The inflight experiments and photographic tasks conducted during the Apollo 15 mission are discussed in this section. The discussion is concerned primarily with experiment hardware performance and data acquisition operations. In instances where preliminary scientific findings were available at the time of report preparation, they are included, but more complete information on scientific results will be found in reference 2.

The experiments located in the scientific instrument module bay of the service module ( fig. 5-1) consisted of a gamma ray spectrometer, an X-ray spectrometer, an alpha-particle spectrometer, a mass spectrometer; and a subsatellite which is the vehicle for a particle shadows/boundary layer experiment, an S-band transponder experiment, and a magnetometer experiment. The subsatellite ( fig. 5-2) was launched successfully just prior to transearth injection on August 4 at approximately 2100 G.m.t., and was inserted into a 76.3-by-55.1-mile lunar orbit with an inclination of minus 28.7 degrees. The three subsatellite experiments are expected to acquire data for a period exceeding 1 year. At the time of launch, the moon was in the magnetosheath (transition) region of the earth's magnetosphere (
fig. 5-3
), one of several data collecting regions of scientific interest. All subsatellite experiments are turned off while the battery is being recharged after each tracking revolution. Both the magnetometer and particle shadows/boundary layer experiments are acquiring data on all revolutions except those when the battery is being charged.

Other inflight experiments consisted of ultraviolet photography of the earth and moon, photography of the Gegenschein from lunar orbit, an S-band transponder experiment using the command and service module and lunar module S-band communication systems, a down-link bistatic radar experiment using both the S-band and VHF communications systems of the command and service module and an Apollo window meteroid experiment.

Photographic tasks that were designated as detailed objectives rather than experiments are also discussed. They are the service module orbital photography employing the panoramic camera, the mapping camera, and the laser altimeter; and command module photography of lunar surface areas and astronomical subjects. A brief description of the equipment used for these experiments and photographic tasks is given in appendix A.


The gamma-ray spectrometer was operated in lunar orbit for over 90 hours. The instrument was operated in the minimum-background mode for prime data collection approximately 65 percent of the time. The remaining of the time it was operated in various non-minimum-background modes to determine the effects of background radiation sources on the prime data. The instrument was also operated for approximately 50 hours during transearth flight obtaining background data necessary for analysis of the lunar data, and to acquire data from galactic sources.

The instrument as well as the deployment boom performed well throughout the mission. However, two anomalous conditions occurred which affected instrument calibration. First, a downward drift in the linear gain of the photomultiplier or pulse analyzer was detected after the first boom extension (prior to undocking in lunar orbit) when several lines in the spectrum of the Apollo lunar surface experiment package fuel capsule were used for calibrations. The drift decreased in magnitude from an initial rate of 1 percent per hour to 0.4 percent per day and, eventually, reached a fairly stable state. The second anomalous condition was noted about 2-3/4 hours after transearth injection, when spectrum zero shifted eight channels, causing loss of the 0.279-million-electron-volt calibration reference. Commencing at 246:56, the problem disappeared for approximately 25 hours, returning at 271:47 and remaining for the rest of transearth flight. These problems are discussed further in section 14.3.4.

The preliminary data indicates variations in radioactivity as the spacecraft passed over different kinds of terrain. The western mare areas are generally the highest in radioactivity, with the eastern maria being somewhat lower. The highlands are the lowest in activity with a slightly lower level in the far-side highlands. The data further indicate a continuum level comparable to that predicted from Ranger 3 and Luna 10 data. Peaks due to potassium, thorium, oxygen, silicon, and iron have been identified. Detailed analysis is expected to show the presence and distribution of uranium, magnesium, aluminum, and titanium.


All X-ray spectrometer objectives were achieved and no hardware problems were noted. About 90 hours of data were obtained from operation of the instrument in lunar orbit, and approximately 26 hours of data were acquired while in transearth flight. During this latter period, the instrument was pointed at six preselected locations to acquire data on possible variations in X-ray intensity. Two observations were coordinated with simultaneous ground-based observations. After 276 hours, the instrument was left on to obtain data for use in the search for new sources of X-ray emission and to improve spectral information on known sources.

Near the end of transearth flight, an engineering test was conducted to determine if the gas-filled proportional counters Would be damaged by direct impingement of solar X-rays. The experiment continued to operate satisfactorily after the test.

The preliminary data shows that the fluorescent X-ray flux was more intense than predicted; that the concentration of aluminum in the highlands is about 50 percent greater than in the maria; and that the ratio of magnesium-to-aluminum in Mare Smithii and Mare Chrisium is about 50 percent greater than in the highlands between, and to the east and west of, the two maria. Analysis of the X-ray astronomy observations made enroute to the earth has shown that the intensity in X-ray output of Scorpius X-1 and Cygnus X-1 fluctuates with periods of several minutes.


