Except for the occasional headline of some important achievement, the general public today is largely unaware of how space technology affects them despite the pervasiveness of its influence. Space technology allows instant knowledge of one’s location, provides near-real time images of weather, is a key part of instant global communications, creates views for online maps along with a growing list of other applications which make up part of the background of every day life.
The launch of Sputnik on October 4, 1957 made headlines across the globe and shocked many Americans.
But long before these applications of space technology ever existed, the situation was very different. Six decades ago at the dawn of the Space Age, the geopolitical and security implications of space were on the minds of almost everyone. With the successful launch of the first Sputnik in October 1957 (see “Sputnik: The Launch of the Space Age”), it became apparent that the Soviet Union and not the United States was taking the lead to explore (and conquer!) this new frontier. The very public failure of America’s first attempt to launch a satellite in December 1957 only heightened these anxieties (see “Vanguard TV-3: America’s First Satellite Launch Attempt”). It took the efforts of a team of talented engineers and scientists lead by German rocket pioneer Wernher von Braun working for the US Army to finally orbit America’s first satellite known as Explorer 1.
Project Orbiter
In the years following the end of World War II, the possibility of space travel experienced a great surge of interest in Europe and America. This was in large part due to the development of the German A-4 (more popularly known as the V-2) rocket during the war. The A-4, developed and built as a weapon for the German Third Reich by a team of engineers lead by Wernher von Braun, was the largest rocket developed up to that time and it paved the way for the building of much larger rockets with significantly increased performance. It was recognized by many after the war (as well as by von Braun during the A-4 development) that this new technology would make space travel possible in the near future. As a result, there were a flurry of studies conducted during the late 1940s and early 1950s on the possibility of launching an Earth orbiting satellite.
The German A-4 rocket better known as the V-2. (MSFC/NASA)
While the bulk of post-war rocket development, both in the Soviet Union and the US, centered on the creation of new weapons systems for the military, many involved with these endeavors still had the possibility of space travel in the back of their minds. Von Braun, who was relocated to the US after the war along with many of his colleagues, was also consumed by his passion for space travel. His writings on this topic in popular magazines like Colliers during the 1950s inspired an entire generation with visions of space stations, trips to the Moon, and large expeditions to Mars. All these missions were still far in the future since they required the development of rockets significantly larger than any in existence.
Von Braun’s articles in popular magazines like Colliers helped to increase public interest in space travel in the 1950s. (NASA)
In the near term, von Braun and his team at what was to become the Development Operations Division of the Army Ballistic Missile Agency (ABMA) at the Redstone Arsenal in Huntsville, Alabama had much more modest goals in mind. Advances in the miniaturization of electronics during the late 1940s and early 1950s made it possible for a small satellite with a mass of just a few kilograms to perform useful investigations from orbit. Such a small payload could be launched using rockets only slightly more capable than those currently under development. During 1954 von Braun’s team, in cooperation with the California Institute of Technology’s Jet Propulsion Laboratory (JPL) and the Office of Naval Research, began work on a proposal to launch an Earth satellite called Project Orbiter.
The Project Orbiter team shown in a meeting on March 17, 1954 in Washington, DC. Wernher von Braun is seated in the foreground to the right. (MSFC/NASA)
This proposal centered on using a modified Redstone rocket in combination with a cluster of existing solid rocket motors to launch a small satellite into orbit. The Redstone started life as a design study called Hermes C in the late 1940s. The Hermes program was a series of experimental rockets that combined proven German A-4 technology with new American innovations. As such, the Redstone is a direct descendant of the A-4. In July 1950 a feasibility study began for a ballistic missile with a 800-kilometer range based on the Hermes C work.
Diagram showing the major components of the Redstone missile and its warhead. Click on image to enlarge. (Chrysler)
As the Korean War dragged on, the Redstone program received the highest priority and was redirected. In order to speed development and make it a mobile field weapon, the range was reduced to 320 kilometers and it was decided to use a smaller Rocketdyne-built engine based on the one used by an early version of the rocket booster employed by the USAF Navaho supersonic cruise missile then under development. On April 8, 1952 this new rocket was designated Redstone after the Redstone Arsenal. Development proceeded quickly and the first of what would be 37 test flights was launched on August 20, 1953.
