The path to Earth

The path of an Apollo spacecraft from the Moon to Earth began with the trans­Earth injection burn around the Moon’s far side. This set the spacecraft on an S – shaped trajectory which, had Earth not possessed an atmosphere, would have caused the spacecraft to loop around at an altitude of about 40 kilometres before returning to deep space on a very long elliptical orbit. Of course, 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 will be dissipated as heat.

N Moon at ‘■ PTEI+2 days

Diagram of Apollo’s trajectory from TEI to re-entry.

If the spacecraft were to penetrate deeply enough into the atmosphere to lose the momentum needed to return to space, then providing that it has appropriate protection it will instead reach the surface. 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 is to slow’ from approach velocity to orbital velocity by the consumption of a large quantity 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 can lose 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.

If Apollo’s targeted perigee w’ere too low. the entry angle would be steeper than ideal. ‘This would increase both the heat impulse that the heatshield had to deal with and the deceleration forces that the crew would have to endure. It would also tend to shorten the entry flight path – perhaps by more than the CM’s flight characteristics could compensate, forcing a landing short of the planned point. In the extreme case, it would be lethal, either by excessive g-load or by incineration. A higher than ideal perigee, and hence a shallower angle, would result in a longer entry path and lower g-forces. but at the risk of the spacecraft failing to shed enough energy to enable it to be captured before it rc-emerged from the atmosphere to coast out into space on a long elliptical orbit. Since the solo command module had no means of propulsion and very limited supplies of power and oxygen, failure to be captured by Earth’s atmosphere at the first attempt would be fatal for the crew’.