This is the description from the
"Yellow Book", NASA SP-473
The Chemistry of Micrometeroids (A0187-1)
Robert M. Housley
Rockwell International Science Center
Thousand Oaks, California
Background
The mineralogy, petrography and chemistry of both "primitive" and more
evolved meteorites recovered on Earth are currently the subjects of intense
laboratory studies. The purpose of these studies, in conjunction with
our knowledge of terrestrial and lunar petrogenesis, is to establish an
observational framework that can be used progressively to constrain and
confine cosmochemical and mechanical-dynamic models of early solar-system
evolutionary processes. Such modeling attempts include the nature and
kinetics of nebular condensation and fractionation, the accretion of
solid matter into planets, and the role of collisional processes in
planetary formation and surface evolution. All of these processes are
known to be highly complex.
Fine-grained interplanetary particles (micrometeroids) of masses as little
as 10E-16 g are, however, largely excluded from models of the early
solar-system evolution because their mineralogic, petrographic, and
geochemical nature is largely unknown. In comparison, however, their
dynamics, orbital parameters, and total flux are reasonably well
established, although still fragmentary in a quantitative sense. According
to current (largely dynamical) hypotheses, a majority of these objects
are derived from comets. This association affords a unique opportunity
to study early solar-system processes at relatively large radial
distances from the Sun (greater than approximately 20 AU). These
cometary solids may reflect pressure and temperature conditions in the
solar nebula which are not represented by any of the presently known
meteorite classes, and therefore may offer potential insight into the
formation of comets themselves.
Objectives
The prime objective of this experiment is to obtain chemical analyses of
a statistically significant number of micrometeroids. These data will
then be compared with the chemical composition of meteorites. Secondary
objectives of the experiment relate to density, shape, mass frequency,
and absolute flux of micrometeroids as deduced from detailed crater
geometries (depth, diameter, and plane shape) and number of total
events observed.
Approach
This experiment is designed to collect micrometeroid residue in and
around micrometeroid impact craters that are produced by hypervelocity
collisions of the natural particles with high-purity targets. After the
return of these targets, the micrometeroid residue will be chemically
analyzed with a large array of state-of-the-art microanalytical tools
(e.g., electron microprobe, scanning electron microscope with
energy-dispersive analyzer, Auger and ESCA spectroscopy, and ion probe
mass analyzer). In favorable cases, precision mass spectroscopy may
be possible. The experiment will involve both active and passive
collection units.
Active Unit
The principles of the "active" unit are described below.
(See fig. 56.)
A clam shell concept allows two sets of clam shells, housed in a
12-in.-deep peripheral tray, to be opened and closed. The figure shows
one set of clam shells in the stowed (i.e., closed) mode and the other
set in a deployed mode. Due to the high sensitivity of the microanalytical
tools and the extremely small masses of the micrometeroid residue to be
analyzed (10E-7 to 10E-12 g), the stowed configuration will protect the
collector surfaces from particulate contaminants during ground handling,
launch, and LDEF deployment and retrieval sequences. The clam shells
will be opened by a timed sequencer some 8 days after LDEF deployment
and they will close at a similar time prior to redocking for retrieval
of LDEF. The basic contamination barrier is a precision labyrinth seal.
Figure 56.-Active micrometeoroid detector unit
Inflight picture of this
tray
The main collector surfaces are made of 99.99-percent-pure gold sheets
0.5 mm thick and totaling some 0.85 square meters total surface area.
Two individual gold panels, each about 57 by 20.6 cm, will be fastened
to each clam shell tray for a total of seven panels. A high-quality
surface finish will be obtained by polishing, acid etching, and
electroplating. The space for the eighth panel is taken up by a
series of experimental collector materials (about 6.5 by 20.6 by 0.05 cm
each) for the purpose of empirically determining collection efficiency
and/or optimum chemical background (i.e., signal-to-noise ratio during
the analytical phase). These auxiliary surfaces consist of Al
(99.999 percent pure), Ti (99.9 percent pure), Be (99.9 percent pure),
Zr (99.8 percent pure), C (99.999 percent pure), Kapton (a polyimide),
and Teflon filters. There are three reasons for selecting gold as the
main collector surface. First, its behavior under hypervelocity impact
conditions is reasonably well known, in contrast to that of some of the
auxiliary surfaces. Second, gold is not an overly abundant constituent
in meteorites, and third, it is a highly suitable substrate for many of
the microanalytical techniques contemplated. For a model exposure
duration of 9 months , a fairly well established mass-frequency
distribution, and a conservatively low flux estimate for micrometeoroids,
the approximate numbers of micrometeorite craters expected on the gold
collector are as follows: 165 craters larger than 5 micrometers, 52 craters
larger than 10 micrometers, and 9 craters larger than 50 micrometers in
diameter. Quantitative analysis is feasible only for craters larger
than 20 micrometers in diameter (approximately 20 events), although an
attempt will be made at qualitative analysis of smaller craters.
Passive Unit
The experiment will use a "passive" collector unit that occupies a
3-in.-deep peripheral tray. (See fig. 57.) This unit will be covered
by six Al (99.9 percent pure) panels (47 by 41 by 0.3 cm each).
These surfaces have no special protection against contamination
because they are rigidly bolted onto a structural framework which in
turn is fastened to the LDEF tray. If contamination is not too significant,
approximately another 25 events larger than 20 mm in diameter will be
available for analysis. Furthermore, an additional gold surface
(approximately 12 by 2.3 by 0.05 cm) will be flown inside the experiment
exposure control canister used in LDEF experiment S0010 (Exposure of
Spacecraft Coatings) for optimum calibration of gaseous and particulate
contamination.
Klaus G. Paul, 4-30-1994