The astronaut Scott Kelly is about to take off for the International Space Station, and if he is like some space travelers, he may temporarily feel a bit foggy or disoriented once in orbit.

Scientists have not had much luck measuring this subtle effect with standard cognitive tests, but now, a group of University of Pennsylvania researchers is trying a new tack.

While Kelly is in space, they will compare his mental performance with that of a uniquely qualified individual who stays behind on Earth. An individual whose brain is about as similar to Kelly's as you can imagine: his identical twin, Mark.

The Penn study of the twins, who grew up in West Orange, N.J., is one of 10 that NASA has approved for the one-year mission, which could launch as early as Friday.

Scientists from other institutions are tackling such topics as space-induced changes in Kelly's immune system, intestinal bacteria, and genetic expression. All of it will be compared with test results from his brother, a retired astronaut.

The goal is to get a better idea of what happens to the human body on a prolonged mission, in preparation for an eventual journey to Mars.

NASA has lots of data on astronauts who have spent six months in space, said Mathias Basner, assistant professor at Penn's Perelman School of Medicine. The impact of a one-year trip is a relative unknown, although a few Russians have done it. And a Mars trip would be far longer, 21/2 to three years.

Astronauts may suffer from sleep deprivation, stress, and elevated levels of carbon dioxide in the air they breathe, among other factors. Then there is the impact of weightlessness, which saps bone strength and causes bodily fluids to accumulate in unfamiliar places.

"It's all about getting the astronaut there and back, healthy," said Basner, who is leading the Penn research with colleagues Ruben Gur and David Dinges.

Not just for the astronauts' own sake, but also for the protection of the robotic arms and other high-tech equipment that they operate.

"This is a super-hostile environment," Basner said. "A tiny little mistake can lead to huge errors."

Studies of twins have long been a hallmark of science, allowing researchers to tease apart the impacts of genes and environment. Since the DNA of identical twins is very nearly the same, differences in their health may be attributed to different exposures - in this case, being in space vs. on the ground.

No one is pretending that the 10 teams of scientists will uncover profound, statistically valid truths about space travel by monitoring one set of twins. But the results from these pilot studies can be compared with those from astronauts on future long missions.

And the various findings from this mission may complement each other, Basner said. Scientists could determine if, say, changes in Scott Kelly's internal bacteria correlate with any altered performance when he takes the cognitive tests.

One reason that scientists have had trouble measuring mental fogginess in space may be that they used tests that are designed more to gauge the effect of significant brain injury, Basner said.

And most cognitive tests were developed for typical people, whereas astronauts tend to be on the higher end of the spectrum in both intelligence and physical fitness. So Gur and his Penn colleagues have developed harder questions in a variety of areas, such as working memory, spatial orientation, and emotion recognition.

In one series, test-takers must pick one from a group of abstract shapes in order to complete a pattern. In another section, which measures the ability to balance risk and reward, they must decide how much to inflate virtual balloons on a computer screen. A bigger balloon earns a higher score, but if it pops, the test-taker gets a zero.

If the test battery is validated in future studies, it might be used to determine if an astronaut is alert enough to engage in a particularly challenging task, such as a space walk. Subpar performance could indicate the need for more rest.

The Penn team says similar tests could be used for people in other high-stakes occupations, such as civilian or military pilots. The conditions in an airplane cockpit are markedly different from those on the space station, but both are dangerous places to experience fatigue.

The 51-year-old Kelly twins have taken the cognitive tests several times here on Earth, and are scheduled to do so 11 times during the mission, which will launch aboard a Soyuz spacecraft from the Baikonur Cosmodrome in Kazakhstan. They will take the tests three more times once Scott Kelly is back on land.

As identical twins, the brothers are ideal test subjects from a genetic standpoint. Yet their exposures and experiences make them different in any number of subtle ways, even though they grew up in the same household and even pursued the same career as astronauts.

What about their brains? Probably quite similar, though in a 2010 conversation with a NASA interviewer, the twins agreed that Mark Kelly, who is six minutes older, had better grades in high school.

Apparently he tried harder.

"I was more interested in, kind of, what was going on outside of the classroom vs. inside," Scott Kelly said.

"I don't think I remember him ever once doing homework," said Mark Kelly, the husband of former U.S. Rep. Gabrielle Giffords (D., Ariz.), who was shot four years ago. Although retired from astronaut duty, he agreed to participate in the twin study.

Both brothers have been to the space station on previous missions.

It is a grueling environment. But it's not Mars. A key difference: the radiation. The space station orbits less than 250 miles from Earth, and is mostly shielded from the harmful soup of cosmic particles in outer space.

To study the possible impact of radiation on Mars, researchers at Johns Hopkins University School of Medicine exposed rats to similar levels of radiation here on Earth.

Some of the rats experienced lapses in attention or delays in reaction times as a result, whereas others stayed the same, said Hopkins behavioral biologists Robert D. Hienz and Catherine M. Davis. In the affected rats, the radiation appeared to impair the brain's ability to transport the neurotransmitter dopamine.

Add that to the effects of sleep loss and weightlessness, and it would not be surprising to see an astronaut lose a step or two on a mission to Mars.

That trip is likely decades away, too far in the future for the astronaut twins from New Jersey to take part. But they will contribute to the Mars mission nonetheless, in the form of test results from their immune systems, their bacteria, and their brains.

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