The initial shock [of acceleration] is the worst part of it, for he is thrown upward as if by an explosion of gun powder…. Therefore he must be dazed by opiates beforehand; his limbs must be carefully protected so that they are not torn from him and the recoil is spread over all parts of his body. Then he will meet new difficulties: immense cold and inhibited respiration…. When the first part of the journey is completed, it becomes easier because on such a long journey the body no doubt escapes the magnetic force of the earth and enters that of the moon, so that the latter gets the upper hand. At this point we set the travellers free and leave them to their own devices: like spiders they will stretch out and contract, and propel themselves forward by their own force – for, as the magnetic forces of the earth and moon both attract the body and hold it suspended, the effect is as if neither of them were attracting it – so that in the end its mass will by itself turn toward the moon.
–Johannes Kepler, Somnium, published posthumously in 1634FREEWAYS IN THE SKY
Not many years ago, the only conceivable use of the beautiful celestial mechanics developed over hundreds of years was to compute the positions of bodies in the heavens. Today that situation has changed radically.
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