Possible Clues:

• Reusable orbiter
• *Former NASA project
• One makes a flying visit, not put off by lack of atmosphere
• Reusable rocket-launched craft flown by astronauts
• Clearance for ferry launch with the really high-flyers
• Discovery, say, of room service making regular trips
• Challenger was one small step closer, almost catching learner
• Former long-distance traveller's small step closer, almost crossing line
• ''Atlantis'' or ''Endeavour''
• Astronauts' vehicle