Recent years have seen a renaissance in lunar exploration with the US and multiple nations around the globe launching and planning missions to our nearest celestial neighbor. While most of these ventures have been successful (or are expected to be so), this is only possible as a result of the hard lessons learned during the earliest years of the Space Age when failures were more common than successes. This was especially true with NASA’s early forays to the Moon.
NASA’s earliest Pioneer lunar probes, a program started by the US Department of Defense and inherited by the agency after it was founded in October 1958, were plagued by a series of launch vehicle failures (see “The First Race to the Moon: Getting Off the Ground”). Out of all of NASA’s initial attempts to launch probes towards the Moon, only the tiny six-kilogram Pioneer 4 built by the Jet Propulsion Laboratory (JPL) and launched on March 3, 1959, by a team at the Army Ballistic Missile Agency (ABMA) headed by Wernher von Braun (which would become the basis of NASA’s Marshall Space Flight Center) managed to escape Earth’s gravitational grasp to make a very distant flyby of the Moon (see “Vintage Micro: The Pioneer 4 Lunar Probe”).
A model of the Block I Rangers flown in 1961. (NASA)
The initial flights of NASA’s first in-house lunar program, Ranger, which was built and managed by JPL, fared little better than the Pioneers. The two flights of the Block I Ranger, which were designed to test the innovative Ranger design in extended Earth orbit, were stranded in short-lived low Earth orbits due to failures of the upper stage of the Atlas-Agena B launch vehicle (see “The Prototype that Conquered the Solar System”). The three Block II Ranger flights, which were designed to hard-land a small probe on the lunar surface, fared little better. While most of the launch vehicle issues were resolved, fatal malfunctions of key spacecraft components resulted in complete failure of all of these missions (see “NASA’s First Moon Lander”).
Model of the Block II Rangers flown in 1962. The spherical hard-lander is the sphere on the top of the spacecraft. (NASA)
As 1962 was drawing to a close, the situation with the American Moon program looked bleak. The failure of the last Block II Ranger, Ranger 5 launched on October 16, 1962, was NASA’s sixth consecutive lunar mission failure in three years. Only 17 months after President John F. Kennedy committed the United States to landing a man on the Moon with Project Apollo, it was beginning to look as though the Americans would never make it. If NASA could not get a simple unmanned probe to the Moon in working order, how could they hope to pull off the much more complicated mission of a manned lunar landing?
Dealing with Failure
Following President Kennedy’s announcement of the goal to land Americans on the Moon, NASA authorized the flight of four Block III Ranger flights. Officially announced on August 29, 1961, the mission of the Block III Ranger was to perform high resolution lunar imaging before crashing on the Moon about 67 hours after launch. But before the Block III could fly, the problems with the spacecraft had to be addressed. Formal investigations into the failures of the Ranger program started on October 30, 1962. Over the course of the next month, several groups inside NASA and out examined every aspect of the project in an attempt to pin down the causes of the failures and recommend changes. On November 30, NASA Headquarters released the findings of its inquiry: In brief, the report recommended streamlining management and changing the mission goals to be more in line with supporting the needs of the Apollo lunar missions. This meant concentrating on lunar imaging and dropping all other experiments on the new Block III Rangers.
The report also called for a thorough reevaluation of the Ranger design, modifying vulnerable systems and the inclusion of more backup systems. More extensive testing of systems and better quality control for components were recommended. Most of all, the report urged the immediate abandonment of prelaunch sterilization of the spacecraft. Sterilization was pinpointed as the cause of many of Ranger’s system failures and it was now felt to be unnecessary, given that the hostile lunar environment was unlikely to harbor any indigenous life forms. Unless these changes were made, the Block III Rangers were likely to suffer the same fate as their predecessors. With these recommendations in hand, JPL set about redesigning and rebuilding the Block III Rangers.
The first flightworthy Block III spacecraft, Ranger A, undergoing checkout. (NASA/GRC)
The first improved Block III Ranger, designated Ranger A, was finally ready by the end of 1963. Much had been changed from the previous design. The Ranger hexagon-shaped bus was similar to previous models with some notable exceptions. First, the framework of the bus was now made of aluminum due to its better thermal characteristics. A second battery to provide additional backup power was added. The course correction system was enlarged to provide a 60 meter per second velocity change capability – a one-third increase over the earlier Block II Ranger. The sequencer which controlled spacecraft functions was redesigned to incorporate components which were not heat sterilized. This included features that increased the chances of a successful mission in case of equipment failure. A second, independent attitude control system was added for redundancy.
