US exploration of Mars, artwork. Astronaut preparing to plant the flag of the United States of America (USA) on the Martian surface. Mars is a rocky desert world with no surface water and a gravity about one third of that on Earth. The atmosphere is mostly carbon dioxide and surface temperatures are well below freezing. The thin atmosphere means that flags need to be rigid rather than made of cloth. Human missions to explore Mars have been planned since the 1950s, usually involving a period of 10 to 30 years to develop the necessary technology and resources. The most recent US plan, NASA's Constellation Program, had a Martian mission as its ultimate aim, but as of 2010 has an uncertain future.