Break out the party hats, because as of today humans have been continuously living on the International Space Station (ISS) for 15 years. 

Assembly of the station began on November 20, 1998 with the launch of the Russian-built (and U.S. funded) Zarya module. Two weeks later the American Unity module launched and was joined to Zarya by Space Shuttle Endeavour. Four further manned shuttle flights, and two unmanned Russian flights, completed the preliminary construction of the station.

It was not for another two years that the first crew (Expedition 1) arrived, entering on November 2, 2000. They were NASA astronaut and commander William Shepherd, and Russian cosmonauts Sergei Krikalev and Yuri Gidzenko. Together they got the station up and running, activating various critical life support systems, during intensive work days.

^{Expedition 1, from left to right: Gidzenko, Shepherd, and Krikalev. NASA.}

''There are so many people who felt maybe we couldn't do it,'' NASA’s administrator at the time, Daniel Goldin, told reporters after the launch of the first crew on October 31, 2000. ''But it's happening, it's here. We're going to be in space forever with people who are circling this globe, and then we're going on to Mars, back to the moon and with bases on asteroids.''

Michael Foale, a now-retired British-American astronaut who was in charge of the first crew on the ground, added: ''I have often thought this may be the last time that there were no humans in space, and I really believe that could be today.''

Since then, 217 additional people from 17 countries have visited the station, and it remains a lasting testament to our successful occupation of low Earth orbit. With Russia's Mir space station also continuously occupied for 12 and a half of its 15 years in orbit, between 1986 and 2001, there have only been four years in the last three decades that humans have not been living in space.

^{Construction of the ISS began with the docking of the Zarya (left) and Unity (right) modules in 1998. NASA.}

The beginnings of the ISS can be traced back to the proposal for the American Space Station Freedom. This complex, very similar to the ISS in appearance and purpose, was to be a U.S.-only outpost. Daunted by the scope and cost of such a project, though, President Clinton made the wise decision to partner with Russia in September 1993.

This would prove essential. Russia would not only supply some of the modules, they would also ferry people and cargo to the later named International Space Station with their Soyuz and Progress vehicles respectively. With the retirement of the Space Shuttle in July 2011, this capability is now more than ever a necessity to keep the station running.

The two superpowers had cooperated in space before, most notably on the Apollo-Soyuz Test Project and the Shuttle-Mir program. The ISS, though, was unprecedented. The $100 billion station, located 410 kilometers (255 miles) above Earth, would ultimately be the size of an American football field, and allow for incredible research to be performed in orbit. 

^{Astronauts including the current ISS commander Scott Kelly have, in recent years, been tweeting remarkable pictures of Earth-like this from orbit. Scott Kelly/NASA.}

Originally scheduled for completion in 2006, numerous delays and the Space Shuttle Columbia disaster in 2003 saw the station essentially completed in 2011. And its importance in spaceflight cannot be understated. Not only has it provided untold scientific data on how humans operate during prolonged spaceflight, it has also been the location for thousands of microgravity experiments with applications across science, technology, medicine, and many other areas.

And the science shows no sign of abating. Currently on the ISS, two astronauts – NASA’s Scott Kelly and Russia’s Mikhail Kornienko – are just over halfway through the first 12-month stay, providing crucial biological data ahead of manned missions to Mars in the late 2030s, which could last more than two years.

The ISS itself is scheduled to remain in orbit until at least 2024, when parts of it – likely just the U.S. segments – will be deorbited. Those segments were carried into orbit by the Space Shuttle and thus cannot be moved anywhere with ease, but the Russian segments had their own thrusters, and thus could be relocated to build a new station, something Russia is keen to do.

^{Former Canadian astronaut Chris Hadfield, pictured inside the Columbus module in 2012, is perhaps one of the most famous occupants of the ISS. NASA.}

Until then, the ISS will remain the cornerstone of our recent endeavors in space. Following the Apollo missions to the Moon, many have bemoaned the situation where humans have not ventured out of Earth orbit since 1972, but the ISS has been vital in advancing our understanding of spaceflight, hardware, orbital procedures, and much more.

Even after the ISS is retired, it will long be remembered as one of the most successful global projects, a time when the world worked together to live and work in space in harmony. Whether there will ever be such a cooperative space project again in the near future, such as an international mission to Mars, remains to be seen.