Four processes serve to illustrate potential areas of study and their implications for general problems in planetary science. First, accretional processes reflect the success of collisional aggregation over collisional destruction during the early history of the solar system. Second, both catastrophic and less severe effects of impacts on planetary bodies survivng from the time of the early solar system may be expressed by asteroid/planetary spin rates, spin orientations, asteroid size distributions, and perhaps the origin of the Moon. Third, the surfaces of planetary bodies directly record the effects of impacts in the form of craters; these records have wide-ranging implications. Fourth, regoliths evolution of asteroidal surfaces is a consequence of cumulative impacts, but the absence of a significant gravity term may profoundly affect the retention of shocked fractions and agglutinate build-up, thereby biasing the correct interpretations of spectral reflectance data. An impact facility on the Space Station would provide the controlled conditions necessary to explore such processes either through direct simulation of conditions or indirect simulation of certain parameters

Ahrens, T.

Alexander, W. M.

Cintala, M.

Gault, D.

Greeley, R.

Hawke, B. R.

Housen, K.

Schmidt, R.

Schultz, P.

English

NASA Technical Reports Server

2.0 SPACE STATION IMPACT EXPERIMENTS
Peter Schultz (Chairman), Brown University; Thomas Ahrens, California
Institute of Technology; W.M. Alexander, Baylor University; Mark Cintala,
Johnson Space Center; Donald Gault, Murphys Center for Planetology; Ronald
Greeley, Arizona State University; B. Ray Hawke, University of Hawaii; Kevln
Housen, Boeing Aerospace Co.; Robert Schmidt, Boeing Aerospace Co.
2.1 Introduction
The impact process is ubiquitous in the Solar System affecting planetary
surfaces from the mlcroscale (regollth evolution) to the megascale (planetary
disruption). Over the last two decades research has largely focused on impact
processes in which gravity plays a key role, whether in returning ejecta to
the surface of the target body or in limiting crater growth. Ongoing research
continues to provide fundamental new insights under such conditions. As
earth-based observations and missions return new data about small bodies,
however, we discover that our understanding of impact craterlng under
low-gravlty conditions is severely limited not because of lack of interest but
because of difficulty in removing this dominant variable in I g experiments.
A Space Station Impact Facility would provide the unique opportunity to
reproduce impact conditions unachlevable on Earth and to explore parameters
"masked" by the gravity term.
Four processes serve to illustrate potential areas of study and their
implications for general problems in planetary science. First, accretlonal
processes reflect the success of collislonal aggregation over collisional
destruction during the early history of the solar system. Asteroids and
meteorites are relicts of this epoch, but many of the conditions leading to
their formation and evolution cannot be achieved under terrestrial gravity
conditions. Second, both catastrophic and less severe effects of impacts on
planetary bodies surviving from the time of the early solar system may be
expressed by asterold/planetary spin rates, spin orientations, asteroid size
distributions, and perhaps the origin of the Moon. Although theoretical
models can be constructed to describe these colllslonal effects, they require
both essential inputs and constraints that could be provided by experiments
under low-gravlty conditions. Third, the surfaces of planetary bodies
directly record the effects of impacts in the form of craters; these records
have wlde-ranglng implications. The size distribution of craters establishes
the relative surface or resurfaclng ages, and the morphology of craters
provides clues to subsurface structure. The removal of the gravity term,
however, results in craters much larger than those on a gravlty-domlnated
surface, thereby modifying the recorded size distribution and the efficiency
of crater destruction. Subtle forces may control final crater size and shape.
Moreover, removal of the gravity term may help resolve fundamental issues for
much larger craters on gravlty-domlnated bodies: for example, the origin of
central peaks and the effect of gravity on the craterlng flow field. Phobos,
the Moon, and Mercury all have craters (basins) and antipodal patterns that
may indicate near-destructlon by a single event, but conditions favoring or
limiting destruction remain poorly constrained. Fourth, regollth evolution of
asteroidal surfaces is a consequence of cumulative impacts, but the absence of
a significant gravity term may profoundly affect the retention of shocked
fractions and agglutinate build-up, thereby biasing the correct
interpretations of spectral reflectance data.
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https://ntrs.nasa.gov/search.jsp?R=19860017665 2020-03-20T14:27:08+00:00Z
An impact facility on the Space Station would provide the controlled
conditions necessary to explore such processes either through direct
simulation of conditions or indirect simulation of certain parameters. The
following discussion outlines a general plan for achieving this goal: I) we
propose a four-phased approach in implementing of both the facility and
experimentation, 2) we review a tentative scenario for a Space Station Impact
