Exploration of the Moon

Transactions of the Royal Society of New Zealand : General, Volume 1, Issue 14, 9 August 1965, Page 153

Exploration of the Moon

W. H. Pickering

By

Director, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California

[Address delivered during the 11th Annual Science Congress at Auckland Town Hall, on 12 February 1965.]

Introduction

The moon, the brightest object in the night sky, has always been a source of fascination to man. It has preoccupied him more constantly than any other celestial object. Surrounded by myth and mystery, worshipped and feared, the moon has inspired poets and lovers, provided the basis for calendars, influenced the tides, stirred the imagination, and aroused the curiosity of man throughout the ages.

The first scientists wondered at the origin and history of the moon, and the first astronomers studied its motions. As early as the sixth century B.G. Hipparchus discovered and measured the eccentricities of the moon’s movements around the earth. Ptolemy in the second century A.D. mapped the moon’s surface. In the 17th century, Galileo built his first primitive telescope, pointed it at the moon, and found the rough, uneven surface of what he considered to be seas and mountains.

Since that time, the moon has been under constant telescopic observation, but until the Ranger VII spacecraft crashed into the moon on 31 July 1964, we had gone about as far as we could with earth-based optics, and the best telescopes could bring us no closer than to within 400 miles of the lunar surface. We know a great deal about our nearest celestial neighbour, but we are on the threshold of learning much more. We are now sending scientific instruments to the moon, and soon man himself will land there and return safely to earth.

This evening I wish to report to you on the flight of Ranger VII, the first spacecraft to take detailed photographs of the lunar surface. But first let me discuss, briefly, some known facts about the moon.

Galileo’s early telescopic observations of the lunar surface sound almost modem. Let me quote: “ The prominences there are mainly very similar to our most rugged and steepest mountains, and some of them are seen to be drawn out in long tracts of hundreds of miles. Others are in more compact groups, and there are also many detached and solitary rocks, precipitous and craggy. But what occur most frequently are certain ridges, somewhat raised, which surround and enclose plains of different sizes and various shapes, but for the most part circular.

In the middle of many of these there is a mountain in sharp relief, and some few are filled with a dark substance similar to that of the large spots that are seen with the naked eye; these are the largest ones, and there are a very great number of smaller ones, almost all of them circular.”

It is interesting to note that, at almost the same time, speculations regarding life on the moon resulted in what was perhaps the first science fiction book, “ The Man in the Moon ”, by Dominguez Gonzales, published in London in 1638. The novel describes the adventures of an explorer who is flown to the moon in a vehicle lifted by a flight of swans. He finds the moon peopled by a race of supermen. He describes many wonders in the lunar world, but pines for the imperfect earth and returns after a few weeks.

Earth-Based Observations

What is the moon really like? Within the past few decades observations of the moon have been made by radio waves of broad range frequencies, and by light waves in both the visible and infra-red portions of the spectrum. There has emerged a picture of a desolate world. The moon has no air, no water. The surface temperature ranges from 140° C. at noon to —lso° at midnight. The surface features vary from the relatively smooth maria to the exceedingly rugged highland regions. Craters of sizes ranging from about 100 miles diameter down to the limit of telescopic resolution abound on the surface, although the maria are relatively free of craters.

The moon has a diameter of about one-quarter that of the earth. Its mass is only l/80th that of the earth, so that its density is only 3.3 grams per cc as compared with 5.5 grams per cc for the earth. Consequently, the acceleration due to gravity on the surface of the moon is only about Ye of its value on the earth. The area of the visible hemisphere of the moon is about twice that of the United States. Since the moon rotates on its own axis at the same rate at which it moves around the earth, only one hemisphere is visible from the earth.

The moon’s orbit relative to the earth is an ellipse of eccentricity 0.055 and mean distance 239,000 miles from the earth. The sidereal period is 27.3 days. The orbit is inclined at 5° 9' to the plane of the ecliptic, and precesses with a period of 18.6 years.

With these basic facts, what detailed information can be learned about the moon’s history, surface features, and surface environment from recent earthbased observations? If man is going to explore the moon, he needs all the information possible before he embarks on his journey. In fact, he needs this information before he can even design the craft to carry him to the lunar surface.

