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https://en.wikipedia.org/wiki/Black_hole#Innermost_stable_circular_orbit_(ISCO)

ISCO is apparently the current theoretical limit for orbiting black holes (as sourced by wikipedia).

Original Question:

I was reading this question and many different answers eluded to the Event Horizon being the theoretical limit for orbiting a black-hole and that "bad things happen" when you dip beyond the event horizon. Is this inclusive or exclusive? Provided we were able to reach the orbital velocity required to orbit a black hole 1 mile away from the event horizon (excluding orbital drift), what would happen if we adjusted the orbit to EQUAL the event horizon, but not exceed it?

I understand the scientific possibility (with current technology) is 0%, but theoretically what would happen if we tried to orbit a black-hole AT the event horizon?

Based on the current answers, I'd like to shift the future answers to a more theoretical realm. I now understand that orbiting a black hole AT the event horizon is the SAME as being beyond it. Theoretically, how close could we approach the event horizon if we were to assume our craft is indestructible as dictated by our current understanding of physics (I realize studying a black hole up-close will likely redefine physics forever).

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  • $\begingroup$ There will be a lot of random gas and dust in close orbit, so you'll encounter a lot of drag. Does our magical tidally-indestructable orbiter also have infinite ∆v capability to shore up its orbit? Is it also immune to frictional heating? $\endgroup$
    – Russell Borogove
    Jun 27, 2018 at 20:02
  • $\begingroup$ Black holes do weird things to our models of special and general relativity. This means that it does weird things to our models of electromagnetism which is what holds your 'indestructible' spacecraft together. $\endgroup$
    – Jack
    Jun 27, 2018 at 20:06
  • $\begingroup$ There are some theoretical configurations that could affect your question eg. Naked Singularities $\endgroup$
    – Jack
    Jun 27, 2018 at 20:09
  • $\begingroup$ @RussellBorogove I would like to make assumptions that are akin to base-line physics questions. I assume we can say that "particulate drag" is akin to "wind resistance" and can be ignored. Also, provided that the black hole "sucks in" light, thermal considerations can be lumped into "indestructible craft", even thermal friction. Seeing as there isn't an experimental base-line for black-holes any theoretical assumptions you would like to make for a theory on how we'd orbit at (or near) the event horizon are up to you. $\endgroup$
    – Magic Octopus Urn
    Jun 27, 2018 at 20:09
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    $\begingroup$ If you're wanting an answer on perfectly idealised spacecraft, you might be better off asking on Physics or Astronomy SE $\endgroup$
    – Jack
    Jun 27, 2018 at 20:13

2 Answers 2

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I believe that outside the event horizon you can orbit it when traveling below the speed of light, while at the event horizon you would have to be traveling at exactly the speed of light. If you slowed down for any reason (such as colliding with particles in the accretion disk) you'd end up within the event horizon with no hope of escape.

Even traveling at exactly the speed of light and being able to maintain it indefinitely, you could only escape from the event horizon orbit by traveling faster than the speed of light, which is bad enough!

More realistically, you could orbit a black hole at a minimum distance from the singularity at which you can form a stable orbit, called the Innermost Stable Circular Orbit. In this orbit, a spacecraft could stay indefinitely without having to provide energy (Assuming no preturbations).

You could theoretically orbit a black hole within the event horizon, but you'd have to travel faster than light which isn't possible (as far as we know).

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    $\begingroup$ I like this answer, as it matches my assumptions; but it is also un-sourced. Do you have a source for this (even wikipedia is fine) I'm just looking for additional research materials on orbital mechanics pertaining specifically to black holes. $\endgroup$
    – Magic Octopus Urn
    Jun 27, 2018 at 20:03
  • $\begingroup$ I haven't found a source for my answer, it comes from things i've picked up over the years. I have found this however, if you want to get very technical, there's a minimum distance from the singularity at which you can form a stable orbit (i.e. not have to provide energy to keep you in your orbit), called the ISCO $\endgroup$
    – sleepingNewton
    Jun 27, 2018 at 20:24
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    $\begingroup$ YES! This is the answer I was looking for man. You killed it :). As Russel previously stated any perturbations would obviously cause catastrophic destabilization (which would've occurred long before this point). But this equation is something that is vastly helpful, provided I can source it somewhere other than wikipedia. $\endgroup$
    – Magic Octopus Urn
    Jun 27, 2018 at 20:26
  • $\begingroup$ Glad I could help mate! $\endgroup$
    – sleepingNewton
    Jun 27, 2018 at 20:29
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    $\begingroup$ Not to impose, but can you add that to your answer? The ISCO part? If I decide not to delete the question, that will be a key proponent to why this is the selected answer over the other. $\endgroup$
    – Magic Octopus Urn
    Jun 27, 2018 at 20:36
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The event horizon is a singularity, one-sided, so there's no difference between being at the event horizon and beyond it.

"Bad things happen" long before that point, though; a million miles from a black hole your orbiter would be shredded by tidal forces, and at one mile from the event horizon the orbiter would be mostly radiation.

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  • $\begingroup$ Not only the orbiter would be shredded by tidal forces, also everything and everybody in it. Yes everybody. $\endgroup$
    – Uwe
    Jun 27, 2018 at 20:00
  • $\begingroup$ I've edited the question to assume an indestructible craft, and am more interested about the answer (in terms of theoretical physics as we understand them today). What is the drop-dead closest we could orbit a black hole, and what is the requirements for this (indestructible craft or otherwise). $\endgroup$
    – Magic Octopus Urn
    Jun 27, 2018 at 20:02
  • $\begingroup$ Also "the orbiter would be mostly radiation" is a hilarious thought. Does this mean that it is possible for atomic structures to orbit the black hole in a stable orbit provided that they encounter no drag? $\endgroup$
    – Magic Octopus Urn
    Jun 27, 2018 at 20:23
  • $\begingroup$ @RusselBorogove Should I delete my question? I don't know that it contributes to the site due to fundamental flaws in the wording. I feel that sleepingNewton provided adequate material to answer my flaws though, and you supplemented them. What is the best way to handle this question now? Sorry you have 65k rep, I take what you say as law hah! $\endgroup$
    – Magic Octopus Urn
    Jun 27, 2018 at 20:31

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