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."

Comments

No

[InvisblePinkUnicorn]

No, it is not. It is the interstellar medium. Read: termination shock.

Re:No

[E 99]

No, it is not. It is the interstellar medium. Read: termination shock.

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.

Re:Remind me again

[Veinor]

Space is not a vacuum. The speed of sound in space is about 100 km/s, according to Wikipedia.

Re:Remind me again

[kimvette]

That's what I was wondering. How can there be a speed of sound in a "medium" which does not have enough mass to transmit sound waves? I mean, I know there are sparsely-distributed particles even in "empty" interstellar space, but is the medium thick enough that there can even be a "speed of sound" associated with it? Can sound transmission in such a medium ever even be measured? I was curious and googled on desity of matter in space and found this:

http://library.thinkquest.org/C0126626/fate/fate%20of%20universe.fate%20of%20universe.mass%20density%20of%20the%20universe.htm

The most obvious technique for discriminating between an open and a closed universe is to measure the average density of matter. The Friedmann equation describes the competition between the attractive gravitational force and the expansion of the universe. The gravitational attraction exerted at the center of an arbitrary sphere cut out of the universe is proportional to the average density of matter. The measured value of the Hubble constant (H) yields the kinetic energy of the expansion of the sphere. If the present density is below the critical value at which the expansion and gravitational attraction balance, gravity cannot halt the expansion, and the universe must be open. The critical density for closure of the universe is

d critical = 3H2/8G = 5×10-30 gram cm-3

[sorry the equation got munched! -Kim]

where G is Newton's constant of gravitation. Another way to express this critical density is in the number density of atoms, which amounts to 3 x 10-6 atoms per cubic centimeter (cm-3), or only 3 atoms per cubic meter.

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

[cnettel]

That number of 3 atoms per cubic meter is the average density of the complete universe, inluding stars, planets and black holes, but also including the vast void between galaxies. Any place in the Milky Way, and especially in the relative vicinity of the Sun, is "much" denser.

Re:And then what?

[DeepBlueDiver]

Other than "we sent something outside the solar system again", does this mean anything? Again? The only man made objects travelling beyond Pluto's orbit are Pioneer 10 & 11, and Voyager 1 & 2.
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".

We already are quite accurate

[junglee_iitk]

What you are refering to is Voyager 1. TFA is about Voyager 2. They are two different vehicles.

<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>

Re:That's Garbage

[kebes]

You're making a few mistakes...

Space is not a total vacuum, it's true. However, the density of particles of matter in space is, for the most part, so low that space can be treated as a vacuum. It's like rounding 0.1xE-25 to just 0. Rounding and approximations cannot be treated as glibly as you are doing. Approximating outer space as a perfect vacuum is a reasonable approximation for many calculations, but not all. For instance when calculating the properties of light traveling through outer space over short distances (e.g. less than a light year) saying it is a "perfect vacuum" is fine. But when doing calculations over long distances (billions of light years), the thin interstellar medium does indeed induce absorption and polarization effects that must be considered.

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.

Re:Worst. Write up. Ever.

[Remus Shepherd]

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.

The interstellar medium has huge effects on light

[glaswegian]

But when doing calculations over long distances (billions of light years), the thin interstellar medium does indeed induce absorption and polarization effects that must be considered. The effects happen on much smaller scales than this, and will depend on the density of matter that the light crosses. You can simply look at an optical image towards the centre of our Galaxy. It is only ~25000 light years away and has a huge concentration of stars. It should be a blazing ball of light but it is obscured by a dark "shadow".

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.