Figure 1. Visual observations of the moon from the earth’s surface reveal that the predominant geographic feature, or perhaps I should say “ stenographic ” feature, is the vast number of craters. Almost all scientists today are convinced that these are formed by meteoric impacts except for a very few, which appear to be volcanic in nature. The typical lunar crater is formed by an explosion, and the energy is almost certainly brought in by a meteoric impact. These impacts occurred over a long period of time. But without wind and water, erosion has not significantly changed the resulting surface. Many examples can be found of overlapping craters, and of craters within craters, clearly attesting to their varying ages and insignificant erosion. The most recent craters show a light coloured halo ’ and a pattern of radiating streaks. These are believed to consist of material splashed out of the crater. With time, the surface is apparently darkened by the action of the sun’s radiation.

Copernicus is a recent crater. Since it is located in one of the mare, it is an excellent example of the results of a large meteoric impact. Figure 2: The crater itself is about 57 miles in diameter. It is polygonal rather than circular in outline. The interior wall reveals a characteristic terraced formation. The floor is relatively flat with some central mountains. The crater rim is 12,000 feet above the floor. There are numerous rays extending as far as 400 miles and hundreds of small craters scattered throughout the area around the crater. These appear to be secondary craters, resulting from rocks falling back from the primary impact.

It is interesting to compare Copernicus with a crater produced by a nuclear explosion on the earth. Figure 3; Note the characteristic shape and the large number of small secondary craters.

The earth, like the moon, has suffered meteoric bombardment during relatively recent geological times. However, erosion has quickly wiped out most of the evidence. An exception is the meteor crater near Winslow, Arizona. Figure 4: This is believed to have been formed about 10,000 years ago. It is 4,000 feet in diameter. A number of craters of this type have been found on earth, as well as evidence of older craters having diameters of the order of 50 miles.

The maria, the smooth, dark areas, are another prominent feature of the lunar surface. Figure 5: This illustrates a portion of the Mare Imbrium. Many of these maria have roughly circular boundaries and are also believed to be the results of very large impacts. The smooth surface is believed to be lava produced either at the time of impact, or welling up from the interior following the impact. Although the surface appears to be quite smooth on the scale of terrestrial photographs, it could actually be very rough, like terrestrial lava, on a scale of a few feet. Photographs of maria taken close to sunrise or sunset reveal low ridges running for many miles. They are typical of flow patterns or pressure ridges formed in viscous material.

The mountainous areas of the moon are perhaps not as rugged as they appear to be in the usual lunar photographs which accentuate the shadows with the sun near the horizon. Careful measurements reveal very few slopes greater than 15°. However, it is obvious that these mountainous areas would, for the most part, be very difficult to explore on foot.

Among the finer details of lunar photographs, it is worth noting that there is almost no evidence of faults or large rock movements of any kind. Here on earth, rocks are typically folded and broken due to movements along faults, and many faults can be found where horizontal displacements of tens of miles are apparent. On the moon this does not appear to be the case.

Lunar observations with infra-red and with radio waves give us information about the thermal and electrical properties of the surface. For example, some recent data show that the surface does not heat or cool uniformly. During an eclipse of the moon, some areas such as the interior of the crater Tycho, remain as much as 40° C. warmer than the surrounding area. Such differences could be indicative of differences in the thermal conductivity of the rock, or could reflect heat transferred from the interior due to a mass of hot magma near the surface.

Both radio and infra-red data indicate that the actual surface has a very low thermal conductivity and a low dielectric constant. A spongy surface for at least a few centimeters is indicated. Possibly a pumice-like material will be found.

Radar data confirm the fact that average slopes on the moon are of the order of 15°. Observations at short wavelengths indicate an increasing roughness as the wavelength decreases. For dimensions of the order of inches, a rough surface is implied.

It should be noted that, in the visible, the full moon appears uniformly bright across the disc. Hence, the moon is a diffuse scatterer at these wavelengths and therefore there are no optically smooth surfaces.

United States and Soviet Moon Programmes

The United States and the Soviet Union have each included an extensive programme of exploration of the moon as a part of their space activity. The U.S. programme has often been described in detail. The Soviet programme has not been published, but certain aspects of their efforts can be deduced from the flights already conducted and from statements of Soviet scientists.

The U.S. programme is centred on the manned lunar-landing project known as Apollo. This was set up as a result of President Kennedy’s statement in April of 1961, to the effect that the United States would attempt to send a man to the moon and return him safely before the end of the decade. Even before this time, however, the National Aeronautics and Space Administration (NASA) had established an evolutionary programme of lunar exploration with unmanned spacecraft.

At the present time the unmanned programme has evolved into three projects: Ranger, Surveyor, and Orbiter. Ranger, the first of these, was set up to exploit the capability of the Atlas-Agena Launch vehicle, and carry out such experiments as were possible with a spacecraft directed to a specific target on the moon.

