there are so many planets to look at but thy are all too fare away to explore. In 1981, the launch of the space shuttle Columbia marked the beginning of reusable space craft. The costs for human spaceflight are enormous, spaceships can cost billions of dollars to develop and launch. I wonder how many planets are not explored yet? I wonder how many people have set foot on the moon? I understand that the sun is so hot that no one can explore it or go near it.