Voyager 2 Set to Reach Termination Shock 308
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."
No (Score:5, Informative)
Re:Remind me again (Score:5, Informative)
Re:Remind me again (Score:5, Informative)
http://library.thinkquest.org/C0126626/fate/fate%20of%20universe.fate%20of%20universe.mass%20density%20of%20the%20universe.htm [thinkquest.org]
3 atoms per cubic meter is actually higher than I expected it would be given the immense (infinite? It certainly can't be definitively measured by any means we have, only theorised and later disproven) size of the Universe. Is 3 atoms per cubic meter enough to even have a "speed of sound" associated with it?
Re:Remind me again (Score:3, Informative)
Re:Remind me again (Score:5, Informative)
Bingo (Score:2, Informative)
Re:And then what? (Score:3, Informative)
Re:Remind me again (Score:4, Informative)
Re:And then what? (Score:5, Informative)
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.
Re:Haven't you heard? (Score:1, Informative)
* Speed of sound in a material is the speed at which information propagates through collisions between constituent particles. In a supersonic flow, no information can propagate upstream through collisions, the flow just changes weirdly and suddenly at "shocks" (the math works out, honest) - a "sonic boom" is a travelling shock attached to a supersonic aeroplane in a generally subsonic medium sweeping past you.
We already are quite accurate (Score:5, Informative)
<wikipedia href="Heliosphere">
Evidence presented at a meeting of the American Geophysical Union in May 2005 by Dr. Ed Stone suggests that the Voyager 1 spacecraft passed termination shock in December 2004, when it was about 94 AU from the sun, by virtue of the change in magnetic readings taken from the craft. In contrast, Voyager 2 began detecting returning particles when it was only 76 AU from the sun, in May 2006. This implies that the heliosphere may be irregularly shaped, bulging outwards in the sun's northern hemisphere and pushed inward in the south.
</wikipedia>
A Few Things (Score:4, Informative)
Next, how does sound transmit? Well, sound is a density/pressure wave, right? All I need is for the free particles to be interacting somehow to set one up. Turns out, the interstellar medium isn't a gas like you're used to thinking of, it's a plasma. The important point here being that because the electrons are not bound to the atoms, the effective "size" of the atoms goes up (that is, the disntance over which they interact with neighboring atoms). Thus you should be able to get sound waves more easily than you would suspect from a regular gas that is that sparse.
Re:Maybe... (Score:2, Informative)
Re:100km/s? Bloody unlikely (Score:4, Informative)
"Why are you so high, for my low density?"
"Because you are plasma, and no stupid ideal gas, slowpoke!"
Unless I'm mistaken (Score:4, Informative)
Re:That's Garbage (Score:5, Informative)
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.
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.
These pressure-wave effects are of course difficult to measure in such a low-density medium, but they are certainly real.
Re:Remind me again (Score:3, Informative)
As for speed in space, you can judge it simply but measuring time it takes you to travel between two fixed points, or by taking measurments againist another known objects. Besides Acceleration in space matters more than speed. To get to the moon you don't travel at a fixed speed but you accelerate half way there, and then decelerate the other half. without fricition to slow you down you need some massive forward firing engines.
Re:Worst. Write up. Ever. (Score:5, Informative)
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.
Re:Why don't we do this more often? (Score:3, Informative)
But I agree, I'd like to see NASA funding going to a lot of smaller projects like this than one behemoth one.
Re:No (Score:5, Informative)
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 [wikipedia.org] and Supercritical flow [wikipedia.org]. 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.
The interstellar medium has huge effects on light (Score:4, Informative)
This is the effect of minute "dust" particles permeating space and absorbing/deflecting light. The effect is less for longer wavelengths which is why we can get a better view of the Galactic centre in the infrared [caltech.edu].
Re:Congrats to Humanity (Score:3, Informative)
We have ever launched 5 spacecraft going fast enough to escape out of the Solar system, at mere 11-ish km/s at the Earth orbit ("-ish" because we cheat a bit using gravitational slingshots).