Instruments
Dust Monitor
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| Apollo 17 astronaut Gene Cernan displays smudges of lunar dust he acquired in 1972 as the last human to walk on the Moon. |
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Eons of bombardment by micrometeorites have pummeled the surface of the Moon to dust, and that presents a problem for future explorers. Larger particles are sharp and jagged, making them very abrasive. It's likely that the particle surfaces are highly reactive. And lunar dust tends to carry an electric charge that makes it very clingy, like Styrofoam packing peanuts.
Apollo astronauts found that regardless of how much they tried to brush the dust off their suits, they couldn't keep from tracking it into their lander after a moonwalk. Breathing the dust caused allergy-like symptoms in at least one astronaut.
The much longer stays envisioned for the next group of lunar visitors could potentially pose more serious respiratory problems. Scientists responsible for the health of future lunar astronauts are especially concerned about the smallest dust particles because they could be easily inhaled and embedded in the lung lining, and possibly absorbed into the body.
Future Moon missions will require systems to keep lunar dust out of the astronauts' habitats. As a key part of this effort, AEMC is developing a monitor to detect lunar dust in the airlock and cabin of the Lunar Surface Acquisition Module (LSAM) that will ferry astronauts between lunar orbit and the lunar surface, and in future lunar habitats.
Returning to an airlock after an outdoor excursion, astronauts would undergo a procedure to clean off the moon dust clinging to their space suits and equipment. They would rely on a dust monitor to determine when it has become safe to open the airlock and enter their cabin. Inside the cabin, another monitor would warn astronauts if dust were to find its way into this area without being removed by the air-filtration system.
AEMC's dust monitor consists of two distinct systems which, together, can detect particles over the entire respirable range. Fine particles from 20 microns (20,000 nm) to 200 nm in diameter are detected by an optical-scattering sensor. Ultrafine particles from 200 nm down to 20 nm are detected by a differential-mobility sensor.
Neither of the two particle sensors determines the composition of the dust -- just their size distribution and quantity. But that is likely to be enough to monitor the performance of the particle filtration and mitigation systems and to assess how serious a health problem may be at hand.
How it works: Optical-scattering sensor
The optical-scattering sensor operates on the principle that when particles of different sizes scatter light, they produce characteristically different patterns.
A compact laser diode, similar to that used in CD and DVD players, shines into a chamber containing an air sample, and sensors situated at various angles around the chamber measure how much light is scattered at each of those angles. A computer sorts out the overlapping scattering patterns and calculates how many dust particles of each size are responsible for creating the patterns.
The sensor needs to be able to measure light over a large range of intensities, which vary with both the size and quantity of particles. AEMC's optical-scattering sensor therefore features autoranging femtowatt detectors which automatically recalibrate for various amounts of light and permit a linear relationship between the amount of light that is scattered and the detected signal (electric current) over 5 orders of magnitude.
Differential-mobility MEMS sensor
AEMC's differential-mobility sensor is a MEMS device which operates on the principle that larger particles are more difficult to deflect than smaller ones. The complete sensor has three main components: the Compact Field Charger, the Differential Mobility Classifier, and the Femtoamp Charge-Sensitive Preamplifier.
The process begins by sending an air sample through the Compact Field Charger to give the particles in the sample a small electric charge. The particles then enter the Differential Mobility Classifier, which consists of two parallel plates with an applied electric field between them. In the illustration, the electric field deflects the charged particles downward as the air sample moves horizontally from left to right.
Since the larger particles have more surface area than the smaller particles (and since they all have about the same density), the larger particles experience more drag as they "fall" through the air in the chamber. This slows their descent toward the bottom plate and enables them to travel farther before hitting bottom.
The yellow strip in the illustration is an opening in the bottom plate. When particles fall through the opening, they land on a sensor that produces an electric current when struck by a charged particle.
When a relatively weak electric field is applied, the larger particles are able to overshoot the opening before hitting the plate. Only the smallest particles fall through the opening and register with the underlying sensor.
As the strength of the electric field is increased, the smallest particles hit the plate before reaching the opening, while somewhat larger particles travel the right distance to fall through and be counted. Eventually, as the electric field progresses to its ultimate intensity, particles of all sizes within the specified range are sampled and counted.
Since the sensor is designed to be as small as possible, the number of particles passing through it will also be small. Thus, a very sensitive preamplifier is needed to convert those little charges to measurable current.
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