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Space shuttle landing
In 2004, then-President Bush announced the Constellation programme would develop new technology to replace the space shuttle. Budget overruns led to the project being cancelled by President Obama in 2010.
Some of Constellation's technology has, however, found its way into the Multi-Purpose Crew Vehicle, intended for journeys into deep space.
Concerns about the cost and purpose of manned spaceflight cloud Nasa's future. "There is every reason to believe that future human spaceflight vehicles will require an investment beyond the level normally granted to Nasa's programmes," said Roger Launius, citing a figure of around $240bn (£150bn).
Nasa continues to explore the idea of partnering with private companies to develop commercial manned spacecraft technology. In April, it advanced funding of $270m to four projects that could lead to working space taxis by 2015.
"I just don't know if we have that audacity now to build something nearly as ambitious as the shuttle," said astronaut Alvin Drew, following Discovery's last mission.
The space shuttle Discovery will be displayed near Washington at the Smithsonian National Air and Space Museum's Steven F Udvar-Hazy Center. Atlantis will remain at the Kennedy Space Center visitors' complex, while the Endeavour will be displayed at the California Science Center in Los Angeles.
Photo credit: Nasa