Lunar Orbiter Micrometeoroid Experiments

The micrometeoroid experiments flown on all five Lunar Orbiter spacecraft provided measurements in the near-lunar environment of the rate of penetration by meteoroids of 0.025-millimeter beryllium copper detectors. Each spacecraft carried twenty detectors, totaling an effective exposed area of 0.186 square meter. The spacecraft flew both equatorial and polar orbits at altitudes ranging from 30 to 6,200 kilometers and. collected data on micrometeoroid impacts for a period of seventeen months.
A primary goal of the experiment was to obtain data for the purpose of comparing the meteoroid hazard near the Moon with that near the Earth. These data would aid the Apollo Program in the determination of the amount of protection necessary for the spacesuits, instruments, and spacecraft. Moreover, they would refine the estimates of the hazard in near-lunar environment which scientists had made and which ranged from somewhat less to greater by several orders of magnitude than the hazard near the Earth. A major uncertainty was the contribution of secondary meteoroids created by the impacts of primary meteoroids on the Moon.
Before the Lunar Orbiter missions, only the Soviet lunar satellite Luna X had measured meteoroid flux near the Moon. The results of its experiment showed that the average rate of flux of micrometeoroids exceeded the average for interplanetary space by about two orders of magnitude.
To arrive at the average rate of flux for the five Lunar Orbiter micrometeoroid experiments, the primary investigators (Gurtler, Kinnard, and Grew) divided the total number of recorded punctures by the total time-area product. The five Orbiters recorded 22 punctures during a time-area exposure of 139.0 square meters per day. These figures gave an average rate of 0.16 puncture per square meter per day (m2 x day-2) in the near-lunar environment, or about one half the average rate of flux recorded by the Earth-orbiting satellites Explorer XVI and Explorer XXIII.

Spacecraft

Punctures

Exposure (m2 x day)

Punctures (m-2 x day-1)

Lunar Orbiter I through V

22

139.0

0.16

Explorer XVI

44

132.9

.33

Explorer XXIII

50

139.9

.36

The investigators found by analysis of the 22 punctures of the micrometeoroid detectors, in relation to spacecraft positions at time of impact, that there was a preponderance of punctures on the side of the spacecraft facing forward in the orbital direction of movement around the Sun. This preponderance agreed with Earth-based radar observations cited by Q. S. Hawkins 23 and indicated that the influx of meteoroids on the side of Earth facing forward in orbit around the Sun was several times greater than influx on the opposite side.
Preliminary estimates of the flux of secondary meteoroids near the Moon indicated that flux was greater nearer the lunar surface and dropped off sharply with increase in altitude.25 Further study of the Lunar Orbiter data indicated no statistically significant variation of hazard with altitude.
Gurtler and Grew concluded in the summary of their analysis of the micrometeoroid experiment data that the penetration rates in the near-lunar environment as well as near the Earth should be accepted as being only tentative since the number of recorded penetrations was statistically small and the meteoroid flux near the Earth’s orbit might vary from one measurement period to another. However, the data did indicate that the penetration hazard for 0.025 millimeter of beryllium copper was no greater near the Moon than near the Earth. Nor was there any substantial evidence that the hazard in the near-lunar environment increased as a result of secondary meteoroid impacts caused by primary impacts on the Moon.
Source: Destination Moon: A History of the Lunar Orbiter Program

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