A suitcase-sized chunk of foam from an area of the shuttle's external fuel tank known as the left bipod, one of the three areas where struts secure the orbiter to the tank during liftoff, broke off 61 seconds into the flight on 16 January last year.

It gouged a large hole in Columbia's left wing.

The damage went undetected during the shuttle's 16-day mission, but caused the spacecraft to break apart under the stress of re-entry into the Earth's atmosphere on 1 February, killing all seven astronauts.

Costly defect

"We now believe, with the testing that we have done, that defects certainly played a major part in the loss. We are convinced of that," said Neil Otte, chief engineer for the external tanks project.

"We now believe, with the testing that we have done, that defects certainly played a major part in the loss"

Neil Otte,
Chief engineer for NASA's external tanks project

The fault apparently was not with the chemical makeup of the foam, which insulates the tanks and prevents ice from forming on the outside when 500,000 gallons of super-cold liquid hydrogen and liquid oxygen are pumped aboard hours before liftoff.

Instead, Otte said NASA concluded after extensive testing that the process of applying some sections of foam by hand with spray guns was at fault.

Gaps, or voids, were often left, and tests done since the Columbia accident have shown liquid hydrogen could seep into those voids. After launch, the gas inside the voids starts to heat up and expand, causing large pieces of insulation to pop off.

NASA says this happens on about 60% of its shuttle launches.

For the bipod foam, the entire ramp was apparently torn away. It weighed only 0.75 kg, but at the speed involved, it hit the orbiter with enough force to shatter the reinforced wing's leading edge.

NASA has made extensive changes in the foam-application process, but still has tests and more procedural changes before the tanks can be certified for flight.