The Challenger class is one of several different ships that were designed at the end of TNG's second season as part of an experiment to see if new ships could be made with existing parts. The model, which doesn't consist of much more than two nacelles, wasn't meant to be seen on screen, but the study model was shown as part of the wreckage the Borg left behind after the Battle of Wolf 359. The model is 4.3" (11 cm) long.