All primary objectives of the alpha-particle experiment were achieved. The spectrometer was operated for approximately 80 hours in lunar orbit to acquire prime data, and approximately 50 hours during transearth coast to acquire background data.

Two of the ten detectors were intermittently noisy. The noise was at a very low rate (approximately 0.5 count per second) with occasional bursts at higher rates. Since the noise was generally restricted to one detector at a time, the loss of data is not expected to have a significant effect on the validity of the analysis.

An engineering test was performed near the end of transearth flight (in conjunction with the test on the X-ray spectrometer). The open experiment covers, which permitted direct sunlight impingement on the instrument, resulted in three of the ten detectors (including the two noisy detectors) showing some evidence of photosensitivity.

The planned coverage of the lunar surface was obtained. The alpha particle spectrometer did not detect any local areas of radon enhancement (An objective of the experiment was to locate craters or fissures by detecting alpha particles emitted by radon isotopes - daughter products of uranium and thorium). The general radon evolution rate of the moon is three orders of magnitude less than that of earth. A refinement of the data, in which summation of counts from successive orbital passes over the same area is made, will be required to make more definitive statements about the lunar distribution of radon isotopes.


Thirty-three hours of prime lunar orbit data were collected with the command and service module minus X axis in the direction of travel, and 7 hours of background data with the command and service module pointed in the opposite direction. During transearth coast, approximately 48 hours of data were gathered, including waste water dumps, oxygen purges, and boom- retraction tests.

The mass spectrometer boom retract mechanism in the scientific instrument module stalled during five of twelve cycles. Data, supported by the Command Module Pilot's observations during extravehicular activity, confirmed that the boom had retracted to within 1 inch of full retraction.

Each of the five cycles in which the boom did not fully retract was preceded by a period of cold soaking of the boom. In each instance, the boom would retract fully after warm-up. The boom was fully retracted for command module/service module separation. This anomaly is discussed further in section 14.1.6.

The instrument operated well, providing good data. Even though the boom retraction problem resulted in failure to collect prime data during one scheduled period, and real-time scheduling problems prevented instrument operation for another scheduled period, an adequate amount of data was acquired.

The mass spectrometer measured an unexpectedly large amount of gas at orbital altitude around the moon. This amount was an order of magnitude greater than that seen during transearth coast. Many gases were detected, including water vapor, carbon dioxide, and a variety of hydrocarbons. Data obtained during transearth coast indicate that a gaseous contamination cloud existed up to a distance of 4 feet from the command and service module, but contamination was not detected at the maximum extension of the mass spectrometer (24 feet).


The charged-particle telescope detectors were turned on immediately after subsatellite launch and are operating normally. Proper operation of the proton detection system was indicated when a large flux of protons in the 35 000- to 100 000-electron-volt range were observed near the magnetopause (fig. 5-3). Twenty-four hours after subsatellite launch, the electrostatic analyzer detectors were turned on, and have operated normally with no evidence of high-voltage corona or arcing.

When the moon is not in the earth's geomagnetic tail, the effect of the moon's shadow on the solar wind electrons is clearly detected. The variation in the shadow shape is rather large. With the moon in the earth's tail, a very tenuous plasma is seen. Within the plasma sheet, intensities increase with some flow of plasma from the earth's direction.


The magnetometer was turned on when telemetry from the subsatellite was acquired, and the instrument has performed satisfactorily. The experiment has operated continuously except for an 18-hour period after the lunar eclipse of August 6, and periods when the power is turned off to enable the batteries to return to full charge.

The magnetometer is returning better-than-expected information in relation to detecting surface anomalies. The principal investigator is carrying out hand calculations on far-side data that indicate excellent repetitive information over the craters Gagarin, Korolev, and Van de Graaff. While in the solar wind, the magnetometer is mapping the signature of the diamagnetic cavity behind the moon. As the subsatellite crosses the terminator, variations in the solar magnetic field by factors of two to three are detected by the magnetometer. These may be caused by interaction of the solar wind with local magnetic regions near the limb. More careful long-term analysis is required to confirm this preliminary finding.


5.7.1 Command and Service Module/Lunar Module

Good gravitational profile data along the spacecraft lunar ground tracks were obtained. The anticipated degradation of the data caused by changes in spacecraft position from uncoupled attitude control engine firings was not significant. Indications are that the gross shapes of mascons in Serenitatis, Crisium, and Smythii can be established. This complements the Apollo 14 results on Nectaris. Detailed gravity profiles of the Apennines and Procellarum regions were also obtained.

