Where No Man Has Gone Before: A History of Apollo Lunar Exploration Missions|
Lunar Science Objectives:
The Rationale for Apollo
Landing Site Selection
Following is a summary
of material presented to the Apollo Site Selection Board at its meeting on July
10, 1969, intended to show how the Site Selection Subgroup of the Group for
Lunar Exploration Planning arrived at their recommended list of primary and
alternate sites (presented at the June meeting of the Board). It relates scientific
experiments to the "15 Questions" tabulated above.
Determination of the absolute
age of lunar surface materials by radioactive-decay methods was of prime importance.
The method dates the time at which a given sample became a closed system for
a particular element; for example, the potassium-40-argon-40 method determines
the time at which a rock containing radioactive potassium-40 cooled sufficiently
to retain the gaseous argon-40 decay product. The age thus determined may reflect
a period of volcanism, melting by meteoric impact, or original accretion of
Ages of primary interest
to lunar scientists were:
- the age of the moon's
formation or of its oldest crust; this age might make it possible to distinguish
among various theories of lunar origin. One key site for locating such material
was the Fra Mauro Formation. This widespread blanket of debris is considered
to consist of debris from the "Imbrian event," the cataclysmic occurrence
that produced Mare Imbrium, the enormous circular mare in the northwestern
quadrant of the Earth-facing side of the moon. The Fra Mauro Formation was
given high priority on all lists of potential landing sites.
- The time at which the
maria became filled with the relatively smooth material, probably once-molten
rock, that characterizes them. This date would be relatively easy to establish,
since mission planners preferred smooth, level landing sites, most often found
in the maria.
- The time of significant
post-mare events, such as the impacts that created the craters Copernicus
and Tycho. Equally interesting was the origin of the sinuous rilles, which
appeared to have been formed largely in this later period of lunar history.
A primary interest of geochemists
was to find "primitive" solar system material to deduce the conditions
under which planets and satellites condensed from the solar nebula. Data from
analysis of terrestrial and meteoritic samples, along with solar and stellar
spectral studies and theoretical physics, comprised the prime source of such
information, but both the earth and meteorites have been modified by subsequent
heating and weathering, which have obscured the original composition. The moon's
small size (possibly resulting in a smaller flow of heat from the interior)
and its lack of an atmosphere suggest that it might still have original material
on the surface. It was generally accepted that this material could most likely
be found in the lunar highlands, which appeared to represent the oldest lunar
Second in importance was
establishing the bulk composition of the moon. The abundance of the major elements
was expected to be important in establishing the moon's origin; if it proved
to be totally unlike the earth in composition it could hardly have split off
from the earth. The abundance of radioactive elements would indicate how much
energy had been available for heat-induced chemical changes in the moon since
its formation. To determine the bulk composition of the moon it was necessary
to sample as many different geologic units as possible. Particularly important
were sites showing evidence of differentiation or the presence on the surface
of deep-seated material, such as would be present in and around impact excavations
or explosive craters and in blankets of material ejected from craters. Radioactivity
could be measured by instruments in lunar orbit, hence the importance of flying
experiments in the service module.
Finally, analysis of any
present or past lunar atmosphere would give clues to lunar origin and evolution.
Gas detectors operating over a long period of time might possibly detect transient
events, such as had been reported in the crater Aristarchus. The sinuous rilles
(e.g., Rima Prinz) were also likely sites for detecting any emission of gases
from the lunar interior.
Major Geomorphic Processes.
The study of the processes
by which lunar landforms have been created and destroyed was important mainly
to second- and third-order questions about the moon, but was essential in holding
the first-order questions (above) together. Knowledge of dominant processes
would provide the basis for selection of samples and determining their place
of origin, as well as providing major clues to past energy expenditure on the
moon. Regions of particular interest in this regard were the sinuous rilles
and areas of volcanic cratering. Since photography covered a large fraction
of the lunar surface, data from a few landing sites would enable geomorphologists
to draw conclusions about most of the moon.
The only source of information
on the moon's internal structure was the emplaced ALSEP experiments, which included
a seismometer and a heat flow instrument. (Later ALSEPs would include different
Seismic data were expected
to yield information on layering in the moon, the rate of release of internal
strain, and the number and energy of meteorite impacts. It would be very useful
to the seismologists to produce an impact of known energy at a known location,
such as by causing a spent S-IVB stage or a lunar module ascent stage to crash
on the lunar surface. It was important to have at least four seismic instruments
active at one time; they should be about 1,000 km apart with as much angular
separation as possible. Thus sites at high latitudes, such as Tycho, were quite
Data on heat flow were
difficult to interpret but could assist in determining whether the moon had
originally been hot or cold.
Lunar gravity and geodesy
would determine the extent of the "mascons" (mass concentrations)
and whether the moon was in hydrostatic equilibrium. These were well suited
to study by orbiting instruments.
The laser retroreflectors
were expected to make it possible to determine earth-moon distances within a
few centimeters, enabling scientists to measure the librations of the moon with
previously unattainable accuracy. Best results would be obtained with widely
Characteristics of the
Recommended Landing Sites.
The short list of landing
sites for the first 10 lunar exploration missions should include:
Following is a description
of the 10 prime sites chosen by the Group for Lunar Exploration Planning. The
sites were selected in order of their preferred execution on one "G,"
four "H," and five "J" missions.
