Click to read the rules at Multiverse 5000. You only have less than a week to sign up. Go.
For those not familiar with the concept, the Multiverse Crossover Ficathon is a fiction exchange where writers sign up to write a crossover based on the requests of another writer, and get a story written for them in return. The crossovers involve a certain set of fandoms, all of which are space shows. (Except for the fandoms that are actually space books or space movies, but you get the idea.)