4. Gather rocks

The Apollo era answered many questions about the Moon. But much was left undone.

Scientists see the Moon as an attic of Earth, a place where rocks long ago blasted from our planet are sitting around waiting to be studied. This history has not decayed much because there is almost no atmosphere and little geological activity on the Moon.

Earth, on the other hand, regularly recycles clues to its past, pulling material inward, and spitting it back out as unrecognizable lava.

"We are talking about finding material from the very early Earth," says John Armstrong of the University of Washington in Seattle. "Samples of the Earth 3.9 to 4.0 billion years ago could tell us a lot about the state of the early atmosphere, what the crust and surface were like, and possibly even when life began to evolve."

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