A full year after the Columbia tragedy, NASA (search) has finally determined how and why the large piece of foam insulation that doomed the spacecraft broke off from the fuel tank at liftoff.

NASA's top spaceflight official, Bill Readdy (search), said Friday that through extensive testing, the agency has learned that air liquefied by the super-cold fuel in the tank almost certainly seeped into a crack or void in the foam, or collected around bolts and nuts beneath the foam. The trapped air expanded as the shuttle rose, and blew off a chunk of foam the size of a suitcase.

Rather than peeling off, as NASA had assumed from past experience, the foam was pushed off with explosive force, Readdy said. The space agency also had assumed the foam would fall down along the tank and miss the shuttle, but in reality, the falling foam shot toward Columbia (search) and the left wing rammed into it, resulting in a large fatal gash.

"That is really the root cause that we've been able to discover here," Readdy said.

In all likelihood, faulty application of the foam created air pockets, Readdy said.

A tank redesign and improved techniques for applying and double-checking the foam should solve the problem, NASA said. But all this will take longer than expected and is one reason space shuttles will not fly again until next year.

By the time Columbia shattered over Texas during re-entry on Feb. 1, 2003, NASA had run only 400 computer models involving flyaway foam and other launch debris. Since then, millions of models have been run and the space agency has a much better understanding of what went wrong and how to fix it, Readdy said.

A newer problem involves the rudder speed brake, a tail assembly that is used to guide and slow the spaceship as it comes in for a landing. In recent months, corrosion and microcracks were found in the machinery that operates Discovery's rudder speed brake.