Launching probes is very expensive as they can be extremely heavy. This means that if we need to speed up a probe or change its course, extra fuel needs to be taken into space and this increases costs. Using planets to ‘slingshot’ probes is something of a ‘cheat’ to increase their speed through the Solar System. When a slingshot around a planet is performed a small amount of momentum is transferred from the planet to the probe. This works by slowing the planet down by a tiny amount, but because it is far larger than the probe, the probe speeds up by a great deal. A good and recent example of this is the New Horizons probe that is currently en route to Pluto. New Horizons got just such a speed boost from Jupiter and gained a 9,000kph (6,000mph) speed increase. These manoeuvres can make space flight for inter-planetary probes much cheaper.
Answered by Josh Barker, National Space Centre
Image courtesy of NASA.