The moon is a hefty 240,000 miles away, but compared with our planetary neighbors, it's just a jump over the garden fence. Here, Apollo 16 moonwalker John Young is collecting samples that would prove that these lunar highlands were shaped more by meteorites than by volcanism.

Of the 731 rock samples the Apollo 16 astronauts brought home in April 1972, nearly all were breccia, composites formed of fragments fused together--probably by the heat and pressure of meteorite impacts. The logistics of sending astronauts on a similar mission to explore the geology of Mars are "complex, to say the least," NASA admits.