5.7.2 Subsatellite

The initial data contained a high level of noise caused by a wobble about the spin axis. The wobble was inherent in the subsatellite deployment and was subsequently removed by the onboard wobble damper.

The subsatellite S-band transponder is working well, and is being operated every twelfth lunar revolution. The tracking data shows that the perilune variation is following preflight predictions and is expected to confirm the predicted orbital lifetime (greater than 1 year). The subsatellite transponder has shown at least one new mascon in the region of the crater Humboldt on the eastern lunar near side. Repeated overflights of the lunar near side from varying altitudes as the subsatellite orbit decays will be necessary before an accurate gravitational map can be made and large anomalies defined.


The experiment data consists of records of both frequencies (S-band and VHF) during the front-side passes on lunar revolutions 17 and 28. During these dual-frequency periods, signals were bounced off the moon and received at Goldstone (210-ft dish antenna for S-band) and at Stanford University (150-ft dish antenna for VHF). On revolutions 53 through 57 (the crew sleep period), only the VHF frequency was reflected from the moon to the earth.

The experiment results will require considerable data processing. Determination of the bulk dielectric constant and near-surface roughness along the spacecraft track appears possible with the present data. S-band data from revolution 17 are not usable because of incorrect spacecraft attitude. However, VHF data from revolution 17 appear to be of high quality. The attitude error was discovered and corrected in time for revolution 28, and all the data for that revolution are of excellent quality. The VHF experiment conducted during revolutions 53 through 57 provided high quality data. Apollo 15 data may be correlated with data obtained from the Apollo 14 bistatic radar experiment since the spacecraft groundtracks of Apollo 15 during both S-band/VHF operation and VHF-only operation intersect the Apollo 14 groundtrack during S-band/VHF operation.


The command module side and hatch windows were scanned at a magnification of 20X prior to flight to determine the general background of chips, scratches and other defects. Postflight, the windows will again be scanned at 20X (and higher magnifications for areas of interest) to map all visible defects. Possible meteoroid craters will be identified to determine the meteoroid cratering flux of particles responsible for the degradation of glass surfaces exposed to the space environment.


Ultraviolet photographs were obtained while in earth and lunar orbit, and during translunar and transearth coast. The following table lists the ultraviolet photography sequences performed on Apollo 15. Each sequence consisted of two exposures without the use of a filter and two exposures each with a 2600-angstrom filter, a 3750-angstrom filter, and a 4000- to 6000- angstrom visual-range filter. In addition, some color-film exposures were obtained, as planned, with the visual-range filter. These are noted in the last column of table 5-1. Preliminary examination shows that the exposures were excellent



Photography of the Gegenschein and Moulton Point regions from lunar orbit was performed twice, as planned, during revolutions 46 and 60, and at least six exposures were obtained during each sequence. However, the photographs are unusable because incorrect signs were used in premission calculations of spacecraft attitudes. Ground-based photography in support of the inflight photography was performed during the mission at the Haleakala Observatory, Maui, Hawaii, and after the mission at the McDonald Observatory, Fort Davis, Texas.

The camera system used for the Gegenschein experiment and other astronomy tasks performed well. A comparison of preflight and postflight calibration exposures with the faintest brightness observed in the Apollo 15 exposures (of the Milky Way) demonstrates that this camera system is very satisfactory for the Gegenschein experiment, now scheduled for the Apollo 16 mission.


5.12.1 Panoramic Camera

The panoramic camera was carried on Apollo 15 to obtain high-resolution panoramic photographs of the lunar surface. The areas photographed included the Hadley Rille landing sites (fig. 4-1 and 4-2), several areas being considered as the Apollo 17 landing site, the Apollo 15 lunar module ascent stage impact point, near-terminator areas, and other areas of general coverage. Anomalous operation of the velocity/altitude sensor (section 14.3.1) was indicated on the first panoramic camera pass on revolution 4 and subsequent passes; however, good photography was obtained over all critical areas.

The delay in lunar module jettison caused cancellation of photographic passes planned for revolutions 58 and 59. These passes were rescheduled for revolutions 60 and 63, but sidelap with adjacent areas photographed on revolutions 33 and 38 was decreased.

All imagery is of very high quality. Examination of the film shows that less than one percent of the total film exposed was seriously degraded by the velocity/altitude sensor malfunction. 5.12.2 Mapping Camera

The mapping camera was carried aboard the Apollo 15 service module to obtain high-quality metric photographs of the lunar surface. Mapping camera operation was desired during all panoramic camera passes and on selected dark-side passes to assist in analysis of data from the laser altimeter. The camera functioned normally and, essentially, the entire area overflown in daylight was photographed. However, the laser altimeter failed (see the following section) and all scheduled dark-side mapping activities subsequent to revolution 38 were deleted. A problem with the mapping camera deployment mechanism was also experienced. The camera extension and retraction cycles varied from 2 to 4 minutes as compared to about 1 1/2 minutes required prior to flight. After the last deployment, the camera did not completely retract. This anomaly is discussed further in section 14.3.3.

