WASHINGTON, March 30. /TASS/. American astranaut Scott Kelly who set out for a year-long mission to the International Space Station last week will participate in a unique experiment that may help prepare the first space flight to Mars. According to NASA, Kelly has an identical twin brother. After spending 342 days in space, Kelly will return to Earth and the condition of his organism will be compared to his brother's to estimate the influence of long space journeys.