Space Travel: would this work?

[Badly Shaved Monkey]

Spacecraft use gravitational sling-shots to gain energy travelling around the Solar System. Could this be used on an interstellar scale?

I'm thinking more science fiction than planning my own emigration to Alpha Centauri.

If the hard work was done to get people and resources out of our Sun's gravitational well, you could set a world-ship on its path to the stars. Over generations it would reach our nearest star where it could sling-shot and gain speed. Could that be done repeatedly to build speed up to relativistic velocities, so that our far-future descendents could zip around the universe at relativistic speed expending little subjective time in crossing vast distances? The inhabitants of the ship would then live 'fast' in the life of the universe, but would be effectively isolated from any 'slow' civilisations with which their paths intersected because there would be so little opportunity for contact.

Is this really feasible? Is it feasible, but only in a vague Start Trek-tech kind of way? Or, is it flawed in a way that would not even get it onto Start Trek?

 

[CFLarsen]

As usual, Arthur C. Clarke thought of it long time ago:

Rendezvous with Rama

 

[Crossbow]

Badly Shaved Monkey:

Uhm, Gravitation Acceleration has been done for the last several years now.

The space probes to Mars, Jupiter, Saturn, and other planets have used it for quite some time.

I can find more data if you are so interested.

 

[drkitten]

Uhm, Gravitation Acceleration has been done for the last several years now.

Given that he specifically mentioned it, I think he's aware of that.


As far as I can tell, the question isn't "does gravitational acceleration work," but "would it work on an interstellar scale?"

In my opinion, the answer is "probably not." Certainly using a star for gravitational acceleration is possible -- we've done it using the Sun. But the amount of acceleration you could get from any given star, and the distances between stars, mean that the number of stars it would take to accelerate to anything near relativistic speed and the amount of time that it would take would be prohibitive.

 

[MRC_Hans]

It's not as easy as that, I think. Just going down a gravity well and back up will leave you with (at best) the same amount of energy (=speed) as you started with. Notice that space probes don't use the strongest gravity field in the solar system, that of the sun, instead they use planets.

The trick is that you need to spend different times going in and out, and this requires your "slingshot" to be moving relative to your starting position. A probe will approach (say,) Jupiter in the same direction as Jupiters orbital motion. This means that it spends a certain amount of time "catching up" with Jupiter, all the time being accelerated by its gravity field. Then it swings by the planet and leaves in another direction, so now Jupiters motion relative to it is much higher, so the time spent going out is shorter. Thus, it is de-accelerated for a shorter time, leaving it with a net increase in energy.

To do this with stars, we would need to use stars that have some motion relative to the sun.

Hans

 

[Overman]

Nice job Hans!

 

[TjW]

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.

 

[Earthborn]

I said it before, and I'll say it again: Wikipedia is yer frien'.

Or, is it flawed in a way that would not even get it onto Start Trek?

I don't think it is that flawed. It is probably not even as flawed as many of the concepts included in Star Trek. But whether it is really feasible is hard to say in a world where getting someone in orbit is already extremely difficult.

Space travellers who use such a system would -- I imagine -- choose to hop between systems with heavy planets, binary stars or even star clusters where they stay a few hundreds of years getting faster and faster orbits. Whether it is really possible to get up to substantially relativistic speeds that way, I don't know.

 

[jmercer]

It's not as easy as that, I think. Just going down a gravity well and back up will leave you with (at best) the same amount of energy (=speed) as you started with. Notice that space probes don't use the strongest gravity field in the solar system, that of the sun, instead they use planets.

The trick is that you need to spend different times going in and out, and this requires your "slingshot" to be moving relative to your starting position. A probe will approach (say,) Jupiter in the same direction as Jupiters orbital motion. This means that it spends a certain amount of time "catching up" with Jupiter, all the time being accelerated by its gravity field. Then it swings by the planet and leaves in another direction, so now Jupiters motion relative to it is much higher, so the time spent going out is shorter. Thus, it is de-accelerated for a shorter time, leaving it with a net increase in energy.

To do this with stars, we would need to use stars that have some motion relative to the sun.

Hans

Something I've always wondered about is that increased acceleration means a net gain in energy for the accelerating object (at some point.) Obviously, the mass of the planet doesn't decrease when the traveller departs, so the gravity well remains the same. I strongly suspect the planet's orbital motion slows down a tiny bit, but I'd rather hear that confirmed by someone who knows better. Otherwise it's just speculation on my part.

