The Airlock Module enabled spacesuited crew members to make excursions outside the Skylab to replace or adjust equipment, change film, or carry out other extra-vehicular activities. This capability was vital to emergency repairs by the astronauts on the first mission. The Airlock Module was attached to the OWS and passage to the module was accomplished through a hatch which connected the module to the interior of the workshop. When an astronaut entered the module, he would vent the atmosphere of the module into space. When the pressure in the airlock reached zero, the crew member could open the outer hatch and float out into space. [Illustration: 75. Airlock Module.] The Multiple Docking Adapter (MDA) was used by crews arriving or departing from the Skylab workshop. The Apollo command/service modules delivered crews to the MDA from which the astronauts could enter Skylab through the hatch in the docking port. In an emergency, two command/service modules could dock at the MDA. The MDA also held equipment for earth resources multispectral photography, materials processing, and astronomy. The Apollo Telescope Mount (ATM) was on top of and controlled by the MDA. It contained six astronomical instruments to obtain information about the Sun. [Illustration: 76. Multiple Docking Adapter.] Solar energy is the prime source of electric power on Skylab. Two systems of solar electric-cell arrays—one wing on the OWS and four panels on the ATM—deployed after the Skylab reached orbit. Principal contractors: OWS—McDonnell Douglas Astronautics Company; AM—McDonnell Douglas Astronautics Company; MDA—Martin Marietta Aerospace. The Skylab components on display were presented to the museum by the National Aeronautics and Space Administration. Apollo-Soyuz Test Project [Illustration: 77. Artist conception of the Apollo-Soyuz Test Project rendezvous.] On May 24, 1972, President Richard Nixon and Aleksey Kosygin, Chairman of the USSR Council of Ministers, signed an agreement “concerning cooperation in the exploration and use of outer space for peaceful purposes.” The signing represented a formal endorsement of negotiations that had been held between the two nations over several years. The agreement established the Apollo-Soyuz Test Project (ASTP) to develop and fly a standardized docking system “to enhance the safety of manned flight in space and to provide the opportunity for conducting joint scientific missions in the future.” On July 15, 1975, the afternoon countdown for the Soviet launch was completed and _Soyuz_ lifted off from the Baykonur complex near Tyuratum in Central Asia, some 3200 kilometers (2000 miles) southeast of Moscow. _Soyuz_ carried cosmonauts Alexey Leonov and Valeriy Kubasov. Taking advantage of _Apollo_’s larger fuel supply for maneuvering, _Apollo_ followed _Soyuz_ into orbit 7½ hours later. _Apollo_ was launched atop a Saturn 1B from Kennedy Space Center, Florida.