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Voyager 2 enters interstellar space


RealKerbal3x

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According to this video posted a few hours ago (complete with unnecessarily dramatic music :P) by the Jet Propulsion Laboratory, Voyager 2 has crossed the heliopause and entered interstellar space, following Voyager 1 which passed this milestone in 2012. So, we now have two spacecraft beyond our solar system!

 

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I have a question though, why is the electromagnetic sphere preferred over the gravitational sphere for the definition of interstellar ?

The probes have entered interstellar gases, but are still orbiting the sun. Even Voyager 1 isn't in the Oort cloud yet, or is it ?

 

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7 minutes ago, Green Baron said:

I have a question though, why is the electromagnetic sphere preferred over the gravitational sphere for the definition of interstellar ?

The probes have entered interstellar gases, but are still orbiting the sun. Even Voyager 1 isn't in the Oort cloud yet, or is it ?

 

They are still influenced by the sun, but they have solar escape velocity, so are not orbiting it.

The definition of interstellar space is the heliopause, while the definition of a solar system is the extent of objects within the primary gravitational influence of a star. So the Voyager spacecraft are both in the solarsystem, and in interstellar space. Odd that the definitions are not coincident, but there you go.

Apparently it's gonna take a few hundred years before they are even at the inner edge of the Oort Cloud.

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20 minutes ago, tater said:

They are still influenced by the sun, but they have solar escape velocity, so are not orbiting it.

A para- or hyperbolic orbit is still an orbit. Orbits aren't limited to captive ones like ellipses or circles.

Quote

The definition of interstellar space is the heliopause, while the definition of a solar system is the extent of objects within the primary gravitational influence of a star. So the Voyager spacecraft are both in the solarsystem, and in interstellar space. Odd that the definitions are not coincident, but there you go.

I see, there is an ambiguity for the term "interstellar space", that can be either interpreted as the surrounding medium or the gravitational influence. I wasn't aware.

Edited by Green Baron
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4 minutes ago, Green Baron said:

I see, there is an ambiguity for the term "interstellar space", that can be either interpreted as the surrounding medium or the gravitational influence. I wasn't aware.

Two definitions that are different. Yeah, it's odd, frankly.

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14 minutes ago, Green Baron said:

A para- or hyperbolic orbit is still an orbit. Orbits aren't limited to captive ones like ellipses or circles.

You're certainly correct, but I can just as well say that it's in an elliptical orbit about the galactic center, too (like everything else, it's all about frame of reference). I tend to reserve "orbit" in common conversation for closed orbits (eccentricity <1), and call parabolic (singular, it's a special case, e=1) and hyperbolics (e>1) trajectories---that's not a definition, though, just what I tend to say myself, my bad.

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The aliens let the first one slide, but now we're definitely getting a fine for littering.

5 hours ago, Green Baron said:

A para- or hyperbolic orbit is still an orbit. Orbits aren't limited to captive ones like ellipses or circles.

An object on hyperbolic trajectory is in an orbit, but I'm not sure if I'd call it orbiting, because then we'd have to say that everything orbits everything else, making it a kind of a useless definition. I wouldn't even be able to claim that Earth orbits the Sun at that point, because there is nothing inherently special about the Sun as far as Earth's track through cosmos goes. So to me, saying "A orbits B" implies that the two objects are gravitationally bound, which is no longer the case for an object above escape velocity. At that point, it's more of a scattering.

The trouble is that semantically the words "orbit/orbiting" can imply both the trajectory and periodic motion. I'd usually go to Latin root as a tie-breaker, but it's the same issue there. 'Orbis' is something round, and 'orbita' is a track made by a round object, such as track left by a cart. And while I'm pretty sure that the noun 'orbit' is derived from the later, and that's consistent with standard scientific usage, when we talk about 'orbiting', things get fuzzier.

Bottom line, I don't know if I'd correct someone when they say that Voyagers aren't orbiting the Sun. Although, if we look at it that way, they haven't stopped orbiting when they left Heliopause, and rather back when they did their final fly-by boost to get the necessary velocity to leave the system. So if they aren't orbiting now, they haven't been for decades.

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6 hours ago, Green Baron said:

A para- or hyperbolic orbit is still an orbit. Orbits aren't limited to captive ones like ellipses or circles.

If you've flushed the crap off the toilet and it successfully flushed off the toilet then it have all escaped the toilet, no longer in it.

We've successfully flushed the Voyagers out of the solar system, but it's just still transiting the toilet bowl first.

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11 hours ago, tater said:

They are still influenced by the sun, but they have solar escape velocity, so are not orbiting it.

Iirc, the hypothetic Oort cloud existence was presumed due to similar periods (so, semi-axes)  of long-periodic comets.
So, probably they are still heliocentric.

When a craft gets out of the Earth atmosphere, it doesn't stop being an Earth satellite. An we don't say that it has reached interplanetary space.

Sedna is ~9 times farther from Sun in its aphelion than Voyagers now.