Diagram showing the major components of the Block III Ranger design. Click on image to enlarge. (NASA)
The bus was also fitted with new rectangular-shaped solar panels similar to the ones carried by Ranger’s successful cousin, the Mariner 2 Venus probe launched in 1962. This design had portions of the solar panels electrically isolated from each other to avoid a repeat of the total solar panel failure experienced by Ranger 5. All of these changes increased the weight of the Block III Ranger. This prompted the deletion of every instrument except for the large television camera package to keep the probe’s mass under 368 kilograms – the limit for the Atlas-Agena B launch vehicle for Ranger’s mission profile.
View of the Block III Ranger camera package. (NASA)
Two independent chains of slow-scan vidicon cameras developed and built by RCA (a leading electronics manufacturer of the day, bought by GE in 1986 and subsequently dismantled) were enclosed in a 1.5-meter tall tower mounted on top of the bus. Clad in polished aluminum for thermal control, the 173-kilogram cylindrical tower tapered from 69 centimeters at its base to 41 centimeters at the top, where the low-gain antenna was mounted. The six cameras viewed the approaching lunar surface through a 33-centimeter square opening on the side of the tower. Their optical axes were canted at a 28° angle from the spacecraft’s long axis. Also enclosed inside the tower were two independent power supplies, camera sequencers, and batteries; one set for each chain of cameras. Each chain also possessed its own sixty-watt transmitter to independently transmit images in real time back to Earth. The bus still carried its own three-watt transmitter which would now only carry engineering telemetry.
A simplified schematic showing the pointing direction of the Ranger cameras as well as the relative sizes and positions of their fields of view. Click on image to enlarge. (NASA)
The first camera chain was the full-scan or F chain, which consisted of two cameras. One camera was fitted with a 35-millimeter lens, providing a 25° field of view, while the other used a 76-millimeter lens with an 8.4° field of view. Each camera would scan the entire 1,152-line vidicon once the exposure had been taken: over twice the resolution of conventional television of the day and comparable to today’s high definition format. As a set, the F chain returned one image every 2.56 seconds. Normally the cameras would be turned on by commands sent from Earth. If this failed, the bus’s onboard sequencer would activate the package at a preset time. If this failed, the F chain had its own timer that was activated by the spacecraft’s separation from the Agena B escape stage. After 67 hours and 45 minutes of flight, the F chain would automatically turn on and start transmitting images. In this way, even if both primary systems were to fail, at least a few hundred full scan images would be returned.
Independent of the F chain was a second set of four partial-scan vidicon cameras called the P chain. Like the F chain, 35 and 76-millimeter lenses were used, but only three hundred partial lines—about seven percent of the vidicon’s face—was read and transmitted back to Earth. This resulted in images with the same effective resolution as the F chain but covered a smaller area. This was done so that images could be returned at a rate of five images per second in hopes of capturing at least a partial image a couple of tenths of a second before impact. At this altitude of only 300 to 600 meters, a resolution of 0.3 meters or better was possible. If the F chain were to malfunction, the P chain could independently return thousands of images after receiving a command either directly from Earth or from Ranger’s central sequencer and timer.
With all these hardware changes, including redundant and more fault tolerant systems as well as five hundred to eight hundred hours of prelaunch testing, the new Block III Ranger was much more likely to reach its target in working order.
The Block III mission profile was very similar to the Block II up until the encounter with the Moon. Since the Block III probe did not have to be concerned with the site and trajectory constraints of a hard lander, the impact point could be over a much larger range of longitude near the lunar equator. Typically, the most easterly aim point was targeted at the beginning of the multiday launch window. The aim point then drifted westward by about thirteen degrees of longitude per day, so that the impact point would have the optimum lighting conditions.
