Off to the Moon

One second prior to ignition, the central light in the indicator cluster came on to tell the crew that the J-2 was about to spring into life. When it did, the crew felt an acceleration of about 0.5 g that gently rose to about 1.5 g over the duration of the burn as the tanks in the third stage emptied. Translunar injection typically lasted just under 6 minutes, increasing their speed from 7.8 to 10.8 kilometres per second. As soon as the burn began, the position at which it occurred became the perigee of the stack's new elliptical orbit. Then as the burn proceeded and as they continued to orbit around Earth, their height began to rise; slowly at first, but at an increasing rate as the apogee was drawn out. The crew continuously monitored their instruments in case the S-IVB showed signs of trouble, but they were not averse to taking a look out of the window and enjoying the view, which Eugene Cernan described on Apollo 17: ''As the S-IVB manoeuvred, we flew through a sunrise during TLI, which in itself was very interesting, very spectacular.''

It was common for commentators of the day to say that the TLI burn accelerated Apollo to Earth escape velocity. This statement implied that when the S-IVB finished its work, the stack was travelling so fast that it would never return to Earth's vicinity without some intervention, whether from the Moon or a rocket engine. Strictly speaking, this was not true, as the stack's long elliptical orbit around Earth would have eventually returned them to perigee if the Moon had not intervened.

Nevertheless, it was well within the capability of the S-IVB to add the few extra metres per second to its speed and attain true escape velocity.

With respect to the ground, the stack's new trajectory was moving less and less parallel to Earth's surface, and instead more and more perpendicular to it as it pulled away from the planet. As it did so, its horizontal speed across the ground diminished - so much so, in fact, that the rotation of the planet began to catch up with the spacecraft, with the result that the ground track, which had been towards the east, slowed, halted and began to travel towards the west, which kept the spacecraft in view of Hawaii.

For a few minutes, as they raced away at about 10 kilometres per second, the crew passed through the van Allen belts, where they received a small dose of radiation. The Apollo flights represent the only example of human spaceflight through and beyond the van Allen radiation belts into interplanetary space. These belts consist of diffuse volumes around Earth within which radiation levels are elevated by the planet's magnetic field trapping energetic particles from the Sun. There is an inner torus populated by energetic protons, which the spacecraft passed through in a matter of minutes, and which was largely shielded against by the spacecraft's skin. The spacecraft took about an hour and a half to traverse the more extensive outer torus, but because this region has mainly low-energy electrons, it was less of a worry to mission planners. Over a complete mission, including exposure to very energetic particles encountered in the solar wind environment beyond Earth's magnetosphere, crews were believed to have sustained a dose of a similar magnitude to that allowed annually for workers in the nuclear industry. There were additional dangers from occasional explosive events on the Sun when huge quantities of particle radiation were spewed out in a solar flare, but the Apollo programme simply ran the gauntlet of these flares, accepting such risks along with the many other risks already inherent in an Apollo mission. Astronauts came from the test-pilot milieu where danger was a given and risks had to be weighed against the gains of mission success.

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