These recent topics got my imagination going:

http://www.badastronomy.com/phpBB/vi...820&forum=2&11

and

http://www.badastronomy.com/phpBB/vi...1837&forum=2&0

How about using passing asteroids as a power-assist for spaceships? Crazy idea?

One method might be to separate the ship into 2 parts, perhaps a spent fuel pod attached to the operating part of the ship by very long but very strong cable.

Position the ship so that a passing asteroid hits the cable, causing the 2 ship pieces to jerk around the asteroid. Cut the cable at the right moment and send the operating section in the desired direction while discarding the spent pod as space junk.

Don't worry... it's "cosmodegradable" [img]/phpBB/images/smiles/icon_smile.gif[/img]

Some problems:
- Accuracy of tracking asteroids. Is the error low enough?

- Will the asteroid just punch through the cable? Another way of asking this: are there any asteroids slow enough not to punch through the cable but fast enough to provide a good power-assist?

- Does the weight of the cable require more fuel to get it up there than it saves during the power-assist?

Any thoughts on this?

--Tommy
www.tommyraz.com

For the moment, just to suggest a name: Project David and Goliath

If the asteroid was going our way, we could land the vehicle on it and hitch a ride.

Or how about landing on it, then when it is pointed in the right direction, blast off again, maybe just a few degrees off the rock's path. Would you then be going blastoff speed plus takeoff thrust?

I realize you'd have to have reached asteroid speed to get to the rock in the first place, but maybe there would be some benefit in repositioning your vehicle or in 'pushing' off from the rock like pushing off the side of a swimming pool?

<font size=-1>[ This Message was edited by: beskeptical on 2002-07-24 13:39 ]</font>

I may be wrong but:

To use an asteroid you first have to approach it. When you get to it you are moving at least as fast as it does, relative to the Sun (Probably you’d be using a transfer orbit). Thus, the asteroid would never punch through the cable.

Both you and the asteroid would be subject to the gravity pull from the Sun. Remember that in the space environment all the things are moving. Rest is only a relative concept. To be able to stay at rest relative to the asteroid you would likely have to a) decelerate your ship. But, in a transfer orbit, there is no way of doing this without falling into the inner solar system (or, at least, changing your trajectory). Once you start decelerating you’re getting away from the point of rendezvous with the asteroid, so you wouldn’t be able to “wait” until it passed. b) accelerate your ship even more. Both ways would demand complicated maneuvers and enormous fuel expense.

Even if you positioned your ship (probably making both orbits cross), so that you could rope the asteroid and be slung, there could be no useful employment of the momentum acquired (if any), unless you’d be satisfied with any direction you followed.

I think it is possible to build a cable light enough and strong enough to withstand the traction force. But the whole thing seems a bit complicated. Your idea has the merit of being original. The problem would be to convince the people at The Hill.

(*) People are using such slingshot nowadays in order to travel the solar system. The only thing is that it is called gravity.


<font size=-1>[ This Message was edited by: Argos on 2002-07-24 14:55 ]</font>

I see it slightly differently.

Why not land on (or more acurately in) the asteroid, and set up home there for a while. You could mine its resources, and use any cavities as shielding agains solar flares. Assuming the asteroid is in near Earth orbit, then perhaps it could be used as a temporary camp by multiple spacecraft.

One major advantage with "taming" asteroids in this way is that we would not need to take so much fuel and resources with us (due to extraction of water and minerals from the asteroid).

Eventually we may even be able to consider the asteroid as another spacecraft which we use for ferrying between planets.

After a quick web search I found the following link to an example article illustrating such an idea.

Phobos


<font size=-1>[ This Message was edited by: Phobos on 2002-07-24 16:07 ]</font>

I know what you mean. There is not much benefit to using an asteroid with a similar orbit to the Earth. What we need is a collision course with an asteroid passing near the Earth with a larger yet more eccentric orbit than the Earth's. Asteroids from deep space with highly eccentric orbits are moving VERY fast compared with the Earth whenever they are near. The difference in speed is where the power assist benefit comes from.

Right. We don't want to "park" the ship in the path of the asteroid. We want to cross paths with the asteroid. An extreme case is when the apogee of our orbit is the perigee of the asteroid's orbit. The two bodies will be at very different speeds!

As a simple illustration, let's say I am moving at speed X relative to the Sun. Asteroid is moving at 2X relative to the Sun in the same direction. For this to happen, my ship's orbital ellipse is smaller than the asteroid's ellipse and the direction is only the same briefly where the ellipses touch.

If I miss the asteroid, then my orbit will be curving more towards the Sun. The asteroid will head out into deeper space.

