allow up to a 28-day mission (although a 28-day mission was never attempted; the record is 17 days, which was set by Columbia). Updated avionics systems that included advanced general purpose computers, improved inertial measurement units and tactical air navigation systems, enhanced master events controllers and multiplexer-demultiplexers, a solid-state star tracker and improved nose wheel steering mechanisms. An improved version of the Auxiliary Power Units (APUs) that provided power to operate the Shuttle's hydraulic systems. Modifications resulting from a 2005–2006 refit of Endeavour included: The Station-to-Shuttle Power Transfer System (SSPTS), which converted 8 kilowatts of DC power from the ISS main voltage of 120VDC to the orbiter bus voltage of 28VDC. This upgrade allowed Endeavour to remain on-orbit while docked at ISS for an additional 3- to 4-day duration. The corresponding power equipment was added to the ISS during the STS-116 station assembly mission, and Endeavour flew with SSPTS capability during STS-118. Final flights Endeavour flew its final mission, STS-134, to the International Space Station (ISS) in May 2011. After the conclusion of STS-134, Endeavour was formally decommissioned. STS-134 was intended to launch in late 2010, but on July 1 NASA released a statement saying the Endeavour mission was rescheduled for February 27, 2011. "The target dates were adjusted because critical payload hardware for STS-133 will not be ready in time to support the previously planned 16 September launch," NASA said in a statement. With the Discovery launch moving to November, Endeavour mission "cannot fly as planned, so the next available launch window is in February 2011," NASA said, adding that the launch dates were subject to change. The launch was further postponed until April to avoid a scheduling conflict with a Russian supply vehicle heading for the International Space Station. STS-134 did not launch until May 16 at 08:56 EDT. Endeavour landed at the Kennedy Space Center at 06:34 UTC on June 1, 2011, completing its final mission. It was the 25th night landing of a shuttle. Over its flight career, Endeavour flew and spent 299 days in space. During Endeavour's last mission, the Russian spacecraft Soyuz TMA-20 departed from the ISS and paused at a distance of . Italian astronaut Paolo Nespoli took a series of photographs and videos of the ISS with Endeavour docked. This was the second time a shuttle was photographed docked and the first time since 1996. Commander Mark Kelly was the last astronaut