Diagram showing Ranger’s terminal maneuver to align its cameras to take images before it impacted the lunar surface. Click on image to enlarge. (NASA)
About one hour before impact, the spacecraft would begin its terminal maneuver and reorient itself. This maneuver would aim the cameras along Ranger’s flight path towards approaching lunar surface with the high gain antenna turned to point towards Earth. Some seventeen minutes before impact, the F chain of cameras would be commanded to warm up for ninety seconds. The P chain then would take its turn and warm up. Finally, fourteen minutes before impact at an altitude of about 1,900 kilometers, the F chain’s sixty-watt transmitter would start beaming images back to Earth, followed by the P chain typically 150 seconds later. Transmission would continue until the spacecraft impacted the lunar surface at 2,600 meters per second. If everything worked perfectly, over 4,200 close-up television images of the lunar surface would be transmitted back to the Earth.
The Atlas-Agena B being prepared for the launch of Ranger 6. (NASA)
Because of various minor schedule slips, the first modified Block III spacecraft, designated Ranger A, was ready for launch by the beginning of 1964. Its primary targets were in the smooth equatorial mare regions, which were considered likely Apollo landing sites. On the first day of the launch window, the site would be a point at 8.5° north and 21.0° east in Mare Tranquillitatis, the Sea of Tranquility. After several short holds, Ranger 6 lifted off on its first attempt on January 30, 1964. The launch and injection into a translunar trajectory went perfectly except for a telemetry channel that inexplicably switched into an unscheduled mode for 67 seconds when the booster engines separated from the ascending Atlas.
Initial tracking of Ranger 6 indicated that it would miss the Moon by about 965 kilometers. More refined calculations later indicated a miss of only 796 kilometers that was corrected by a 67-second burn of the course correction engine about 16 hours and 41 minutes after launch. This 41.2-meter per second change of velocity placed Ranger 6 on course for an impact on the western edge of Mare Tranquillitatis 65 kilometers south of the crater called Ross.
Diagram highlighting the steps needed for Ranger to perform a midcourse maneuver on its way to the Moon. Click on image to enlarge. (NASA)
On February 2, as Ranger 6 passed the 2,076-kilometer altitude mark moving at 1,998 meters per second, the television cameras were switched into warm up mode with all systems functioning normally. When the time came for the cameras to switch to full power and start returning images, however, only static was received. Quickly a series of emergency commands were sent from Earth, but to no avail. Ranger 6 crashed into the lunar surface at 9.39° north, 21.51° east at a speed of 2,658 meters per second without returning a single picture.
Lunar Reconnaissance Orbiter image from October 29, 2009 showing the crater made by the impact of the unsuccessful Ranger 6 over 45 years earlier. (LRO/NASA)
Ranger 6 was definitely a very successful engineering test. With the exception of the cameras, all systems worked perfectly. In addition, the navigation accuracy was the best ever attained; the spacecraft impacted the Moon only 31 kilometers from its aim point and only 0.3 seconds before its post-mid-course maneuver predicted impact time. Still, from the point of view of the public as well as the scientific community, this was NASA’s seventh consecutive lunar mission failure. NASA Headquarters formed another board of inquiry to investigate this mishap. The March launch of Ranger B was postponed pending the outcome of this new investigation. The pressure was on NASA, and JPL was fighting for its life.
The NASA investigation into the failure of the Ranger 6 camera package was released on March 17, 1964. The 75-page report pinned the problem squarely on the RCA camera package itself. The completely redundant camera system was found not to be perfectly so. There was a single line that carried commands to both camera chains. Somehow a command was sent to the camera package during ascent that turned it on, hence the anomalous telemetry reading during launch. The cameras were turned on and, in the relatively dense atmosphere, both camera power supplies arced and shorted out.
While the source of the errant command was not known at the time, several changes in the RCA camera package were suggested. These included changes to simplify ground testing and in-flight operation, telemetry system modifications to increase failure mode coverage, inclusion of additional noise suppression in the camera command circuitry, and a more rigorous prelaunch inspection of the television circuitry. These changes also included an interlock that would prevent the cameras from being turned on during launch. In addition, the tower temperature would be lowered by 11° C.
While these changes would further increase Ranger’s chances of success, the blame did not totally lie with Ranger. It was later discovered that the jettisoning of the Atlas booster engines caused Ranger’s cameras to turn on. When the Atlas 199D dropped its booster engines, about 180 kilograms of unburned propellant were expelled and subsequently ignited by the sustainer. This small detonation had caused some problems during the development of the Atlas E/F ICBM but was never a problem for the Atlas D. The detonation wave produced during the flight of Ranger 6 worked its way into a mechanically sealed umbilical door on the Agena. The umbilical pin that controlled the camera package was 6 millimeters from another pin carrying twenty volts. The burning fuel vapor was conductive enough to short the two pins briefly, cause the camera package to turn on prematurely and, as a result, burn out.
