Gravity assist

A technique used to boost a vehicle's velocity via a planet or sun's gravitational field. Simply flying close to a planet can give a substantial velocity increase. Flying close to a sun and applying thrust at perihelion can result in a much greater increase, by virtue of the fact that kinetic energy increases as the square of velocity. The closer you can get to the sun, the greater a boost you can get, but getting too close means tidal forces would probably tear your ship apart.

Gravity swingbys have been a staple in written science fiction since as far back as the Sixties (at least), and have actually been used by NASA to send Galileo to Jupiter and the Pioneer and Voyager probes out of our solar system. I have yet to see them used in cinematic "SF", but this doesn't surprise me. I do seem to remember them being used in a couple of Star Trek episodes.

This is also known as a "slingshot maneuver" - thanks, Lord Brawl!

The Slingshot Effect (also known as Gravitational Assist) is a great way for a spacecraft to use a planet to gain or lose a great deal of velocity for free.

An example of the slingshot effect is a vehicle such as Voyager swinging behind Jupiter and gaining enough speed to make Saturn.

It's really much simpler than you would think- the effect is much the same as the way a ping pong ball goes so fast when hit by a bat; although in this case the spacecraft gets pulled by gravity rather than pushed.

Physicists refer to a body getting perturbed in this way as a collision, even though the craft never physically touches the planetary body; because the end results are very much the same.

OK, so the spacecraft ends up going much faster. But where does this speed come from? Well, Jupiter is moving, and the probe has used the gravity of Jupiter to grab hold of Jupiter and be towed along by it for a while; gaining from Jupiter's orbital speed. If you time it just right, and choose the angles you arrive and leave at, you can go swinging off with much greater speed.

Jupiter is minutely slowed by this encounter and it falls into a lower orbit about the Sun; but the amount of speed lost by Jupiter is immeasurably small because the probe is so much less massive than Jupiter.

Interestingly, relative to Jupiter the closing speed and departure speed is identical, albeit at different angles, but of course Jupiter itself is moving. So, from the Sun's point of view, the probe's speed has very much changed. (The same is true of the ping pong ball and the bat compared to the table, in fact, it's just the same thing.)

The slingshot effect can both increase as well as decrease speed, depending on whether the probe swings infront of or behind the planet.

A gravity assist maneuver is when a spacecraft uses a planet to boost (or lower, but this is less common) its speed. To get a gravity assist, the orbit of the spacecraft is timed such that it passes near the planet with just the right velocity. The planet then pulls the spacecraft along in the direction of its orbit for as long as the spacecraft is near (if the planet weren't moving, it would only change the direction of the craft, not the magnitude of its velocity). After the momentum of the spacecraft carries it past the planet, it gets to keep the "free" velocity. The timing and velocity must be very precise, of course, if you want to be headed in just the right direction afterwards.

The Cassini-Huygens spacecraft is the record holder in gravity assists. It took two assists from Venus, one from Earth and another from Jupiter in order to get enough velocity to reach Saturn. Other notable assists include Voyager 2's hat trick: it took assists from Jupiter, then Saturn and then Uranus in order to reach Neptune. Neither of the Voyager craft had the energy to leave the solar system on their own; it took a big kick from Jupiter to give them solar escape velocity.

Of course, there's no such thing as a free lunch; mother nature's books must balance. The "free" energy comes at the expense of the planet's orbital velocity. Any momentum transfered to the spacecraft is lost by the planet, slowing its orbital velocity infinitesimally.