 

[drkitten]

Something I've always wondered about is that increased acceleration means a net gain in energy for the accelerating object (at some point.) Obviously, the mass of the planet doesn't decrease when the traveller departs, so the gravity well remains the same. I strongly suspect the planet's orbital motion slows down a tiny bit, but I'd rather hear that confirmed by someone who knows better. Otherwise it's just speculation on my part.

My understanding is that the loss actually comes from the planet's rotational speed. Again, a real physicist is welcome to correct me.

 

[Ziggurat]

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.

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.

 

[Ziggurat]

I strongly suspect the planet's orbital motion slows down a tiny bit, but I'd rather hear that confirmed by someone who knows better.

Yes, that is exactly what happens.

 

[Badly Shaved Monkey]

The mass-energy of the planet must have been reduced if the velocity of the probe has increased.

 

[Badly Shaved Monkey]

this requires your "slingshot" to be moving relative to your starting position.

Thanks for that. It's the first time I've seen that aspect of the slingshot mentioned. I'd not seen how the energy got 'borrowed' before.

It does look like a limitation for my interstellar scheme. I'm not giving the money back for the tickets I've already sold.

 

[Badly Shaved Monkey]

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.

Further to what Hans said, it's the requirement for specific velocities and angles that scuppers my scheme. Or maybe it just extends its timescale as my ship needs to go round a large part of the Galaxy like a pinball. The Universe has a long time left to run, so maybe it could still work.

 

[casebro]

Did I catch the gist of Hans' post, that the acceleration is due to the gravitational pull on the way in towards the planet/sun? So the "slingshot effect" would have more to do with the trajectory, since the acceleration would have happened before the " crack the whip" effect. Or, not the rubber-band type sling shot, but the old fashioned string-whirled-around-your-head kind of sling. Now I think I understand...

 

[Ziggurat]

The mass-energy of the planet must have been reduced if the velocity of the probe has increased.

Yes, but you don't have to get relativistic for this. The whole scheme works fine with Newtonian mechanics, where the mass and energy are independent quantities. In both cases, though, the body used for the slingshot effect gets slowed down. I think it's actually easier to figure out this requirement conceptually by thinking about conservation of momentum, rather than energy.

 

[jmercer]

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?)

 

[chance]

from the wiki
http://en.wikipedia.org/wiki/Gravitational_slingshot

Limits to slingshot use

The main practical limit to the use of a slingshot is the size of the available masses in the mission.

Another limit is caused by the atmosphere of the available planet. The closer the craft can get, the more boost it gets, because gravity falls with the square of distance. If a craft gets too far into the atmosphere, the energy lost to friction can exceed that gained from the planet. This effect can be useful if the goal is to lose energy. See aerobraking.

Interplanetary slingshots using the Sun itself are impossible because the Sun is at rest with respect to its own frame of reference and is therefore incapable of donating any angular momentum. However, thrusting when near the Sun has the same effect as the powered slingshot described above. This has the potential to magnify a spacecraft's thrusting power enormously, but is limited by the spacecraft's ability to resist the heat.

An interstellar slingshot using the Sun is conceivable, involving for example an object coming from elsewhere in our galaxy and slingshotting around the Sun to boost its galactic travel. The energy and angular momentum would then come from the Sun's orbit around the Milky Way. The time scales involved for such an operation are considerably beyond current human capabilities, however.

There's also another, theoretical limit based on general relativity. If a spacecraft gets close to the Schwarzschild radius of a black hole (the ultimate gravity well), space becomes so curved that slingshot orbits require more energy to escape than the energy that could be added by the black hole's motion.

Also, a spinning mass produces frame-dragging. A spinning black hole actually is surrounded by a region of space, called the ergosphere, within which standing still (with respect to the black hole's spin) is impossible, as space itself slips in the same direction as the black hole's spin at the speed of light. Suffice it to say that there is a subtle relativistic effect (the Lense-Thirring effect) which can transfer angular momentum between any spinning mass and a passing object.

 

[Badly Shaved Monkey]

An interstellar slingshot using the Sun is conceivable, involving for example an object coming from elsewhere in our galaxy and slingshotting around the Sun to boost its galactic travel.

Well, if it's in Wikipedia it must be true.

I do like the concept of the relativistically 'fast' and the 'slow' and how their different existences would interact.