Probably, the proper definition depends on if there is really Oort cloud, as an obvious outer rim

Edited by kerbiloid
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12 hours ago, tater said:

Apparently it's gonna take a few hundred years before they are even at the inner edge of the Oort Cloud.

I suspect this is the main reason for the definition. "Voyager 2 passes heliopause" isn't nearly as punchy as "Voyager 2 enters interstellar space"

Also, Matthew McConaughey and Anne Hathaway didn't make a billion-dollar movie named "Heliopause" so you gotta think of the SEO.

Edited by 5thHorseman
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48 minutes ago, kerbiloid said:

When a craft gets out of the Earth atmosphere, it doesn't stop being an Earth satellite. An we don't say that it has reached interplanetary space.

If it is given escape velocity, we don't call it an Earth satellite, we call it New Horizons, Insight, etc.

:)

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Hey guys,

i simply meant Keplerian Orbits, the ones we love and hate, depending on whether they bring us to were we want to go or not.

If you prefer trajectory for the open orbits, no problem. We all know now where the V'gers are and i have learned new definitions of interstellar space or medium.

I mean, in the end, we all "orbit" the Great Attractor, until expansion of space us parts.

:-)

Edited by Green Baron
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On second thought, the concept of the heliosphere in comparison to the hill sphere may be more convenient for this "use case". If we took the hill spheres of every sun in a galaxy, there'd be hardly any interstellar space, only areas with very low gravity.

Though, if somebody planned to travel to the next star he/she/they would have to do that along the "lines" of gravity, exiting one system on a para- or hyperbole and enter the next one on a similar trajectory/orbit/stone's throw.

 

Edited by Green Baron
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Sphere of influence.
The region around a celestial body where the primary gravitational influence on an orbiting object is that body.

r ~= R * (m / M)2/5

Hill sphere.
The region in which it dominates the attraction of satellites.

r ~= R * (m /(3 * M))1/3

The region of stability: stable satellite orbits exist only inside 1/2 to 1/3 of the Hill radius.

***

Sun orbit: V = 220 km/s, R = 27.2 kly.

Spoiler

V = sqrt(GM/R);

Mgal = R * V2 / G = 27 200 * 9.5*1015 * (2.2*105)2 / 6.67*10-11 = 1.9*1041

Rinf ~= 27 200 * (2*1030 / 1.9*1041)2/5 = 1.1 ly.

Rhill ~= 27 200 * (2*1030 / (3 * 1.9*1041))1/3 = 4.1 ly.

So, the Sun's gravitational feud is 1-2 ly in radius.

***

Alpha Centauri M ~= 2 Msun.

So, relative to them Sun prevails inside ~= 1.5 ly sphere.


***

Say, escape condition for gases is EscapeDeltaV ~= ThermalVelocity.

Spoiler

Luminosity = 3.83*1026 W.

EquilibriumTemperature = (Luminosity / (4 * pi * 5.67*10-8 * Distance2 ))0.25 ~= 34.4 Luminosity0.25 / Distance0.5.

ThermalVelocity = (3 * 8.31441 * EquilibriumTemperature / MolarMass)0.5 = (3 * 8.31441 * 34.4 Luminosity0.25 / (Distance0.5 * MolarMass))0.5 =
    = (3 * 8.31441 * 34.4)0.5 * (Luminosity0.25 / (Distance0.5 * MolarMass))0.5 ~= 29.3 * Luminosity0.125 / (Distance0.25 * MolarMass0.5)
    = 29.3 * Luminosity0.125 / (Distance0.25 * MolarMass0.5) = 29.3 * (3.83*1026)0.125 / ((9.5*1015 * DistanceLY)0.25 * MolarMass0.5) = 
    = 29.3 * (3.83*1026)0.125 / ((9.5*1015)0.25 * DistanceLY0.25 * MolarMass0.5) = 6.25 / (DistanceLY0.25 * MolarMass0.5) m/s.    
EscapeDeltaV = (sqrt(2) - 1) * sqrt(GM / Distance) = (sqrt(2) - 1) * sqrt(6.67*10-11 * 2*1030 / (9.5*1015 * DistanceLY)) ~= 49 / sqrt(DistanceLY) m/s
    
6.25 / (DistanceLY0.25 * MolarMass0.5) ~= 49 / DistanceLY0.5

DistanceLY0.25 ~= 7.84 * MolarMass0.5

DistanceLY ~= 3800 * MolarMass2

Atomic hydrogen 
MolarMass = 1*10-3 kg/mol.
DistanceLY ~= 0.004 ly ~= 250 AU

Molecular hydrogen 
MolarMass = 2*10-3 kg/mol.
DistanceLY ~= 0.016 ly ~= 1000 AU

Helium
MolarMass = 4*10-3 kg/mol.
DistanceLY ~= 0.06 ly

Water & Ammonia
MolarMass = 17..18*10-3 kg/mol.
DistanceLY ~= 1.2 ly

***

So, radius of the Sun feud is ~1 ly for anything except H & He, but anyway ~1000 AU even for the hydrogen.

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