Gravity boost

The Juno spacecraft is on it’s way to Jupiter. It was sent here from Earth in 2011, out to past the orbit of Mars, it then fell back towards Earth, gaining speed, just miss us and is now on it’s way to Jupiter. It gained a lot of speed. Exactly how does this work? Why does it not lose all the gained speed as it moves away from Earth?

Interesting question. As the craft approaches earth, it will clearly accelerate towards earth due to the gravitational force applied to the craft as dictated by the inverse square law. And then, as it misses earth and slingshots past, one would expect it to decelerate accordingly, since there are no separate laws for coming and going. I too cannot spot any glaring reason why there should necessarily be a net gain in velocity. I do recall that Voyager 1 used similar gravitational tricks during its very productive exploration of our solar system.

Maybe our assumption is wrong and “accelerate” simply means a change in direction, not speed, since it can mean either or both.

ETA: OK here’s a thought: Imagine as a stationary reference point the sun instead of the earth. Earth moves around the sun, right? What if the craft approaches the earth from such an angle that the velocity of the earth, or some component thereof, is added to that of the craft in addition to the expected gravitational gain in speed. Then, after the gravitational acceleration is cancelled by the gravitational deceleration, the net gain in speed will equal this “earth speed” component.


Yes, the speed gained through a gravitational slingshot is the orbital velocity component of the planet (or other celestial body) used to effect the slingshot since the planet drags the object along by gravity as it orbits another body. The object typically gains speed relative to the Sun, which is taken as an essentially fixed point in the solar system. The Moon or an asteroid could also be used as a slingshot. You can read a fuller, more technical account here.


Thanks, now it makes sense. 8)