Diagram showing Juno's path to Jupiter:

Juno path to Jupiter 

 

Hohmann transfers

What is the most efficient way to get from one planet to another?  It's not the way Buck Rogers (or Luke Skywalker) would do it, blasting off in a straight line path. The most efficient transfer orbit was discovered by the German engineer Walter Hohmann in 1925.  This diagram shows the way to get from a planet close to the Sun to a planet farther from the Sun with the minimum cost in fuel.  This is how we could get the largest payload from Earth to Mars at the lowest cost.  Click here for a short discussion of why energy is so important for spaceflight!

hohmann

Why is space travel a lot more difficult than it seems in the movies?

Getting off Earth and into orbit requires vast amounts of energy (the equivalent of a car-bomb of explosive power is needed to put 100 kg in Earth orbit).

The power can't be applied suddenly (as in an explosion) but must be regulated so that the g forces aren't too large (< about 5 g, preferably).  This leaves us with rockets.

At present, it costs about $10,000 per pound to put a payload in Earth orbit.  That's too expensive for many commercial enterprises.