Astronaut Scott Kelly Begins Historic Mission

Scott Kelly blasted off on a space mission today that won't end until around this time next year. Kelly will spend 342 days aboard the International Space Station, the longest mission by far ever undertaken by a US astronaut, reports CNN. Kelly was riding up in a Russian rocket along with two Russian cosmonauts, and one of them, Mikhail Kornienko, also will be on the ISS for a year. Part of the reason for the extended stay is so NASA can get a better sense of how lengthy journeys might affect space travelers. Kelly, though, also is participating in a unique study about twins.

Identical twin Mark Kelly, also an astronaut and the husband of Gabby Giffords, will remain on Earth, and researchers will keep intricate tabs on both men to measure how Scott's time in space changes his body in relation to his brother's. Quartz assesses one bizarre aspect: Scott will essentially "age faster" thanks to the effect of radiation, weightlessness, and other factors on sections of his DNA called telomeres. "Yet even as Scott Kelly ages faster biologically, he’ll age more slowly in physical time," writes Gideon Lichfield. Figure about 28 microseconds a day. Click for the full explanation, which involves the ISS orbit speed and an Einstein theory known as the twin paradox. (More Scott Kelly stories.)