• Quick note - the problem with Youtube videos not embedding on the forum appears to have been fixed, thanks to ZiprHead. If you do still see problems let me know.

Space Travel: would this work?

How would you slow down at the other end? Airobraking in the photosphere of a red giant?
 
Ziggurat said:
It doesn't work that way. You cannot slingshot in arbitrary directions. Within the solar system, you only gain speed by coming in with low angular velocity compared to the planet you're using for your slingshot and leaving with much higher angular velocity (best case scenario: about twice the angular velocity of the planet). Once you've done that with, say, Jupiter, any further slingshots around other planets are probably going to slow you down, not speed you up, because you'll be approaching from the wrong direction.
Click to expand...
I'm still not sure I understand how this works. If the increase in velocity is only in an orbital direction then that component in the satellites velocity would simply continue orbitting. To decrease the travel time to the outer planets would one not need to increase the velocity directed away from the sun? How is the increase in that component of velocity achieved?

(I can see how one might, with the right timing, but its hard to visualize.)
 
John Hewitt said:
I'm still not sure I understand how this works. If the increase in velocity is only in an orbital direction then that component in the satellites velocity would simply continue orbitting.
Click to expand...

Jupiter has orbital motion with respect to the sun. So does any satellite we throw out there. We can use the slingshot to increase the velocity of the satellite, but ONLY in the same direction as the orbital motion of Jupiter (again, with respect to the sun).

To decrease the travel time to the outer planets would one not need to increase the velocity directed away from the sun?
Click to expand...

If you're in a circular orbit, and you get a speed boost in the direction of that orbit, you will not remain on that circular orbit: your new orbit will be elliptical, and will bring you farther outward, away from the sun. If the boost is large enough, the orbit will actually become hyperbollic, and you'll escape the solar system.

One way of thinking about this is in terms of angular momentum (with respect to the sun), rather than velocity. The farther out an orbit is, the larger its angular momentum per unit mass (elliptical orbits have angular momentum between that of a circular orbit with a radius of their farthest point and a circular orbit of their nearest point). To escape the solar system completely requires very large angular momentum, and if you have sufficient angular momentum you will escape. You can only really GAIN angular momentum from a planetary slingshot if you begin your trajectory with less angular momentum (per unit mass) than the planet you're using to slingshot past, and you'll end the slingshot with more angular momentum per unit mass than the planet. There's a maximum amount of angular momentum per unit mass that can be transfered in this process, one good slingshot will bring you close to that limit, and repeating slingshots within the solar system after that will not really help. You might be able to use it to help get from inner planets to outer planets for less energy, and then use a final slingshot from an outer one to get maximum benefit, but you cannot just keep building up speed within the solar system, since if you're already at high angular momentum, you'll be approaching planets from the wrong side and will lose speed, not gain it, from a slingshot.
 
jmercer said:
Hmm... so what is it? Orbital velocity or planetary rotation that loses energy? (And Dr. K - wouldn't a loss in rotation mean that the traveler slings around the world counter-rotation?)
Click to expand...

It's the orbital velocity of the planet, NOT the rotation of the planet, which is important for the slingshot effect, and it's the orbital motion, not the rotation, which will be slowed down. It is incredibly difficult to even detect the gravitational effect of a planet's rotation, known as frame dragging (see Gravity Probe B for an experiment designed to do this). You'd need something like a spinning black hole to get any appreciable transfer of angular momentum from the rotating body to the passing object from frame-dragging effects.
 
drkitten said:
My understanding is that the loss actually comes from the planet's rotational speed. Again, a real physicist is welcome to correct me.
Click to expand...
I don't know whether I'm a real physicist. But I'm pretty sure that's not the way it works. You can slingshot off a nonspinning planet as easily as off a spinning one.

The simplest way to analyze a slingshot is to use the reference frame in which the planet is stationary. Since the planet is so massive compared to the spaceship, it basically stays stationary throughout the maneuver, and the spaceship leaves the planet at the same speed that it approached it, but in a different direction. Then, you can just figure out how this looks in any other reference frame you're interested in.
 
