text: Space Radiation May Harm Astronauts' Blood Cells In the time it takes you to read this sentence, more than 10 million red blood cells in your body will die. Don't be alarmed; it's natural, and stem cells in your bone marrow are constantly making enough new cells to replace the dying ones. But what if those blood-making cells stopped working? This could be a concern for astronauts taking long trips beyond Earth orbit.