The path to Earth

The path of an Apollo spacecraft from the Moon to the Earth began with the trans-Earth injection burn around the Moon's far side. This set the spacecraft on an S-shaped trajectory that was targeted so that, had the Earth been without atmosphere, the spacecraft would have looped around the planet and passed about 40 kilometres above the surface before returning to deep space on a very long elliptical orbit. Of course, the Earth does have an atmosphere and any spacecraft on a trajectory with a 40-kilometre perigee is bound to plough into its gases where the immense kinetic energy would be dissipated as heat. If the spacecraft went deeply enough into the atmosphere and lost enough energy, it would no longer have the momentum to return to space, and would instead be captured by Earth. The return trajectory of the Apollo spacecraft was designed to achieve this in a highly controlled manner.

A modification of this technique is commonly used by unmanned spacecraft as a means of arrival at other planets. The conventional technique slows to orbital velocity by consuming large quantities of propellant in a long burn. However, substantial weight savings in propellant can be made if the spacecraft carefully dips into the upper reaches of a planetary atmosphere, where it loses small increments of velocity. The initial insertion burn can then be much shorter and the resulting mass reduction will also reduce overall mission costs by enabling the spacecraft to be launched by a smaller, cheaper rocket.

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