A technician checks out Shepard's communications gear just ahead of launch, while the astronaut shows himself to be the embodiment of nonchalance. At about 5:20 a.m. ET, he would enter the capsule, carrying a portable air conditioner. "Afterward he told how his heart quickened as the hatch was closed," NASA's "This New Ocean" account relates. "The sensation was brief; his heartbeat soon returned to normal. At 6:25 a.m. he began a denitrogenation procedure by breathing pure oxygen. This was to prevent aeroembolism, or decompression sickness, the airman's equivalent of the deep-sea diver's bends."