2 Astronauts Again Try Out Vacuum Device : Space: Pumping blood to legs seen as solution to faintness crew members feel on return to Earth. One is too skinny to fit in machine properly.

Two Atlantis shuttle astronauts Thursday again spent time inside a vacuum container having their blood redistributed after struggling to keep the device sealed.

The medical experiment is intended to reduce the faintness that astronauts feel when returning to Earth. Scientists believe that shifting blood from the top of the body, where it accumulates in weightlessness, back into the legs may ease that transition.

The experiment had to be cut short because of the additional time it took to tighten the seal around the small waist of G. David Low. He is 5 feet 9 inches tall and weighs 145 pounds.

NASA reduced Low’s four-hour decompression session by nearly an hour so pilot Michael Baker could get in his four hours of container time.

Baker tried to get his session shortened.

“The second subject wants to know if he can also have a 2 1/2-hour soak,” Baker asked Mission Control.

“Negative,” was the reply.

“It’s worth a try,” said Baker, who expressed little enthusiasm for the test before the flight began.

Although the seal around Low’s waist leaked Thursday, the data collected still will be useful, said Dr. Sam Pool, chief of the medical sciences division at Johnson Space Center in Houston.

“We are holding the pressures that we want to hold, and that’s the important thing in spite of the fact that it’s leaking,” Pool said.

National Aeronautics and Space Administration officials said no leakage was evident during the tests on Baker, whose waist is larger than Low’s.

Low sat for about an hour in the waist-high container, waiting for the procedure to begin, as the crew tried to seal the device. Baker stuffed towels around Low’s waist and, at one point, even pressed his hand against the part of the seal covering Low’s stomach.

The test finally began, and the pressure inside the container gradually was reduced until the stress on Low’s heart was about the same as if he were standing on Earth, officials said.

Crew mate Shannon Lucid conducted echocardiographs on Low and Baker during the experiment, providing images of their hearts to physicians on the ground.

Low and Baker are scheduled to slip back into the container for hourlong sessions today and Saturday. The nine-day flight ends Sunday.

Low and Baker had tried out the depressurization chamber for an hour Sunday.