Nasa has confirmed that an unprecedented experiment involving identical twins will take place in March 2015.
Twins Mark and Scott Kelly have announced they will be participating in an experiment which will see one of the brothers sent into space, while the other stays on Earth.
Next spring, Scott will embark on a 12-month mission on board the International Space Station to explore the effects of prolonged weightlessness and long-term spaceflight. The record-breaking year-long mission will be the longest period an American has spent in space, and will help the space agency prepare for future Mars missions.
Mark, a retired astronaut, will remain on Earth to receive medical tests before, during and after his brother's stay on the ISS. It is hoped the experiment will provide an accurate analysis, as the siblings are identical twins.
"In typical investigations, we usually have a specific outcome in mind and are goal-oriented," explained John Charles, head of HRP's International Science Office, as quoted by The Space Reporter. "In this case, the slate is essentially blank. I am anxious to see what proposals we receive from the scientific community."
The research will be conducted by Nasa's Human Research Program and the National Space Biomedical Research Institute, which aims to study and minimise the risks carried by astronauts on long-term missions.
Planned studies on the twins include taking blood samples from Scott before, during and after his year-long stay on the ISS, as well as corresponding samples from Mark.
Overall, Nasa has selected ten parameters for the study including fluid shift in space, reaction time and gut bacteria, which will be examined using cheek swabs, saliva and stool samples. Psychological evaluations may also be carried out.
Scott has had two previous shuttle missions and from 2010 to 2011, he spent five months on board the ISS. He will embark on next year's mission with Russian cosmonaut Mikhail Kornienko.
In a recent interview with the Associated Press, he said he had heard reactions ranging from "Oh, that would be really cool to be in space for a year" to "What, are you out of your mind?"
"No second thoughts - I'm actually getting kind of excited about the whole idea as we get closer," Scott enthused.
Mark, who lives in Arizona, has participated in four shuttle missions and has a total of 54 days in low-Earth orbit. While Scott is on the space station, Mark will live a normal life on Earth.
Mark said he draws the line at mimicking his brother's extreme exercise in orbit or eating "crappy space station food".
"It's not bad when you're in space," Mark told Fox News, adding he will not carry around "a can of Russian lamb and potatoes when I'm out to eat with my friends".