Gravitational Slingshot

Gravitational sling shot

The gravitational slingshot is when an object such as a space ship, a rocket, an asteroid or even a comet changes its speed and trajectory because of the gravitational pull of a planet. If an object passes near a planet, it will change direction but because of its initial velocity it will not pull into a planetary orbit. Instead the object will take small fraction of the orbital energy of the planet as it travels past. The planet will lose some amount of energy but it impossible to see sometimes because of the planets large mass. Many spacecrafts use the effect of the gravitational slingshot to increase their speed but the thing is that to do this the spacecraft must be flying close to the planet and it must be travelling in the opposite direction as the planets orbit around the sun. Even some space probes sometimes use the gravitational slingshot to propel them to high velocities.  For example, Voyager 2(a space probe launched in 1981) had a close flyby of 

Saturn on the 27th of August in1981, which helped in slinging it toward its flyby of Uranus on the 30th of January in 1986.  

Since gravity is a conservative force, it may seem strange that an object can achieve a net gain in speed due to a close encounter with a large gravitating mass.   We might imagine that the speed it gains while approaching the planet would be lost when receding from the planet.   However, this is not the case, as we can see from simple consideration of the kinetic energy and momentum, which shows how a planet can transfer kinetic energy to the spacecraft.