Mass spectrometer

Mounted on the end of a boom to get it clear of the spacecraft was the mass spectrometer. It was designed to characterise any lunar atmosphere by measuring the atomic weight of the atoms and molecules that entered an aperture on one side of the instrument. Upon entering, they were electrically charged, or ionised, by electrons from a filament source. A magnet then diverted the path of the resultant ion stream towards two detectors. Simply stated, the heavier an atom or molecule, the more resistant is its motion to change by the magnetic field. By measuring the deflection of the particle stream, the masses of its constituent parts could be determined.

When deployed out of the SIM bay, its inlet aperture faced away from the bulk of the CSM and in the same direction as the engine bell in an attempt to shield it from gases emanating from the spacecraft. During its time in lunar orbit, the instrument was flown with the inlet either facing the direction of travel or facing backwards. The hope was that differences between the two modes of operation would allow scientists to discriminate between atoms that were genuinely part of the Moon's atmosphere (which should tend not to enter when the inlet was facing backwards) and those that were coming from the spacecraft (which would enter from either direction).

In practice, little difference was detected whichever way the inlet faced, implying that most of what was being detected was essentially pollution from the spacecraft. This supported, on a global scale, the same results that researchers were finding from experiments placed on the surface at various sites. These experiments, placed by Apollos 12, 14, 15 and 17, were deluged with contaminants from the Apollo spacecraft, which made it very difficult to extract natural data from their results. This was hardly surprising considering that estimates for the total mass of the natural lunar atmosphere were around 10 tonnes - a figure very similar to the quantity of gases released during each Apollo mission, mostly from operation of the descent and ascent engines. Essentially, each Apollo flight temporarily doubled the mass of the entire lunar atmosphere.

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