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Guru
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Subsonic Speed in Outer Space?

05/29/2006 11:14 AM

Voyager 2, one of that pair of incredibly durable spacecraft, could pass beyond the outermost layer of our solar system, called the "termination shock," sometime within the next year, says NASA. Compared to Voyager 1's passing, this is way too early, indicating an odd shape for the edge of our solar system. Read more in Space.com

The "termination shock" happens where the supersonic solar wind particles meet incoming particles from interstellar space and are slowed down to subsonic speeds. Similarly, it is postulated that where the incoming supersonic interstellar particles meet the (now subsonic) solar wind particles, a "bow shock" occurs as they are also slowed down to subsonic speeds.

The intermediate region, where both types of particles are subsonic is called the heliosphere. Read more about these shocks and regions in Termination shock and Bow shock

But what is meant by traveling at subsonic speed in empty space?

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The Engineer
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#1

Termination Shock

05/30/2006 10:18 AM

As I understand it, all stars gives off high energy particles in all directions. If our sun was the only sun, these particles would travel outward as a ever growing sphere forever. The problem is the sun is in a galaxy and the galactic center is ejecting a ton of high energy particles outward. The farther you get from the sun, the lower the density of solar particles, the more easily they get swept up in this "galactic wind".

So long story short (too late), the termination shock is an area surrounding our star where the solar wind and galactic wind are in balance and neither dominates. Before you hit it solar wind dominates and after you hit it cosmic wind dominates.

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Guru
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#2
In reply to #1

Re:Termination Shock

05/30/2006 2:08 PM

Very well stated and quite right, but what do supersonic and subsonic mean in "empty" space? Energy particles with a speed greater or less than sound? What sound?

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The Engineer
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#4
In reply to #2

Re:Termination Shock

05/30/2006 2:39 PM

I agree. The speed of sound is dependent upon the material it's passing through. Supersonic and Subsonic doesn't make much sense to me in this case. Is it possible the description is a mistake. Could it mean the speed of sound through air just being used as a reference velocity?

Sorry to answer your questions with questions.

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#3
In reply to #1

Re:Termination Shock

05/30/2006 2:17 PM

OK Mr. Pink, if sound does not travel in a vacuum, what does the term "sonic" mean in space? If the speed of sound is proportional to density, then the the speed of sound in space must be very, very small. Does this mean that virtually any movement in space is supersonic?

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Guru
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#5
In reply to #3

Re:Termination Shock

05/30/2006 2:57 PM

"Sonic" (the speed of sound) is defined as in plasma physics - see my original reference to Wikipedia (bow shock).

The whole of the interstellar and intergalactic mediums are considered to be plasma of very low density and low pressure. According to the formula, the speed of sound is not necessarily very low, due to the ratio pressure/density.

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#6

Sound in space?

05/30/2006 7:41 PM

Here is a reply to a very similar question that I had, from Dr.Hathaway at NASA: Hope this helps. Space is not a perfect vacuum - sound waves can travel through it but at very different speeds than that of sound at Earth's surface (1100 ft/s or 330 m/s). Sound speed near the termination shock is about 300000 m/s. __________________________ Dr. David H. Hathaway Mail Code VP62 NASA/MSFC/NSSTC Huntsville, AL 35812 Tel: 256-961-7610 Fax: 256-961-7216

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Guru
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#7
In reply to #6

Re:Sound in space?

05/31/2006 3:02 AM

Could you perhaps post a reference where one can see what parameter values are used for the speed of light at the termination shock? (Maybe a link was intended, but just a underscore line came out?)

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Guru
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#8
In reply to #7

Re:Sound in space?

05/31/2006 4:48 AM

Correction on post #5421 - must read: "..... where one can see what parameter values are used for the speed of sound at the termination shock?"

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#9
In reply to #8

Re:Sound in space?

05/31/2006 9:40 AM

Here is Dr Hathaway's e-mail address. Please post his reply if possible and agreeable with him. I am sure others will be interested. David.Hathaway@nasa.gov

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Guru
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#10
In reply to #9

Re:Sound in space?

06/03/2006 1:43 PM

Thanks, I have e-mailed Dr. Hathaway, but had no response to date. Will post any reply here if he answers and agrees to a posting.

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