Ranger 4: NASA’s First (Unintentional) Impact on the Moon

Often forgotten, even by many space enthusiasts today, are NASA’s Block II Ranger missions launched by the Jet Propulsion Laboratory (JPL) in 1962. Frequently confused with the later Block III Ranger lunar impact missions of 1964-65, one of the objectives of the three Block II Ranger spacecraft was not a destructive impact on the Moon, but to hard land an instrument package on the lunar surface – a feat that the Soviet Union would not attempt for the first time until January 1963 with their more complex E-6 Luna spacecraft (see “The Mission of Luna 5” for more details on these first E-6 missions). With the disappointing failure of the first landing attempt, Ranger 3 launched on January 23, 1962 (see “NASA’s Ranger 3: The First Attempt to Land on the Moon”), attention turned to JPL’s Ranger 4 scheduled for launch in April.

The Spacecraft & Its Lander

The main bus of the Block II Ranger consisted of a hexagon-shaped magnesium frame bus 1.52 meters across. The various compartments of this bus contained the spacecraft’s central computer and sequencer which controlled the spacecraft, a battery to provide backup power, a radio transmitter, and the attitude control system. Attitude reference was provided by six Sun sensors, two Earth sensors, and three gyros. Extending from the sides of the bus were two solar panels holding 8,680 solar cells to provide up to 210 watts of electrical power for the spacecraft. Also extending from the base was a hinged, dish-shaped high-gain communications antenna 1.22 meters across, which would be pointed at Earth with the aid of a light sensor. The spacecraft could communicate with the Earth either through this high gain antenna or a cone-shaped omnidirectional antenna at the top using a 3-watt transponder. The spacecraft maintained its attitude using a set of ten gas jets supplied by 1.1 kilograms of compressed nitrogen gas held in three tanks.

Diagram of the Ranger Block II spacecraft designed to hard land a capsule on the Moon. Click on image to enlarge. (NASA)

Also included in the main bus of the Block II Ranger spacecraft was a 16-kilogram, hydrazine-fueled course correction engine which generated 222 newtons of thrust to fine-tune Ranger’s aim as it approached the Moon. This engine could be fired for a maximum of 68 seconds, giving a total velocity change of 44 meters per second. Since any torques imparted during this engine’s operation could not be negated using the small attitude control jets, this engine was fitted with steering vanes at the exit nozzle.

A cutaway diagram showing the major components of the Ranger Block II lander package. Click on image to enlarge. (NASA)

Mounted on top of the main bus was a 149-kilogram package consisting of a small hard lander with a 97-kilogram retrorocket that provided 22.6 kilonewtons of thrust. The hard lander, built by the Aeronutronic Division of the Ford Motor Company in Newport Beach, California, was a 64-centimeter diameter sphere with a mass of 43 kilograms. The exterior was composed of balsa wood designed to absorb the force of impact. Inside this impact limiter was a smaller 31-centimeter, 26-kilogram sphere that housed the lander’s systems. The primary instrument carried by the lander was a seismometer sensitive enough to detect the impact of a 2.3-kilogram meteorite on the opposite side of the Moon. The sensitive components of the seismometer were protected from the impact forces by a cushion of liquid heptane. Also included in the capsule was a 50-milliwatt L-band transmitter, six silver-cadmium batteries, and a temperature-sensitive voltage oscillator. The lander was designed to survive an impact of 67 meters per second.

The hard lander’s interior temperature was controlled by a capsule containing 1.7 kilograms of water. During the hot lunar day, the interior would heat up to 30° C, at which point the water would start to boil under the ambient conditions. The temperature would rise no further until all the water had boiled away. During the cold lunar night, the warmed water, along with the heat generated by the lander’s internal electronics, would keep the interior temperature above freezing. The battery-powered lander package had an expected 20 to 30-day lifetime on the lunar surface.

Photograph of the television camera carried by the Block II Ranger missions. (JPL)

The bus of the Block II Ranger also carried its own set of instruments. A radar altimeter provided ranging information for lander deployment as well as data on the lunar surface’s radar characteristics. A gamma ray spectrometer sensitive to the 0.1 to 2.6 MeV energy range was mounted on an extendable 1.8-meter boom. This would allow the characterization of gamma rays being emitted by the surface of the Moon allowing the concentration of key radioactive elements to be determined. The science package was rounded out by a slow-scan television camera which would return images during the final 40 minutes before impact with the lunar surface. The camera was fitted with a JPL-designed 1020-millimeter focal length lens which provided a 0.65° field of view in combination with the vidicon tube and supporting electronics manufactured by RCA. The image would be broken up into 200 scan lines (just 40% of the NTSC video standard of the day) and transmitted in analog form back to Earth in ten seconds. Following a three-second interval to erase the latent image from the camera’s vidicon plate (when other data would be transmitted), the camera would snap another image and begin transmission. Over 150 images were expected to be transmitted before the lander would ignite its retrorocket. For the Block II Ranger missions, the highest priority was given to the gamma ray spectrometer followed by the TV camera and the hard lander.

