favor of the Constellation program and its crewed Orion spacecraft. The Constellation program was later canceled with the NASA Authorization Act of 2010 signed by President Barack Obama on October 11. Construction milestones First operational orbiter Weight As the second orbiter to be constructed and the first able to fly into space, Columbia was roughly heavier than subsequent orbiters such as Endeavour, which were of a slightly different design and had benefited from advances in materials technology. In part, this was due to heavier wing and fuselage spars, the weight of early test instrumentation that remained fitted to the avionics suite, and an internal airlock that, originally fitted into the other orbiters, was later removed in favor of an external airlock to facilitate Shuttle/Mir and Shuttle/International Space Station dockings. Due to its weight, Columbia could not have used the planned Centaur-G booster (canceled after the loss of Challenger). The retention of the internal airlock allowed NASA to use Columbia for the STS-109 Hubble Space Telescope servicing mission, along with the Spacehab double module used on STS-107. Due to Columbia's higher weight, it was less ideal for NASA to use it for missions to the International Space Station, though modifications were made to the Shuttle during its last refit in case the spacecraft was needed for such tasks. Thermal protection system Externally, Columbia was the first orbiter in the fleet whose surface was mostly covered with High & Low Temperature Reusable Surface Insulation (HRSI/LRSI) tiles as its main thermal protection system (TPS), with white silicone rubber-painted Nomex – known as Felt Reusable Surface Insulation (FRSI) blankets – in some areas on the wings, fuselage, and payload bay doors. FRSI once covered almost 25% of the orbiter; the first upgrade resulted in its removal from many areas, and in later flights, it was only used on the upper section of the payload bay doors and inboard sections of the upper wing surfaces. The upgrade also involved replacing many of the white LRSI tiles on the upper surfaces with Advanced Flexible Reusable Surface Insulation (AFRSI) blankets (also known as Fibrous Insulation Blankets, or FIBs) that had been used on Discovery and Atlantis. Originally, Columbia had 32,000 tiles – the upgrade reduced this to 24,300. The AFRSI blankets consisted of layers of pure silica felt sandwiched between a layer of silica fabric on the outside and S-Glass fabric on the inside, stitched together using