Vintage Micro: The Second-Generation Explorer Satellites

While interest in miniaturized satellites for a range of applications has been growing in recent years, as a class these satellites are hardly new. By necessity, America’s first satellites would be categorized today as “microsatellites” because of the limited payload capability of the first satellite launch vehicles. Despite their small size and employing technology that would be considered primitive by today’s standards, these early satellites were able to accomplish much. Perfect examples of this would be NASA’s “second generation” of Explorer satellites launched between 1959 and 1961 by the ABMA (Army Ballistic Missile Agency) which addressed important questions about the Earth, the space environment and the universe beyond.


The Beginnings

In the chaos that swept the United States after the launching of the first Soviet Sputniks, a number of separate satellite programs were sponsored by the Department of Defense (DoD) to supplement (and in some cases supplant) the country’s flagging “official” satellite program called Vanguard (see “Vintage Micro: The Original Standardized Microsatellite” and “Vintage Micro: The Original Nanosatellite”). One of the stronger programs was sponsored by ABMA headquartered in Huntsville, Alabama (which would become NASA’s Marshall Space Flight Center in July 1960) with its engineering team led by the German rocket expert, Wernher von Braun. Using a highly modified Redstone rocket called the Juno I, the ABMA team launched America’s first satellite, Explorer 1, which was built by Caltech’s Jet Propulsion Laboratory (JPL) (see “America’s First Satellite… Almost“).

From left to right, William Pickering (then head of JPL), James Van Allen and Wernher von Braun triumphantly holding a model of the Explorer 1 satellite during a post-launch press conference. (NASA/JPL)

While Explorer 1 and the Explorer-series satellites which followed returned a wealth of new data, they were limited by the tiny 11 kilogram payload capability of the Juno I launch vehicle. In order to orbit heavier payloads carrying a larger range of instrumentation, von Braun’s team developed the Juno II. While the Juno I upper stage cluster of solid rocket motors was retained, the Juno II used a modified Jupiter IRBM instead of the smaller Redstone as a first stage. As 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.

Diagram of the Juno II launch vehicle. Click on image to enlarge. (NASA)

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 (the latter of which did not include the single motor fourth stage) 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. This new combination was first used to launch the Pioneer 3 and 4 lunar probes in December 1958 and March 1959, respectively (see “Vintage Micro: The Pioneer 4 Lunar Probe”).

Diagram illustrating the upper stages of the Juno II launch vehicle with a second-generation Explorer satellite. Click on image to enlarge. (NASA)

ABMA planners started working with JPL under the aegis of ARPA (Advanced Research Projects Agency) to develop a second generation of new and larger Explorer-series satellites to fly on the Juno II. But long before the first of these new satellites was even launched, political decisions changed the landscape of America’s fledgling space program. With the formation of NASA in October of 1958, all ARPA-sponsored space science satellite programs were eventually transferred to the new space agency. Among these programs were the second-generation Explorer satellites that ABMA and JPL were developing.


The First Launches

The first of the new series of larger Explorer satellites was the 39.7-kilogram satellite NASA designated as S-1. The spin stabilized S-1, similar to the other JPL-built payloads of this series, consisted of a pair of fiberglass cones joined at their bases with the weight distributed so that the satellite would spin stably in orbit after separation from the final stage of the Juno II. The S-1, with a diameter and height of 76 centimeters each, had a scientific payload consisting of instruments to study cosmic rays, solar X-ray and ultraviolet emissions, micrometeorites, as well as the globe’s heat balance. This wide ranging payload and its support systems were powered by a bank of 15 nickel-cadmium batteries recharged by 3,000 solar cells mounted on the satellite’s exterior. This advanced payload was equipped with a timer to turn itself off after about a year in orbit.

The failure of Juno II AM-16 on July 16, 1959 carrying Explorer S-1 is one of the most spectacular in the early history of Cape Canaveral. (NASA)

Explorer S-1 lifted off from Launch Complex 5 at Cape Canaveral, Florida on July 16, 1959 on Juno II Round AM-16. Immediately upon launch an electrical problem in the Jupiter first stage doomed the mission to failure. In one of the Cape’s more spectacular early launch failures, the Range Safety Officer detonated the rocket’s destruct package 5.5 seconds after lift off as the rocket pitched over towards the ground shortly after getting off the launch pedestal. The burning debris impacted just 75 meters from the pad and 90 meters from the block house filled with launch personnel (see a video of this failure in the Related Video section at the end of this post).

