For decades, people have fantasized about going to Mars, yet a human expedition to the red planet remains just that—a dream. NASA, other space agencies, and private entrepreneurs are taking concrete steps toward that goal, but science journalist David Whitehouse says we must still overcome many constraints and risks—biological, technological, and social—before we see the first footprint in red Martian soil. Whitehouse is author of “Space 2069: After Apollo: Back to the Moon, to Mars, and Beyond.”