Florida ? The Atlantis astronauts uncovered a 21-inch stretch of nicks on their space shuttle Tuesday, but NASA said the damage did not appear to be serious.

The damage was likely the result of debris that came off the fuel tank shortly after liftoff Monday. The astronauts were inspecting their ship while racing to the Hubble Space Telescope when they came across the nicks spread over four to five thermal tiles.

A NASA photo shows what appears to be about 10 white scuff marks — officials hadn’t counted how many yet — around the edge of the shuttle where the right wing joins the fuselage and the belly curves up to the top of Atlantis.

“It doesn’t look very serious,” Mission Control said. “Those tiles are pretty thick. The nicks look to be pretty small.”

This repair mission is especially risky — a rescue shuttle is on standby for the first time ever — because of the debris-littered orbit of Hubble. Unlike other space flights, the astronauts can’t reach the international space station because it is in a different orbit than the telescope.