> If we could somehow "bounce" a spacecraft off a planet's surface, it would impart momentum similar to that of a gravity assist. When my kids were little, I s

> If we could somehow "bounce" a spacecraft off a planet's surface, it would impart momentum similar to that of a gravity assist. When my kids were little, I showed them by holding a basketball in one hand and a tennis ball in the other, with the tennis ball on top of and touching the basketball. When I dropped them simultaneously, they fell together and bounced off the driveway — and the tennis ball took off like a rocket (no pun intended).