Context: While the United States continued missions to the Moon in the early 1970s, the Soviets worked to build a space station. A space station is a large spacecraft. People can live on this craft for a long period of time. Between 1971 and 1982, the Soviets put a total of seven Salyut space stations into orbit. Figure 23.22 shows the last of these, Salyut 7. These were all temporary stations. They were launched and later inhabited by a human crew. Three of the Salyut stations were used for secret military purposes. The others were used to study the problems of living in space. Cosmonauts aboard the stations performed a variety of experiments in astronomy, biology, and Earth science. Salyut 6 and Salyut 7 each had two docking ports. One crew could dock a spacecraft to one end. A replacement crew could dock to the other end. The U.S. only launched one space station during this time. It was called Skylab. Skylab was launched in May 1973. Three crews visited Skylab, all within its first year in orbit. Skylab was used to study the effects of staying in space for long period. Devices on board were and for studying the Sun. Skylab reentered Earths atmosphere in 1979, sooner than expected. The first space station designed for long-term use was the Mir space station (Figure 23.23). Mir was launched in several separate pieces. These pieces were put together in space. Mir holds the current record for the longest continued presence in space. There were people living on Mir continuously for almost 10 years! Mir was the first major space project in which the United States and Russia worked together. American space shuttles transported supplies and people to and from Mir. American astronauts lived on Mir for many months. This cooperation allowed the two nations to learn from each other. The U.S. learned about Russias experiences with long-duration space flights. Mir was taken out of orbit in 2001. It fell into the Pacific Ocean. The International Space Station, shown in Figure 23.24 is a joint project between the space agencies of many nations These include the United States (NASA), Russia (RKA), Japan (JAXA), Canada (CSA), several European countries (ESA) and the Brazilian Space Agency. The International Space Station is a very large station. It has many different sections and is still being assembled. The station has had people on board since 2000. American space shuttles deliver most of the supplies and equipment to the station. Russian Soyuz spacecraft carry people. The primary purpose of the station is scientific research. This is important because the station has a microgravity environment. Experiments are done in the fields of biology, chemistry, physics, physiology and medicine. NASA wanted a new kind of space vehicle. This vehicle had to be reusable. It had to able to carry large pieces of equipment, such as satellites, space telescopes, or sections of a space station. The new vehicle was called a space shuttle, shown in Figure 23.25. There have been five space shuttles: Columbia, Challenger, Discovery, Atlantis, and Endeavor. A space shuttle has three main parts. You are probably most familiar with the orbiter. This part has wings like At the end of the mission, the orbiter re-enters Earths atmosphere. The outside heats up as it descends. Pilots have to steer the shuttle to the runway very precisely. Space shuttles usually land at Kennedy Space Center or at Edwards Air Force Base in California. The orbiter is later hauled back to Florida on the back of a jet airplane. The space shuttle program has been very successful. Over 100 mission have been flown. Space shuttle missions have made many scientific discoveries. Crews have launched many satellites. There have been other great achievements in space. However, the program has also had two tragic disasters. The first came just 73 seconds after launch, on January 28, 1986. The space shuttle Challenger disintegrated in mid-air, as shown in Figure 23.27. On board were seven crew members. All of them died. One of them was Christa McAuliffe, who was to be the first teacher in space. 
 Question: While the U.S. flew missions to the moon in the early 1970s, the Soviets worked to build space
Output: stations.