The purpose of this study is to perform ground-based research to study what happens to the heart muscle in space. People who have problems with their aortic valve have an increase in the amount of muscle in the left ventricle of the heart. After valve surgery, the amount of muscle should decrease and return to normal. Astronauts lose heart muscle mass during space flight. Our study will look at these changes in your heart, which we believe are similar to what happens during long term space travel.