The assembly of the hard lander at Ford Aeronutronic taking place under sterile conditions. (NASA)

In order to minimize the chances of Earth organisms reaching the Moon (which was still a concern in some circles during these early days), the spacecraft components were sterilized first by baking them for 24 hours at 125° C and then all its parts were cleaned with alcohol before they were assembled. Finally, the entire spacecraft was saturated with ethylene oxide gas for 24 hours while in its launch faring.

An artist’s depiction of spacecraft sterilization on the pad using a gas like ethylene oxide. (NASA)

The launch vehicle which would send the Block II Ranger and its lander to the Moon was the Atlas-Agena B. The Atlas-Agena launch vehicle family was originally designed to place large payloads, such as the MIDAS experimental early warning satellite and the SAMOS reconnaissance satellite, into medium altitude Earth orbits. The three-meter in diameter Atlas D booster was a modified version of the ICBM built by Convair. In order to handle heavier payloads, the forward bulkheads of the rocket were stiffened and the Atlas’ original MA-2 propulsion systems was replaced with the uprated MA-3 system being used on the improved Atlas E/F silo-based ICBM then under development. This boosted the liftoff thrust of the 30-meter-tall Atlas-Agena B to 1,820 kilonewtons.

Diagram showing the Atlas-Agena B launch vehicle and the Block II Ranger spacecraft. Click on image to enlarge. (NASA)

The Agena B upper stage, built by Lockheed Missiles and Space Company (a corporate antecedent to today’s aerospace giant, Lockheed Martin), was an upgraded version of the original Agena A first flown in 1959 (see “The First Discoverer Missions: America’s Original (Secret) Satellite Program”). While the B-model kept the original 1.5-meter diameter of the stage, it was lengthened by two meters to 6.30 meters to support a larger propellant load. The original A-model’s Bell Aerospace Hustler 8048 engine was replaced with an upgraded 8081 which generated 71 kilonewtons of thrust and possessed an in-orbit restart capability. This capability allowed the Agena ignite at exactly the right moment during its coast in an Earth parking orbit to ensure an accurate injection into its desired trajectory. The Atlas-Agena B was capable of launching payloads of 2,300 kilograms into a 480-kilometer orbit or up to about 330 kilograms towards the Moon.

Diagram showing the major features of the Agena B upper stage used to launch NASA’s early lunar and planetary spacecraft. Click on image to enlarge. (NASA)

The Mission Plan

There were many variables involved in choosing a proper launch window for the Block II Ranger. First, the length of the trip to the Moon was set to about 66 hours to maximize the payload while ensuring that Ranger would be in clear view of the Goldstone tracking antenna in California’s Mojave Desert— the most sensitive in NASA’s network of tracking stations — when it reached the Moon. Ranger also had to approach the Moon almost vertically at a precise speed because of the fixed velocity increment of the lander’s retrorocket. Because of the imaging requirements and the position of celestial references, the landing could only take place on the Moon’s visible face during a four- or five-day period centered on the Moon’s last quarter phase. Finally, the requirement that the hard lander’s antenna have Earth in view meant that it could not be placed more than 45° from the center of the Moon’s visible side. All of these constraints limited impact sites to near the lunar equator in the eastern part of the mare known as Oceanus Procellarum.

A schematic showing a typical Ranger launch window to the Moon. Click on image to enlarge. (NASA)

A typical mission sequence for the Block II Ranger started with the modified Atlas D rocket placing the Agena B upper stage and Ranger spacecraft into a parking orbit after a short burn from the Agena B. Following a preprogrammed time delay, the Agena B would reignite to place the spacecraft on a path to the Moon. Once its job was completed, the Agena B would separate from the probe and fire small retrorockets to distance itself from the Ranger. About five minutes after separation and around 48 minutes after launch, Ranger would then unfold its solar panels and high-gain antenna to begin its search for its first attitude reference, the Sun. Once acquired, Ranger would switch from its battery to its solar panels for power. It would then begin a slow roll until its antenna locked on to Earth, its final reference point, about four hours after launch.

