Houston, we have a heart problem. Apollo lunar astronauts are four to five times more likely to die from cardiovascular disease than astronauts who never left Earth’s orbit or who never flew at all, according to a study published in Scientific Reports today that considered about 100 astronauts, seven of them Apollo. When the Apollo crew members, pictured above performing prelaunch tests on Earth, ventured to the moon between 1968 and 1972, they exited a magnetic shield around Earth called the magnetosphere, which protects us from cosmic radiation. A team of researchers believed those rays, which come from outside our solar system, may have caused the Apollo astronauts’ circulatory systems to function incorrectly. To test this, they put mice in tiny harnesses to simulate weightlessness and bombarded them with the same type of cosmic radiation that the Apollo astronauts received. What happened in the mice confirmed the researcher’s fears: Although weightlessness had no apparent effect on the rodents, the radiation impaired enzymes that control blood vessels’ ability to relax. As a result, the vessels had a tendency to stay small and tight, which made it harder for blood to flow. In humans, this could lead to high blood pressure and, in some cases, heart disease. The findings, the authors say, highlight the dangers for future astronauts as they explore Mars and beyond.