xviii Figure 5. Size comparison of the ISS to a US football field. The following statistics provide additional information to offer a sense of scale. • Size: 51 m (167.3 ft) from front to back (PMA2 to Service Module) and 109 m (375.5 ft) from one tip of the truss to the other. That is equivalent to the length of an American football field including the end zones (a football field measures 110 m [360 ft] in length). The ISS is almost four times as large as the Russian space station Mir and about five times as large as Skylab, the first US space station. • Power Generation: Eight solar arrays on the US Segment are capable of producing a total of 84 kilowatts of solar power. The solar array wingspan (73 m [240 ft]) is longer than that of a Boeing 777-200/300 model, which is 65 m (212 ft). The total ISS solar array surface area is nearly 4,050 m 2 (1 acre) in size. Thirteen km (8 miles) of wire connect the electrical power system. • Mass: 419,400 kg (924,700 lbs), the equivalent of more than 320 automobiles. • Pressurized Volume: 916 m 3 (32,333 ft 3), or equal to that of a Boeing 747. • Habitable Volume: 388 m 3 (13,696 ft 3), roughly the same living space as a 158 m 2 (1,700 ft 2) house that has 2.5 m (8 ft) walls. During the assembly phase, some missions were purely logistical in nature, bringing up equipment, supplies for the crew (e.g., food and water), or research payloads. Russia transports supplies to the ISS using its unmanned autonomous Progress vehicle. On 12 shuttle flights, the orbiter transported temporary Multi-Purpose Logistics Modules (MPLMs) containing approximately 4,500 kg (~10,000 lbs) of materials. The MPLM would ride up in the cargo bay of the Space Shuttle. After the shuttle docked, the robotic arm would take the MPLM out of the cargo hold and berth it to the ISS where the astronauts could then exchange cargo. Before the shuttle left, the MPLM would be stowed in the cargo bay. It was later realized it would be of significant benefit to leave one of the MPLMs permanently on the ISS. One MPLM, nicknamed Leonardo, was retrofitted with additional debris shielding for a continuous life in space. Conceptually designed to act as a storage closet for the ISS, Leonardo was renamed as the Permanent Multipurpose Module (PMM) and installed on the space station in 2011. NASA and the international partners also had their own autonomous cargo vehicles. These included the ESA Automated Transfer Vehicle, the Japanese H-II Transfer Vehicle, and the American commercial vehicles Dragon and Cygnus. Later, when the ISS Program needed to ensure two berthing ports for cargo vehicles and two docking ports of the new US crewed vehicles, the PMM was moved from its position on the nadir side of Node 1 to the forward side of Node 3 in 2015. In 2016, the Bigelow Expandable Activity Module was installed on the aft side of the Node 3 module as part of a demonstration of such technologies. Several other modules are planned for the Russian Segment.