Sally Ride (tennis player, engineer, physics PhD, and astronaut) said,

"The view of Earth is absolutely spectacular, and the feeling of looking back and seeing your planet as a planet is just an amazing feeling. It's a totally different perspective, and it makes you appreciate, actually, how fragile our existence is. You can look at Earth's horizon and see this really, really thin royal blue line right along the horizon, and at first you don't really quite internalize what that is, and then you realize that it's Earth's atmosphere, and that that's all there is of it, and it's about as thick as the fuzz on a tennis ball, and it's everything that separates us from the vacuum of space."