Technicians were replacing a vent valve near the top of the 47 metre tall tank when a Halogen work lamp fell and hit the tank's foam insulation.
Preliminary inspections show the impact left five small indentations, with the largest about the size of a stick of gum, and a 15 cm scratch, said Marion LaNasa, spokesman for tank manufacturer Lockheed Martin.
A detailed inspection of the area was underway but the incident was not expected to affect the shuttle's targeted July 1 liftoff.
The affected area is not among the sections of tank foam insulation that were redesigned after the 2003 Columbia disaster and again after the July 2005 flight of Discovery, the only launch since the accident.
A piece of foam insulation that fell off the tank and hit Columbia's wing during liftoff was responsible for heat shield damage that led to the ship's destruction and the loss of seven crew members during atmospheric re-entry on February 1, 2003.
A similar problem occurred during Discovery's liftoff two and a half years later, though the shuttle escaped damage.
NASA is preparing Discovery for launch again, but it first must prove that the new tank design is safe to fly and its undergoing a series of wind tunnel tests and analyses.
Safety has been a top priority for NASA, particularly at the shuttle processing centre in Florida, where a series of mishaps have resulted in a death, equipment damage and several near-disasters in the past month.