TABLE 1.1 U.S. Human Spaceflight Programs

Program Name Vehicle Development Flight Operations Number of Flights Crew Size
Mercury 1958-1961 1961-1963 6 1
Gemini 1958-1965 1965-1966 10 2
Apollo 1960-1968 1968-1975 12 3
Skylab Space Stationa 1966-1973 1973-1974 3 3
Space shuttle 1972-1981 1981-2011 135 2-8
Space shuttle-Mirb   1995-1998 9 7+
International Space Stationc 1993-2011 1998-TBD 27 3-6
Constellation Explorationd 2004-2011 Canceled 0  
Orion/multipurpose crew vehicle 2005-? NLT 2016? 0 4

aFirst long-duration missions for the United States. Longest of 84 days set world record and established high standard of biomedical data for long-duration missions.

bDocking between the space shuttle and the Russian Space Station, Mir. Flight of U.S. astronauts on Russian Soyuz to Mir and Russian cosmonauts onboard the space shuttle. Seven astronauts were left on the Mir for long-duration missions (greater than 90 days). Program used existing Astronaut Corps (no new selections) but required new vehicle training on Russian Soyuz and Mir Space Station and Russian language training. In addition, shuttle crews were trained in docking operations with Mir. All Russian vehicle training was executed at Gagarin Cosmonaut Training Center in Star City, Russia.

cFlight is defined as “increment,” which is a multimonth mission and separated from the vehicle used to reach or return from the ISS.

dThe Exploration Program to the Moon and Mars was canceled in 2010, but the Astronaut Office had been closely involved in requirements and design.

at the peak of space shuttle flights and preparation for the ISS in 2000. Vehicle habitable volume and flight rate also increased. The Mercury capsule flew with a crew of one, and the later space shuttle could accommodate a maximum crew of eight. The ISS hosts an international crew of six. The current size of the active U.S. Astronaut Corps is 61, and an additional nine astronauts are in training (astronaut candidates, referred to as ASCANs). NASA has projected a minimum required Astronaut Corps size of 55 to 60 astronauts through 2016.

The size of the Astronaut Corps has historically been aligned not just with the spacecraft being flown at the time but with future human spaceflight programs in development but not yet flying. Because of the lead time required to train an astronaut through a basic and then a mission-specific training syllabus—often up to 4 years for the ISS—the Astronaut Office and the Mission Operations Training Division were required to develop reliable forecasting algorithms for personnel and facilities. The forecasts also typically tried to accommodate anticipated astronaut attrition due, for example, to retirements, large gaps in flight opportunities, health issues, and, more recently, lifetime radiation limits and Russian anthropometric size requirements.

The U.S. human spaceflight program history, from program development to flight operations, is summarized in Table 1.1. The table does not reflect the current dependence on the Russian Soyuz capsule (three cosmonauts) to reach the International Space Station after shuttle retirement. With retirement of the space shuttle, flights of U.S. astronauts to the ISS will decrease from about 28 a year to fewer than 6. However, the time to train for a mission increases from approximately 1 year for a shuttle to 3-4 years for the ISS. Furthermore, individual astronauts will not be able to fly as frequently on the ISS as on the space shuttle, because of lifetime radiation dose limits currently imposed on them.


The Mercury, Gemini, and Apollo programs required that a full backup crew be trained for each flight. The backup astronauts were required when medical problems, or even death, affected the crew manifest. The early schedules were driven by the Cold War space race, so resources were available to support the missions fully.

Backup crew members were used in the early flights of the space shuttle (STS-1 through STS-4), when the

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