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Voyager 2 Set to Reach Termination Shock
Posted by
CmdrTaco
on Wed Nov 28, 2007 12:28 PM
from the cool-stuff-you've-never-heard-of dept.
from the cool-stuff-you've-never-heard-of dept.
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."
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Remind me again (Score:3)
Re:Remind me again (Score:5, Informative)
Parent
Re:Remind me again (Score:5, Funny)
Parent
Re:Remind me again (Score:5, Funny)
Parent
Re: (Score:3, Funny)
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!"
Parent
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?
Parent
Re:Remind me again (Score:5, Informative)
Parent
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.
Parent
Re: (Score:3, Informative)
Re:Remind me again (Score:5, Funny)
It's full of stars...
Parent
Re: (Score:3, Funny)
Re:Remind me again (Score:4, Informative)
Parent
Re: (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. Besid
cool (Score:3, Interesting)
Re:cool (Score:5, Insightful)
No matter what happens, it can't negatively affect the mission, because it is the mission. (well part of the mission, anyway) As a useless analogy, if Space Aliens came down and ray-gunned all of SETI's equipment, you wouldn't say that SETI's mission would be negatively impacted, would you?
Parent
Re: (Score:3, Insightful)
can this negatively affect the mission/spacecraft itself?
Technically, the mission was to study the outer planets, so its already been accomplished. NASA keeps listening to Voyager even though its mission is technically complete because its one of a select few probes on course to exit the solar system. In other words, its mission right now is to go to the edge of the solar system and report back what it sees. In this sense, Voyager is close to accomplishing its (2nd) mission.
Will its speed change? (Score:3, Interesting)
Voyager 2... (Score:5, Funny)
Maybe... (Score:5, Interesting)
Nov. 6, 2003: Voyager Spacecraft Approaches Solar System's Final Frontier
Dec. 19, 2000: Most Distant Spacecraft May Reach Shock Zone Soon
May 25, 2005: Voyager Spacecraft Enters Solar System's Final Frontier
Besides the speculation, will we even know when the boundary is crossed? Do they expect data to indicate a transition, or do we even know if the instruments can detect such a thing?
Dan East
Re:Maybe... (Score:5, Funny)
Parent
My solar system, let me show it to you... (Score:5, Interesting)
If you go here:
http://code.google.com/p/nmod/downloads/list [google.com]
and get the windows installer or linux source for my nbody modeling kit, and then download this:
http://www.politespider.com/nbo/time_series.zip [politespider.com]
And unzip it to save you the bother of having to actually generate your own time series (3d time series model of the solar system), which can take a while. You can then watch both Voyager probes follow their orbits (with 24th august 2006 as their starting date), for 20,000 days of travel time.
This isn't a program with a scrummy easy interface I'm afraid, the viewer is console opengl. But there are instructions here:
http://code.google.com/p/nmod/wiki/nbview [google.com]
And it's not too hard once you get the hang of it.
The orbits do not take termination shock into account, this is pure Newtonian motion. The dataset for the solar system has taken months to put together. It's incomplete, It only has our moon (zoom in for ages with Earth centred and you'll see it), the others have been tricky to get right.
Parent
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>
Parent
Re:Maybe... (Score:5, Funny)
Parent
Re:Maybe... (Score:4, Interesting)
Parent
hrm... (Score:3, Funny)
The edge of... (Score:4, Funny)
...and then... (Score:3, Funny)
Make your own termination shock. (Score:5, Interesting)
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.
Re:And then what? (Score:5, Insightful)
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.
Parent
Re: (Score:3, Funny)
I think you will find he is an american, and therefore that doesn't apply.
Re:And then what? (Score:5, Interesting)
The reason why many Americans speak only one language or don't spend a lot of time in other countries is based in this. For a majority of Americans there is only one language besides English that is of any utility, and that is Spanish. I was once fluent in both German and Italian, but since I've been back in the US I have yet to run into a situation where I needed to speak either language. It's not like we can day trip to France or that most businesses can deliver finished products to a foreign country with a simple truck ride of 3 or 4 hours.
Parent
Re: (Score:3, Informative)
Re:And then what? (Score:5, Funny)
Why do you go on vacation to foreign places...
OMG, underage Taiwanese hookers...in space?Parent
Re: (Score:3, Insightful)
It's worth pointing out that the primary missions of the Voyager probes were to explore the outer planets, which they did with great success many years ago (and we have sent more probes since). The fact that they are still active now and sending back useful information about the termination shock is just a bonus, so what you say about only sending two being a risk isn't really valid.
Re: (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:And then what? (Score:5, Funny)
Parent
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.
Parent
Unless I'm mistaken (Score:4, Informative)
Parent
Re:And then what? (Score:5, Insightful)
Once to get outside the boundary, twice if the boundary expands and catches back up with it, and thrice to once again get outside the boundary.
Just a thought.
Parent
Re:speed of sound (Score:4, Interesting)
Parent
That's Garbage (Score:3, Insightful)
That's garbage. 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.
And as for the whole thing about sound travelling faster in space, you just made that up. Light (and other electromagnetic phenomena) do t
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.
Parent
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].
Parent
Re:That's Garbage (Score:5, Interesting)
Think of it this way, if your sitting in the next cubicle over and I whisper something and you are unable to hear it, does that mean that the speed of sound doesn't exist, or simply that the amplitude of the signal was too small? Similarly, in order to transmit sound in space I'd need some serious lungs. More to the point the speed of sound is a critical parameter when examining how two flows (such as the solar wind and the interstellar medium) interact. Simply put if the speed of sound in the interstellar medium were undefined it would not interact with the solar wind, and there wouldn't be a termination shock at all. Every particle of the solar wind would not interact with any particles of the interstellar medium.
It's been a while since I've done any fluid dynamics, so some of the details may not be precisely right, and I am not knowledgeable enough on rarefied systems to comment on why the speed of sound is so high in the interstellar medium, but suffice to say that many things behave in counter-intuitive ways for rarefied systems.
Parent
Re:Price Fixing (Score:4, Funny)
Today only! Closeout prices! Everything in the store is 3 cents off! That's right, 3 cents off! That's lower than sale price! Even lower than clearance sale price! Don't miss this amazing opportunity!
Parent
No (Score:5, Informative)
Parent
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.
Parent
Re: (Score:3, Funny)
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.
Parent