The International Space Station has housed people for 20 years. From elusive air leaks to 3,000 experiments, here's how the $150 billion project has evolved.

The International Space Station has housed people for 20 years. From elusive air leaks to 3,000 experiments, here's how the $150 billion project has evolved.Twenty years after Expedition 1 arrived on the International Space Station, here are some crew members' favorite moments and biggest challenges.