Vintage Micro: The Pioneer 4 Lunar Probe

Not all that long ago, the web was filled with articles and postings in celebration of the anniversary of the Apollo 11 Moon landing. What is often forgotten is that just a decade earlier, the US was struggling to get its first lunar probes off the launch pad. And unlike the Apollo 11 spacecraft with a total mass of almost 44 metric tons, these first lunar probes launched by NASA had masses as small as six kilograms – just nanosatellites by today’s standards. But despite their tiny size and the comparatively crude nature of their technology, they were trailblazers for the successes that were to follow years later. They should also serve as an inspiration for today’s mission planners and satellite builders with much more advanced technology at their disposal of what could be done with nano-size probes to the Moon or beyond.

The First “Nano” Lunar Probes

The project that would launch what would become known as Pioneer 3 and 4 towards the Moon had its origin in a proposal by the Jet Propulsion Laboratory (JPL) and Army Ballistic Missile Agency (ABMA which would later become NASA’s Marshall Space Flight Center) to launch a pair of small space probes on escape trajectories using a modified version of the Jupiter IRBM during the International Geophysical Year – a cooperative international scientific study of the Earth and its interaction with the Sun running from July 1957 to December 1958. JPL and ABMA were already involved in a number of cooperative space-related programs at this time including one that culminated in the launch of America’s first satellite, Explorer 1, on January 31, 1958 using a highly modified Redstone rocket known as the Juno I launch vehicle. After the launch of the Soviet Union’s first two Sputnik satellites in the fall of 1957, development of a pair of lunar probes using what would become the Juno II launch vehicle started in November 1957 under the codename “Red Socks”.

Diagram showing the high speed stage and trajectory to the Moon for JPL’s Project Red Socks proposal. Click on image to enlarge. (JPL)

For the first stage of the Juno II launch vehicle, Jupiter’s kerosene and LOX tanks retained their original 2.67 meter diameter but were lengthened by a total of 0.92 meters making the rocket 16.84 meters long. This prolonged the burn time of the 668 kilonewton thrust Rocketdyne S3D engine by 20 seconds to a total of 182 seconds. Mounted on top of the modified Jupiter first stage under an aerodynamic shroud was the “high speed assembly” consisting of an instrument compartment and a three-stage solid rocket cluster developed by JPL similar to that used on ABMA’s Juno I rocket. This cluster of rocket motors was a spin-stabilized tub of 11 scaled-down JPL Sergeant rockets with seven used for the second stage, three for the third and a single motor for the fourth stage. Modifications of this cluster from the version used on the Juno I and earlier Jupiter-C used for high speed reentry tests included filling the third and fourth stages with a higher performance propellant and changing the original stainless steel casing of the fourth stage to a lighter weight titanium casing. The Juno II was designed to place 43 kilograms of payload in a 480-kilometer high Earth orbit or up to 7 kilograms of useful payload on a direct ascent escape trajectory.

Diagram of the Juno II lunar mission launch sequence and the “Juno IIA” payload. Click on image to enlarge. (JPL/NASA)

On March 27, 1958 President Eisenhower approved a plan for the Defense Department’s new Advance Research Projects Agency (ARPA) to launch a series of five missions to the Moon over the following year as part of “Operation Mona” with the hope of beating the Soviet Union to the Moon. The first missions were a USAF proposal to use their Thor-Able rocket, which had been originally developed for high speed reentry tests, to launch three probes with the ambitious goal of orbiting the Moon. Following these missions, the more modest ABMA-JPL lunar probes would be launched on lunar flyby missions.

An exploded view of an unflown Juno IIA payload (serial no. 3) currently in the Smithsonian National Air & Space Museum collection. (NASM)

On May 2, 1958 ABMA officially contracted JPL to develop and build four of the tiny lunar probes of which two would be launched on missions designated “Juno IIA” and “Juno IIA-Prime”. Weighing just 6.67 kilograms each (which would qualify them as “nanosatellites” by today’s definition), the probes were a 23-centimeter wide cone with a 8-centimeter spike antenna on the top. They were 51 centimeters long with an exterior housing constructed of gold-washed fiberglass. The exterior was gold plated and striped with paint for passive thermal control. The electrically conductive gold plating on the cone also served as an unsymmetrical dipole antenna element in conjunction with the spike antenna. At the squat cylindrical base of the probe was a despin mechanism consisting of two 1.5-meter long weighted wires. A hydraulic timer would release the wires ten hours after launch. As the wires unwound, the payload’s spin rate would decrease from 400 to 11 revolutions per minute. Located inside the probe was a 500-gram UHF transmitter operating at a frequency of 960 MHz with an effective power of 180 milliwatts. The power supply for the transmitter and instruments consisted of a set of eighteen mercury cells.

