 Complete the following phrase:  During its 22 years of operation, Columbia was flown on 28 missions in the Space Shuttle program, spending over 300 days in space and completing over 4,000 orbits around Earth. While it was seldom used after completing its objective of testing the Space Shuttle system, and its heavier mass and internal airlock made it unideal for planned Shuttle-Centaur launches and dockings with space stations, it nonetheless proved useful as a workhorse for scientific research in orbit following the 