NASA Cutting Short Spacewalk Outside Station

The third and final spacewalk during Endeavour's mission to the International Space Station is going to be cut short by about 30 minutes as a result of elevated levels of carbon dioxide building in one of the astronaut's spacesuits.

Endeavour mission specialist Bob Behnken has been working so hard that the carbon dioxide levels in his spacesuit are threatening to eclipse the suit's ability to remove the gas, which is created as an astronaut inhales oxygen and then exhales carbon dioxide.

Flight surgeons monitoring the spacewalk have noted that the supply of lithium hydroxide in the suit -- a substance used to scrub carbon dioxide -- is being exhausted more quickly than expected. Build-ups of carbon dioxide can case headache, dizziness, sweating, restlessness, disorientation, visual distortion, and loss of consciousness. High concentrations can be fatal in worst cases.

The spacewalk had been planned to last six hours and 30 minutes. Flight controllers have instructed the astronauts to finish up work and start preparing to reenter the station about 5.5 hours into the excursion, which began at 9:15 p.m.

Look for the astronauts to be back inside the U.S. Quest airlock by 3:15 a.m.