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Voyager 2 Set to Reach Termination Shock
Invisible Pink Unicorn writes "A computer model simulation developed at UC Riverside has predicted that in late 2007 to early 2008, the interplanetary spacecraft Voyager 2 will cross the termination shock, the spherical shell around the solar system that marks where the solar wind slows down to subsonic speed. At the termination shock, located at 7-8.5 billion miles from the sun, the solar wind is decelerated to less than the speed of sound. The boundary of the termination shock is not fixed, however, but wobbly, fluctuating in both time and distance from the sun, depending on solar activity. Because of this fluctuation, the spacecraft is also predicted to cross the boundary again in middle 2008. The article abstract is available from The Astrophysical Journal."
this is the kind of thing scientists predict all the time and observe in lab experiments... but this device is actually GOING to the edge of a solar system... it's someplace human made instruments haven't been. Science at it's very purest form, simply going and observing something nobody has actually seen before.
Why do you go on vacation to foreign places.. aren't postcards and Discovery channel good enough? It's a whole lot different to say "we were there" than guessing what it would be like from a long distance.
There's a Voyager 2?! Oh God no; come back Enterprise, all is forgiven...
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It means Janeway's going to have to pretend to be thrown all over the place while bits of the ship fall off.
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Space is not a vacuum. The speed of sound in space is about 100 km/s, according to Wikipedia.
Much like killing Rasputin, leaving the Solar System is apparently an ongoing process marked by significant milestones.
So, in space, they can hear you scream?
Why do you go on vacation to foreign places...
OMG, underage Taiwanese hookers...in space?I swear to God...I swear to God! That is NOT how you treat your human!
Four, just four small space probes.
Sorry dude, all the space ships you see on TV are just FX. We are not (yet) exploring the galaxy. Will we get any new data about "termination shock" or whatnot? Yeah! We may confirm that there exists this termination shock we expect to find there, or we may find our theories are wrong and there is not such "thing".
So you cannot always assume that "near vacuum" and "perfect vacuum" are the same thing. In the case of solar wind interacting with the interstellar medium, you can't approximate either as having zero density: to do so would ignore some very real physics that occurs when the pressure of the high-velocity solar wind impinges on the nominally static interstellar medium. And as for the whole thing about sound travelling faster in space, you just made that up. Every material (even low-density materials like the interstellar medium) have a "speed of sound." The interstellar medium is no different. It has a "speed of sound" on the order of 10 km/s to 100 km/s (by comparison the speed of sound for air on Earth is 0.3 km/s). Sound travels faster and farther through more solid materials. You're being imprecise by saying that sound travels faster in more "solid" materials. The equation is:
v = sqrt( C/d )
where v is the speed of sound, C is the coefficient of stiffness, and d is the density. So, actually, more dense materials have a lower speed of sound (all other things being equal). The reason that liquids and solids have higher speed of sound is not because they are dense, but rather because they have strong cohesive forces binding the constituent atoms/molecules together (that's why they are condensed into a solid or liquid, after all). These strong forces lead to a very high coefficient of stiffness, compared to a gas (more than enough to offset the higher density).
For something like the interstellar medium, the stiffness is quite low, but the density is exceedingly low, which produces a correspondingly large speed of sound.
Sound, however, is caused waves of physical compression. In other words, one particle bumps into the next, which bumps into the next, and so on. You're quite right. However nothing prevents compression waves from traveling in low-density materials. The atoms of the material are free to fly large distances, and they will indeed statistically bump into each other, transfer momentum, and so on. This collective motion will indeed be compression waves. Of course you will not be able to set up very large-amplitude compression waves using, e.g. your vocal cords in such a low-density medium... but the high-speed collision of the solar wind with the interstellar medium will most certainly lead to all kinds of expanding pressure waves, whose behavior is dependent on the local speed of sound.
These pressure-wave effects are of course difficult to measure in such a low-density medium, but they are certainly real.
A termination shock/shockwave/bore/hydraulic jump occurs when the bulk speed of a fluid drops below the wave propagation speed.
Run a tap in to a flat sink (like a kitchen sink) and you see a circular pattern (if the sink is flat) some distance from around where the water hits the sink. The pattern should have shallow fast moving awater close to where the jst hist the sink, and deeper slower water on the other side of the circle.
The "jump" where the water goes from fast to slow is the same kind of object as a termination shock. For extra fun, stick an object in the slow water, and see how waves propagate ahead of it (against the flow). Then see how it doesn't happen in the fast water.
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The writeup didn't bother me at all. But then, I *am* a scientist.
Are you SURE that it fluctuates in time from the sun, or do you actually mean that it fluctuates (only) in distance from the sun?
The termination shock fluctuates in distance because it's an interaction between the heliosphere of the sun and the interstellar medium. Parts will experience more drag due to magnetic fields, and thus be closer to the sun than other parts of the shock. It fluctuates in time because the sun's output fluctuates in time -- when the solar winds are stronger, the corresponding parts of the termination shock will be further away. So it fluctuates in both time and distance, and depends upon solar activity. Just as the writeup said.
Ignoring the poor English, care to explain the logic behind this? Surely, going from inside to outside, Voyager 2 will have to cross the boundary an odd number of times?
Nope. The solar winds overlap each other. A weak wind will create a shock terminator nearer to the sun, while a stronger wind will create one further away. And they hang out there for a long time after they were generated. Apparently the astronomers looked at solar activity and calculated that Voyager 2 will hit two shocks -- one from a weak, but earlier wind and one from a stronger but more recent wind. Makes perfect sense.
And you have some sort of problem with them describing the terminator shock as the boundary where the solar wind decelerates to the 'speed of sound'? That's accurate. Remember that the solar wind is composed of charged ions, and that we're talking about the speed of sound in a plasma. When the particles go below the speed of sound, then random magnetic fields suddenly become a greater influence than the outward driving force of the sun. There will probably be lots of magnetic turbulence, although nobody really knows what to expect.
The writeup was technical but accurate as far as I can tell. Sorry it if was too geeky for you.
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No it's not. The Wikipedia article is wrong were it implies that, and right where it says "At the termination shock, a standing shock wave, the solar wind falls below its speed of sound and becomes subsonic." Read Hydrolic Jump and Supercritical flow. The termination shock happens when the solar wind transitions from supercritical to subcritical, which is dependent on its own density, and its own wave speed (speed of sound), not the wave speed of the interstellar medium, which is much further out. While the wave speed of the interstellar medium is given by the article as 100km/s, the density and wave speed of the solar wind can't be expressed as a constant, as it is a function of distance from the sun and heliolatitude.