NASA said much of the research and technology that makes the ISS possible was still just theory prior to launch of Skylab. "I think the greatest achievement is that we pretty much proved that the human body can stay weightless for a very long time," astronaut Gerald Carr said. "This was our first opportunity to go up and settle in." He said that the Skylab crews also helped develop countermeasures to help astronauts better endure long-duration flights. "I don't see any reason we couldn't go to Mars without artificial gravity," he said.