THE MOON

    The landing of Apollo astronauts, and their return to earth with lunar soil samples, will help solve some of the mysteries of the moon. What is known about the moon, from centuries of astronomical observation and from the recent space mission, is this:

Terrain    Mountainous and crater-pitted, the former rising thousands of feet and the latter ranging from a few inches to 180 miles in diameter. The craters are thought to be formed by the impact of meteorites. The surface is covered with a layer of fine-grained material resembling silt or sand, as well as small rocks.

Environment    No air, no wind, and no moisture. The temperature ranges from 250 degrees in the two-week lunar day to 280 degrees below zero in the two-week lunar night. Gravity is one sixth that of earth. Micrometeoroids pelt the moon (there is no atmosphere to burn them up). Radiation might present a problem during periods of unusual solar activity.

Dark Side    The dark or hidden side of the moon no longer is a mystery. It was first photographed by a Russian craft and since then has been photographed many times, particularly by NASA's Lunar Orbiter spacecraft.

Origin    There is still no agreement among scientists on the origin of the moon. The three theories:   

1. the moon once was part of earth and split off into its own orbit,   

2. it evolved as a separate body at the same time as earth, and   

3. it formed elsewhere in space and wandered until it was captured by earth's gravitational field.

Table 4. Facts about the moon.

    Possible landing sites for Apollo's lunar module have been under study by NASA's Apollo Site Selection Board for about two years. Thirty sites originally were considered, and these were later narrowed down to eight.

    Selection of the final five sites was based on high resolution photographs returned by Lunar Orbiter, plus close-up photos and surface data provided by Surveyor.

    All of the original sites were on the visible side of the moon within 45 degrees east and west of the center of the moon and 5 degrees north and south of its equator.

The final five choices were based on these factors:

• Smoothness (relatively few craters and boulders)

• Approach (no large hills, high cliffs, or deep craters that could cause incorrect altitude signals to the landing radar)

• Propellant (selected sites allow the least expenditure of propellant)

• Recycling (selected sites allow for necessary recycling time if the Apollo/Saturn countdown is delayed)

• Free return (sites are within reach of the spacecraft in a free-return trajectory)

• Slope (there is little slope-less than 2 degrees in the landing area and approach path)

    Three of the five sites will be chosen for a specific lunar landing mission so that a three-day period each month will be available for the launch.

(P-41) The five Apollo moon landing sites

Table 5. The Apollo lunar landing sites: