When the project was announced in 2014, Craig Kundrot of NASA's Human Research Program at the Johnson Space Center said the team would take various measurements and samples of the Kelly twins before the launch, during and after the one-year mission.
The Twins Study includes a number of tests, with collaborations among various universities, including Stanford University, Colorado State University, Johns Hopkins University and Cornell University. The study results will help better inform NASA about future plans for sending astronauts on deep space missions, such as to Mars.
The astronauts will be subject to a battery of tests looking at things such as muscle mass, bone loss and even the shape of their eyeballs. In a previous NASA study, some astronauts reported a change in vision after the physical shape of their eyeballs changed.
Researchers will look at how genes go "on and off" during space flight and if being away from Earth affects proteins in the body.
NASA wants to know how the stressors of space flight could change the body. This includes living in the micro-gravity environment at the space station and how radiation changes affect the proteins and metabolic systems in the body.
NASA also wants to discover how blood flow changes -- a result of micro-gravity -- can have unexpected effects on the body. One hypothesis is that astronauts' eyes change shape in space because blood volume on their upper body increases without gravity.
Mark Kelly told ABC News last year he wanted to help NASA in its quest to send astronauts on future deep space missions.
“We need to figure out how people are going to live in space for really long periods of time," Kelly said. "Especially if we want to send somebody to Mars or maybe we want to build a base on the moon."