The Ranger programme consists of a total of nine flights. The first two were engineering tests of the basic spacecraft. The next three were designed to land a seismograph on the moon, and the remaining four were to take TV photographs as the spacecraft impacted the moon.

The two engineering test flights gave a reasonable check on the spacecraft design concepts, although rocket difficulties precluded the attainment of all test objectives. The three seismograph flights did not attain the scientific objective, but much was learned about spacecraft engineering and lunar trajectories.

Ranger number VI, the first television attempt, made a perfect flight to the target point on the moon, but the TV system failed. Ranger VII was completely successful. Rangers VIII and IX will be launched in the near future.

Figure 6: Surveyor is a spacecraft designed to make a soft landing on the moon, and conduct a number of experiments on the lunar surface. It will carry a TV camera and instruments to conduct physical and chemical surveys of the surface. Perhaps the most difficult part of the development is the retrorocket system which must conduct a controlled deceleration and descent to the lunar surface. This project will utilize the Atlas-Centaur launch vehicle, and the first test flight of Surveyor will occur before the end of 1965.

Project Orbiter is intended to take TV pictures of the lunar surface from a spacecraft which will orbit the moon at a distance of the order of 100 km. Orbiter was started last year in order to provide additional information for the Apollo project, and will make detailed maps of possible landing sites. The first flights of Orbiter will occur next year.

The Apollo mission concept, very briefly, is as follows: A three-man spacecraft will be placed in orbit around the earth, then injected on to a lunar trajectory. Near the moon the spacecraft will be decelerated so that it will orbit around the moon. At an appropriate time, a two-man lunar excursion module (LEM) will be separated and brought down to the lunar surface. After conducting certain experiments, the LEM will take off from the moon and rendezvous with the orbiting spacecraft. The craft will then be accelerated into an earth trajectory. Return to the earth will be accomplished with retrorockets and parachutes, as has already been done in earth orbital flights. The Saturn V rocket with a thrust of 7,500,000 pounds at take-off, is being developed for Project Apollo.

MOON

The Soviets, so far, have launched four moderately successful lunar spacecraft. The first merely flew by the moon. The second impacted the moon but performed no close-in experiments. The third flew past the moon and took some photographs of its back side which were later transmitted to the earth. Figure 7: These photos were taken from a distance of 40,000 miles, and the signal to noise ratio on the communication system was very poor, so that the resulting pictures produced only a very crude impression of the side of the moon not visible from the earth.

The last Soviet attempt, Lunik IV, was launched on 2 April 1963. It apparently missed the moon by a large distance and failed in its mission.

The Soviets have made no positive announcement about an Apollo-type programme, but their current interest in manned space flight, and in rendezvous techniques, suggest that they also expect to send a man to the moon.

The Flight of Ranger VII

Ranger VII was launched 28 July 1964. Figure 8: The launch sequence required the spacecraft to be accelerated to satellite speed and placed in a 120mile altitude circular orbit. It coasted for 20 minutes, then was accelerated to the lunar trajectory velocity of 10.949 km/sec, or 24,500 m.p.h.

Figure 9: The required accuracy of this manoeuvre and the complication arising from the fact that the earth is rotating is illustrated in this figure. The path to the moon remains reasonably constant over any one day, hence the trajectory from the launch in Florida to injection on to the lunar path must change with time as the earth rotates. On any one day, launch constraints restrict the permissible launch period to about two hours. The duration of the flight from earth to moon is selected so that the arrival at the moon will be visible from the Goldstone tracking station in California. Figure 10: The trajectory is illustrated in this figure.

After injection on to the correct lunar trajectory, Ranger VII was separated from the final stage rocket and was then ready to perform its operations in space. Figure 11: The first operation was to unfold its solar panels and stabilize itself with the long axis pointed at the sun. The roll angle of the vehicle was then turned so that the high-gain antenna was pointed at the earth. Photoelectric sensors were used to detect sun and earth. The spacecraft was moved by small gas jets exerting forces of the order of 4 grams. The gas is supplied from vessels containing 4 pounds of air at a pressure of 3,500 psi. Sun and earth acquisition were completed in 6-1/2 and 23 minutes respectively. After being stabilized, the craft oscillated through a 1/2° limit cycle within a period of 15 minutes.

The spacecraft communication system, operating in the region of 2,300 me, was phase-locked from an earth-based transmitter. Two antennas were used, an onmidirectional and a four-foot parabola. The switch over to the parabola, by command from the earth, was done about 20 minutes after the spacecraft had stabilized on the earth. The transmitter power was 3 watts.

