Why are we asking this now?

Yesterday marked the 10th anniversary of the launch of the first module to become the International Space Station, the largest man-made structure in space. But it is still incomplete, with another four pressurised modules yet to be added to the 10 that are already there in orbit about 200 miles above Earth. As well as being at least five years over its deadline, the space station is also heavily over budget, with estimated costs hovering around $100 billion (£65bn).

Where did the idea of an International Space Station come from?

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It came out of the collapse of the Soviet Union in the early 1990s. Until then, the United States and the Soviet Union each had their own space-station initiatives. The Russians called their station Mir, which means "peace", and it flew successfully for many years, notching up records for the length of time humans had spent in orbit.

The American station was called Freedom but it never got off the ground because of technical and funding constraints. After the collapse of the Soviet Union, US President George H W Bush and Russian President Boris Yeltsin signed an agreement in June 1992 to collaborate on joint space missions. A year later, in September 1993, the American vice-president Al Gore and Russian Prime Minister Victor Chernomyrdin announced plans for a new space station, which later became the International Space Station.

The idea, broadly, was that Russia could provide some of the pressurised modules, and its heavy-lift capabilities using its Proton rockets, while the Americans could provide frequent servicing missions using its re-usable Space Shuttles.

What was the station designed to do?

It is supposed to be there to offer a permanently manned presence in space and to conduct scientific research. It also had the political role of bringing nations together in a joint effort designed for peaceful purposes. This was particularly important in terms of Russia's involvement because there were genuine fears that its rockets scientists may go and work for rogue states if they were not offered legitimate jobs in the space industry. The International Space Station project now involves nearly 20 nations, mainly the US, Russia, Japan, Canada, Brazil and those countries signed up with the European Space Agency.

How big is it?

When finished the space station will be as big as a football field, which includes of course the vast solar panels that provide the structure's power supply. The 14 pressurised modules will have a combined volume of a thousand cubic metres and they will include laboratories, docking compartments, airlocks, nodes and living quarters. The modules were launched either by the American Space Shuttle, or on the Russian Proton or Soyuz rockets, and are fitted together in space. It is possible to see the space station with the naked eye – it appears as one of the brightest objects in the sky when it catches the light at the right angle.

Who lives there and for how long?

The first resident crew on the International Space Station arrived in November 2000, two years after the launch of the first module. Since then, the space station has been home to successive three-person crews who overlap on their six-month missions. In total, more than 160 astronauts have visited the station, some more than once.

Sleeping can be difficult as the station experiences 16 sunrises and sunsets each day, which is why the windows are covered at "night" to give the impression of darkness. A typical day for the crew starts with wake-up calls at 6am followed by a morning inspection and breakfast before starting work at around 8am. Lunch is a one-hour break about 1pm and "pre-sleep" activities start at 7.30pm, before lights out at 9.30pm. The crew performs regular exercises during the 10-hour working day to counteract the effects of low gravity, which can cause muscles and bones to waste away.

What sort of science is carried out?

There are a number of scientific laboratories on the International Space Station. The American lab is called Destiny and was launched in February 2001, and has the largest window in space – a 20-inch, optically perfect window called nadir for viewing the Earth. The European lab is called Columbus and carries out experiments in biology as well as fluid physics. It also carries out research on solar activity and new materials, as well as having an atomic clock. The Japanese science module is called Kibo, which has an external platform for exposing objects to space. Finally, a second American lab called Express Logistics Carriers will carry out research into the vacuum of space.

What have we learned from this research?

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Conducting experiments in the near-weightlessness of low-earth orbit has been useful but it has not resulted in earth-shattering conclusions. Indeed, many scientists believe it could all have been done at far lower cost by using unmanned spacecraft, which are far cheaper because they do not require the high level of safety needed for a manned mission.

One of the station's leading critics is Lord Rees of Ludlow, the president of the Royal Society and the Astronomer Royal, who has described it as a giant and very expensive "turkey in the sky". His point is that it has cost tens of billions of pounds to put the space station into low-earth orbit when the same amount of money could have been spent on sending relatively simple and cheap exploratory robots to the Moon, Mars and other planets.

Lord Rees, like many scientists, is not necessarily against manned space flight if it inspires and enthuses the wider public, but he does draw a line at an International Space Station set up for political rather than scientific reasons. As Lord Rees says: "My view about manned space flight is that, as a scientist and practical man, I'm against it, but as a human being, I'm in favour of it."

What happens next to the International Space Station?

The Space Shuttle programme is due to be phased out in 2010, although there is some talk of extending it beyond that date rather than relying solely on Russian rockets to get to and from the space station. No doubt there will be many more crews who will spent their six-month tours of duty aboard the station, but the focus for the coming decades is going to be on a permanently manned base on the Moon after a return there in 2020, and a possible manned mission to Mars by mid 21st Century. The space station is being seen as little more than a stepping stone to those far more ambitious horizons of the high frontier.

What have been the station's highs and lows?

The blackest moment was the Space Shuttle Columbia disaster of February 2003. It resulted in a two-and-a-half year suspension of the shuttle programme, which almost ended the construction of the space station. On happier note there have been the six space tourists and a golf shot that went around the world – in addition the space wedding of space station cosmonaut Yuri Malenchenko to his bride Ekaterina Dmitrieva, who was in Texas at the time.

Was the International Space Station worth the money?

Yes...

* It ensured that Russian rocket scientists did not stray to rogue states with nuclear ambitions.

* It cemented a bond of scientific collaboration between nearly 20 nations involved in the space station.

* It is a brilliant scientific and technological achievement that will be useful for further space missions.

No...

* The little useful science it has provided could have been gathered more cheaply using robot spacecraft.

* It has given science a bad name by involving commercial gimmickry such as space tourism and space golf.

* Low-earth orbit is intrinsically less exciting than the exploration of the Moon and planets.