- two types of mare material,
"older" or eastern and "younger" or western;
- regional stratigraphic
units, such as blanket (ejecta) deposits around mare basins;
- various types and sizes
of impact craters in the maria and in the highlands;
- morphological manifestations
of volcanism in the maria and in the highlands; and
- areas that may give clues
to the nature and extent of processes other than impact and volcanism, which
may have acted on the lunar surface.
The Group for Lunar Exploration
Planning recommended this list of sites after considering the expected evolution
in capability as well as the constraints imposed by operations. Those selected
for "J" missions, for example, were picked on account of the additional
time on the surface that would be available and the increased mobility that would
be provided by a powered vehicle. Tycho, accessible only in the early part of
the year because of operational limitations, was switched from the third to the
second "J" mission for that reason, although it was better than the
Marius Hills site for exploration on foot.
- (1) Landing Site 2
("older" or eastern mare).
- This site is entirely
within relatively old mare (Imbrian) material. It includes many large subdued
craters 200 to 600 meters in diameter but comparatively few in the size range
50-200 meters, a distribution common to many apparently old surfaces. Determination
of the age and nature of this Imbrian mare material was a primary object of
landing at this site.
- (2) Landing Site 5
("younger" or western mare).
- Landing Site 5 is located
within relatively young (Eratosthenian) mare material and displays many craters
50 to 200 meters in diameter and relatively fewer of the larger (200-600 meter)
craters. It is surrounded by well developed rays from Kepler, making it likely
that it contains material derived from considerable depth. The chief goal
of a landing at Site 5 was to determine the age and composition of Eratosthenian
- (3) Fra Mauro Formation.
- This extensive geologic
unit covers large portions of the surface around Mare Imbrium and is thought
to be material ejected when Imbrium was formed. Samples from the Fra Mauro
Formation would help to understand its nature, composition, and formation
and its relation to the "Imbrian event."
- (4) Rima Bode II.
- Rima Bode II, a single
linear rille running close to a fresh, elongate crater and a crater chain,
was of interest because both the rille and the crater were possible sources
of several dark geologic units most probably of volcanic origin. The site
was selected as an example of a region where material of deep-seated origin
was expected. An alternative site was Hyginus Rille, similar in characteristics
but apparently less fresh. Another site, Littrow, would meet part of the objectives
of a mission to Rima Bode H.
- (5) Censorinus
- Censorinus is a 3.8-km
crater located within and near the edge of a highland block south-southeast
of Mare Tranqillitatis. It offered the opportunity, early in the exploration
program, to sample both highland material and features associated with a fresh
impact crater. The proposed site was within the ejecta blanket about 1 km
north of the crater rim and allowed investigation of the crater on foot, without
mobility aids. If Censorinus presented operational difficulties, Littrow could
be considered as an alternative site for this mission.
- (6) Copernicus (peak).
- This bright crater, 95
km in diameter, is the source of visible rays of ejected material extending
for several hundred kilometers. Its walls expose a 4-km vertical section of
the lunar crust. The floor, some 60 km across, contains multiple peaks with
a maximum height of 800 meters. A mission to the central peaks would be mainly
a sampling mission with the objective of bringing back material that once
lay at considerable depth.
- (7) Marius Hills.
- Marius Hills, a group
of domes and cones near the center of Oceanus Procellarum west-northwest of
the crater Marius, are part of a ridge system stretching some 1,900 km through
Oceanus Procellarum. The variety of features in this area and their similarity
to terrestrial volcanic structures strongly suggests intensive and prolonged
- (8) Tycho (rim).
- Tycho, like Censorinus
a fresh impact crater, is in the southern highlands. It is much larger than
Censorinus and offers an opportunity to study many features common to large,
fresh impact events, including associated volcanism. The proposed landing
site was near the Surveyor VII spacecraft, offering the option
of returning some Surveyor parts. In that area are several generations of
flows, a pond or pool, ejected blocks (probably from Tycho), and other ejecta
features and structures.
- (9) Rima Prinz I,
- Rima Prinz I, in the
Harbinger Mountains northeast of the Marius Hills, is a double sinuous rille
- a small meandering rille enclosed within a larger sinuous rille. The origin
of the rilles is of great interest because they resemble channels carved by
a flowing fluid. A landing near the mouth of Rima Prinz I, selected because
of the freshness of its details, would allow examination of the lower part
of the eroded valley, sampling the materials and studying the exposed structures.
An alternative, Schroter's V alley , displays similar characteristics but
appears older than Rima Prinz I.
- (10) Descartes.
- The area of the southern
highlands north of the crater Descartes and west of Mare Nectaris is characterized
by hilly, groovy, and furrowed deposits reminiscent of terrestrial volcanoes.
A mission to a region of intensive and prolonged volcanism within the lunar
terrae was considered most important, from both the geological and geochemical
viewpoints. An alternative to this site was Abulfeda, just to the southwest.
The Apollo Site Selection
Board accepted this list for planning purposes at its meeting on July 10, 1969.
As was to be expected, the list underwent considerable revision during the next
three years, both as to the choice of sites and the order in which they would
be explored, as mission planning became more detailed and operational capability