The sequence of events for Ranger’s midcourse correction maneuver. Click on image to enlarge. (NASA)

If its trajectory needed refining, Ranger would be commanded to make a single mid-course correction about 15 hours after launch at a distance of around 146,000 kilometers to ensure a lunar impact at the proper location and terminal velocity. During this time, the internal gyroscopes were used as an attitude reference as the spacecraft was pointed in the required direction. After the burn and cruise attitude had been reestablished, the relatively fragile gamma ray spectrometer boom would be deployed.

The sequence of events leading to the touchdown of the Ranger hard lander. Click on image to enlarge. (NASA)

As Ranger approached the Moon, it would begin its terminal descent maneuver. The spacecraft would switch to its internal battery and turn so that its bottom was aligned with the Moon. After the high-gain antenna was once again pointed at Earth, the probe would begin to acquire television images starting about 32 minutes before impact at an altitude of 3,900 kilometers. Images would be taken every 13 seconds down to an altitude of 59 kilometers. Transmission of this last image, which was expected to reveal features as small as 3 meters across, would have been completed as the probe reached an altitude of only 24 kilometers.

Only 8.1 seconds before the bus crashed into the lunar surface at a speed of 2,900 meters per second, the radar altimeter would generate a fusing signal at an altitude of 21.4 kilometers. At that moment, bolt cutters would free the hard lander and retrorocket from the bus. A three-nozzle spin motor would fire, lifting the package 0.8 meters above the bus and imparting a spin of 300 RPM. The retrorocket would then fire, slowing the capsule to a virtual stop at a height of only 335 meters above the lunar surface. Explosive bolts would cut the clamp holding the lander to its retrorocket and the two would be separated by springs. The hard lander would then free fall to the surface with an impact speed of 45 meters per second, give or take 9 meters per second.

An artist’s depiction of the Block II hard lander firing its retrorocket before landing. (NASA)

Protected from the force of impact by its balsa wood shell, the lander would roll to a stop. The capsule floating freely inside the shell on a layer of liquid Freon was made to be bottom heavy so that it would settle into a horizontal position. After twenty minutes, plugs are blown out, allowing the 225 milliliters of liquid heptane protecting the seismometer as well as the Freon to evaporate into the lunar vacuum, thus fixing the capsule in place and allowing the single-axis seismometer to operate correctly. The package would then transmit its findings on lunar seismic activity for the next month. If it worked, the United States would have the first high-resolution pictures of the Moon as well as the first hard landing on its surface.

Preparing for the Mission

One of the more vexing issues for the Ranger program centered on the 6000-series Agena B stages Lockheed was building specifically to support NASA missions. The first two launches of the Ranger program involved Block I engineering prototypes that were to test the spacecraft’s systems in a highly extended Earth orbit. Ranger 1, launched on August 23, 1961, was stranded in a low Earth orbit when Agena 6001 failed to reignite properly for its second burn because of an overheated pressure switch circuit. Agena 6002 fared no better with the launch of Ranger 2 on November 18 when a malfunction in its roll gyro prevented a second burn to send the spacecraft into its high apogee orbit (see “The Prototype That Conquered the Solar System”). Modifications were made to the Agena B to correct these and other problems encountered during early USAF flights of the new upper stage.

Agena 6001 being readied for stacking at LC-12 on July 25, 1961 in preparation for the launch of Ranger 1 (Convair/SDASM)]

The problems with the Agena B continued with the first Block II mission to land on the Moon, Ranger 3. Launched on January 26, 1962, Ranger 3 encountered trouble 49 seconds after liftoff when the radio guidance link to the Atlas 121D booster was lost. Forced to rely on its less accurate autopilot as a backup, the launch vehicle was travelling higher and faster than desired when Agena 6003 made its first burn. Without a proper guidance update and as a result of an incorrect constant used by the Agena’s velocity meter which guaranteed an overburn, Ranger 3 was placed in the wrong trajectory. The excess velocity meant that Ranger 3 would miss the Moon by 39,000 kilometers – far in excess of the spacecraft’s midcourse correction capability which was the equivalent of about 10,000 kilometers. New procedures were established to recheck the velocity meter calculations and programming in future flights.

Launch of Ranger 3 on January 26, 1962 from LC-12 using Atlas 121D/Agena 6003. (NASA/KSC)

To compound the issues with the Atlas-Agena B, the Ranger 3 spacecraft and its mission operations also experienced a frustrating series of problems. When ground controllers commanded Ranger 3 to perform a midcourse maneuver 14 hours after launch to optimize probe’s path past the Moon to support an improvised flyby imaging mission, an undetected sign inversion in the uplinked commands pointed Ranger’s engine in the wrong direction. Ground controllers were also experiencing issues deriving an accurate path for the outbound spacecraft due to excessive noise in the tracking data. Most disappointing of all, the probe’s central computer and sequencer failed just as Ranger 3 was about to begin imaging the Moon (see “NASA’s Ranger 3: The First Attempt to Land on the Moon”).

