The goal is to get Discovery ready for a launch now set for February 24, on a mission to the International Space Station. The shuttle has been delayed since an initial expected launch date in the latter half of 2010.
The upcoming flight will be Discovery's final mission (STS-133) after 25 years of service, as NASA puts an end to the shuttle program.
NASA expects to complete the repairs by the last week of January.
That outer layer of foam serves as insulation to help keep the chill in the super-cold propellants. It also protects against damage by what NASA delicately calls "aerodynamic environments" during lift-off.
(The shuttle seen lifting off here is the Atlantis.)
Discovery has had a notable career. NASA says that it has flown more missions than any other spacecraft--38, not counting the upcoming flight. It has gone around the Earth 5,600 times and carried 174 people into space, and its payloads have included the Hubble Space Telescope.
Technicians also used computing radiography scanners which move faster, scanning as many as five stringers per hour.