Columbia's weary astronauts worked feverishly on troublesome freezers Thursday to save blood and urine samples collected during their nine days in space. They interrupted their sleep and last full day of orbital research to fix one overheating icebox, then another, and to shuffle the frozen specimens. Shuttle commander Bryan O'Connor called it "playing musical refrigerators."
The biomedical research mission, which began June 5, is scheduled to end today. Columbia is to land at Edwards Air Force Base, Calif., at 11:40 a.m. EDT.
The mission has provided the first in-depth medical testing of Americans in space since the Skylab flights of 1973 and 1974. It was the first flight to scrutinize astronauts' organ function immediately after lift-off, NASA said.
Mission Control awakened the astronauts four times late Wednesday and early Thursday _ three times to work on the freezers and once to fix a water-cooling system for some of the rat cages. There are 29 white rats aboard Columbia as well as 2,478 tiny jellyfish.
The freezers still were not working right when the astronauts got up. They readjusted the temperature of the empty freezer and restocked it with blood and urine samples. Scientists believe the specimens are fine.
O'Connor and pilot Sidney Gutierrez got ready for the trip home by successfully firing the orbiter's steering jets and checking flight control surfaces.
NASA instructed O'Connor to start closing the cargo bay doors a little earlier than normal Friday to make sure a loose seal does not hamper the procedure.
In the unlikely event of trouble, NASA said it would extend the flight a day and send two astronauts outside to fix the seal.
The cargo bay doors must shut tightly for the fiery plunge through the atmosphere. Otherwise, hot gases could force them open and the ship could burn up.