Possible Clues:

• Never backward with experiment forming part of space shuttle mission
• Never return to attempt the resumption of possession
• A spacecraft's coming down, never rising on test
• What happens if you run out of space
• Never turn and attempt to come again like this
• Return of spacecraft to earth's atmosphere
• Astronauts' descent to Earth
• Astronaut's homecoming
• NASA mission phase
• "No ___" ("You can't go back in" rule)