The International Space Station celebrated its birthday on 20 November, 15 years after Russia launched the first module, Zarya.

Three weeks later, on 4 December, the Space Shuttle delivered the second module, Unity, and the two were joined together on 6 December 1998. Subsequent expansion – including in 2007-2008, with pressurised modules supplied by the Italian, European and Japanese space agencies – has created the world’s largest spacecraft, which has been continuously occupied for more than 13 years.

The notion of an international station was born of 1980s budget realities, which ended go-it-alone plans by the USA, Russia and Europe. The ISS is arguably the most expensive structure ever built, so far costing the US, Russian, Canadian, Japanese and European governments some $150 billion in up-front and operating costs.

Critics question the safety and efficiency of the ISS, noting that the majority of an astronaut's time on board is spent on maintenance, and argue that the money could be better spent on other projects. However, supporters point to the station's contribution to scientific research and Earth observation, not to mention international collaboration.

Under current budget plans the station will be deorbited in 2020, but it could remain spaceworthy until well into the next decade, if the partners choose to keep it orbiting the Earth.