If my cable hits the asteroid and I let go of the cable exactly 180 degrees into the swing, (assuming the ship's mass is negligible compared with the mass of the asteroid), I would attain a speed roughly 3x relative to the sun and would hurtle into much deeper space than the asteroid. Now my orbital ellipse is bigger and outside the asteroid's ellipse, but still touching. (Also, my brain is a thin piece of crusty goo stuck to one side of my skull, but that's besides the point) [img]/phpBB/images/smiles/icon_wink.gif[/img]

How can I benefit from this? Good question.

In order to benefit from this, I would need to cut or let go at a mind-bogglingly (is that a word?) precise moment (no pun intended)!

I agree. This would require light yet strong and shock-resistant materials. This would require high precision trajectory and timing. And I have to wait for an asteroid to cooperate just to get the darn thing tested! Not an easy project at all.

We'd need an application that makes it worth it, such as it being a quick/cheap way to get massive supplies to a remote colony.

Is this really original? Cool [img]/phpBB/images/smiles/icon_smile.gif[/img]

BTW... I like "David and Goliath".

Another name might be Project T.A.R.Z.A.N.

"Towards Asteroid RendeZvous, Accelerating Nervously?"

--Tommy

Now I see your considerations make some sense. But it's still a very complicated project.

Phobos has got a more conservative view. I think that's the way things will go.

As to T.A.R.Z.A.N., man, what a perfect acronym to your project! I see you thought of everything!

Let's see what the other fellows say.[img]/phpBB/images/smiles/icon_wink.gif[/img]

Numbers, numbers... I'll write up an example scenario.

Okay, let's assume an asteroid about 1km in length - pretty big relative to a shuttle. From the asteroids perspective, the ship comes in at speed x, and goes flying away at x - net difference in speed vector is 2x.

Let's have an acceleration of say 10g (98 m/s^2), so that the people don't turn into hamburger, and the ship doesn't fall apart. (An unmanned ship designed for this operation could take a much higher acceleration.)

Let's say the asteroid and ship are heading towards eachother at 1500 m/s (1/20 as fast as earth moves w/resp the sun). We know that the centripetal acceleration a=v^2/r for this circular motion, so we need r=v^2/a=1500^2/98= 23 km of cable that can hold the mass of the ship at 10g acceleration. And if we double the speed we have to quadruple the length.

Is hauling the cable worth the benefit of a rope assist? How common are these 1km asteroids (a bigger asteroid needs a bigger rope to wrap around it)? How big of an assist would actually be wanted? I'm not sure.

<font size=-1>[ This Message was edited by: xriso on 2002-07-27 07:45 ]</font>

Very nice example, xriso. I'm going to give some thought to your questions. Here are some of my initial thoughts...

Comparing cable to fuel:

There's no benefit unless the delta-v from TARZAN is greater than the delta-v obtainable from fuel of the same weight as the cable.

The delta-v from fuel greatly depends on the weight of the cargo+fuel. However, we can enhance it by use of gravity assist.

The delta-v from TARZAN is minimally dependant on the weight of the cargo+cable. Therefore you get the most out of TARZAN for very heavy payloads. You get the best results by figuring out the cable's maximum load and putting something similar at each end. You have to also take into consideration sharp-edged rocks on the asteroid's surface in figuring the cable's strength.
The delta-v from TARZAN is also dependant on how close the asteroid's heading is to your desired heading. Chances are, the asteroid's heading is not what you want so you have to let go earlier than 180 degrees.
So in summary, parameters for the feasibility are:
1. Orbit of asteroid.
2. Orbit of Earth.
3. Destination of ship.
Compute the best rendezvous point.
Compute the delta-v resulting from TARZAN.

4. DIfference in velocity
5. G force limit

Compute the length of cable required to get the G force limit.

6. Cable strength

Compute the maximum load for the cable. Subtract the cable weight at G force limit for the weight of the payload. (The other half of the cable is supporting the counterweight).
Compute the fuel required to bring the payload + counterweight + cable to the rendezvous. Cable will be twice the length needed for G force limit since it needs to go to the payload and counterweight.

7. Orbit of available gravity-assist bodies.

Compute the amount of fuel required to send the same payload to the destination, taking advantage of gravity-assist where available.

Then you can compare the costs.

Looks like a lot of homework to do!

(Let me know if I mentioned any Bad Astronomy in my thoughts... thanks!)
--Tommy

I was talking with a friend of mine and had another thought about this. I'm imagining a system where TARZAN helps send a massive shipment of raw supplies to the remote colony, while a conventional rocket with no payload is sent separately to meet with the shipment en route and provide course corrections. A space tug-boat, so to speak.
--Tommy