As part of NASA’s year-long science experiment, twin astronauts Mark and Scott Kelly will be very separated for a very long time.

Astronaut Scott Kelly will spend one year in residence at the International Space Station while his now retired twin brother Mark will remain on Earth.

The concept for the year-long study was proposed to the organization by the two brothers themselves. The experiment will permit for an additional collection of blood, tissue, and stool samples from Scott Kelly during his time at the ISS. The grounded Mark Kelly will serve as the scientific control to any of NASA’s findings.

The experiment is part of NASA’s ongoing initiative to determine the long-term durational effects of human bodies in space… and not in space. Sorry, Mark.