CFLarsen said:
As usual, Arthur C. Clarke thought of it long time ago:

Rendezvous with Rama

:)
Click to expand...

I read this book when I was a kid, and it made me angry. I kept waiting for the climax of the story, and...it never happened. Where were the long-hibernating aliens awoken from their slumber, only to wreak havoc on their discoverers? Where was the artificial intelligence left behind by the dead aliens, continuing to act on its now-pointless programming by killing the intruders?

Bah....

Maybe if I read it again today, I would get more out of it.
 
aggle-rithm said:
I read this book when I was a kid, and it made me angry. I kept waiting for the climax of the story, and...it never happened. Where were the long-hibernating aliens awoken from their slumber, only to wreak havoc on their discoverers? Where was the artificial intelligence left behind by the dead aliens, continuing to act on its now-pointless programming by killing the intruders?

Bah....

Maybe if I read it again today, I would get more out of it.
Click to expand...

You could try the sequels which Clarke wrote in collaberation with Gentry Lee. If you enjoy watching paint dry.
 
I thought it was the angle of the planets orbit that changed instead of the speed of the orbit or am I incorrect? (wouldn't suprise me)
 
Even if you get your spacefraft up to relitivistic speed you then have the problem of what to do about being hit by hydrogen atoms at signifcant fractions of the speed of light. Problem gets worse if you travel outside the Local Bubble.
 
One can think of the slingshot as being a special type of "collision" in which the two objects never actually touch.

Badly Shaved Monkey said:
I am the editor of Encyclopaedia Britannica. It is not. :)
Click to expand...
The editor? Not just an editor?

TjW said:
Using the gas giants and the sun to give an interstellar probe a higher initial velocity might be interesting, though.
Slingshot around the moon.
Slingshot around the Earth.
Then spend a fifty or a hundred years slingshotting around the system, occasionally burning fuel or solar sailing to increase speed or set up for the next encounter.
I wonder what sort of velocity would be achievable?
It might make an interesting setting for a science fiction story, since, although they could be in communication with Earth, a large difference in velocity would make them inaccessible.
Click to expand...
I weould think that it would be rather obvious that once one exceeds the exscape velocity of the sun, one will no longer be in orbit.
 
geni said:
Even if you get your spacefraft up to relitivistic speed you then have the problem of what to do about being hit by hydrogen atoms at signifcant fractions of the speed of light. Problem gets worse if you travel outside the Local Bubble.
Click to expand...

That's what navigational deflectors are for, der. ;)
 
Ziggurat said:
It's the orbital velocity of the planet, NOT the rotation of the planet, which is important for the slingshot effect, and it's the orbital motion, not the rotation, which will be slowed down. It is incredibly difficult to even detect the gravitational effect of a planet's rotation, known as frame dragging (see Gravity Probe B for an experiment designed to do this). You'd need something like a spinning black hole to get any appreciable transfer of angular momentum from the rotating body to the passing object from frame-dragging effects.
Click to expand...

Makes sense to me, thanks. :)

So it appears that the slingshot effect is a result of the conservation of angular momentum, correct?
 
Soapy Sam said:
How would you slow down at the other end? Airobraking in the photosphere of a red giant?
Click to expand...

:eek: :flamed: That sounds HOT!

I suppose a revers sling-shot is equally applicable: You approach at an angle to the orbital motion of some heavy body, then climb out the long way.

As for reaching relativistic speeds, there is the problem of orbital velocity: You need to have your direction changed by the pass, so the faster you go, the less deflection you get. At relativistic speeds, you would need to pass very close to a very strong gravitiational field to get sufficient deflection, and even not counting the many weird effects around black holes (which are the only conceivable sources of a sufficiently strons gravitational field you can get really close to without colliding with it), the tidal forces would probably rip anything apart.

Hans
 
How do you define "interstellar space travel"?

Voyager 1 passed through heliopause some time ago, and I think Voyager 2 is about to (if it hasn't already--I haven't been following). I thought they only used thrusters for changing trajectory, and relied on the "gravitational sling-shot" to escape the solar system.
 
You must log in or register to reply here.

Back
Top Bottom