‘With current technology, nothing larger or heavier than [the Mars Science Library, touching down on Mars in August 2012] can be put on the surface of Mars. Anything more massive, including a human mission, which NASA estimates would require landing at least 40 to 80 tons of machinery, is completely out of the question.

“We’ve maxed out our ability to take mass to the surface of Mars,” said engineer Bobby Braun, former NASA chief technologist and co-author of a 2005 research paper highlighting this problem.

The basic obstacle for large-scale missions is Mars’ tenuous atmosphere, which is more than 100 times thinner than that of Earth. The pressure of the Martian atmosphere at its surface is equivalent to what someone would experience flying at 100,000 feet on Earth.’ (via Wired.com).