The 26-meter tracking antenna at the Goldstone Station that was used on early lunar and planetary missions as it appeared shortly after its completion in 1958. (JPL/NASA)

Tracking of the probes would be performed using a tracking facility employing a 3-meter antenna in Mayaguez, Puerto Rico during the early phases of the mission. As the probe receded farther from the Earth, a 26-meter antenna at the new Deep Space Instrumentation Facility known as the “Goldstone Station” at Camp Irwin in California’s Mojave Desert would be used. This latter facility would become the basis of the Deep Space Tracking Network that would communicate with NASA’s lunar and planetary spacecraft for decades to come. The probe’s coordinates, Doppler velocity and telemetry data would be channeled directly from these tracking stations to JPL in Pasadena, California for reduction and analysis.

Originally these JPL-built lunar probes were to carry a tiny photographic package capable of obtaining a single photograph of the Moon’s far side during a lunar flyby. A photoelectric triggering device would trip the camera’s shutter when the Moon was in the detector’s field of view and closer than 32,000 kilometers . But with the discovery of the Van Allen radiation belts surrounding the Earth by the first Explorer satellites during 1958, the JPL Moon probes’ primary instrument was changed to a pair of Geiger-Mueller tubes to obtain data on the radiation environment between the Earth and Moon. The original photoelectric triggering device was retained as an engineering test for future systems.

Cutaway diagram of the Juno IIA lunar probe showing its major components. Click on image to enlarge. (JPL/NASA)

With the new instrument, the mission of this probe was also changed. Instead of a close flyby, the probe was now intended to impact the lunar surface 33 hours and 45 minutes after launch when the Moon was best positioned to be seen by the Goldstone tracking station. But given the inherent inaccuracy of direct ascent trajectories, the relatively crude nature of the Juno II solid rockets and guidance system, as well as the lack of any course correction capability, the JPL-ABMA probes would be lucky to make it anywhere near the Moon never mind hit it. Nonetheless, at this early stage of space exploration a lunar flyby was just as valuable scientifically as a direct hit. Since ABMA had to wait until after the USAF probe launches, September of 1958 was initially set as the tentative launch date for their first Moon shot. With luck the ARPA-sponsored Moon probes, in addition to providing valuable IGY data, would get America ahead of the Soviet Union in the Space Race.

The Pioneer 3 & 4 Missions

As was originally planned, the first of the ARPA-sponsored lunar missions was the USAF-built probes. The first Thor-Able carrying a lunar orbiter was launched on August 17, 1958 but exploded only 77 seconds into its flight due to an engine failure on the Thor first stage (see “USAF Project Able-1: The First Attempt to Reach the Moon“). But before the second launch was attempted, control of all of the ARPA-sponsored scientific space projects including the lunar probes was transferred to the newly formed NASA in October with the military relegated to an advisory role. NASA’s first official space mission, the second USAF Thor-Able lunar orbiter, was launched on October 11, 1958 and subsequently designated Pioneer 1. Unfortunately, Pioneer 1 ended up with a 152 meter per second velocity shortfall and reached a peak altitude of only 114,000 kilometers before falling back to Earth 43 hours after launch (see “Pioneer 1: NASA’s First Space Mission“). The final lunar orbiter mission was launched on November 7 but Pioneer 2, as it was called, reached a peak altitude of only 1,550 kilometers due to an ignition failure of the Thor-Able third stage.

The first of the JPL-ABMA lunar probes, the Juno IIA mission, lifted off from Pad 5 at the Atlantic Missile Range at 5:44:52.3 UT on December 6, 1958 – just four seconds after the optimum launch time in its 50-minute launch window. While at first the launch of what was designated Pioneer 3 looked good, a review of the telemetry showed that the Jupiter AM-11 booster had cut-off 3.7 seconds too early due to a failure in the tank depletion sensors and that the trajectory was lower than planned. Like its USAF predecessors, Pioneer 3 failed to reach Earth’s escape velocity. Early telemetry from Pioneer 3 also indicated that its despin mechanism failed to operate as intended leaving the probe spinning at the rate of 415 rpm instead of the slower 11 rpm required for the mission.

Diagram showing the actual trajectory of Pioneer 3 through Earth’s Van Allen radiation belts with the distances measured from the center of the Earth. Click on image to enlarge (JPL/NASA)

Pioneer 3 reached a peak altitude of only 102,300 kilometers before it arced back to Earth and burned up over what was then French Equatorial Africa 38 hours and 6 minutes after launch. Despite the failure, Pioneer 3 still made useful measurements that confirmed the extent of Earth’s Van Allen radiation belt and discovered a second belt between 16,000 and 64,000 kilometers above the Earth. While scientifically important, it still did not make up for the fact that yet another American spacecraft failed to reach the Moon (see “Pioneer 3: JPL’s First Moon Shot Attempt“).