For the next 12 hours the spacecraft reported its operating conditions through a telemetry system which measured 110 quantities such as temperatures, voltages, etc.

Figure 13: The tracking stations at Goldstone, California, Johannesburg, South Africa, and Woomera, Australia, collected these data and also measured the spacecraft position and velocity. Position measurements were made by measuring the pointing angles of the 85-foot parabolic antenna. Radial velocity relative to the tracking station was obtained by measuring the doppler shift in the phase-locked loop from ground to spacecraft to ground.

These data were sent to the control centre in Pasadena, and a trajectory was calculated. It was determined that Ranger VII would impact on the back side of the moon at 204° east longitude. A correction manoeuvre was then calculated to bring the spacecraft to the desired target at 21° west longitude, 11° south latitude. This called for the spacecraft to roll 5.6°, pitch 86.8°, and change its velocity 29.9 m/sec or 66 miles per hour. The necessary commands were sent when the craft was 105,000 miles from the earth. The spacecraft rolled and pitched as commanded, then ignited its midcourse correction rocket motor and changed its speed as directed. Figure 12; Shown here schematically are the necessary manoeuvres.

With the completion of the midcourse manoeuvre, the spacecraft returned to the cruise orientation. Further tracking showed that the trajectory had been corrected as desired, and the final impact point was about 5-1/2 miles from target.

About 68 hours after launch and 17 minutes before impact on 31 July, the TV system was turned on. The spacecraft approached the moon in a direction which permitted the cameras to look approximately along the velocity vector. Therefore, a terminal orientation manoeuvre, which could be performed if necessary, was not required.

Two of the six cameras, the full scan or F cameras which transmitted the complete image, were turned on first. Four minutes later four partial scan or P cameras were turned on which transmitted only a portion of the image but which permitted the transmission of photographs at a faster rate. The full scan F cameras sent pictures alternately at 5 second intervals, so that one picture was sent each 2-1 /2 seconds. The partial scan P cameras transmitted at a rate of 0.8 seconds each to give a picture every 0.2 seconds. Thus, over 4,000 pictures were transmitted before impact.

Now, let us consider some of the problems to be solved before such a Ranger flight. The launching rocket must be reliable and well tested. Its guidance system must be flexible enough to accept the changing trajectory requirements while the vehicle is on the launching pad. The Atlas used had an initial thrust of 368,000 pounds. Figure 14: This is a photograph of the take-off from Cape Kennedy.

The second stage rocket (in this case the Agena) must be designed to operate its engines, on command, twice. The second burn, after a coasting period, is a requirement for a lunar trajectory unless the launch time can be restricted to a single precise moment. The time and duration of this second burn must be adjustable with the launch time. During the coasting period the second stage rocket must maintain its orientation with respect to the earth. The final injection occurs when the spacecraft is over South Africa.

The primary power supply for the spacecraft comes from solar cells. A total power of 200 watts is generated. When the solar cells are not sun-orientated, as in the initial phases of the flight and during manoeuvres, a battery is used to provide power. Batteries are also used to provide power for the television system.

Programming of operations abroad the spacecraft is accomplished by a unit known as the central controller and sequencer or CC & S. This device is essentially an electronic clock which closes switches at designated times. The CG & S also contains a computer which receives commands, routes them to the proper destination, and, in some cases, stores command information for future reference.

One of the most interesting systems aboard the Ranger is the midcourse motor. This is a monopropellant rocket engine exerting 50 pounds of thrust. The fuel is stored in a bladder inside a closed tank and pumped into the motor

by gas pressure. The starting and stopping of the motor must be precisely controlled. During the operation of the rocket motor the attitude control system is not able to control the orientation of the vehicle. Hence a set of vanes are mounted in the jet and controlled by an autopilot which uses gyroscopes for its reference directions.

Figure 15: The television system consists of two separate camera chains and transmitters. The two systems are as independent from each other as possible for increased reliability. The set of six cameras consists of two with one-inch focal length FI lenses, and four three-inch focal length F2 lenses. The short focal length, wide angle images are focused on to vidicon tubes having sensitive areas of 0.44 x 0.44 in. In the case of the two full scan cameras, the image is scanned by 1,100 lines. The P cameras scan 300 lines across an image 0.11 x 0.11 inches. The six cameras are orientated to give overlapping coverage with the P camera fields in the centre. Figure 16.

