If, as proposed by President Obama, a manned mission is on its way to Mars in the mid-2030s, and if perhaps in the distant future mankind attempts to journey beyond the limits of our solar system, scientist will want to know some answers to a most pressing question: How does long-term exposure to the zero gravity of space affect the human body?

Now NASA is preparing to take advantage of the unique opportunity to study an individual living simultaneously in zero gravity in space and on the surface of the Earth -- a seeming impossibility made possible by the fact the "individual" in this case is in fact two people, identical twin astronauts Scott and Mark Kelly.

Beginning in March 2015, NASA astronaut Scott Kelly will begin a year-long stint aboard the International Space Station while his genetically identical retired twin Mark will be living at the Arizona home he shares with his wife, former U.S. Rep. Gabrielle Giffords, still recovering from being shot at a Tucson campaign event in January 2011.

The Kellys, the only twins and only siblings to have traveled in space, are both veterans of missions to the ISS and have readily agreed to allow NASA to conduct experiments as each lives at a different end of Earth's "gravity well."

In fact it was their idea, NASA said.

"This opportunity originated at the initiative of the twin astronauts themselves," NASA said in a solicitation for research proposals released last week.

Scott Kelly is a veteran of two space shuttle flights and a six-month space station stint while Mark Kelly has flown four space shuttle missions delivering equipment and crews to the ISS.

Twins have long fascinated scientists, and have often been used in studies on Earth, usually into the "nature verses nurture" question in which twins long separated and living in completely different environments are examined for differences and similarities.

There can be few environments more different than zero gravity aboard the ISS and a home in suburban Tucson, and NASA is eager to have the chance to compare long-term data collected from genetically similar astronauts to observe the human effects of spaceflight.

Because nothing of this kind has been attempted -- or even possible -- before, just what sort of experiments might be conducted has been left open, and NASA has issued a call to the academic and research communities to propose investigations for observational comparison of the brothers during the course of the study.

One tentative plan is for blood sampling on Scott at regular intervals before, during and after the one-year mission on the space station and corresponding blood sampling on Mark, who will be living his normal lifestyle in Arizona, to gauge the effects of the space environment on human DNA.

NASA said it will consider any proposals focusing on analysis of human molecular responses to the physical, physiological and environmental stressors associated with human spaceflight.

The experiments could help determine the role genetics plays in health problems observed in astronauts spending long periods in low gravity, including loss of bone and muscle mass and vision issues, plus an increased risk of cancer from cosmic radiation and sleep problems, the space agency said.

While the NASA call for proposals document goes by the somewhat unwieldy title of Differential Effects on Homozygous Twin Astronauts Associated with Differences in Exposure to Spaceflight Factors, its aim can be simply stated: keeping astronauts safe and healthy as humankind breaks free of the gravitational bonds of Earth and begins journeys to new worlds and beyond.