On reentry the tiles burn and degrade, and thus had to be tested and replaced on Earth if necessary. They are all numbered with serial numbers that uniquely ide

On reentry the tiles burn and degrade, and thus had to be tested and replaced on Earth if necessary. They are all numbered with serial numbers that uniquely identifies them and their location on the surface of the shuttle [1]. Like a real life hash table ;) Smaller tiles are easier and cheaper to replace, and thus the uneven look. [1] http://artifacts.nasa.gov/tile_serial_numbers.htm http://artifacts.nasa.gov/tile_serial_numbers.htm