We haven't sent any space missions to planets very fast. I think people just forget how long it takes. Cassini was launched in 1997 and won't arrive at Saturn until 2004. It takes a long time to travel in space right now because it would be a lot more expensive to send a spacecraft more quickly to its destination. Why do you think this is the case?

We make use of what is called the "sling shot effect" to increase the speed and redirect the motion of a satellite bound for distant planets.When the planets are in just the right positions in their orbits, their momentum can be used to accelerate a satellite, giving it much greater velocity than its original rocket booster. It works this way.

The satellite and the planet both have mass and therefore gravity.
They both pull on each other with equal force. Each also has a velocity (speed and direction) and momentum (the tendancy to keep moving in its current direction). If we aim the satellite so it passes just behind the planet as the planet moves in its orbit "around" the Sun, the planet will pull on the satellite, and the satellite will pull on the planet. The result is that the planet will slow down in its orbit just the very tiniest amount, and the satellite will speed up a lot and go in a different direction. The satellite's path will bend slightly towards the direction the planet is moving. In keeping with the law of conservation of momentum, the planet gives up a little of its momentum to the satellite, which then speeds off to a distant destination.

You can experience the same sort of thing on a basketball court. If you run with the ball, then throw it ahead of you, can you sense your speed decreasing as you release the ball?