There is, however, no firm evidence that Mars has ever hosted life.
American astronauts have performed 239 spacewalks, each of which typically lasted six hours or more.
The endurance trial will simulate the 250-day flight to Mars, a landing and surface stay of up to 30 days, and then the 240-day trip back to Earth.
Japan's Hayabusa spacecraft is scheduled to touch down in Australia on June 13. The probe might be carrying a little piece of asteroid.
A mockup of the James Webb Space Telescope, billed as the successor to the Hubble Space Telescope, took center stage at New York's World Science Festival.
A life-size model of a huge new space observatory described as the successor to the Hubble Space Telescope is on display at the World Science Festival at New York City's Battery Park.
Under NASA's new space plan, commercial spacecraft and services have a larger role than before.
When galaxies merge, some black holes will suddenly emit 10 billion times the energy of the sun. Using data from NASA's Swift satellite, scientists now know why this happens.
Originally set to launch on May 28, the privately built Falcon 9 rocket will take flight no earlier than June 2, SpaceX officials said.
Scientists previously favored the idea that a natural force carved both the canyon and pinwheel pattern.