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 At this point the spacecraft is about one hour past the translunar injection point at an altitude of about 9, 000 nautical miles. Inertial velocity has decreased to about 23,000 ft/sec. or about 13,000 ft/sec. less than at injection cutoff. The velocity will, of course, continue to decrease for most of the trip until the spacecraft nears the moon.

The spacecraft is now being tracked by one of the three deep space stations with 85 ft. dishes (Madrid, Canberra, or Goldstone), and two of the unified S-band stations with 35 ft. dishes. Similar tracking coverage will be available throughout the rest of the mission back to entry, except for periods when the spacecraft is behind the moon.

The first midcourse correction will be made in about two hours, after the spacecraft's trajectory has been accurately determined by extensive ground tracking. During this period, the crew will make a series of star-landmark sightings to check out their space mode of navigation, which is a backup to the ground navigation.

The time of the first midcourse correction is not a critical event. Delaying the correction will, of course, allow the initial injection errors to grow, so that a larger delta V will be required for the correction once it is made; however, it is not extremely sensitive in the range of 3 to 5 hours after injection. In some cases, it will even be preferred to wait; if the injection has been particularly good, the delta V required for the early correc­tion may be so low that it could not be performed accurately with the 20,000 lbs. thrust SPS engine (less than about 4 ft/sec.). Such small corrections could be made with the SM RCS engines but it would be preferred to conserve RCS propellant wherever possible, even at the expense of SPS propellant, where the reserves are considerably greater.

The typical midcourse correction, then, will be done with the SPS (Figure 31) about three hours after translunar injection, and will require a delta V of about 25 ft/sec., based on analyses of expected injection accuracies. This corresponds to about 3 seconds burn time by the SPS, and could occur in any direction.

Following the midcourse correction, the spacecraft is set up for the long coast period ahead. Another correction is not expected to be required until the spacecraft nears the moon, about 2½ days later.

The first operation to be performed is to orient the spacecraft for passive thermal control. The object of passive thermal control is, of course, to insure that critical components in the spacecraft do not get too hot or too cold during the long coast period, as a result of either looking directly at the sun or directly away from the sun for long periods of time. For