Facility with anticipated hardware requirements, 3) we identify possible
commonality with other experiments, and 4) we offer recommendations.
2.2 Approach
A phased approach to mlcrogravity impact experiments is necessary in order
to refine specific experiment parameters and to gain practical experience.
The subgroup identified four distinct phases: I) Earth-based feasibility
experiments; 2) STS (Shuttle) experiment package; 3) IOC (Space Station)
experiment package; and 4) the Space Station impact facility. This phased
approach will not only lead to further definition for the final goal--the
Space Station Facility--but inevitably will also lead to new scientific
results. Although the first three phases may be driven primarily by only a
few research groups in order to maximize efficiency and minimize costs, an
operational Space Station Impact Facility is envisioned as a national facility
for a wide range of qualified users.
Earth-based Feasibility Studies
Constraints on the size of the impact chamber size restrict the range in
micro-gravity experiments. Experience with existing low-gravlty craterlng
data and extrapolations of data from higher g-levels indicate that crater
dimensions and formation time increases dramatically under micro-gravity
conditions. For example, at i g a 20-cm-diameter crater formed in a
particulate target takes 0.2 seconds to form. If the current understanding of
gravity effects is correct, then extrapolation indicates that at 0.001 g the
crater will be 64 cm in diameter and take I0 seconds to form. The expected
increase in crater size requires large containers; the increase in formation
time would also permit numerous shock-wave reflections from container walls
that could possibly affect crater growth. Consequently, the subgroup
recognizes that initial experiments dealing with crater growth up through the
IOC phase may be severely restricted and that the most promising results would
first come from impact experiments focusing on free-floating targets. Further
experience is needed, however, in order to assess the severity of such
constraints and to identify the crucial parameters for specific IOC
experiments. Such experience can only be gained from properly designed
experiments at existing facilities (e.g., NASA-Ames Vertical Gun and Johnson
Space Center Vertical Impact Facility) and exploratory low-gravity experiments
at the Ames facility, the KC-135 Reduced Gravity Aircraft, and drop-towers.
The latter two facilities also provide essential experience with target
design, preparation, and design of the IOC Impact Facility.
STS Impact Experiment Package
This phase offers proof-of-concept and design resulting from earth-based
feasibility studies. The subgroup recognized enormous benefits that would
accrue from experience on the Space Shuttle. This experience includes
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problems in target preparation and handling (e.g., construction of particulate
spheres, free-floating liquid targets) and in testing preliminary designs of
the IOC package.
IOC Impact Experiment Package
The first phase of Space Station experiments most likely will concentrate
on impacts of free-floatlng targets in order to better understand phenomena
associated with collision processes. The subgrouppresently envisions this
facility to be equipped with an accelerator permitting impact velocities from
0.I to at least 2.5 km/s, monitoring systems (launch diagnostics, film/vldeo
records, etc.), and an ability to prepare, analyze, and exchange targets. The
IOC module w-Ill be a direct outgrowth of the STS package but with increased
sophistication to anticipate requirements for a larger impact chamber and to
ensure meaningful scientific results. An impact chamber size of at least 1.0
m 3 is required.
Space Station Impact Facility
Key requirements for this final operational phase include variable gravity
levels (10 -5 g to 0.2 g), a large impact chamber (4 m diameter minimum), full
range of impact velocities (0.1 km/s to at least 6 km/s), variety of targets
(particulate, liquid), and experimenter interaction. More specific
requirements and descriptions are deferred to a subsequent section. The
expanded dimensions and capabilities permit systematic analyses of crater
scaling, crater flow fields, crater relaxation, regolith evolution, accretion
studies, and more elaborate free-floatlng targets.
2.3 Space Station Impact Facility: A Scenario and Requirements
In order to visualize the ultimate configuration of this facility and to
identify problem areas, a tentative scenario with hardware requirements was
compiled. The reader is reminded that this is a preliminary view that will
evolve with further experiments and experience gleaned from the phased
approach. Five key requirements, however, have emerged: I) variable g, 2)
large impact chamber, 3) wide range in impact velocities, 4) flexibility in
target composition and structure, and 5) "hands-on" experiment operation.
Impact Chamber
A large chamber is necessary in order to reduce reflections of shock waves
from the walls of the target container, to contaln/capture eJecta products,
and to ensure a variety of more complex free-floatlng targets. A minlmum
chamber size of 4 m by 4 m by 4 m is envisioned. In early stages of
implementation this might be achieved within the primary module; nevertheless,
the subgroup strongly urges the availability of airlocks at least i m in