In operation, the vidicon image must be erased after being scanned. In the case of the F cameras, scanning and erasing each take 2.56 seconds. The partial scan cycle takes 0.2 seconds each. Pictures from the cameras are thus transmitted in turn, the F cameras on one and the P cameras on the other. The two 60 watt transmitters are diplexed into the high-gain antenna. The signals occupy band widths of 200 kc.

Another interesting problem to be solved is that of vehicle temperature control, in the vacuum of space, heat enters the spacecraft by radiation from the sun and leaves by radiating out to empty space. Hence temperature is established by the radiation properties of the surfaces of the vehicle. By proper coating of these surfaces, Ranger maintained temperatures within the range of 75° to 100° F. in all critical areas.

Another point of interest is that the launching rocket capability places a constraint on the total weight which can be sent to the moon. Therefore the design of the structure must be as efficient as possible. The Ranger spacecraft weighed 806 pounds and contained approximately 30,000 parts.

Ground-based elements which are needed during the flight are the tracking and telemetry receiving stations and the control centre. Each of the three stations in the network has the capability of sending commands to the spacecraft.

The control centre at Pasadena has the task of analyzing all the incoming data. The tracking data were analyzed by digital computers to determine the actual trajectory and the necessary corrections. The telemetry data were analyzed to determine the engineering performance of Ranger.

The most accurate tracking data were the doppler measurements. These were generally accurate to better than 0.01 cycle over a 60 second period. The equivalent accuracy in the radial velocity measurement is about .001 metres/sec. An example of doppler data is shown in the next figure, Figure 17. The solid lines indicate the calculated velocities during the midcourse manoeuvre; the actual measured data are shown as dots.

Some Ranger Results

The principal result of the Ranger flight is, of course, the series of close-up photographs. However, a great deal of additional data was obtained from study of spacecraft telemetry and the analysis of the actual trajectory.

The telemetry data confirmed the design concepts for a deep space vehicle. Ranger demonstrated that a spacecraft can be accurately controlled in attitude during a long journey through space. The stabilized spacecraft made possible both an efficient power supply from solar cells and efficient communications to earth through a high-gain directional antenna.

Ranger also proved that precision space guidance is possible through the midcourse manoeuvre technique. This requires an excellent radio tracking system on the earth and a large electronic computer to calculate the trajectory and make the necessary correction in a matter of a few hours.

Analysis of Ranger trajectory data has resulted in several major improvements in our knowledge of physical parameters of the earth-moon system. The mass of the moon, or rather the value of G times the lunar mass, was determined as 4902.58 in units of kilometres cubed per second squared. This compares with the old astronomical value of 4900.76. The astronomical value had an estimated standard deviation of =±= 5.0, while the Ranger value is ± 0.17. Figure 18: Note in this figure that the lunar mass was also determined from Mariner II data. This calculation rested upon the measurement of the position of the barycentre of the earth-moon system. Thus Ranger data have improved the accuracy of our measure of the lunar mass by a factor of 30. Figure 19: Similarly, the uncertainty in the mass of the earth was reduced to about Vs of the previous astronomical value.

Determination of the impact times of Ranger VI and VII have resulted in a new estimate of the lunar radius of 1,735 kilometres, 0.3 kilometres less than the previously accepted value.

Correlation of tracking data from the three ground stations gave a measurement of the relative locations of the stations with respect to the spin axis of the earth. These data have resulted in small changes in the surveyed locations of the stations. The accuracies are better than 30 metres.

The TV pictures taken by Ranger are of excellent quality. This figure (Figure 20) shows the manner in which Ranger approached the moon, and the next Figure (Figure 21), the area actually photographed. Interpretation of these photographs is still under study, but some interesting observations can be readily made from the final pictures. For example, this (Figure 22) is the last photograph taken with the A camera. It covers an area about miles on a side. Superimposed on this photograph is a mosaic of pictures taken with the P cameras and showing increasing detail.

Figure 23: This is an enlargement of this mosaic. The dimensions of the last photograph were about 60 x 150 feet. You will see that the surface of this particular lunar area is relatively smooth. There are no large scattered boulders. All of the surfaces appear round. Scientists believe that this is clear evidence of the extent of erosion on the surface. This erosion is due to bombardment by meteoric dust and by debris from the large meteor impacts.

The violent temperature extremes and the solar radiation, particularly in the X-ray and ultra-violet regions, have also modified the surface.

Dr W. H. Pickering, Jet Propulsion Laboratory, Pasadena, California, U.S.A.