When spacecraft use the gravity of a celestial body to "slingshot" itself, how does this actually work?

Question

When spacecraft use the gravity of a celestial body to "slingshot" itself, how does the gravity actually help?

Answer 1

The key is that the celestial body is not stationary, and imparts some of it own momentum to the spacecraft.

Please see this link:
http://en.wikipedia.org/wiki/Gravitational_slingshot

By the way, crabby's answer below is simply wrong. It doesn't matter how long you spend in a gravitational field. Gravitational potential energy is determined solely by your position - if you fly out faster than you fly in, you'll just gain the same amount of gravitational energy back faster, but you won't gain any "extra" energy.

Answer 2

There's a reason it's called rocket science.

Imagine the approaching spacecraft and the other "celestial body" are alone in a 'system'. This system has a certain amount of energy. The craft is on a predetermined trajectory approaching the celestial body determined by "rocket scientists"; however, the spacecraft might have to do some minor adjustments to arrive there. As the spacecraft approaches the celestial body the kinetic energy (energy of motion) of the craft increases because the gravitational pull of the celestial body is having a greater effect on the craft the closer the two bodies get (inverse-squared law). The craft's velocity (speed and direction) changes continuously because of the gravitational acceleration that the body is experiencing. Depending on the initial velocity of the craft, the amount of 'slingshot-redirection' it experiences could be slight to an almost U-turn maneuver.

Such a maneuver causes the spacecraft to have more kinetic energy than it began with because it essentially stole it from the other body. Each time a spacecraft performs such a maneuvre it is removing orbital energy from whatever planet (usually Mars, Jupiter or Saturn) that it encounters. This removal of energy causes the planet's orbit to very, very, very slowly move farther away from the sun. If a spacecraft is launched on such a trajectory so that it slows down after encountering another celestial body the opposite is true and energy is taken from the craft and given to the other body.

Answer 3

Sorry but the other answers are wrong--or incomplete. Here's how a "slingshot" maneuver works:

As a spacecraft falls into the gravity well of a planet, it's velocity increases. Now, if it simply stays in free fall, it will either be captured (trapped in an orbit) or, more often, fly on past. But as it flies past and out of the planet's gravity it will slow down just as much as it sped up falling toward the planed.

But (BTW--if the angles and/or effect of other bodies is just right this can occur naturally--but we're talking about how the spacecraft maneover works)--suppose just as you start to get near the closest approach to the planet (the closer the better) you fire the spacecraft's engines, causing it to accelerate to an even higher speed. When the spacecraft starts travelling back out of the planet's gravity, it will be moving faster --and will spend less time in the planet's gravity thanit did coming in--so there is less time for gravity to slow the spacecraft down. The result is that the final velocity is increased not only by thet burst of acceleration, but an additional residual amount that is the speed the spacecraft would have lost if it hadn't gotten out of the planet's graity more quickly.

Not only that, but that extra residual velocity works as a second-order (square) formula--so the extra velocity can be quite large--often many times the amount of acceleration the spacecraft itself added by firing its engines.

Answer 4

Yes (if we follow your assumption), we know this from Newton's law of gravitation. Gravity is an inverse-squared field that gets more powerful the closer you get to it's center (if treating it as point mass, unfortunately we cannot treat the Earth this way). F = GMm/r^2 ... the smaller r is the bigger F is. Mind you, if you assume the core to be 80% of Earth's total mass then you definitely would feel a greater and greater pull as you got closer to it. Most definitely. This is because if you use Newton's law above, it is easy to see that if the greatest amount of mass gets closer and closer it will simply overtake any gravity from lesser mass. At least until you breached the core and moved into its center. The college physics professor is right, ONLY if you assume a uniform mass distribution. The first guy was wrong completely about one thing and sort of right about another. If you ever got to the center of the Earth, the exact center of gravity actually which would be offset from the geometric center due to mass concentration differences, you would remain in lingo and not move. This is because all the force of gravity of the Earth is pulling you equally in all directions. In actuality, you'd probably be ripped apart by such force... and the immense pressure and heat of the Earth's core... but that's another story.

Answer 5

The slingshot maneuver uses the gravity of the celestial body, as one of the forces acting on the spacecraft. In order to move an object, one needs a force acting on that object; if you need it to change direction then you need another force acting on it. When spacecrafts do the slingshot maneuver they use the gravitational pull as one of the forces acting on it. It is also a way to conserve energy.

Answer 6

There's an initial momentum leading past the body, and a force of gravity pulling toward the center of the body (which is a very different direction).

Hence the force of gravity causes the path of the spacecraft to curve.

Answer 7

it's exactly how the earth and sun relationship goes, with the earth's orbit being elliptical, there needs to be two foci instead of one centerpoint which would be in a circle...

the sun is on one focal point, and as the earth gets close to the sun, the sun pulls it faster and faster, but because of the speed and centrifugal force acting against the gravity it doesn't fall directly into the sun, but flows around it and gets flung around the other side with enough speed that it gets away before it slows down enough to be drawn back immediately. instead, it slows down later when it's further away and begins to return to repeat the process.

think of when you roll a penny into a huge funnel for those charity people... =)

Answer 8

they get into an orbiting position, things in orbit around earth are in constant freefall, going over 26000 kmh. when they reach the point they want toleave from they fire the rockets and the speed is added to an orbiting speed.
you can orbit faster too but your orbit will be longer the faster you go.

Answer 9

Good one Tom, I learned something tonight.

Answer 10

weight, volume!