Juno II Round No. AM-19B shown being prepared for launch on August 14, 1959. (NASA)

Soon thereafter, the ABMA team was ready for another launch attempt. Instead of one of the new JPL-developed satellite, Juno II Round AM-19B carried a USAF-sponsored payload called Beacon. This was a 12 kilogram package which included a telemetry beacon, an aluminum-coated Mylar balloon and hardware designed to inflate the balloon to 3.7 meters across once in orbit. The balloon was meant to study the properties of the upper atmosphere by noting how its orbit changed as a result of atmospheric drag and other influences. Unfortunately, a malfunction in the rocket’s guidance system shortly after launch on August 14, 1959 and an estimated 5% to 10% velocity shortfall from the first stage prevented Beacon from reaching orbit.

Juno II Round No. 19A shown at the moment of ignition during the launch of Explorer 7 on October 15, 1959. (NASA)

Undeterred, von Braun’s team studied the causes of the Juno II failures and made corrections for the launch of the next satellite designated S-1a on Round AM-19A. Payload S-1a had a mass of 41.9 kilograms and was a slightly improved version of the ill-fated S-1 payload. But unlike S-1, S-1a was successfully launched into a 557 by 1,069 kilometer orbit inclined 50.3° on October 15, 1959 to be officially designated Explorer 7. NASA’s newest Explorer returned much new information on the spatial and temporal structure of the inner edge of the Van Allen radiation belts that complimented earlier data and that taken concurrently by later satellites. Explorer 7 returned continuous real-time data through February 1961 and then intermittently until August 24 when the satellite finally fell silent. Data returned by this flight showed a definite correlation between solar activity levels and the intensity of radiation in the Van Allen belts.

Diagram showing the experiments carried by payload S-1a known as Explorer 7 after it reached orbit. Click on image to enlarge. (NASA)

Next up was payload S-46. This 10.2 kilogram satellite was 18 centimeters in diameter and 53 centimeters long. Similar in design and mission to the earlier pencil-like Explorer satellites launched by the Juno I, S-46 carried instruments to study the Earth’s radiation belts. But unlike the first-generation Explorer satellite, S-46 also carried four banks of solar cells mounted on a rectangular box to recharge its batteries for up to one year. By using the more powerful Juno II, this new payload could survey the Van Allen belts from a more highly elongated orbit than was possible with the Juno I that was designed to survey the breadth of the newly discovered belts. The new payload mounted atop of Juno II Round AM-19C was launched on March 23, 1960 but all telemetry was lost shortly after first stage burn out. The Juno II had failed again.

Diagram showing the experiments carried by payload S-30 known as Explorer 8 after it was launched on November 3, 1960. Click on image to enlarge. (NASA)

The next attempt by ABMA to launch a JPL-built Explorer came later that year. Payload S-30 was similar in shape and design to the earlier S-1 satellites except it did not carry solar cells and had a life of only 45 days. Weighing in at 40.26 kilograms, this satellite was designed to make in situ measurements of upper atmospheric properties such as electron density, temperature, composition and how they vary in the 200 to 1,200 kilometer altitude range. The solar cells were excluded from this payload because asymmetric charging on the cell surfaces would produce electric fields that could affect experiment results. Three different sensors to measure micrometeorites were also carried.

The night launch of Juno II Round AM-19D on November 3, 1960 carrying Explorer 8 into orbit. (NASA)

This new payload was successfully launched into a 459 by 2,289 kilometer orbit by Juno II Round AM-19D on November 3, 1960 to become Explorer 8. As expected, Explorer 8 operated until December 28 when its batteries were finally exhausted. During its useful life, the satellite returned a large volume of data but unfortunately there were problems processing the raw telemetry by computer into usable measurements. Because of these problems, most of the data had to be processed by hand. Nonetheless many important new observations were made including the discovery of a helium layer in the ionosphere.

Explorer 8 shown deploying weighted lines after entering orbit to slow its rate of spin for its orbital mission. (NASA)


The Final Juno II Flights

From the start, NASA policymakers knew that the Juno II was only a stopgap measure. Kludged together from a variety of preexisting hardware, the Juno II was hardly an optimum design for the task of satellite launches. And its high failure rate only underscored the need for a replacement. By the end of 1960, the all solid-rocket motor Scout had already started test flights. Promising lower costs and better reliability, the Scout was designed to launch small Explorer-class payloads into low orbits and would gradually replace Juno II in that role during 1961. Larger payloads to be launched into distant Earth orbits would use an upgraded version of the Thor Able called the Thor Delta (later known as just Delta). But in the mean time the remaining Juno II rounds had payloads to launch.

