Landing near a Robot

For all its glory and accomplishment Apollo 11 lacked one thing: accuracy. Apollo 12 set a new goal and a new task for the human pilot: demonstrate accurate, ''pinpoint'' landing capability, defined as within one kilometer of the aim point. As with Apollo 11, NASA chose a relatively smooth area in a flat region, called the Ocean of Storms.

Ironically, Apollo 12 would not be the first landing in this area. The goal was to land near where the unmanned Surveyor III spacecraft had come down two and a half years before in April 1967. Surveyor III was the second in its series to land on the moon after Surveyor I (Surveyor II crashed on the moon after a control thruster failed). When landing, Surveyor III bounced several times on the moon because the engines failed to shut down on time;only a command from earth finally turned off the thrust and stabilized the vehicle. Simply landing on the moon did not require human presence.

Apollo 12 was to show, however, that precision landing called for piloting skill.1 NASA considered numerous changes to make Apollo 12 a more precise touchdown than Apollo 11.

What did it mean to land with accuracy? ''Accuracy'' for geologists might mean proximity to a known feature of scientific interest like a crater or mountain. By contrast, engineers defined accuracy by an error circle around precise coordinates. They also distinguished between accuracy, as integrity to a physical quantity, and precision, as a consistent numerical measure.

The Apollo program employed various maps based on different types of data (earth observations, Lunar orbiter data, previous Apollo missions, etc.) and their grid coordinates did not always line up. The unique requirements of the landings meant that the Apollo guidance and control system had its own coordinate system into which the Surveyor III coordinates were translated to target the Apollo 12 landing.2 ''It is not meaningful to compare stored landing coordinates with the actual site location,'' one mission report warned, ''because of the various transformations and targeting biases which have necessarily taken place.''3 In other words, the Apollo computer's coordinate system was not necessarily compatible with those used by lunar scientists. Landing near the Surveyor, however, an arbitrary but unequivocal point ( perhaps the only such point on the moon, given the monotony and homogeneity of the lurain), would serve as proof of a pinpoint landing (hence Apollo 12 is better described as a precise landing than an accurate one).

Only two changes were made to Apollo 12 software on the basis of Apollo 11, both to improve precision. A new command, Noun 69, enabled the astronauts to update the coordinates of the landing site early in the braking phase, based on improved data from ground tracking. Using Noun 69 (and about twenty-eight key presses) the crew could update the landing site position (up or down range, but not cross range) to correct any errors, thus increasing precision. By contrast, the Apollo 11 crew knew they were going long several minutes before pitch over, but could do nothing about it once the computer was in control.4 The second software change enabled the astronauts to calibrate the LPD against a star during powered descent, fine-tuning the optics.

A few procedural changes were made as well. First, during undocking, Command Module Pilot Dick Gordon moved his ship away from the LM using the larger craft's thrusters, rather than the LM's, which had been used during Apollo 11. This maneuver avoided adding additional velocity to the lander, one possible source of 11's long landing. Second, the Apollo 12 LM would spend the entire braking phase in the face-up attitude. Unlike Armstrong and Aldrin, Commander Pete Conrad and LMP Alan Bean would not check their altitude visually against the ground—thus eliminating the necessity of a yaw-around that could introduce uncertainty into the trajectory.5 Third, a modification to the trajectory in the approach phase allowed the crew more time for redesignation with the same amount of fuel and allowed the LPD angle to change more slowly and smoothly to make it easier for the commander to follow.6

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