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At the end of the tenth lunar revolution, the Apollo crew will fire the SPS engine again—this time for 206 sec. —boosting their speed to 6,060 m.p.h., more than enough to break the moon's gravitational hold and start the spacecraft back toward the earth. About 57 hours later, accelerating under the pull of terrestrial gravity, the astronauts will position their craft properly and then jettison the service module. Streaking into the earth's atmosphere at an angle of 6.5° and a velocity of 24,765 m.p.h., the 11,700-lb. command module—all that will remain of the 3,100-ton vehicle that left Cape Kennedy—will glide downward along a curving 1,300-mile path, deploy its main parachutes at 10,000 ft., and drop gently into the Pacific. Elapsed time for the great lunar adventure: six days, 2 hr. and 52 min.
Acting NASA Administrator Thomas Paine believes that risks to Apollo 8's astronauts "will be within the normal hazards of test pilots flying experimental craft." The careful design, redesign and check-out of rockets and spacecraft, the policy of including duplicate systems wherever possible, and the logical, step-by-step progression of unmanned and manned Saturn and Apollo space shots, he says, "give us a great deal of assurance" about the moon flight.
No matter how carefully it is planned and executed, however, the December flight of Apollo 8 will involve some chilling perils. Besides anticipating the kinds of problems that could occur in a simple near-earth orbital flight, lunar-mission planners must plan realistically for troubles that would be magnified by sheer distance from earth. Should life-support or power systems begin to fail on earth-orbital flights, astronauts are usually within half an hour to three hours of recovery on land or water; a relatively small thrust from a retrorocket can lower their orbit into the atmosphere, where friction provides the additional braking necessary to return them to earth. In the vicinity of the moon, the astronauts might be as long as a three-day journey from home. They could fall victim to minor malfunctions —like a deteriorating oxygen supply—that would not necessarily be fatal in an earth-orbital flight.
There are other moon flight worries, most of them centered around the functioning of the Apollo command module's SPS engine. Should the SPS fail to ignite, or should it burn for less than 80 sec. during the attempt to place Apollo in lunar orbit, there would be little difficulty; the spacecraft would simply continue around the moon and be whipped back toward the earth and safety. But if the SPS should fail between the 80-and 110-sec. marks of its scheduled 246-sec. burn, Apollo would enter what NASA euphemistically describes as an "unstable orbit." After rounding the moon, it would begin heading back toward earth, but not fast enough to escape the moon's gravitational pull. Depending on the length of the abortive burn, Apollo would sooner or later fall back and crash into the lunar surface.