NASA will send the space shuttles back to work after repairing cracks in their hydrogen fuel lines. These cracks are caused by stress as the engines cycle on and off during flight. While the cracks can be repaired, they raise serious doubts about NASA's plan to fly the shuttles another 20 years. Doing that would require a very expensive and time-consuming replacement of the fuel lines. It could take over two years to manufacture the parts and another year or more to install them in each orbiter. NASA sold the shuttle to Congress on the grounds that it would fly cheaply and often. Instead, the expensive engines have to undergo major overhauls after every flight, and the planned 24 flights per year have turned into about four.--Stephen V Cole