Five months before the flight of Ranger 3, assembly of the P-35 spacecraft that would become Ranger 4 had started on August 22, 1961 at JPL. Despite a few issues encountered following heat sterilization, like a slight warping of the spacecraft’s frame, power-on testing commenced on September 26. Meanwhile, assembly of the Agena 6004 stage at Lockheed was completed on November 7 followed by a series of preshipment tests.

Technicians working on the Ranger 4 spacecraft. (NASA)

Not long after P-35 completed its final systems checks at JPL on February 4, 1962, a series of changes were recommended based on the experience with Ranger 3. Although the malfunctions in the Ranger 3 spacecraft could not be reproduced on the ground with a proof test model, the heat sterilization procedure was suspected of contributing to the failure in the probe’s central computer and processor. All new components added to the central computer and processor of P-35 would no longer require heat sterilization. Because of elevated temperatures the Block II landing package experienced during the flight of Ranger 3, a new saw-tooth paint pattern was added to the lander along with a new aluminized Mylar thermal curtain design that shielded the package from direct sunlight during the flight. This new thermal curtain, along with issues with transmission strength during the Ranger 3 mission, necessitated changes to the conical omnidirectional antenna on top of the spacecraft to modify its transmission pattern. The P-35 spacecraft was shipped from JPL on February 19 and arrived at Cape Canaveral a week later.

Arrival of Atlas 133D at Cape Canaveral on March 17, 1962 in preparation for the Ranger 4 mission. (Convair/SDASM)

As engineers unpacked and tested the next Ranger spacecraft, preparations for the launch vehicle were well on their way. Following the completion of final systems checks, Agena 6004 was accepted by the USAF on March 1 clearing the way for its shipment. On March 17, the Atlas 133D booster arrived at Cape Canaveral and was erected on the pad at Launch Complex 12 (LC-12) two days later. Following the addition of the Agena 6004 stage, testing of the launch vehicle began. Final assembly and preflight preparations of the spacecraft were completed on March 28. Meanwhile, modifications to the deep space station in Johannesburg, South Africa were completed during March allowing the station to command the Ranger spacecraft in the first hours of the flight to the Moon. The lack of this capability complicated the recovery of the wayward Ranger 3 back in January. The number of tracking ships was also increased from one to four in order to provide better telemetry coverage of the Atlas-Agena B flight.

The Ranger 4 spacecraft being prepared for launch at Cape Canaveral. (NASA)

The target point for the Ranger 4 lander was not far from that of Ranger 3 just south of the lunar equator near the eastern rim of Oceanus Procellarum. The launch window to reach this target under the lighting conditions required for the lander and camera opened on April 21 but this conflicted with a scheduled launch by the USAF. With Easter Sunday in 1962 falling on April 22, the decision was made to make the first launch attempt on Monday, April 23. On April 3, final assembly and preflight sterilization using ethylene oxide gas was completed and the spacecraft was sealed inside its launch shroud. The encapsulated Ranger 4 was mounted on top of its Atlas-Agena B launch vehicle for a round of testing which were completed on April 9. Following removal of the spacecraft from LC-12 for its final prelaunch preparations, the Flight Readiness Test was completed on April 20 with Ranger 4 added back to the stack. Everything was now ready for launch.

Ranger 4 being prepared for launch from pad at LC-12. (Convair/SDASM)

The Mission

The countdown for the launch of Ranger 4, which started during the morning of April 23, 1962, proceeded without incident with lift off from LC-12 coming at 3:50:15 PM EST (20:50:15 GMT). Unlike during the ascent of Ranger 3 three month earlier, Atlas 133D performed as expected with good radio guidance until the shutdown of the booster’s engines. Following the separation of the Agena B from the Atlas and the jettisoning of the payload’s launch shroud, the Agena coasted for a short time with its attitude maintained to the local horizontal by the stage’s horizon sensors and gyros. After the initial burn of its engine that ended 8 minutes and 18 seconds after launch, Agena 6004 and its Moon-bound payload were traveling 7,800 meters per second in a 185-kilometer-high parking orbit. Downrange tracking of the launch vehicle and the spacecraft showed that all systems were operating as expected.

