The five-decade long history of human spaceflight reached another milestone Monday with the 15th anniversary of the first crew taking up full-time residency in the International Space Station.
More than 220 people representing 17 countries have flown aboard the ISS since American astronaut William Shepherd and Russian cosmonauts Sergei Krikalev and Yuri Gidzenko first floated into the station, which only had three working sections.
Several years of U.S. space shuttle flights delivered various sections to the $100 billion orbital laboratory, which now has a mass of nearly 1 million pounds and as much pressurized volume as a Boeing 747.
The anniversary comes as U.S. commander Scott Kelly and Russian cosmonaut Mikhail Kornienko spend a full year aboard the ISS. Scientists will study the effects that weightlessness, radiation exposure, and isolation on the human body during a long-duration spaceflight, in preparation for a future manned flight to Mars.
"The space station really is a bridge," American crewman Kjell Lindgren told reporters Monday. "It's a testbed for the technologies that we need to develop and understand in order to have a successful trip to Mars.
NASA Administrator Charles Bolden says the ISS "should be considered the blueprint for peaceful global cooperation."