The quest to land humans on Mars by the 2030s bears the reminder of a decidedly Earthly challenge: water. 26,417.205 gallons, to be exact. Though any site where NASA lands craft and crew will require no more water than fills an in-ground home swimming pool, can that water get there from here? No. That would be the rough equivalent in weight of an additional crew complement of at least 150 astronauts.
Good touchdown sites will allow crews to land safely and carry out operations; offer a wealth of interesting science activities; and provide resources that the astronauts could use. For example, any favored exploration zone should allow expeditionary crews to tap into at least 100 metric tons (110 U.S. tons) of water, NASA officials have said.
Ironic that the biggest problem posed by landing a small crew of astronauts on Mars may be the same problem already faced by 800,000,000 residents on Earth: lack of water. The question is: which problem will be solved first?