OPM: Overview
The OPM contained three instruments that measured
how various materials samples deteriorate
in space. All three of the instruments worked
by measuring optical properties; that is,
how the materials transmit, absorb, or reflect
light. Optical properties measurements have
an advantage over other types of measurements
that involve physical contact: they don't
touch or disturb the sample. At regular intervals (approximately one week), the OPM took measurements
on the samples and produced a data file
for downlink to Earth; the rest of the time,
it exposed the samples and waited in a passive
mode until the schedule again called for
measurements.
What did the OPM look for? It looked for
changes in materials that are caused by
exposure to the various factors of the space
environment: vacuum, extreme hot and cold,
solar radiation (light and charged particles),
reactive ions from the upper fringes of
Earth's atmosphere, and contaminants from
the spacecraft.
These changes include: color changes, darkening
(which is particularly bad because it causes
the material to absorb more solar radiation,
raising its temperature and making the darkening
worse), flaking, surface roughening, and
spalling. These are all indications that
the material is being physically or chemically
altered by the space environment, and may
indicate that the material is unsuitable
for use in building spacecraft.
A critical aspect of the OPM's operation
was its package of environmental monitoring
instruments. These continuously monitored
the space environment around the OPM, so
that when scientists examine the sampled
measurements, they know what kind of conditions
the samples were actually exposed to during
the exposure interval. The space environment
near a spacecraft can be effected significantly
by variations in solar activity (including
sunspots and flares), the orbit parameters
of the spacecraft, and operations on board
the spacecraft (such as water dumps and
gas venting). Every fifteen minutes, the
OPM took measurements on solar radiation,
atmosphere components, and the amount of
heat being absorbed by selected samples.
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