The space shuttle's external tank is used for just eight minutes, then ditched over the Indian Ocean. It holds more than half a million gallons of liquid hydrogen and liquid oxygen, the highly volatile gases the craft's main engines burn on the leap into orbit. The tank is covered with plastic foam; without that, it would ice over with moisture sucked from the Florida air.

At the dawn of the shuttle program, NASA rules said no foam at all should be allowed to hit the shuttle and possibly damage the fragile heat-resistant tiles that cover its aluminum skin.

But fidelity to those standards was relaxed over time; in fact, foam fell from a PAL ramp in two early missions, including the one in June 1983 on which Sally Ride became the first American woman in space. There may have been many more incidents, but dozens of shuttle missions have been launched in darkness, with no visual record of foam, and the tanks themselves cannot be retrieved from the ocean for analysis.

As the early tank was replaced with two lighter successors, the PAL ramps remained -- one a 19-foot baffle along a channel for cables and pressurized lines along the forward end of the tank and the other the 37-foot strip along the flank of the cylindrical midsection of the fuel tank. And as experience showed NASA that shuttles returned safely despite well over 100 nicks and gouges requiring repair on many flights, the concerns abated over time.

Until Feb. 1, 2003, the day the Columbia disintegrated on its way home to Cape Canaveral.

It turned out that on liftoff, 16 days earlier, a 1.67-pound piece of foam had fallen from the tank and struck the leading edge of the shuttle's left wing. Despite years of assurance that such a strike could do no serious damage -- a mind-set the Columbia Accident Investigation Board would call the "normalization of deviance" -- the foam had cracked open a hole that admitted superheated gases when the shuttle re-entered the atmosphere, burning it up like a torch from within.

After the accident, NASA examined all possible sources of liftoff debris, eventually identifying more than 170. Engineers recognized that they could not eliminate all risk from debris, but they could do a much better job of reducing it.

The PAL ramp became a focus of attention: like the bipod arm ramp, the part of the tank implicated in the Columbia disaster, it is covered with foam by hand. NASA conducted extensive wind tunnel tests to see whether the ramp could be removed.