diameter in order to accommodate an add-on chamber of larger size and an
accelerator mount. We suggest that airlocks on opposite sides of the module
be considered in order to anticipate possible extensions for advanced
accelerator designs. It is virtually certain that the desired variation in
gravity levels will be obtained through some application of centrifugal force.
The geometry that would yield the lowest rotation rate--presentlng the
smallest induced Coriolis effects--would find the target chamber at one end of
a free-floating habitable module. On the other hand, other arrangements might
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be more desirable. A set-up that would find the chamber and accelerator
attached to the two airlocks or docking ports, for instance, might be handled
better by spinning the module along its major axis, i.e., perpendicular to the
axis running through the chamber and accelerator. The chamber should be
capable of operating in conditions ranging from vacuum to ambient atmospheric
levels. It is possible that the Impact Chamber would be used also as a target
preparation area. The large size of this chamber may be useful for other
experimenter groups.
Accelerators
In order to achieve the desired range in impact velocities (0.I to 7
km/s), the subgroup presently envisions the use of llght-gas gun, powder-gun,
and alr-gun technology. Light-gas gun technology has been used for over
thirty years and could be adapted easily. The subgroup also recognizes,
however, that new technological advances in rail guns, mass drivers, and
electrothermal guns may provide feasible alternatives and must not be
designed-out. Additionally, experiments involving low-yleld explosives will
be desired. Post experimental analysis envisions the need for holographic
systems, binocular microscopes, still photography, mass determination, and
sample curatlng. The subgroup envisions real-tlme telemetry of experimental
data to the earth for analysis by ground-based personnel.
Instrumentation
Three primary groups of instrumentation support are envisioned:
launch/environment diagnostics, impact recording instrumentation, and
post-impact analytic diagnostics. Launch/envlronment diagnostic
instrumentation includes computer-controlled gun operatlons/sequencer,
projectile velocity, sequencer, flash x-ray generators and detectors, pressure
and temperature transducers, and accelerometers. This instrumentation is
necessary to record projectile velocity and integrity, to monitor conditions
at impact, and to coordinate instrumentation and sequence of use. Impact
recording instrumentation includes a range of film recording devices from 24
fps to 35,000 fps (movie and video), lighting, pressure transducers, ejecta
sensors, computer-controlled 20 channel digital transient recorder, and
holographic recording. Several experimental recording instruments will be
integrally tied into mlcro-computers, and we envision that one of these will
be used to program experimental theories and backup for each mission.
Target Preparation and Housekeeping
An assortment of additional requirements include target containers, molds,
sieves, vacuum systems, target preparation equipment, small lathe and other
power tools, "scales" for mass determination, microwave heater, transducers,
ice-crusher/freezer, oven, and miscellaneous "furniture" (benches and
accessories). We envision such materials to be needed by other experimenters
and anticipate a mutually shared facility.
Personnel
In this advanced experimentation phase, we recognize the need for 2
payload specialists and 2 back-up specialists on the ground.
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2.4 Summary
It must be emphasized that this scenario depicts a fully operating
facility more than a decade in the future. Such an exercise was performed in
order to envision more limited needs and requirements during earlier phases
and to anticipate design requirements at much later phases.
Commonality
The large impact chamber, certain recording instrumentation (video, film
recorders, microcomputer, etc.), and shop facilities all should be conducive
to shared-use with other experiment groups, although not necessarily at the
same time. The unique section of this facility is primarily the accelerator
system; however, proper design should permit portability. It is possible that
the accelerator system, however, may be useful for certain studies in shock
dynamics (shock tube) in addition to the more typical impact experiments.
2.5 Recommendations
The Impact Experiment Subgroup recommends a phased approach in order to
gain insight and experience with impact processes under low to mlcro-gravlty
conditions. This phased approach includes pilot studies and research
involving existing impact facilities, the KC-135 Reduced Gravity Aircraft, and
drop-towers. The subgroup feels that the most useful first experiments at
mlcro-gravlty (IOC) will involve impacts of free-floating targets in order to
understand the effects of momentum transfer and disruption on realistic or
modeled planetary bodies (e.g., regollth-covered bodies, fluid spheres, etc.).
However, continued studies may identify additional configurations during the
I0C phase.
The subgroup also recommends that additional studies involving meteorite
impact detectors and cosmic dust collectors be encouraged since the results of
such studies will be complementary.
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