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Meteoroid and Exposure Module

Scientific interest in meteoroids has existed for many centuries, since man first observed meteor showers and meteorites. Interest in meteoroids and meteor phenomena significantly intensified in the United States during the early 1950's for several reasons. Meteoroids were studied under defense research to gain knowledge about high speed entry into Earth's atmosphere, and, as satellites and other spacecraft were being designed, researchers needed a clearer insight into the probability and hazards of meteoroid impacts. During the 1960's and 1970's additional meteroid work in space gave provided invaluable experimental data.

One early method for measuring meteoroid impacts was the pressurized-cell detector, in which a sensor would read and report the loss of pressure that resulted from penetration of space-exposed surface of the cell. Different cells had different skin thicknesses, which were pre-calibrated in ground tests to indicate different penetrating masses. Data transmission equipment relayed information about impacts and penetrations down to Earth.

Another method that evolved was the capacitor detector, in which each impact generated an electrical signal. Meteoroid researchers also studied the bumper concept, a shielding technique in which an external covering causes a impacting object to break apart and distribute its energy over a larger area, reducing the likelihood of spacecraft penetration.

The Meteoroid and Exposure Module (MEM) was proposed as the first Shuttle payload in 1970, and it was planned as a cylinder sized to fit within the Shuttle's payload bay. The Shuttle would place it in orbit, where its large surface area would collect a comprehensive sample of meteoroid data. MEM was to include a thick-skin, thin-skin, and bumper configurations. After several months, the Shuttle would retrieve MEM and return it to Earth for data analysis. This retrievability feature, both for MEM and for LDEF, was especially important.

In almost all previous space research, the only measurements available were those that could be transmitted to Earth. Data transmission equipment was expensive, took precious room in a spacecraft, and was not always absolutely reliable. Retrievability eliminated the need for this equipment for the MEM or LDEF. Retrievability also placed hard experimental evidence, not just transmitted signals, into researcher's hands. This evidence allowed in-depth analysis, use of a variety of analytical equipment, and participation by an increased number of investigators. Retrievable, MEM-type experimentation was also attractive for other space research areas.

In 1974, MEM was renamed LDEF, and LDEF officially became a NASA project managed by Langley Research Center for the Office of Aeronautics and Space Technology. Meteoroid research was initially seen as the primary mission, although eventually LDEF became a vehicle to study:

• changes in materials properties over time in the space environment,
• performance tests of spacecraft systems,
• evaluations of components used in powering spacecraft,
• experiments in the growth of crystals in low gravity, and
• scientific investigations in space physics and related fields.

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