For a large party, make cakes in various sizes and decorate them like planets.
For a small party, serve cupcakes in the shape of a constellation.
Make comets out of balls of cream cheese and pretzel sticks.
If you can find gummy aliens, it would be fun to freeze them in ice cubes for drinks.
Cut sandwiches or cookies into stars and moons shapes.
Mini cheese balls covered with chopped nuts can be meteorites.
Get some real astronaut food.
Cover black balloons with yellow or white nail polish stars.
Decorate walls with posters.
Make a model of the space shuttle for a centerpiece.
Cut out circles of varying sizes and make a model solar system for the wall.
Cut out cardboard stars, paint and cover with glitter and hang from the ceiling.
Oriental Trading Company is a great source for Alien and Celestial goodies.