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Rocket design is all about trade-offs: every extra pound of cargo that a rocket needs to lift off the surface of Earth requires more fuel, while every new bit of fuel adds weight to the rocket. Weight becomes an even bigger factor when trying to get a spaceship somewhere as far away as Mars, land there, and come back again. Accordingly, mission designers have to be as judicious and efficient as possible when figuring out what to pack on a ship headed for space and which rockets to use.