Explorer 9 was successfully launched on February 16, 1961 using the Scout X-1. The Juno II was phased out of use in favor of this and other rockets purpose-built for satellite launches. (NASA)

The next payload ready for launch was S-45. It was similar to the proven design of Explorers 7 and 8 with a mass of 34.1 kilograms. This solar-powered satellite would transmit low power, phase-coherent radio signals at six different frequencies between 20 and 960 MHz which would be monitored by ground stations. Measuring how the transmissions were affected at different frequencies would allow scientists to determine many key parameters of Earth’s ionosphere. On February 24, 1961 what would have become Explorer 10 (Explorer 9 was successfully launched on a Scout X-1 eight days earlier) was launched on Round AM-19F. Unfortunately a malfunction prevented the motors on the last two stages from igniting and S-45 failed to reach orbit.

Shown here is the antenna configuration of the S-45 payload which failed to reach orbit on February 24, 1961. (NASA)

Next was payload S-15 whose mission was to observe the heavens making it the first true astronomical satellite. This satellite, which was the responsibility of NASA’s Goddard Space Flight Center founded just two years earlier, consisted of an octagonal box 31 centimeters across and 59 centimeters long mounted on top of a 52 centimeter long cylinder with a diameter of 15 centimeters. The satellite’s principle instrument was designed to detect gamma rays with an energy greater than 50 MeV over a field of view of 34 degrees. The tumbling motion of the satellite in orbit would allow this directional detector to scan most of the celestial sphere with emphasis along the galactic plane. The satellite would also measure the Earth’s reflectivity to gamma rays. This 43.2 kilogram satellite had a life expectancy of four months which was limited primarily by the deleterious effects of radiation on the solar cells mounted on the exterior of the payload.

NASA’s Explorer 11 satellite, known as payload S-15, was the first orbiting gamma ray observatory. (NASA)

Launched on April 27, 1961, S-15 became Explorer 11 when Round AM-19E sent it into a 497 by 1,793 kilometer orbit. Despite the loss of its data tape recorder, Explorer 11 was able to return a large amount of real-time data during its unexpectedly long 224-day life. One of the more important findings was the lack of evidence to support steady state cosmology. This theory proposed that new matter was being continuously created to fill the expanding Universe. This process should have generated a gamma ray signature that Explorer 11 could detect. The absence of these gamma rays was a boost for the popular alternative theory called the Big Bang which, with modifications, remains the preferred theory for the origin of the Universe to this day (for a fuller description of the satellite, its mission and findings, see “Vintage Micro: The Original Gamma Ray Observatory”).

Depiction of Explorer 11 in Earth orbit. (NASA)

The last payload that Juno II launched was S-45a. Essentially a duplicate of S-45 lost three months earlier, the last ABMA-JPL Explorer lifted off on Round AM-19G on May 24, 1961. But fate struck again when a failure of the guidance system’s power supply prevented upper stage ignition dooming the mission. With this anticlimactic finale, the Juno II was quietly retired after just ten launches. Although the Juno II had its problems, it did successfully launch three satellites (Explorers 7, 8 and 11) and one lunar probe (Pioneer 4) which added significantly to our knowledge during the early Space Age. While the successful Explorer program would expand and continue for decades to come, NASA would now use newer launch vehicles purpose-built for the task.


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Related Video

Here is a video showing footage of the spectacular failure of Juno II Round AM-16 on July 16, 1959.


Related Reading

“Vintage Micro: The Original Standardized Microsatellite”, Drew Ex Machina, July 5, 2014 [Post]

“Vintage Micro: The Original Nanosatellite”, Drew Ex Machina, February 5, 2015 [Post]

“America’s First Satellite… Almost”, Drew Ex Machina, October 4, 2015 [Post]

“Vintage Micro: The Pioneer 4 Lunar Probe”, Drew Ex Machina, August 2, 2014 [Post]

“Vintage Micro: The Original Gamma Ray Observatory”, Drew Ex Machina, June 13, 2014 [Post]


General References

Josef Boehm, Hans J. Fichtner, and Otto A. Hoberg, “Explorer Satellites Launched by Juno 1 and Juno 2 Space Carrier Vehicles”, in Aeronautical Engineering and Science, Ernst Stuhlinger, Frederick I. Ordway III, Jerry C. McCall, and George C. Bucher (editors), pp. 218-239, McGraw-Hill, 1963

Ray V. Hembree, Charles A. Lundquist, and Arthur W. Thompson, “Scientific Results from Juno-Launched Spacecraft”, in Aeronautical Engineering and Science, Ernst Stuhlinger, Frederick I. Ordway III, Jerry C. McCall, and George C. Bucher (editors), pp. 281-297, McGraw-Hill, 1963

C.F. Mohl, Juno Final Report Volume III: Juno II Earth Satellites, JPL Technical Report No. 32-31, June 28, 1962