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fifiela
2010-Jul-16, 06:26 AM
Newbie here, so my apologies if this is common knowledge or a silly question or posted in the wrong spot.

As I understand it, deep space probes are often programmed to pass by planets to increase their speed.

It is obvious to me that a probe would gain speed from the pull of gravity as it approached a large object. Why does a probe not loose an equal amount of speed to the tug of gravity after it passes a large object? Shouldn’t the speed gained on approach equal the speed lost on departure?

The net gain of speed after passing an object seems to be “free” energy and I know that can not be. Perhaps the increased momentum comes from a longer approach glide path toward the plant and a sharper exit after passing the object.

pzkpfw
2010-Jul-16, 08:43 AM
The energy comes from the object that is passed.

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

Jens
2010-Jul-16, 09:36 AM
Newbie here, so my apologies if this is common knowledge or a silly question or posted in the wrong spot.

It's actually a very perceptive question -- it shows that you are thinking about this logically. The answer is that in fact, you are correct. If you are looking at the perspective of the planet in question, the object cannot gain any speed. So from the perspective of Jupiter, a probe passing by Jupiter has gained or lost nothing.

Where it gets interesting is that by flying past Jupiter, you can make a gain or loss with respect to the Sun. Think about it this way. Suppose you speed toward Jupiter following it in its orbit, and then go around 180 degrees and are moving the other way. You've obviously lost speed with regard to the sun. So what has happened? Apparently, what's happened is that you've stolen some of Jupiter's speed, or in this case you've given speed to Jupiter.

Jeff Root
2010-Jul-16, 03:05 PM
And although your speed relative to Jupiter is the same at the end
of the encounter as it was at the beginning, the direction is different.
So instead of moving in a nearly circular orbit around the Sun at
some particular speed, you will be in an elliptical orbit moving away
from the Sun at that speed.

-- Jeff, in Minneapolis