The mapping camera was turned off during the panoramic camera pass over the landing site on revolution 50 in a test to determine if the velocity/altitude sensor anomaly might be related to the mapping camera operation. This resulted in a minor loss of coverage. Also, the photographic pass planned for revolution 58 was deferred until revolution 60 because of the delay in lunar module jettison. The consequence of this was a decrease in sidelap below the desired 55 percent.

Approximately 6 hours of mapping camera operating time remained at transearth injection. About 1 1/2 hours of this were expended photographing the receding moon, and 3 1/2 hours were used photographing selected star fields with the stellar camera associated with the mapping camera.

Image quality is excellent throughout the entire sequence of 3400 frames. The entire portion of the lunar surface which was overflown by Apollo 15 in daylight has been covered by excellent stereoscopic photography which is as well suited to detailed analysis and geologic interpretation as it is to mapping.

5.12.3 Laser Altimeter

The laser altimeter was flown to accurately measure lunar topographic elevations in support of mapping and panoramic camera photography, and inflight experiments. The altimeter was designed to supply a synchronized altitude measurement for each mapping camera exposure on light-side photography, and independent altitude measurements on the dark side to permit correlation of topographic profiles with gravity anomalies obtained from spacecraft tracking data.

Operation of the altimeter was nominal through revolution 24, but improper operation was noted on the next operation (revolution 27). The performance of the altimeter became progressively worse until, on revolution 38, the altimeter ceased to operate (sec. 14.3.2). Consequently, the altimeter was not operated on subsequent dark-side passes, although operation on lightside mapping camera passes was continued. On revolution 63, an attempt was made to revive the altimeter through a switching operation by the Command Module Pilot, but the effort was not successful.

Approximately 50 percent of the planned altimeter telemetry data were actually obtained before the instrument failed. The data from the early orbits have been correlated with S-band transponder data for the frontside pass, and show the shape of the gravity anomalies as related to mare basins. The complete circumlunar laser altimeter data show that, relative to the mean lunar radius, the average lunar far side is about 2 kilometers (1.1 mile) high and the average near side is about 2 kilometers low.


While in lunar orbit, photographs were taken from the command module of lunar surface sites of scientific interest, and of specific portions of the lunar surface in earthshine and near the terminator. Also, while in lunar orbit, photographs were taken of low-light-level astronomical subjects including the solar corona, the zodiacal light, lunar libration point L4, and of the moon as it entered and exited the earth's umbra during lunar eclipse. During translunar and transearth coast, photographs were taken of a contamination test and star fields were photographed through the command module sextant.

In accomplishing some of the tasks, the crewman obtained extra frames and some with longer exposures than required. This will enhance the value of the total data desired. The only 16-mm data acquisition camera magazine containing very-high-speed black-and-white film was lost. About 35 percent of the magazine had been exposed during lunar orbital flight and transearth coast for solar corona and sextant star field photography. The most probable cause of the loss of the magazine was that it floated through the hatch during the Command Module Pilot's extravehicular activity. This required a substitution of a slower black-and-white film magazine for the final sextant star field photography and real-time update for contamination photography but, because premission-planned exposure settings were used with the much slower film, the sextant star field photographs are not clear.

Photographs were obtained of 21 of 23 specific lunar surface targets, the solar corona, the moon during lunar eclipse as it entered and exited the earth's umbra, star fields through the command module sextant, lunar libration region L4, and specific areas of the lunar surface in earthshine and in low light levels near the terminator. Near-terminator strip photography scheduled on revolution 58, and 2 of the 23 lunar surface targets scheduled on revolutions 58 and 59 were deleted because of the delay in lunar module jettison due to problems during tunnel venting operations and subsequent extension of the crew's sleep period. Based on preliminary examination of the dim-light photography, it appears that excellent quality imagery was obtained of the solar corona, the zodiacal light and the lunar surface in earthshine.


Visual observations from lunar orbit was an objective implemented for the first time on this mission. The Command module Pilot was asked to make and record observations of special lunar surface areas. Emphasis was to be placed on characteristics difficult to record on film, but which could be delineated by the eye, such as subtle color differences between surface units. All of the scheduled targets were observed and the results relayed. These results are documented in reference 2. Significant observations were as follows: