrupt change occurs at the point when the solar wind's speed decreases into the subsonic range (speed of sound in the interstellar medium is approximately 100km/s and the sun emits the particles that makeup the solar wind at approximately 400km/s). This transition to a subsonic regime causes compression waves to form, and causes the rapid drop off.
- simonjgreen 3y agoThat’s fascinating, and going to send me down a learning hole! I had never before considered speed of sound in space.
- puzzledobserver 3y agoI didn't know that the speed of sound in the interstellar medium is 100 km/s. That seems surprisingly high, given that there's more atmospheric material here on the surface of Earth, and the speed of sound is only about 330 m/s. How can sound travel so fast in the interstellar medium?
- ben_w 3y agoIn an ideal gas, speed of sound depends on the temperature and molar mass, but not density as the density terms cancel out: https://en.wikipedia.org/wiki/Ideal_gas#Speed_of_sound https://en.wikipedia.org/wiki/Ideal_gas#Speed_of_sound It's hot, so the speed of sound is high.
- mcv 3y agoCan you use it to transmit actual sound? Would it be possible to use this for short-range communication in space? Or is this only a very theoretical kind of sound?
- ben_w 3y agoGiven the supersonic flow, one directional communication only. Given the impedance mismatch[0], even the parts of the solar system outside Kármán lines where the interplanetary medium can support pressure levels equivalent to normal speaking (including low Earth orbit), I'm told human ears can't respond to that pressure change properly. Sensors can be built to pick it up, but that may not be in scope for your question as we can also do that for acoustic waves in the CMB. [0] https://en.wikipedia.org/wiki/Impedance_matching#Acoustics https://en.wikipedia.org/wiki/Impedance_matching#Acoustics
- Otek 3y agoMy favourite analogy: Heliosphere is like water flowing from a faucet into a sink: the water represents the solar wind emanating from the Sun, and the point where it meets the sink's surface illustrates the heliosphere's boundary with the interstellar medium. Just as water changes direction and slows down upon hitting the sink, the solar wind decelerates and changes direction at the heliospheric boundary, where it interacts with the gases and particles of interstellar space. Photo to illustrate: https://en.wikipedia.org/wiki/Heliosphere#/media/File:Heliosphere-washbasin.svg https://en.wikipedia.org/wiki/Heliosphere#/media/File:Helios...
- zaik 3y agoInverse quadratic would have been my guess.
- lttlrck 3y agoI need an eink display on my office wall showing the current location/status. I always get a tremendous sense of wonder and wellbeing thinking about these probes/achievements, maybe it'd help keep me centered before the daily onslaught.
- hackernewds 3y agothe current status is that we do not know where it has fared to, according to the title
- fnordpiglet 3y agoUnless aliens grabbed it or something hit it, I’m pretty sure we know precisely where it is. What we don’t know is what it’s experiencing there.
- Vecr 3y agoNot exactly, the model for the various slowing/drag effects are not known along even its near future course. Edit: disregard, I think it's probably still measurable, just not as well.
- ant6n 3y agoNono, it's Voyager _6_ that gets destroyed by the Klingons.
- soylentcola 3y agoI thought 6 was the one that became V'Ger.
- cafeinux 3y agoIt is, and afaik no Voyager probe has been destroyed by the Klingons.
- dotancohen 3y agoMaybe you're the right person to ask. How do we know that the sun-particle-sensor or its wiring harness or one of its connectors simply hadn't failed?
- NhanH 3y agoI am guessing the number comes from multiple systems of independent sensors. So the assumption is that they won’t all fail at the same period in the same way
- cshimmin 3y agoA very valid question. In this case IIRC this drop-off in low-energy solar wind particles was correlated with observed changes in the magnetic field and also an increase in higher-energy cosmogenic particles all around the same time. These three phenomena (observed by different instruments) were theoretically predicted to occur at the heliopause transition. So it lends much more confidence to the interpretation of the data.
- ferw 3y agoI believe that the direction of the magnetic filed didn't change, contrary to expectations. The explanation was that the galaxy's magnetic field is aligned with the magnetic field of our sun. https://web.archive.org/web/20130913162459/http://news.nationalgeographic.com/news/2013/13/130911-voyager-interstellar-solar-system-nasa-science-space/ https://web.archive.org/web/20130913162459/http://news.natio...
- superjan 3y agoAnd hopefully Voyager 2’s measurements will confirm this. I don’t know how long we need to wait though.
- Tommstein 3y agoThe Voyagers leaving the solar system is so popular that they've done it like 10 times!
- wongarsu 3y agoAnd in another 300 years or so they will leave the solar system again when they reach the Oort cloud, and in another 30000 years or so they will finally leave the solar system for the last time when they leave the Oort cloud