Since construction started, the International Space Station programme has had to deal with several maintenance issues, unexpected problems and failures. These incidents have affected the assembly timeline, led to periods of reduced capabilities of the station and in some cases could have forced the crew to abandon the space station for safety reasons, had these problems not been resolved. 2003 – Waste accumulation after the Columbia disaster The Columbia disaster did not involve the ISS, but did impact the ISS construction schedule and maintenance. The disaster on 1 February 2003 (during STS-107, a non-ISS mission) resulted in a two-and-a-half-year suspension of the US Space Shuttle program. Another one-year suspension following STS-114 (because of continued foam shedding on the external tank) led to some uncertainty about the future of the International Space Station. All crew exchanges between February 2003 and July 2006 were carried out using the Russian Soyuz spacecraft; a STS-114 visit in July 2005 was purely logistical. Starting with Expedition 7, caretaker crews of just two astronauts were launched, in contrast to the previously launched crews of three. Because the ISS had not been visited by a space shuttle for over three years, more waste had accumulated than anticipated, which temporarily hindered station operations in 2004. Automated Progress transports and the STS-114 mission were able to eliminate this waste build-up. 2004 – Air leak and Elektron oxygen generator failure On 2 January 2004, a minor air leak was detected on board the ISS. At one point, five pounds of air per day were leaking into space and the internal pressure of the ISS dropped from nominal 14.7 psi down to 14.0 psi, although this did not pose an immediate threat to Michael Foale and Aleksandr Kaleri, the two astronauts on board. Using an ultrasonic probe (CTRL UL101), Foale traced the leak on Sunday 10 January to a vacuum jumper hose connected to a multipaned window in the US segment of the station. The search for the leak had been hampered by noise emitted from scientific equipment on board. Successful identification and repair of the leak narrowly averted a planned lock down of the station in an attempt to isolate the leak, which would have affected station operations. Experts believe the leak was caused by astronauts using the hose as a handhold. In this same year, 2004, the Elektron unit shut down due to (initially) unknown causes. Two weeks of