Scientists have learned a great deal from manned space flights. One unexpected discovery has been that humans are not well adapted to life in space. Life without gravity causes the fluid in a human body to float upward, which not only creates uncomfortable pressure in the head and chest, but also weakens the leg muscles. Long space missions can also cause astronauts to develop brittle bones, vision problems, and difficulties eating and sleeping enough. NASA hopes to send astronauts to Mars in the 2030s. Scientists calculate a trip to Mars would take around 2.5 years. To date, the longest a human has lived in space is 438 days, a feat achieved by Russian cosmonaut Valeri Polyakov on the Mir space station in 1994–95. Write a one to two sentence conclusion appropriate to the purpose and audience of the passage.