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» Hatrack River Forum » Active Forums » Books, Films, Food and Culture » pseudomayfly: What's the planetary equivalent of a black hole's "event horizon"?

   
Author Topic: pseudomayfly: What's the planetary equivalent of a black hole's "event horizon"?
James Tiberius Kirk
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I'm trying to settle a scifi question. I know, technically, planets don't really have an equivalent - but I figure there has to be some term (or a set of terms) for the outer limit of an object's gravity well (inasmuch as one exists). Google isn't helping me here. Can some one point me in the right direction?

[ May 18, 2012, 02:27 AM: Message edited by: James Tiberius Kirk ]

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Hobbes
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There is none. You could set a reasonable threshold beyond which you, or your ship, or a comet or whatever wouldn't move significantly as a result of the gravity but it's not a fundemental property like the event horizon.

Also, the event horizon doesn't represent the outer limits of the gravity well of a blackhole, which I'm guessing you know, but maybe if you clarified what you were looking for...

Hobbes [Smile]

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James Tiberius Kirk
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quote:
Originally posted by Hobbes:
There is none. You could set a reasonable threshold beyond which you, or your ship, or a comet or whatever wouldn't move significantly as a result of the gravity but it's not a fundemental property like the event horizon.

Also, the event horizon doesn't represent the outer limits of the gravity well of a blackhole, which I'm guessing you know, but maybe if you clarified what you were looking for...

Hobbes [Smile]

Sure - I'll try to clarify.

Say you have a probe flying past Jupiter. You don't want to get too close, or else you'll end up in orbit, or crash. I suppose the minimum distance you could fly past without getting trapped in orbit depends on any number of the physical properties of the probe, like its velocity.

What is that minimum distance called? Not quite the same as 'event horizon' but maybe you see why I made the analogy.

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Shigosei
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I think the event horizon is the point at which the escape velocity is equal to the speed of light. You could probably draw similar boundaries around a planet for a particular escape velocity lower than the escape velocity at the planet's surface.

On the other hand, supposedly there's lots of weird space-time warping or something going on inside the event horizon, so I don't think it's quite as mundane as a simple "at this point you must go this speed to escape orbit.*"

*Note that you don't actually have to exceed escape velocity to escape a planet's gravity as long as you're under powered flight and not coasting. As far as I know, this does not apply to the event horizon -- no amount of propulsion will let you escape.

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Shigosei
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JTK, this article might be helpful with the example you give: http://en.wikipedia.org/wiki/Escape_velocity
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El JT de Spang
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In this case, I think he's just looking for the outer boundary of the gravity well of any particular planetary body.

My very meager knowledge of astrophysics leads me to guess that there's no hard and fast line for that, that it will depend on the masses of the planet and the satellite/spaceship/whatnot.

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mr_porteiro_head
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For any distance from a planet, there's a speed which would give you a stable circular orbit. This is known as the escape velocity.
If you are traveling faster than that, you'll naturally leave orbit unless. If you're going slower, you'll naturally crash into the planet. If you're going exactly that speed you'll have a stable orbit, either circular or elliptical, depending on the direction you're traveling.

Provided you don't run into the planet or anything else, and that no other forces act on it (like thrust).

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James Tiberius Kirk
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Thanks for all the help folks.

The relation between escape velocity and distance is closest to what I'm looking for, I think - but there just doesn't seem to be a discrete term for that. I propose we invent one.

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King of Men
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Conceivably you are thinking of something like the specific orbital energy?

If I took the thread title literally (and I understand that this doesn't actually answer the real question) I would probably refer you to the Schwarzschild radius. For any given object, the Schwarzschild radius is the distance such that compressing the mass to that radius would make it a black hole.

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El JT de Spang
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That's an interesting concept, KoM. I'd forgotten there was a name for that.
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aspectre
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What goes up must come down. And when talking about orbital mechanics, what falls down must come back up...
...unless something more complex than a simple Newtonian two-body gravitational interaction blocks the process.

Except for a BlackHole* -- for which the escape radius of matter falling from an infinite distance away (ie the definitional equivalent of escape velocity) is 2times the radius of the EventHorizon -- all naturally falling bodies (ie system-orbiting bodies, or those with system-escape or greater velocity) will gain enough (speed) kinetic energy from the fall toward a planet to use that same kinetic energy to climb back out of its gravity well...
...Unless their initial trajectory is such that they hit the body...
or they interact with the planet and the moon in just the right manner that a gravitational slingshot effect takes place.

When talking about a planet orbiting a star, the closest to an effective "end" to a planet's gravity well is defined by the radius of its HillSphere.

* And possibly some maximal NeutronStars, hypothetical QuarkStars, and other hypotheticals. The equations of state for such hyper-dense states aren't sufficiently well-defined to say that the results even approximate the reality of "stars" made of the stuff, or even IF the stuff can be made.

[ May 13, 2012, 12:48 AM: Message edited by: aspectre ]

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aspectre
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Gotta ask... Why would you think that this thread might be worth mayflying (ie deleting after a brief lifespan)?
Or did you just mean that you needed some answers quickly?

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King of Men
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There may be a certain amount of selection bias involved; but I've never actually seen a thread marked 'mayfly' disappear.
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Dan_Frank
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I can imagine, if there was a word that fit exactly what he wanted, and that was the very first post, then the thread might simply end and "die" at that point... but probably just by dropping off the front page rapidly, not being deleted.

I agree with KoM, I can't remember seeing a mayfly thread actually be deleted... ever. The only threads I've seen deleted were... not mayflies. And caused a lot of controversy.

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rivka
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quote:
Originally posted by King of Men:
I've never actually seen a thread marked 'mayfly' disappear.

They do, sometimes.
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Samprimary
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a mayfly thread is a thread that will probably be around forever and have its data preserved through the heat death of the universe. you can tell because it says mayfly at the front
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Annie
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quote:
What's the planetary equivalent of a black hole's "event horizon"?
For this planet at least, it's yo momma.

Glad I could help.

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BandoCommando
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I always figured "mayfly" referred as much the the acceptance that it would "fly" off of the front page as much as the term referred to the possibility of deletion...
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odouls268
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I'm just shooting from the cuff here, but I've decided to call it the "Orbital Well."

Just made it up, and have made the decision completely capriciously. Thanks for making me think about it though, it was a fun exercise. Any others?

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aspectre
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Ifn ya'd thought it through, ya'da called it ***O'Doul's*Deep*Draught*** cuz it pulls ya in even though ya haven't volunteered.

[ May 17, 2012, 08:09 PM: Message edited by: aspectre ]

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James Tiberius Kirk
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quote:
Originally posted by aspectre:
Gotta ask... Why would you think that this thread might be worth mayflying (ie deleting after a brief lifespan)?
Or did you just mean that you needed some answers quickly?

Didn't anticipate much of a discussion.
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