NASA astronaut Scott Kelly spent a year in space while scientists monitored changes in his body, as well as that of his twin, Mark Kelly, who remained on the ground. Kelly came back to Earth in good shape, the experiment showed. And, some Russian cosmonauts have also spent even longer than Kelly in space without obvious long term ill effect from the low gravity or increased radiation exposure.
But humans ultimately want to venture far beyond Earth. And, so far, all long-term human spaceflight experiments have happened on space stations that orbit within Earth’s protective magnetic field, providing shielding from the highest-energy radiation. To travel to Mars, we’ll need to be prepared for an entirely different environment.
The longest Apollo trips to the the moon lasted less than two weeks. A journey to Mars and back would take months, if not years. And NASA’s latest plans for the moon would have humans stay there perhaps months.
