It's entirely possible to build a rocket that doesn't need help from gravity assists, but rocket fuel is very expensive to send to the outer planets. Better to

It's entirely possible to build a rocket that doesn't need help from gravity assists, but rocket fuel is very expensive to send to the outer planets. Better to roll downhill towards Jupiter rather than uphill away from the sun. There are other fun things that we can do with gravity assists as well: A rocket burn is significantly more effective when close to a large gravitational source. http://en.wikipedia.org/wiki/Oberth_effect http://en.wikipedia.org/wiki/Oberth_effect If you are willing to wait long enough there is a way to get almost anywhere in the solar system for very little fuel. http://en.wikipedia.org/wiki/Interplanetary_Transport_Network http://en.wikipedia.org/wiki/Interplanetary_Transport_Networ...