We are now Interstellar Travelers (er.. so to speak)

 

Although there are no real, live humans on board, the Voyager I spacecraft has now passed out of our own solar system.  One of the things that I found fascinating about this news is that it took a couple of months for the scientists working on the Voyager mission to confirm that it had passed out of the solar system.  This is because it is not a simple matter of looking out the window and seeing a "you are leaving the solar system, come again soon" sign.  There is data that is being sent back from the craft that must be meticulously analyzed to determine it's position.

Now... a question I would ask my Physics class (we are studying Newton's Laws of motion), or anyone who has seen the film "Gravity":  Why does the craft use so little power as it travels through (and now out of) the solar system?