What’s Up When There’s No Down? : Science: Atlantis-Mir mission helps NASA study how zero gravity affects body. Tests find that the brain reprograms itself to adapt to sensory confusion.
In some ways, the recent docking of the Russian space station Mir and the U.S. space shuttle Atlantis conjures up visions of the fictional starship Enterprise on “Star Trek” and its international, two-gender crew.
But in other ways, the images beamed down by the real-life astronauts and cosmonauts couldn’t be more different from the visions of space travelers embedded in the popular imagination.
Capt. James T. Kirk and his crew did not struggle to stay anchored to their chairs. Their faces did not puff up with body fluids that rushed to their heads in the absence of gravity. They did not appear to suffer from nausea, insomnia or any of the other ills that affect people who try to get along without gravity. In fact, for NASA the main purpose of the current mission--other than learning how to work in space with the Russians--is to study the effects of zero gravity on humans.
“The absence of gravity is devastating to the human body,” National Aeronautics and Space Administration chief Daniel S. Goldin said in an interview. “[Normally,] we study abnormal physiology in a normal environment. Now [with U.S. astronauts on Mir] we can study super-healthy astronauts in an abnormal environment.”
To find out exactly what lack of gravity is doing to their bodies, the astronauts and cosmonauts have been and will be prodded, poked and measured in every conceivable way--sometimes two or three times a day.
Perhaps most interesting is research into the central role gravity plays in the human neurosensory system--that is, how the brain, nerves, muscles and senses work together to allow people to stand, walk, reach for objects, turn their head to follow motion and perform simple tasks.
From what researchers know so far, the brain has a hard time knowing which end is up when there isn’t any down.
“Your brain is used to the idea that it knows where down is,” said NASA neuroscientist Deborah Harm. “[In space], you don’t know . . . [so] the nervous system begins to send out commands differently than it does on Earth. When you return to Earth, you have difficulties.”
Researchers are particularly concerned with what happens to the space travelers when they return because the effects of relearning how to deal with gravity can be more hazardous than learning how to live without it.
The brain uses gravity to orient itself. If someone is trying to pick out a can of soup from a supermarket shelf, the eyes and head move back and forth to scan the shelves. As the head moves, sensory organs in the inner ear record the motion. In that way, the brain knows that the head motion is making the cans of soup move across the visual field, not the motion of the cans themselves.
This helps keep your visual world steady while your head moves, Harm said.
Other sensors in the soles of the feet and the buttocks also contribute to a sense of how a person is oriented in space.
At the beginning of a mission, Harm said, “all of your physiological systems are designed to function in a gravity environment. When you take that away, everything tries to readapt.”
But readapting is not a smooth process, and the brain gets easily confused.
“All the sensory systems are coordinated,” Harm said. “The eyes, the inner ear, the pressure signals from the feet or seat. When there’s no gravity, the combinations are different.”
One result of this sensory conflict, she says, is space sickness, or nausea. Some astronauts adapt rapidly, but others remain a little out of sorts the entire trip.
Simply standing up can be a very complex problem. The sensors in the ear tell you whether you’re tilting. The soles of the feet give pressure clues. Muscles constantly adjust to keep the upright position steady.
On Earth, people constantly work bones, muscles, the heart and nerves simply to stand up. In space, the need to work against gravity simply disappears, which leads to two effects: First, the nerves, muscles and bone atrophy from lack of use. Second, the brain reprograms itself to stand, reach, tilt and so forth without gravity.
“You reach for something, but your hand goes too far to the right, so you reach to the left. After a few trials, you learn. You get feedback. The brain says, oh, I know what to do now, and you’ve got a new program.” When a space traveler returns to Earth, Harm said, “you’re a zero-g animal.”
But all the programs learned in space are useless for dealing with life on Earth. It normally takes a shuttle crew three or four days to readapt, but people who have been in space for many months may take many months on Earth before they are completely back to normal.
The good news is that astronauts who make many trips into space tend to readapt faster than first-timers.
“The more you go back and forth [between gravity and no gravity],” Harm said, “the more easily the brain is able to call each program up.”
She and her colleagues are working on ways to teach astronauts to be pre-adapted. Essentially, she says, it’s just like being bilingual. Except in this case, the astronauts will, it is hoped, become fluent in working with gravity and without it.
