NASA will conduct an experiment using its twin astronauts to assess the effects of space travel on humans. Here's why one of the twins is older than the other.
Beginning in March 2015, NASA astronaut Scott Kelly will spend one year at the International Space Station. His twin brother, a former astronaut, will spend that time at home in Arizona.
So, at the twin’s request, NASA plans to use the opportunity to measure the effects of space flight on Scott’s body, using his twin’s earthbound body as a baseline for assessments. Last month, the organization opened a public call for research proposals under the topic "Differential Effects on Homozygous Twin Astronauts Associated with Differences in Exposure to Spaceflight Factors."
NASA’s experiment – more public relations than ground-breaking science, as Scott has already logged significantly more time in space than Mark and, technically, the two could already be test subjects without one of them going back to space – will likely show that Scott is biologically "older" than Mark, given the toll that spaceflight is believed to take on astronaut’s bodies.
But even if space travel has made Scott biologically older than Mark, it has also make him in a different sense younger – thanks to special relativity.