It creates a starfield by copying a prototype and basically the 'stars' will loop using built in Etoys commands like Wrap under the movement category. The playfield is in a book, so I can hide the edges of it where wrap isn't 'pretty'. Then for hte planets, I set it up so that if you click them, they memorize their 'home' location to reset the level and they don't wrap. They all respond to an event called 'ScrollSpace' which is broadcast by the 'player's' sphere in the center of the screen.
Basically, the planets are free to move forever inside of the playfield but the stars repeat, giving the impression that space and these stars go on forever, but as you can see at the bottom of the screen, there's only a limited number of stars in the playfield.