The seesaw of theories on why space shuttle Columbia broke up during re-entry has tipped back to a focus on the polyurethane foam used on its external tanks.
NASA officials are studying whether the foam could have damaged the shuttle's thermal tiles on the leading edge of its left wing significantly enough to cause the catastrophe.
Further, NASA spokeswoman Melissa Motichek confirmed the agency is studying changes to the way it applies the foam. NASA first decided to revisit the process after the launch of Atlantis last fall. In that incident, a piece of foam from the bi-pod - a point on the external tank where two rods connect the tank to the shuttle's underbelly - ripped away and struck a ring attaching the left solid-rocket booster to the external tank.
``The foam incident on Columbia gave new urgency to this inquiry,'' she wrote in an e-mail.
There is speculation that as many as three pieces of foam damaged Columbia's tiles, allowing gases of at least 2,000° F to enter the orbiter. That could account for the sudden temperature spike observed in Columbia's left wheel well moments before the breakup.
According to news reports, e-mails from NASA engineers during the days leading to Columbia's planned landing speculated that the shuttle's left wing could burn off because of the possible damage caused during its launch. The engineers concluded, however, that any damage incurred during liftoff was minimal, therefore they did not forward their concerns to NASA brass.
The shuttle disintegrated Feb. 1 over the western United States during its re-entry, killing all seven aboard.
Motichek said NASA has not decided yet how it might change foam application. Lockheed Martin Corp. currently uses a computerized, mechanized system to spray on the foam, and does finishing touches by hand.
She indicated NASA has been in touch with Lockheed Martin regarding possible alterations to the process.
``They were key members of the team that evaluated this process change,'' Motichek wrote.
Lockheed Martin spokesman Harry Wadsworth said the company would not comment on any possible changes until the investigation into what caused Columbia's disintegration is complete.
The most recent significant change to the process came as a result of the 1990 Clean Air Act, eliminating the use of freon during the application process. Motichek said the agency has tinkered with the application to prevent the loss of chunks of the material during missions since, but ``nothing new in this area was tried on Columbia.''
Charles Eastlake, former summer research fellow at NASA's Structures and Propulsions Laboratory in Huntsville, Ala., said foam was not a major point of concern for the agency until recently.
``Almost everyone thought the foam on the external tank was of a low enough density that it wasn't a serious threat to the tiles,'' he said. ``As time passes, they aren't finding evidence to lead them in another direction. Maybe it's capable of more damage than we ever thought before.''
Bill Kauffman, an aerospace engineering professor at the University of Michigan, cited the e-mails exchanged by NASA engineers as caveats NASA should have heeded.
``There were all kinds of warning flags raised, just like with the Challenger's O-ring,'' he said, in reference to the failure that doomed the 1986 Challenger flight.
Debris, in most cases falling foam, has damaged an average of 100 tiles during shuttle flights.
``Some of these had impact depths of 4-5 inches, and then you're down to the aluminum,'' Kauffman said.