The Doom machine by Mark Teague.   With surprises around every corner, Teague takes readers on a funny, outrageous adventure when the small, sleepy town of Vern Hollow is invaded by a flying saucer filled with ridiculously inept aliens.–catalog summary
This is a great science fiction story complete with UFO’s!  The illustrations are in the style of movies from the 50’s.  If you love Doom Machine, you’ll also enjoy reading Whales on Stilts by M.T. Anderson.  Need more ideas for Science Fiction books? 
Check out the Golden Duck Awards for the best in science fiction for children.