Dan laughed a little. "You have no idea..." he chuckled out loud, "you really don't..." He thought of how to explain to Simba what it will be like, and why it's so dangerous. "Alright, let me see, how to describe it..."
While waiting for the plane to land, Dan began to tell Simba and Nala how rockets work, and why space is not a very good place to be in. "...Everything that keeps you alive here. The warmth of the sun, the very air you breath, all gone..." As he described those, Dan got the feeling that, for the most part, Simba did not really understand much. But when the lion did understand what Dan was describing, it was like he was opening up a whole new world.
After some more talking, Dan decided that it was enough for now. "You will learn a lot more soon enough, and not from me. I'm supposed to help you with when you actually get up there, not as far as actually how to get you there."