International Space Station allows people to live in space for long periods of time. Supporters believe that space exploration benefits people on Earth. Other people are worried about the cost of a space station and whether the discoveries are worthwhile.

The International Space Station (ISS) will be the biggest structure ever built in space. Due to its size, people can live and work on the ISS for much longer than ever before. This allows scientists to gather information about the effects of living in space for long periods. This information could be helpful in working through the challenges of travelling to Mars.

Astronauts on the ISS conduct experiments in an environment where there is very little gravity. They can compare these results with what happens on Earth where gravity affects everything. It increases our knowledge about why things behave the way they do.

Some scientists question whether the small number of crew on the ISS can devote enough time to conducting serious experiments. They say there is a limit to the resources on a space laboratory which reduces the number and usefulness of experiments. It is estimated that the cost from the start of the project to its completion in 2010 will be over $170 billion. Critics argue that this money could be better spent on Earth.

The first part of the International Space Station was launched in 1998. It won't be complete until at east 2010.

Having astronauts living on the International Space Station will provide data which can be used to further space exploration.

When the ISS is complete, it will be the size of a football stadium — 109 metres across and 80 metres long.

Sixteen countries are building the ISS — USA, Russia, Canada, Japan, Brazil and the 11 countries of the European Space Agency.