How high can one go?

Today Lucas made a airplane out of craft sticks ("See, I'm a mechanic just like U. Dan") and was filling me in on it's capabilities. It was faster than a jet plane and probably a rocket. It could fly to the moon and Grandpa and Grandma's house in like one minute. It could also fly really really high, like as high as heaven. I commented that that was as high as one could hope to go and he said " yeah, if you try to go past heaven it's just a dead end."