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As twins, Mark (left) and Scott (right) may provide scientists valuable information about how outer space affects a person’s body.  
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Twins in Space!
Two brothers from the United States are the first identical twins to travel into outer space

By Sara Goudarzi

Mark and Scott Kelly, NASA’s only identical twin astronauts, are set to be test subjects for studying the effects of traveling in outer space over long periods of time.

Scott will spend a year on the International Space Station (ISS), a habitable satellite that orbits Earth. This will be the longest mission in outer space by any NASA astronaut. Scott has been on two space shuttle flights and has previously spent six months on the ISS.

While Scott is living in space, Mark, who retired from NASA in 2011, will stay on Earth. In the past, he took part in four space shuttle missions and he has spent a total of 54 days in space.


Living in outer space is not easy. Observing astronauts who have the same genetic makeup will give researchers a chance to investigate how such a big difference in living conditions affects the body.

Genes are the basic units of living organisms—from people to plants. They define how those organisms grow and react to the world around them. Genes are passed from parents to children, so families share many genes. Identical twins are born with nearly the same genes. It should be easier for scientists to compare the two astronaut brothers than it would be to compare people with very different genes.


The ISS is stationed in low Earth orbit (LEO), some 250 miles above our planet’s surface. Astronauts aboard the ISS experience weightlessness (zero gravity).

In weightless conditions, muscles and other bodily systems don’t work as hard as they do on Earth. For this reason, astronauts experience a greater loss of muscle tissue and bone density. A person also experiences a rise in heart rate and blood pressure while in space.

To gain knowledge for future space missions, researchers will look for any genetic differences that occur between the twins because of their different habitats.

“This is a once-in-a-space-program opportunity,” says John Charles of NASA. “The mission of [NASA’s Human Research Program] is to reduce the risk to astronauts during long-duration space flight.”