The first Redstone test flights, like round RS-6 shown here at launch in October 1954, lifted off from the then new Launch Complex 4 (LC-4) at Cape Canaveral. (MSFC/NASA)
For Project Orbiter, a modified version of the Redstone would be employed. While the diameter of the missile remained at 1.8 meters, the propellant tanks would be lengthened by 1.65 meters to increase their volume. This increased the burn time for Redstone’s engine from 121 to 155 seconds. The Redstone’s Rocketdyne A-5 production engine, which normally burned alcohol and liquid oxygen to produce 334 kilonewtons of thrust, was modified to become the A-7 which used what the US Army called “Hydyne” (a corrosive mixture of unsymmetrical dimethylhydrazine and diethylene triamine) as a fuel to produce 369 kilonewtons of thrust. The larger propellant load, greater thrust, and improved engine efficiency increased the performance of the rocket sufficiently to make it part of a viable satellite launch vehicle.
The final configuration of ABMA’s Jupiter C in its satellite launch configuration that was later known as the Juno I. Click on image to enlarge. (NASA)
Mounted on top of this modified Redstone would be a high speed assembly consisting of an instrument compartment and a cluster of off-the-shelf solid rocket motors. Originally 37 Loki antiaircraft missiles were considered for this application. But in order to simplify the design and increase the assembly’s reliability, it was later decided to use a cluster of 15 scaled-down versions of the JPL-developed rocket motor used in the Sergeant surface-to-surface tactical missiles. Each of these motors would be 15 centimeter in diameter, 1.2 meters long and would generate 7 kilonewtons of thrust for five to six seconds. The cluster would be held in an electrically-driven spinning tub mounted on top of the modified Redstone. The cluster was spun to provide gyroscopic stability and to even out any performance variations in the solid rocket motors which were quite primitive by today’s standards. This Redstone-based launch vehicle had a total length of 21.7 meters and a lift off mass of 29,000 kilograms.
This diagram shows the arrangement of stages of the Jupiter C in its satellite launcher configuration. Click on image to enlarge. (JPL)
The upgraded Redstone first stage would loft the high speed assembly above the atmosphere whereupon it would separate from the cluster. The spinning aluminum tub would then coast for a predetermined period of time using variable thrust, compressed gas thrusters to maintain proper attitude. At the right moment, the outer ring of 11 rocket motors would ignite to send the other upper stages and payload on their way. Immediately after burnout, a third stage made up of a cluster of three motors mounted inside this ring would ignite. The final stage in this rapid fire sequence, consisting of a single rocket motor, would then ignite to place the small payload into Earth orbit. In theory this rocket could place 9 kilograms into orbit but further improvements later raised this to 11 kilograms – the lower end of the mass range for what are considered microsatellites today. The primary advantage of this proposal over its competitors was that it made use of existing technology and proven hardware. As a result, many felt that Project Orbiter would be available to launch a satellite before any other project.
Death & Rebirth of Project Orbiter
In September 1954 the joint Army-Navy Project Orbiter proposal to launch a single satellite was submitted to the Department of Defense (DoD) for consideration. At about this same time, there was a building effort in scientific circles to organize the International Geophysical Year – an international scientific cooperative effort to study the Earth and its interaction with the Sun that would run from July 1, 1957 to December 31, 1958. With the US considering a commitment to launch a satellite during the IGY, the US Air Force (USAF) and the Naval Research Laboratory (NRL) submitted their own satellite proposals as well. With three choices before him, Assistant Secretary of Defense Donald A. Quarles deferred the decision to an Advisory Group on Special Capabilities.
NRL’s Vanguard satellite project got the go ahead to be America’s first official satellite program over ABMA’s Project Orbiter. Click on image to enlarge. (NASA)
On September 9, 1955 this group choose the NRL proposal which was eventually called Vanguard (see “Vintage Micro: The Original Standardized Microsatellite”). While Project Orbiter made the greatest use of off-the-shelf hardware and had the best chance to get a satellite into orbit first, the Eisenhower administration made it clear that they wanted to use as little military hardware as possible to launch America’s IGY satellite. This was to give the project as civilian a look as possible to ease establishment of the concept of overflight rights for Earth-orbiting satellites (making it easier for later military satellites, then secretly under study, to fly their missions – see “The First Discoverer Missions: America’s Original (Secret) Satellite Program“). There were those in the government who also wanted to minimize any potential interference between the satellite program and vital defense projects like the Army’s Redstone or the USAF proposed use of their Atlas ICBM then under development (see “The First Atlas Test Flights”). Another perceived weakness in the Project Orbiter proposal was that it would launch only a single satellite with no follow up. Of course this could have been easily remedied with additional resources to build hardware for more flights but it was felt that this could have had an impact on the Redstone development program.