With the failure of Pioneer 3 to reach the Moon, two additional probes – serial numbers 3 and 4 which were flight spares from the original Juno IIA mission – were modified and prepared so that one could be launched on the Juno IIA-Prime mission. But before this mission could be launched, the Soviet Union scored yet another space first. After three unsuccessful launch attempts on September 23, October 12 and December 4, 1958, Soviet engineers finally succeeded in launching an E-1 lunar probe called “Metcha” or “Dream” in Russian (and later better known as Luna 1) on January 2, 1959. Although the intended mission of the 192-kilogram E-1 No. 4 was to impact the Moon, it managed to pass within 6,000 kilometers of the lunar surface 34 hours after launch before heading out into solar orbit. Tracked out to a range of half a million kilometers before its batteries gave out 28 hours later, Luna 1 was able to beat its much smaller American competitor to the Moon.

Pioneer 4 mounted on the top of the fourth stage of its Juno II rocket before it was encased in its aerodynamic shroud prior to launch. (MSFC/NASA)

The success of the Soviet Luna probe had an incredible impact on NASA. Under increasing pressure, the Juno IIA-Prime mission just had to succeed. Except for some additional lead shielding on one of the Geiger-Muller tubes and other modifications to improve the performance of the telemetry system, the 6.08-kilogram probe for the Juno IIA-Prime mission was identical to its unsuccessful predecessor. The tiny probe, serial number 4 (number 3 was unlaunched because of last minute issues uncovered during testing), was finally sent on its way by Juno II Round AM-14 at 5:10:56.7 UT on March 3, 1959 and was designated Pioneer 4.

The successful launch of Pioneer 4 towards the Moon on Juno II AM-14 in the early morning hours of March 3, 1959. (MSFC/NASA)

A slightly longer than planned burn of the second stage along with a nearly nominal performance of the other stages guaranteed this time that Pioneer 4 had surpassed escape velocity. But calculations based on early tracking of the receding probe quickly showed that this excess velocity and the inevitable aiming errors conspired to place Pioneer 4 on a trajectory that would not pass within 32,000 kilometers of the Moon as planned. Instead it would pass the Moon at a distance of 60,000 kilometers or around 35 lunar radii – a wide miss by any measure.

The trajectory of Pioneer 4 past the Moon. Click on image to enlarge. (NASM)

On the afternoon of March 4, 41½ hours after launch, Pioneer 4 passed the Moon and continued to relay its measurements back to the 26-meter tracking antenna at the Goldstone Station. After 82 hours of operation, the probe’s batteries were finally exhausted as Pioneer 4 passed a range of 655,000 kilometers on its way into solar orbit – a new long distance communications record at the time. Engineers were confident that they could have tracked Pioneer 4 out to a range of 1.1 million kilometers if the batteries had not given out.  Although it was quite small and failed to beat the Soviet Union to the Moon, Pioneer 4 was a much needed success. Unfortunately, it would be another 5½ years before the next NASA lunar mission succeeded with the flight of Ranger 7 (see “The Mission of Ranger 7”).

Related Video

Here is an excellent NASA short-film provided by the Smithsonian National Air & Space Museum about the Pioneer 4 mission to the Moon.

Related Reading

“USAF Project Able-1: The First Attempt to Reach the Moon”, Drew Ex Machina, August 15, 2022 [Post]

“Pioneer 1: NASA’s First Space Mission”, Drew Ex Machina, October 11, 2016 [Post]

“Pioneer 3: JPL’s First Moon Shot Attempt”, Drew Ex Machina, December 6, 2022 [Post]

“The Mission of Ranger 7”, Drew Ex Machina, July 28, 2014 [Post]

General References

M. Eimer, A.R. Hibbs and R. Stevens, “Tracking the Moon Probes”, in Space Research: Proceedings of the First International Space Science Symposium, edited by Hilde Kallmann Bijl, Interscience Publishers, pp. 518-531, 1960

Conrad S. Josias, “Radiation Instrumentation Electronics for the Pioneers III and IV Space Probes”, Proceedings of the IRE, Vol. 48, No. 4, pp. 735-743, April 1960

William H. Pickering, “History of the Juno Cluster System”, in Astronautical Engineering and Science, edited by Ernst Stuhlinger, Frederick I. Ordway III, Jerry C. McCall, and George C. Bucher, McGraw-Hill Book Co., pp. 203-214, 1963

Robert Reeves, The Superpower Space Race, Plenum Press, 1994

Allen E. Wolfe, “Juno Final Report Volume II Juno II: Space Probes”, JPL Technical Report No. 32-31, September 12, 1961