Success at last!
The Ranger 7 spacecraft undergoing inspection at JPL. (NASA/JPL-Caltech)
By the summer of 1964, Ranger B had been modified and was ready to be launched during the next launch window in late July. There were some who wanted to target Ranger B close to the impact point of Ranger 6 to observe the crater it produced. Unfortunately, the trajectory constraints of this launch window would not allow an impact that far east. Instead, several targets were considered for the first day of the launch period on July 27 along 7° west longitude between 21° north and 14° south latitude. The launch on this first day was scrubbed due to problems with the ground-based portion of the guidance system. Finally, on July 28, Ranger 7 successfully lifted off only 7.9 seconds into its launch window aimed at 11° south, 21° west in the northwest portion of Mare Nubium.
Launch of Ranger 7 on July 28, 1964 for what would be NASA’s first completely successful lunar mission. (NASA)
With a good injection burn from the Agena B, it was calculated that Ranger 7 would skim over the leading edge of the Moon and impact on its far side. A 50-second course correction burn the day after launch brought the predicted impact point within the intended target area. When Ranger 7 was 2,277 kilometers above the lunar surface traveling at 1,917 meters per second, the F chain cameras were placed into the ninety-second warm-up mode followed later by the P chain. Much to the relief of JPL and NASA officials, pictures from the F chain cameras started streaming back to Earth seventeen minutes and thirteen seconds before impact, followed three minutes and 33 seconds later by the P chain.
First image of the Moon returned by Ranger 7 (or any American spacecraft) on July 31, 1964. (NASA)
By the time Ranger 7 plowed into the lunar surface 68 hours, 35 minutes, and 42 seconds after launch, 4,316 pictures had been transmitted back to Earth. The last image, only a portion of which was transmitted before the probe’s destruction, was acquired at an altitude of only 300 meters, showing features as small as one meter across. Ranger 7 had impacted at 10.7° south, 20.7° west, only 13 kilometers from its aim point. It was the first major American lunar mission success after almost six years of attempts.
The pictures returned by Ranger 7 confirmed that the lunar mare regions are quite smooth and apparently free of major hazards for the Apollo Lunar Module. Because of the size and shape of the craters and the topography observed during the approach, it seemed unlikely that the lunar surface was coated with a deep dust layer that could bury a lunar lander upon touchdown, as some had feared (for more details, see “The Mission of Ranger 7”).
The next mission, Ranger 8, was successfully launched on February 17, 1965. Like its predecessors, it was targeted for the most promising class of Apollo landing sites: the smooth equatorial mare regions. For this mission, the selected aim point was 3° north, 24° east in Mare Tranquillitatis, about 210 kilometers south of the impact point of the unsuccessful Ranger 6.
The launch of Ranger 8 on an Atlas-Agena B from LC-12 at Cape Kennedy on February 17, 1965. (NASA)
After injection into a translunar trajectory, tracking indicated that Ranger 8 would miss the Moon by 1,828 kilometers. This was negated by a 59-second mid-course correction burn at a distance of 159,743 kilometers from the Earth. During the burn, however, controllers were alarmed by a loss of telemetry from the receding spacecraft. Concerned about attempting any more maneuvers, it was decided that Ranger 8 would not perform the terminal descent maneuver to align Ranger’s cameras with its flight path. While this slightly angled viewing geometry would smear the last few images returned by the quickly descending probe, it did offer the opportunity to take a swath of images over a wider area that would partially overlap with the earliest images returned by the previous Ranger 7 mission. Stereo images would also be procured in the process.
One of the images returned by the Ranger 8 television system showing the craters Ritter (left) and Sabine. The craters are each about 30 kilometers in diameter and located in the southern part of Mare Tranquillitatis. (JPL/NASA)
As the probe approached the Moon, the cameras were turned on 23 minutes before impact, almost ten minutes before normally planned. The resolution of these first images was comparable with the best Earth-based telescopic photographs. As Ranger 8 hurtled towards its destruction, the robot craft continued returning a stream of pictures that were very similar to those returned by the previous probe. The maria all seemed to have similar topography and presented no major problems for a landing, human or otherwise. Ranger 8 then crashed into the Moon, producing a 14-meter diameter crater at 2.59° north, 24.77° east, only 23 kilometers southeast from its aim point. Ranger 8 returned a total of 7,137 pictures, the best of which showed features as small as 1.5 meters across. The American lunar program finally seemed to be on the road to success (for more details, see “The Mission of Ranger 8”).
