Like everything else in the 1960s, NASA's Saturn V rocket set a mark for extreme. At 363 feet tall, with 7.5 million pounds of liftoff thrust, it lifted six moon-bound missions into space. Retired in 1973, it remains the tallest, heaviest, and most powerful rocket our species has ever built. With moon missions on hold, we haven't needed anything close to its capacity. Until now. As governments and private companies race to send astronauts to Mars, bigger is once again better—and necessary. Whose heavy-lifter is the biggest and baddest? Here's how they stack up.