The final positioning of "Harmony" allowed for the later installation of the European "Columbus" and Japanese "Kibō" research modules, which were attached to the side ports of "Harmony". During STS-120, the P6 solar arrays were moved from the Z1 truss on top of "Unity", to their final position at the port end of the truss. STS-120 carried the signatures of over 500,000 students that participated in the 2007 Student Signatures in Space program, jointly sponsored by NASA and Lockheed Martin. In celebration of Space Day in May 2007, students from over 500 schools around the world signed giant posters. Their signatures were scanned onto a disk, and the disk was manifested on the STS-120 mission. Also during STS-120, the lightsaber used by actor Mark Hamill in the 1983 film "Return of the Jedi" was flown to the station and returned to Earth. Stowed on-board "Discovery" for the length of the mission, the prop was flown in honor of the 30th anniversary of the "Star Wars" franchise. On 28 August 2007, Chewbacca from the "Star Wars" films presented the lightsaber to NASA officials from Space Center Houston, in an official ceremony at Oakland International Airport. The lightsaber was then flown to Houston, where it was greeted by Stormtroopers. The lightsaber will be displayed at Space Center Houston. At the time of the "Columbia" disaster, STS-120 was scheduled for early 2004. It was delayed repeatedly until 23 October 2007. NASA originally scheduled the launch for 20 October 2007, but due to the International Space Station program requirement, launch was delayed to 23 October 2007. In light of the small gouge to the underside of "Endeavour" that occurred on STS-118, in August 2007, NASA managers announced that they expected to add a fifth spacewalk to the STS-120 mission, designed to test a heat-shield repair tool. The repair technique was originally scheduled for a flight in 2008, but following STS-118, it was decided to move up the testing. The repair tool, called a TPS (thermal protection system) repair ablator dispenser (T-RAD), has never been tested in space, so the spacewalk would have allowed managers to evaluate its effectiveness in low gravity environments. During the course of the mission, issues with the S4 starboard Solar Alpha Rotary Joint (SARJ), and P6 Solar Array, deferred this test objective to a future mission.