# Regarding Black Holes

• November 28th, 2007, 10:32 AM
Regarding Black Holes
Speculation: Possibility of using the gravity of a black hole for the "slingshot" effect of boosting speed during space travel.

The event horizon of a black hole is the "point of no return," past which you can never return to normal space and are inexorably drawn into the black hole.

Is there a point, or line, at which one could use the gravity to slingshot a spacecraft, boosting speed, like current spacecraft do? (This process has been used to send unmanned spacecraft outwards-- ie using Mars' and Jupiter's gravitational field.)

In other words, is there a region around a black hole that, if you had sufficient speed, you could enter this region, zoom around-- picking up (phenominal, obviously) speed-- without getting sucked into the vortex?

This is the kind of stuff that keeps me awake at night. Lol.

Thoughts? Opinions?
• November 28th, 2007, 12:33 PM
Janus
The first thing you need to learn is how a gravity slingshot works. It works by transfering momentum from one body to another. Gravity is not the cause of the increased speed, it only acts as the means by which the momentum is transfered. How much momentum is transfered and which way it is transfered (Gravity slingshots can also be used to reduce the velocity of a probe) depends on the relative trajectories of the two objects.

For example, with using Jupiter as a slingshot, we first launch a craft that ends up a little ahead of and moving with a slower orbital speed than Jupiter. Assuming the craft's orbital speed is 3 km/sec and that Jupiter's is 13 km/sec, this gives them a relative velocity of 10km/sec with Jupiter moving towards the craft. (imagine you are looking down on the pair with their orbital velocities moving the to the left.)

As Jupiter's gravity begin to express itself on the craft, it begins to fall faster and faster towards Jupiter. (From our perspective, as the distance closes begin Jupiter and the craft, the craft begins to at first lose and then reverse orbital speed. (it slows to a stop and begins moving to the right)

Assuming we placed the craft correctly, it will whip around Jupiter in a hairpin curve and begin to climb back out of Jupiters gravity well it has taken a hyberbolic path around Jupiter. As it pulls away from Jupiter, it is moving in a different direction than it was when it was falling in. (where as before it was moving to the right, it is now moving to the left. As first it will be moving fast but will slow as it pulls away from Jupiter. When it reaches its orginal distance from Jupiter it will have the same relative velocity with respect to Jupiter, but now will be moving in the opposite direction with respect to Jupiter. Assuming it made a perfect 180 degre turn it is now moving 10km/sec to left with respect to Jupiter plus Jupiter's 13km/sec orbital velocity, or from our perspective it is moving at 23km/sec to the the left, compared with the 3km/sec to the left that it was moving before the encounter. In the exchange, Jupiter loses a little orbital velocity, but due to the large differences in mass it is minute.

Now in real life we can't quite do that 180 degree hairpin turn and we do shallower curve, as a result with don't get that full 20 km/sec change in velocity but a little less.

Once you've read and digested this, we can discuss the pluses and minuses of using a blackhole for such a slingshot.
• November 28th, 2007, 12:53 PM
I (mostly) Understand
I get the general idea of your explanation. (I can close my eyes and imagine it, more or less).

So! Could the same method be used to slingshot around a black hole, or is the case that if you even graze the gravitational pull of a black hole you're lost forever.

Is there a region (theoretically) that you can fly into and then back out of, or are you simply damned from the get-go. Is there, say, a just-outside-the-event-horizon-where-you're-still-okay type of situation?
• November 28th, 2007, 01:34 PM
Janus
As long as you don't cross the event horizon you can orbit in as close as you want to a black hole, with a couple of caveats. Depending on the size of the black hole, tidal forces could tear you apart before you even get that close.
The region near an event horizon might not be too healthy due to x-rays emitted by infalling dust and gas.

What would be the advantage of using a black hole? Primarily just one. You can more closely approach that 180 degree hairpin turn and get the most out of your velocity difference.

Other than that, there is no real advantage. You still have to set up a proper relative velocity difference, and the maxiumum velocity gain will be limited by that difference.

So you either have to have a blackhole orbiting another body that you are also orbiting and get your velocity change relative to that body (like we do with Jupiter). Or you have to be sitting in space waiting for a black hole heading in the right direction to pass by closely enough and at high enough speed to give you your boost.
• November 28th, 2007, 01:54 PM