When the space shuttle Discovery returns from orbit on Wednesday, concluding its 39th flight in its 27-year space-traveling career, its journey still won’t be over. According to NASA, 21 institutions across the country have put in proposals to land either the Discovery, or one of its soon-to-be-retired mates, the Endeavour and the Atlantis, to their collections. The cost of the Discovery, which has logged 150 million miles traveled, will run museums a hefty $28.8 million. Some museum campaigns are more energetic than others. The Intrepid Sea, Air and Space Museum in Manhattan has collected 150,000 names on a petition urging that one of the shuttles be housed there, while The Museum of Flight in Seattle has erected a $12 million wing just on the hope that they might land a shuttle. The Kennedy Space Center in Florida, however, may boast the most appealing offer. They plan to suspend one of the shuttles horizontally, as if it were in flight, with the payload doors open, and have it serve as the centerpiece of a $100 million, 64,000-square-foot exhibit slated to open in 2013.