Well, the answer was obvious,gravity. I should have figured that out! When the craft gets closer to a planet, its speed increases because of the gravity of it, right? And if the course is just right, the craft doesn't crash into the planet but "swings by" it with the same accelerated speed, and nothing comes out less than the original amount. So you can say it's a REALLY clean energy source!
And, anyway, I recalled something a guy told me in a space museum a few years ago.
The guy: "what would you think will happen if you lit a candle in the space station"
me: "uh... blow up?"
guy: "no, the ISS isn't filled with pure oxygen though the shuttle is. So it won't blow up. But it would *******"
And then he taught me that astronauts sleeping on thee ISS have to be careful because they would ********* if there was no *****
What do you think will go in the ***? Please answer by commenting. I'll post the answer as a new post in a few days so check back!