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Space Shuttles usually take off with a seven-man-crew. Such a large crew allows to divide the astronauts into two categories. There are the pilots who are responsible for flying and maintaining the orbiter and there mission specialists for experiments and payloads. These specialists are not necessarily career-astronauts.
During the early missions the Space Shuttles often carried communications satellites into the sky. Sometimes three satellites were place in orbit during one single mission.
This policy changed when the greatest catastrophe in human space flight so far took the lives of seven astronauts. On January 28, 1986, the Challenger exploded just 73 seconds after lift-off. Later it was determined that a seal in one of the two Solid Rocket Boosters did not work properly due to the freezing cold temperatures that day. The Shuttle program was suspended for over two years in order to make several improvements to the other Shuttles.
The number of flights was greatly reduced as a result to the accident. Today the Shuttles are only used when a payload requires them or when human presence is absolutely necessary. The majority of the missions were of defensive or scientific nature. Most recent payloads include the decade's most important space science projects such as the Hubble Space Telescope , the Galileo Jupiter spacecraft, and the Gamma Ray Observatory. The most recent Shuttle mission was launched on December 4, 1998. The Endeavour brought a module for the International Space Station into orbit and the crew connected it to the Functional Energy Block that is already in space. The next Shuttle launch is going to be on May 20. The Discovery crew is also going to work on the International Space Station. This mission will be the 95th Shuttle mission.
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