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This month, NASA revealed new details of the plan to send humans to Mars by 2030. It's an elaborate and expensive mission, involving a giant deep-space rocket, and roping an asteroid into the moon's orbit to use as a stepping stone to Mars.

But there are still some serious questions about a manned expedition to Mars. Namely, is it safe? That's where astronauts Scott and Mark Kelly come in. The Kelly brothers are identical twins, and the only siblings ever to both fly in space.