A Discussion About Black Holes

[Pikachu8091]

Hello, everyone. I had a question about black holes, but this is also a thread for other people's thoughts about black holes.

I was wondering, could a spacecraft use black holes as a catapult sort of propulsion? For example, it's well known that modern-era spacecraft will use temporarily orbit a planet, hit the periapsis of the orbit, then burn their way out of orbit, with the extra speed given from hitting the periapsis. My question, again, is, could a spacecraft do this some type of manuever with a black hole? And where would they go about it? On the edge of the black hole's event horizon?

[Godot]

Gravitation is gravitation, so I guess the answer is yes.

Guess you just have to be careful to do this with a save distance of the black hole, so the gravitation gradient doesn´t rip your ship apart

[kStrout]

Be careful of tidal forces, send probes or Jeb first.

[Rakaydos]

Read the Larry Niven short story "Neutron Star" for the dangers of something like this.

It's only a neutron star in the story, they've got FTL drives and indestructable hulls... and effectively a murder mystery. (The makers of the indestrucable hulls wants the main character to investigate what bypassed the hull, so they know if they need to update their guarentees.)

[Pikachu8091]

Read the Larry Niven short story "Neutron Star" for the dangers of something like this.

It's only a neutron star in the story, they've got FTL drives and indestructable hulls... and effectively a murder mystery. (The makers of the indestrucable hulls wants the main character to investigate what bypassed the hull, so they know if they need to update their guarentees.)

I actually heard about that a while ago, heard it was a good read. I love sci-fi stuff, but I prefer relatively near future stuff. Hyper-warp drives and FTL just isn't fun to imagine!

[Rakaydos]

I actually heard about that a while ago, heard it was a good read. I love sci-fi stuff, but I prefer relatively near future stuff. Hyper-warp drives and FTL just isn't fun to imagine!

The Known Space series covers a huge span of time- a few of the stories follow the first astronaut on Venus and Mercury, for example. But though alien tech shows up fairly early, Niven like to explore the fridge logic of these devices, like a teleportation booth that works like a phone booth, making alibis no longer a thing in criminal cases as you can murder someone halfway around the world and be back in the time it takes to use the restroom.

[Aethon]

Read the Larry Niven short story "Neutron Star"

Agreed! One of my fav sci fi short stories.

One problem I see with your question Pikachu. Gravity assists use the orbital motion of a body to gain momentum relative to that body. Black holes are usually at rest to their local systems. I imagine you could get a nice Oberth effect though.

[K^2]

Black holes are usually at rest to their local systems.

That's a strange way of saying what I think you're saying. But yes, the situations in which you could actually make practical use of black hole gravity assist are bound to be rare.

On the other hand, if we're talking about using black holes for boosts, we probably aren't talking about things on scale of a star system. There are plenty of black holes going around with the other stars in our galaxy, and these can still be used for significant boosts. One of the limiting factors of boosts from planets or even stars is that there is only so much angle change you can get without hitting the thing. With black holes, that limitation does not exist. Your trajectory can have any exit angle you like. Just watch out for tidal forces.

[Pikachu8091]

In relation to my first question, do black holes have a gravitational force outside of the event horizon? I've heard that both once you hit the event horizon, there's no going back as well as you can escape the event horizon depending on how close you are to the actual black hole.

[SelectHalfling0]

The event horizon is merely the point at which light can not escape (and therefore nothing else since energy is the fastest known force in existence) and is simply sucked into the black hole. There are still huge forces (gravity of the black hole) outside the event horizon, which can still effect anything from radio waves to spacecraft (that is why light is bent by the black hole). In relation to your second question, there is no escape from the black hole once you have passed the event horizon. To make it simpler, imagine you are on the edge of a sinkhole. The crossing the event horizon would mean you "jumped" into the sinkhole (without any means of flying back up).

[lajoswinkler]

It could be done if the periapsis is high enough to avoid tidal forces in case of smaller and medium sized holes, and to ensure it's way above the horizon event.

In case of very large black holes, tidal forces are unimportant and they're catastrophic inside the event horizon.