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Thread: Do we really understand the "Slingshot Effect"?

  1. #1 Do we really understand the "Slingshot Effect"? 
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    We have been using the Slingshot Effect in our space exploration, but do we really understand how it works. I have looked at some of the explanations and they leave me questioning the accuracy. Thanks for comments. Joe L. Ogan


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    Veracity Vigilante inow's Avatar
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    The precision and success of the Voyager and Pioneer missions, as well as the numerous other successful orbital missions in which we've engaged suggest that... yes, we do.


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


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    Yes, I understand. We use it all of the time. But do we really know why it works?
    Do we understand how Gravity affects it to make it speed up? I still have some doubts that we really understand the phenomenon. Thanks Joe L. Ogan
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    Veracity Vigilante inow's Avatar
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    Hmmm... I'll try to convey this in a way which makes the most sense using my own limited understanding.

    As the object nears the massive body (the satellite nears the planet, for example), the strength of the gravity increases (actually, the distance decreases, so the effect of gravity is more prominent... the strength of gravity remains the same) and the object speeds up due that stronger influence from gravity. The object is accelerated toward the massive body.

    They do this on a very particular trajectory... they get near to the massive body, but skim past it, now with all of this extra momentum, and the object arcs around the body since it's still caught in it's gravity well (like a piece of dirt circling the drain in your bathtub... it will flow past the drain quickly, and then change direction and come back toward it).

    The object then passes by the massive body again on the other side of it and picks up more speed (just like it did the first time, only now it's going even faster) and it has so much speed that it is able to escape the gravitational pull of the massive body and continue on its path with all of this extra velocity.

    That's about the best I can do. Someone like Janus would be the best to make this both clear and accurate to the reader, as he's really gifted at explaining this stuff in simple terms and without inaccuracies.
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    Moderator Moderator Janus's Avatar
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    Quote Originally Posted by Joe L. Ogan
    Yes, I understand. We use it all of the time. But do we really know why it works?
    Do we understand how Gravity affects it to make it speed up? I still have some doubts that we really understand the phenomenon. Thanks Joe L. Ogan
    It behaves like an elastic collision.

    Imagine you have a space probe placed in front of Jupiter in its orbit. And let's say that it is traveling in the same direction as Jupiter but 5 km/sec slower with respect to the Sun.

    This means it has a relative speed of 8 km/sec towards Jupiter. We've put this probe at just the right starting distance and position from Jupiter that it makes a parabolic orbit around Jupiter. It falls in toward Jupiter, whips around the planet and heads back out the way it came in. Since it is a parabolic orbit, it is traveling at escape velocity and can't be captured by Juptier.

    When it reaches the same distance from Jupiter as it started, it will being moving at 8 km/sec relative to Jupiter. It will be in front of Jupiter again, but moving away from Jupiter not towards it. It relative speed with respect to the Sun will now be 8+13= 21 km/sec.

    At this distance from the Sun, escape velocity from the Solar system is 18.39 km/sec. So the probe goes from below solar escape velocity to above it. The probe's gain in velocity comes at the expense of Jupiter's orbital velocity. But since Jupiter is so many times more massive than the probe, it is an immeasurably small loss.

    The basic mechanism is the same as tossing a ball slowly in one direction and then hitting from behind with a bat moving much faster in the same direction. The collision transfers momentum from the bat to ball and the ball ends up going much faster.

    The difference is that in a gravity slingshot, the planet's gravity field provides the mechanism for the momentum transfer without the need of a collision.

    Now in actual practice, we can't achieve that perfect parabolic trajectory around the planet, but a hyperbolic orbit works also, you just don't get quite as large a velocity change.
    "Men are apt to mistake the strength of their feelings for the strength of their argument.
    The heated mind resents the chill touch & relentless scrutiny of logic"-W.E. Gladstone


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    With today's technology, could we chain slingshots to attain very high speeds? I'm thinking like thousands...
    A pong by any other name is still a pong. -williampinn
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  8. #7  
    Moderator Moderator Janus's Avatar
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    Quote Originally Posted by Pong
    With today's technology, could we chain slingshots to attain very high speeds? I'm thinking like thousands...
    Once you attain solar escape velocity (in the order of 10's of km/sec) you are limited as to the number of bodies that you will pass on the way out and the number of additional slingshots available.

    but more importantly, the higher the velocity of the probe, the flatter the hyperbolic path past the planet will become. The flatter the hyperbola, the less effective the slingshot will be. So there is an effective limit to the speeds you can attain with planetary slingshots.
    "Men are apt to mistake the strength of their feelings for the strength of their argument.
    The heated mind resents the chill touch & relentless scrutiny of logic"-W.E. Gladstone


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