The US space shuttle combines the duties of a city bus and a freight truck for space, capable of carrying a crew of eight and a heavy cargo. But keeping the Nasa workhorse going a quarter century after it was first deployed has required a huge amount of technological sophistication together with more than a dash of risk-taking. The first almost entirely reusable space vessels, the shuttles went into service on April 12, 1981, exactly 20 years after the first manned space flight by Russian cosmonaut Yuri Gagarin. Since then five shuttles