Last image taken by Ranger 8 camera-A from a distance of 4.2 km, 2 seconds before impact on February 20, 1965. The area shown is at 2.7 N, 24.55 E and the image is about 1.4 km across. The right side of the image is missing because Ranger 8 crashed before completing transmission. (JPL/NASA)
By mid-March 1965, the last Block III Ranger spacecraft was being prepared for launch. Unlike its sisters, Ranger D was going to be targeted for more scientifically interesting sites. The first two days of the lunar launch window did not offer any promising targets and no launch attempts were made. A launch on March 21 would allow an impact in the crater Alphonsus, which had shown some signs of outgassing hinting at geologic activity in the recent past. A March 22 launch would land in the bright rayed crater Copernicus. March 23 would allow Kepler to be targeted, while a launch on either March 24 or 25 would permit an impact near Schroter’s Valley, a sinuous rille formed by volcanic activity.
The launch of Ranger 9 from LC-12 at Cape Kennedy on March 21, 1965. (NASA)
As it turned out, Ranger 9 lifted off on its first attempt on March 21, bound for a point at 13° south, 2.5° west, located in the crater Alphonsus. After Agena 6007 completed its ninety-second injection burn, Ranger 9 was heading for a point only 640 kilometers north of its target. A 31-second burn of the course correction engine 38 hours, 26 minutes after launch added the 18.1 meters per second needed to put Ranger 9 back on course.
As the last Ranger was hurtling towards the Moon, the probe aligned its cameras with its flight path. Twenty minutes before impact, controllers sent commands to begin warming the cameras. Starting at an altitude of 2,100 kilometers, Ranger 9 began transmitting the first of 5,814 pictures. The resolution steadily increased to as good as 25 centimeters before the spacecraft slammed into the floor of Alphonsus at 13.3° south, 3.0° west, only 6.5 kilometers from its target (for more details, see “The Mission of Ranger 9”).
A sample of images of Alphonsus returned by Ranger 9. The spacecraft’s impact site is indicated by the small white circle. Click on image to enlarge. (JP/NASA)
Surprisingly, the images returned by Ranger 9 indicated that while the lunar highlands were rougher than the maria, they were still smooth enough to be considered viable landing sites for future lunar landing missions. Tracking of all four Block III Rangers also indicated that the Moon’s geometric center was displaced from its gravitational center. This fact helped to improve the navigational accuracy of future lunar missions. After six years of effort, a total of $267 million in funding (about $2.3 billion in today’s money), much heartache over six failures, and much relief on three eventual successes, NASA’s first major lunar exploration program was over. Efforts now turned to the other two legs of NASA’s unmanned lunar triad, Surveyor and Lunar Orbiter.
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This excellent NASA-produced period documentary, “Lunar Bridgehead”, tells the story of the Ranger 7 mission.
This NASA-produced video prepared by JPL, “Ranger VII Photographs of the Moon”, shows 17 minutes of television images returned by wider-angle “A” camera of the F-chain cameras on Ranger 7 at four-times speed right up to impact.
Here is a film showing the images acquired by the various cameras on Ranger 8 as it approached and impacted the Moon.
This film shows the TV images returned by Ranger 9.
For more articles on Ranger, see the Ranger Mission Page.
R. Cargill Hall, Lunar Impact: The NASA History of Project Ranger, Dover Publishing, 2010
Raymond L. Heacock, “Ranger: Its Mission and Its Results”, TRW Spacelog, Summer 1965
Michael M. Mirabito, The Exploration of Outer Space with Cameras, McFarland, 1983
H. M. Schurmeier, R. L. Heacock, and A. E. Wolf, “The Ranger Missions to the Moon”, Scientific American, January 1966
Paolo Ulivi with David M. Harland, Lunar Exploration: Human Pioneers and Robotic Surveyors, Springer-Praxis, 2004