Funded by a grant from NASA, Science Approach's "Astro Calves" lesson explores the extent to which astronauts lose muscle mass during extended stays in space. While in the zero gravity of space, astronauts can lose from 5 to 40 percent of muscle mass and function. Reduced muscle function can lead to falls and broken bones, hardly an appealing scenario if the crew members are en route to Mars where they'll spend three years away from Earth. In this activity, students use Image-J to measure the diameter of an astronaut's calf muscles before and after an extended space flight. Real, spatially calibrated MRI images provided by NASA are used in the inquiry. To download a free copy of "Astro Calves," register and log in at Science Approach's Web site. After clicking Enter, complete a brief profile, and click the Free Software and Materials link.