"NASA knew from the second day of Columbia's 16-day research mission that a piece of the insulating foam on the external fuel tank had peeled off just after liftoff and struck the left wing, possibly ripping off some of the tiles that keep the ship from burning up when it re-enters Earth's atmosphere."
http://www.washtimes.com/national/20030203-87326768.htm
http://ltp.arc.nasa.gov/space/team/journals/katnik/sts87-12-23.html
"Damage numbering up to forty tiles is considered normal on each mission due to ice dropping off of the external tank (ET) and plume re-circulation causing this debris to impact with the tiles. But the extent of damage at the conclusion of this mission was not "normal."
The pattern of hits did not follow aerodynamic expectations, and the number, size and severity of hits were abnormal. Three hundred and eight hits were counted during the inspection, one-hundred and thirty two (132) were greater than one inch. Some of the hits measured fifteen (15) inches long with depths measuring up to one and one-half (1 1/2) inches. Considering that the depth of the tile is two (2) inches, a 75% penetration depth had been reached. Over one hundred (100) tiles have been removed from the Columbia because they were irreparable.
During the STS-87 mission, there was a change made on the
external tank. Because of NASA's goal to use environmentally
friendly products, a new method of "foaming" the external tank
had been used for this mission and the STS-86 mission. It is
suspected that large amounts of foam separated from the external
tank and impacted the orbiter. This caused significant damage to
the protective tiles of the orbiter."
http://www.arnold.af.mil/aedc/newsreleases/1999/99-041.htm
"According to NASA, during several previous Space Shuttle flights, including the shuttle launched Nov. 29, 1998, the shuttle external tank experienced a significant loss of foam from the intertank. The material lost caused damage to the thermal protection high-temperature tiles on the lower surface of the shuttle orbiter.
Although the AEDC Tunnel A tests did not replicate the in-flight failures, they did provide detailed measurements to better understand the flight environment and fundamental failure mode. From these tests, NASA determined the failure is caused principally by foam cell expansion due to external heating at approximately Mach 4 combined with pressure change and aerodynamic shear. Specialized miniature shear gages and other instrumentation were installed during the test to measure these forces."
IMO .. they need to scrap that foam stuff and fast