NASA’s Twin Experiment

In partnership with

Identical twins are already rare—but having both become astronauts? Almost unheard of. So when NASA realized they had Mark and Scott Kelly, they saw a unique opportunity. In 2015, Scott Kelly spent 340 days on the International Space Station (ISS) while Mark Kelly stayed on Earth. This allowed scientists to track exactly how space changes the human body.

After a year in orbit, 7% of Scott’s gene expression changed forever—meaning the way his genes work is now different from his brother’s. Although their DNA sequence is still identical, the way Scott’s body reads and uses that DNA is altered.

With less gravity in space, Scott’s spine stretched, making him 2 inches taller while on the ISS. But there’s a catch—he also lost bone density and muscle mass, making him physically weaker when he returned to Earth.

Receive Honest News Today

Join over 4 million Americans who start their day with 1440 – your daily digest for unbiased, fact-centric news. From politics to sports, we cover it all by analyzing over 100 sources. Our concise, 5-minute read lands in your inbox each morning at no cost. Experience news without the noise; let 1440 help you make up your own mind. Sign up now and invite your friends and family to be part of the informed.

Sign up today!

One strange effect of microgravity is Spaceflight-Associated Neuro-Ocular Syndrome (SANS)—a swelling of the optic nerve that affects vision. Scott experienced blurred vision, a problem many astronauts face after spending time in space.

The ISS orbits outside much of Earth’s protective atmosphere, exposing astronauts to more radiation. After his time in space, Scott became 12 times more likely to develop cancer than his twin.

Scott’s body aged faster due to stress, radiation, and cell damage. But thanks to time dilation—a concept from Einstein’s theory of relativity—Scott actually moved through time slightly slower. He is now 5 milliseconds younger than his twin!

The NASA Twin Study revealed both the dangers and possibilities of long-term space travel. As humanity looks toward Mars and beyond, understanding how space affects the human body is crucial—and this rare twin experiment brought us one step closer.

Stay tuned,

BREEFX ✨

P.S if you enjoyed this fact and found it interesting, why not share it with a friend!

If you’re that smart friend, subscribe here!