The launch of Ranger 4 from LC-12 on April 23, 1962. (NASA)

After coasting in its parking orbit for four minutes and 14 seconds, Agena 6004 reignited its main engine for the final burn which would propel Ranger 4 to the Moon. At the end of the Agena’s second burn almost exactly 14 minutes after launch, it was 197 kilometers above the Earth travelling at 10,958 meters per second. Subsequent tracking showed that Ranger 4 had been placed into a geocentric 193 by 600,036-kilometer orbit with an inclination of 29.7° which would intercept the Moon on its outbound leg, even without the planned course correction. The Atlas-Agena B had performed almost flawlessly for a NASA mission after three earlier failed attempts. Following injection, Agena 6004 separated from Ranger 4, made a preprogrammed yaw maneuver and fired its retrorockets in order to move away from the spacecraft and prevent any interference.

This map shows the ground track of the Ranger 4 spacecraft for the first 25 hours after launch. About 1½ hours after launch, the angular rate of the receding Ranger 4 became slower than that of the rotating Earth resulting in the ground track shifting westward. Click on image to enlarge. (NASA/JPL)

About 23 minutes after launch, Ranger 4 rose above the horizon at the tracking facility in Johannesburg, South Africa. Ranger 4 had powered up its UHF transmitter but no telemetry was being received, just an unmodulated carrier signal. The health of the spacecraft could not be determined save for a slow tumble inferred from the slowly varying transmission strength from the spacecraft. The solar panels and high gain antenna had not deployed and the commutator, which switched the telemetry input sequentially from one source to another, was not functioning. A series of commands were sent from Johannesburg in order to troubleshoot the problem but to no avail. Ranger 4 was unresponsive.

Apparently, the clock in the central computer and sequencer had failed and, without that 25 pulse per second signal, the commutator, command decoder and other equipment could not function. It was believed that debris had somehow shorted a couple of pins in the spacecraft’s separation connector disabling the computer in the process. The malfunction meant that there would be no scientific data returned from this flight. Without the solar panels to recharge them, Ranger’s batteries were finally depleted at 07:32 GMT on April 24 about 10½ hours after launch. With the main spacecraft dead, tracking continued using the L band beacon onboard the small lander.

This diagram illustrates the trajectory of Ranger 4 to the Moon. Because of the accuracy of the injection by the Atlas-Agena B, an impact on the lunar surface was guaranteed even without a midcourse correction. Click on image to enlarge. (NASA/JPL)

Tracking showed that Ranger 4 would impact on the far side of the Moon about 64 hours after launch. The one bright spot was that the Atlas-Agena B had injected its payload accurately – good news not only for the Ranger program but for the Mariner flights to Venus to be launched using the Atlas-Agena B in July and August. The only other consolation was that Ranger 4 would have the distinction of being the first American payload to impact the Moon three years after the Soviet Luna 2 mission (see “The First Race to the Moon: Reaching Our Neighbor”).

This diagram illustrates the approach trajectory of Ranger 4 resulting in an impact on the far side of the Moon. Click on image to enlarge. (NASA/JPL)

Tracking of Ranger 4 showed that it was on a hyperbolic approach trajectory inclined 13.5° to the lunar equator. The signal from Ranger’s lander was lost at 12:48:50 GMT on April 26 when the spacecraft passed behind the limb of the Moon at an altitude of 965 kilometers and out of sight from the Earth. Impact occurred at 12:50:00 GMT with a speed of 2,669 kilometers per second at 15.5° south, 130.7° west in the 124-kilometer wide, far side crater known today as Paschen after German physicist, Friedrich Paschen (1865-1947). NASA and JPL would have only one more chance to land a Block II Ranger on the Moon in October before the program transitioned to the Block III impact missions.

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Related Video

Here is a brief newsreel about the Ranger 4 mission (as well as the successful launch of the second Saturn I on April 25, 1962).

Related Reading

For more articles, see the Ranger Program page.

References

Sarah A. Grassly, “Agena Flight History as of 31 December 1967 Volume 1”, SMEH-100, Air Force Systems Command Office of Information, June 1969

R. Cargill Hall, “Project Ranger: A Chronology”, JPL HR-2, April 1971

R. Cargill Hall, Lunar Impact: The NASA History of Project Ranger, Dover Publishing, 2010

T. W. Hamilton, W.L. Sjogren, W.E. Kirhofer, J.P. Fearey, and D.L. Cain, “The Ranger 4 Flight Path and Its Determination from Tracking Data”, JPL Technical Report No. 32-345, September 15, 1962

Paolo Ulivi with David M. Harland, Lunar Exploration: Human Pioneers and Robotic Surveyors, Springer-Praxis, 2004

“Space Flight Operations Memorandum – Ranger IV”, JPL EPD-91, July 5, 1962