Apollo 15 Mission
Science Experiments - Soil Mechanics Investigation
The Soil Mechanics Investigation studied the properties of the lunar soil.
Soil mechanics is the study of the mechanical properties of soils and the way these properties affect human activities. Soil mechanics studies were performed on all six of the Apollo Moon landings. The goals of these studies were to improve our scientific knowledge about the properties of lunar soil and to provide the engineering knowledge needed to plan and perform lunar surface activities.
The soil mechanics studies took a variety of forms. These included crew commentary while collecting geologic samples and deploying experiments and postmission analysis of photography of these activities. Several experiments were performed specifically to study soil mechanics. These include use of penetrometers, which are rods that measure the force required to penetrate to various depths in the soil. Also, several small trenches were dug to study the soil properties along the trench walls. Finally, studies were performed on samples returned to Earth. For example, analysis of core tubes allows properties such as density, average grain size, strength, and compressibility to be measured as a function of depth.
During landing, the impact of rocket exhaust with the surface produced dust clouds. On some missions, dust became visible 30 to 50 meters above the surface, and during the final 10 to 20 meters of descent, the surface was largely obscured by the dust cloud. On other missions, the dust cloud was not as dense and the surface remained clearly visible throughout the landing.
The soil on the Moon is very fine-grained, with more than half of all grains being dust particles less than 0.1 millimeters across. Some of these particles become electrostatically charged and cling to objects (such as space suits and other equipment) that they come in contact with. The dark dust grains absorb sunlight, and equipment that became dust-coated sometimes became excessively hot. Despite the fine-grained nature of the surface, it provided good traction for astronauts as they moved about. Crew mobility, both on foot and in the Lunar Rover, was affected more by local topography such as craters and ridges than by soil properties.
The lunar surface easily supported the weight of the astronauts and their equipment. Typically, astronaut boots and the Lunar Rover's wheels only penetrated 1 to 2 centimeters into the surface, with penetration reaching five centimeters in some places. The lunar module footpads sank 2 to 20 centimeters into the soil. When astronauts inserted sampling tubes into the soil, they typically found penetration was easy for the first 10 to 20 centimeters and increasingly difficult below that depth. The deepest penetration achieved on a hand-driven core tube was 70 centimeters, which required about 50 blows with a hammer. For sampling at greater depths, the Apollo 15, 16, and 17 crews used a battery-powered drill. This allowed sampling to depths of 1.5 to 3 meters, which was achieved easily on Apollo 16 but with much more difficulty on Apollo 15 and 17.