View Full Version : slingshot effect

2004-Dec-25, 03:24 AM
Hi ppls, just a quick question, how does slingshot effect works? What is the different between slingshot effect and gravitational pull?

P.S : Merry Christmas to all of you people here.

2004-Dec-25, 03:49 AM
A gravity assist maneuver is just the fact of passing near a massive object and having it alter your velocity. You can slow down, speed up, change directions, or a combination.

A slingshot maneuver is a gravity assist that speeds you up.

2004-Dec-25, 03:54 AM
Gravitational pull is the attraction of objects towards one another. The slingshot effect is when you use the motion of a planet to change the speed and direction of a spacecraft (like alainprice said).

Merry Christmas to you too!

2004-Dec-25, 04:00 AM
Thanks galaxygirl and alainprice :) One more question, does it had to be close or farther away from the planet in order to speed up? Aproximately what distance from the planet?

Peace :)

2004-Dec-25, 04:18 AM
I don't know how close you have to be to a planet to make it move by slingshot, but my guess is that you'll have a bigger boost if you're closer to the planet. I think it also depends on the planet's size- you would have to be closer to a small planet to get a big enough boost. Because larger planets have more gravitational pull you wouldn't have to be as close to them to get the same slingshot effect.

2004-Dec-25, 06:37 AM
The slingshot effect works something like this...

Imagine you are in orbit around the sun on an elliptical path.
At some point on your orbit you arrive in the vicinity of a planet.

When you get close enough to the planet for it's gravity to matter more than the Sun's gravity, you can change coordinate systems and imagine yourself in orbit relative to the planet.

The orbit shape relative to the planet might not be an ellipse. If you catch up to the planet, your path is probably hyperbolic.

When you depart the planet's sphere of influence and are back to orbiting the sun your momentum will have changed from your original so you will be on a new orbital path relative to the Sun. Any momentum you gain or loose will be made up for by the planet.

Do all this right and you can pick up a momentum boost from the planet and kick yourself into a higher orbit relative to the Sun. The more you change your momentum around the planet, the bigger the affect. To do this, you have to get as close to the planet as you need.

2004-Dec-25, 04:11 PM
The net velocity effect from the reference frame of the planet is equivalent. The increase in velocity from the slingshot effect is from the reference frame of the Sun.

2004-Dec-26, 06:41 PM
The greatly simplified answer is that the slingshot effect is the by-product of entering the gravitational field of a massive orbiting body. As mentioned earlier, the spacecraft's velocity relative to the planet is the same outbound as inbound. However, relative to the Sun, the objects exchange momentum; there's no free lunch, so the spacecraft gains momentum, and the planet loses some. Because the planet is so much more massive than the spacecraft, however, the spacecraft gains a LOT of velocity, while the planet loses very little.

Here is an easy-to-understand mathematical treatment of the phenomenon, in PDF format:

Here is a Scientific American Q&A explanation of the phenomenon:
http://www.sciam.com/askexpert_question.cf...EB7809EC588F2D7 (http://www.sciam.com/askexpert_question.cfm?articleID=0001B3B9-8D66-1C72-9EB7809EC588F2D7)

Finally, the difference between the slingshot effect and gravitational pull is that the slingshot effect describes the exchange of momentum, while the latter describes the gravity of any body, irrespective of the frame of reference.

2004-Dec-30, 07:44 AM
Without going to these links and reading screeds of text. Could you please explain why this works? According to me as you aproach a masive object, (planet) the gravatational effect would speed you up conciderably, yes I can see that, but ( remembering I'm not the brightest candle on the cake ) Once past the planet would not its gravaty slow us up and negate any advances made.?

2004-Dec-30, 01:42 PM
If it accelerated you fast enough to pass the escape velocity you'd naturally get out of the planet's attraction and it's pull.

The trajectory you approach the planet on and the one you have when you leave it's vicinity may also influence this I think: if you approach the planet from behind, you'll gain speed and the distance between you shortens at a specified pace. If you leave the planet on a path that makes 90 to it's orbit, then the distance between the 2 of you will increase faster than it was decreasing and the gravity pull will weaken much faster as well... (this makes sense to me but I'm just guessing this as I write it here... I really haven't read that much about the subject... but the first statement is true).

2004-Dec-30, 01:46 PM
oh, and the effect shouldn't depend on the planet's size, but of it's mass...
I think :huh:

And if you get close enough to enter the planet's atmosphere you'll get just the opposite effect: a giant brake
Funny how to similar manouvers lead to such opposite results. This universe has been really well designed :)

2004-Dec-30, 02:02 PM
Originally posted by astromark@Dec 30 2004, 07:44 AM
Could you please explain why this works? ... Once past the planet would not its gravaty slow us up and negate any advances made.?
It is fairly easy to write equations that show how two bodies will orbit each other, but it is very difficult to completely solve the three [or N-] body problem precisely.

That being said, imagine that most of the time, the two bodies [Jupiter & Cassini for example] in the slingshot are too far apart to be affecting each other's motion around the Sun. Each are on an nearly elliptical path, with one more circular than the other.

As the smaller body approaches the bigger body, it makes more sense to think about the two body problem of these two bodies, and less about their movement with respect to the Sun, which for the time being can be approximated as nearly linear. The lighter body falls toward the larger body, passes it, and falls away from it. Eventually it is far enough from the larger body that we should look at its motion around the Sun again. But, during this falling in and falling out process, the smaller object added to its speed up to twice the relative speed between it and the larger body at a large distance, relative to the Sun. This is the slingshot effect.