Fifteen years ago astronauts opened the hatch between US Unity node and Russian-built Zarya module. That, in itself, was nothing new, but that docking formed the basis of what is now the most expensive thing ever built: the football-pitch-sized International Space Station.

Its construction cost around $150bn. Keeping the mission running costs about $2bn each year. But has the money been well spent? And has that investment paid off?

"This is an investment in the long term," says Dr Francisco Diego, Senior Research Fellow at the Department of Physics and Astronomy University College London. "It's giving amazing results, amazing outcomes and still years to go with more experiments."

Construction of the space station was only completed in 2011, so research has only been the focus for the past two years. Now there are a host of experiments exploring the scientific potential of weightlessness. These range from growing tissue cultures, to observing what effect a lack of gravity has on flames, fluids and metals.

"It's a really good environment for understanding how materials behave in zero gravity and also how astronaut's bodies change," says Robert Massey, of the Royal Astronomical Society. "If we ever want to go and explore the wider solar system for example on a mission to Mars we need that zero, that micro gravity environment to understand the effect that it would have on astronauts over that sort of distance."

The ISS has also been an achievement of international cooperation. Spaces agencies from United States, Russia, European Union, Canada and Japan have all been involved. Astronauts from more than 15 countries have visited the station.

"So many nations are involved building components for it," says Robert Massey. "In that sense it's been a good prototype for anything more ambitious. If we go and explore the wider solar system or return to the moon, and we do that on a large scale, the ISS has been a good way of understanding how that might work."

Putting cost in perspective

So how does the station's $150bn cost compare with other big scientific projects?

The Large Hadron Collider (LHC) in southern France cost $10bn to build. This year, it discovered the Higgs Boson particle, a significant advance in the world of physics.

Cancer kills more than 7.5 million people each year, but its global research funding amounts to $9bn.

And then there is NASA's robotic Mars rover, Curiosity. It is still exploring the Martian surface. and the mission cost $2.5bn.

"In the context of the global financial crisis, the amount of money that went into bank bailouts and so on, it suddenly looks a bit less expensive," says Robert Massey. "You haven't seen a sort of Nobel prize winning discovery coming off the back of this, at least not yet. But what you have seen is the construction of a permanent lab in Earth orbit and that's valuable to a whole range of sciences."

The station is expected to be operational for the next 15 years. In that time it is hoped its scientific findings, and the new technologies developed, will eventually justify the cost.