Major General Bruce Medaris (left) and Wernher von Braun were ultimately responsible for getting America’s first satellite into orbit. (US Army)
With Project Orbiter officially shelved, development on von Braun’s proposed satellite launch vehicle was redirected in September of 1955. In addition to the Redstone, the ABMA, under the command of Major General Bruce Medaris, was developing the Jupiter IRBM (Intermediate Range Ballistic Missile). With a range of 2,800 kilometers, Jupiter’s warhead would have to withstand much more extreme conditions upon reentry into the Earth’s atmosphere than previous ballistic missile reentry vehicles (RVs). In-flight testing of a new RV was needed to verify its design but a purpose-built rocket for this task was not yet available. As a stopgap measure, a modified version of von Braun’s satellite launcher was proposed to fill the role. While it was not powerful enough to loft the actual RV, the rocket would be capable of accelerating a one-third scale model with a mass of 140 kilograms to hypersonic velocities. The only major change required was the removal of the final stage and the installation of an adapter for the reentry test vehicle (RTV).
The Jupiter C shown configured for RTV flights in support of the Jupiter IRBM program. Click on image to enlarge. (ABMA)
From the start, the development of this modified Redstone proceeded so that the satellite launch option would be preserved. This rocket was designated Jupiter C (“C” standing for “Composite”) to help disguise its heritage under the Jupiter program umbrella. This would not be the first Redstone to fly in support of Jupiter development, however. Starting in March 1956, modified Redstone missiles designated “Jupiter A” commenced flight testing key Jupiter IRBM components such as the guidance system in preparation of the first actual Jupiter test flights a year later. As development of the Jupiter C proceeded ostensibly to support the US Army’s IRBM project, Medaris and von Braun continued to lobby civilian and military leaders in Washington to allow them to launch a satellite.
Diagram illustrating the high speed assembly and payload for the first Jupiter C, Round 27. Click on image to enlarge. (ABMA)
Development of the Jupiter C proceeded quickly during the next year with the first test vehicle, Round 27, ready for launch only a year after authorization. This vehicle was essentially the same as the proposed satellite launch vehicle except that it carried an inert fourth stage. The 39.2-kilogram payload developed by JPL for this flight, a lightweight transmitter and some ballast, was the forerunner of the satellite payloads to come. According to some popular accounts of this initial test flight, the ballast was intentionally carried on this flight to prevent von Braun from “accidentally” launching the payload into Earth orbit. Having been turned down twice again during 1956 for authorization, von Braun’s desire to launch a satellite was hardly a secret. Still, there is no evidence to suggest that von Braun and his team had any intention of violating their explicit orders not to launch a satellite and the ballast was carried simply to approximate the mass properties of a loaded fourth stage for this test flight.
Jupiter C Round 27 being prepared for launch on September 20, 1956 from LC-5 at Cape Canaveral, Florida. (US Army)
The first launch of the Jupiter C went off without a hitch on September 20, 1956. The inert fourth stage and its dummy satellite payload reached a record peak altitude of 1,097 kilometers and landed 5,366 kilometers down range in the Atlantic Ocean. The principle objectives of the flight were met and the miniature transponder and microlock instrumentation payload was tracked throughout the flight. Von Braun and his team now had the means of launching a satellite into orbit (see “America’s First Satellite… Almost”).
A Jupiter C RTV being prepared for a pre-launch spin test. (US Army)
On May 15, 1957 Jupiter C Round 34, the first to carry a scaled Jupiter IRBM RTV payload, successfully lifted off. Although the rocket operated as intended, a guidance system malfunction caused the RTV to overshoot the target area and the payload was not recovered. Radar tracking, however, indicated that the test vehicle’s new ablative heat shield worked as intended and that the RTV survived reentry. With this second successful flight, the press started taking great interest in von Braun’s Jupiter C satellite proposal. Of course von Braun was all too eager to extol the virtues of his system in comparison to the America’s “official” IGY satellite program, Vanguard, which was making painfully slow progress. Since the public position of the Defense Department was that they could see no military benefit in space exploration (never mind that the satellite “problem” was already being solved by Vanguard), they did not want defense dollars being spent on space programs. As a result, military leaders took a dim view of von Braun’s proselytizing. Finally on July 29, 1957 the Pentagon issued a directive to the three branches of the military forbidding them from discussing with the press space, space technology and space vehicles. The NRL’s Vanguard program was of course exempt from the directive since, on paper at least, they received their money and direction from civilian agencies like the National Science Foundation and the National Academy of Sciences.
President Eisenhower shown with the RTV recovered from the August 8, 1957 flight of Jupiter C Round 40. (US Army)
The third flight of the Jupiter C, Round 40, was launched just ten days later on August 8, 1957. The RTV payload reached a peak altitude of 600 kilometers before arcing back into the atmosphere at a velocity of 5.4 kilometers per second. The RTV came down by parachute 2,140 kilometers downrange within the prescribed 400-meter diameter target circle. It was quickly recovered and within days presented by President Eisenhower on national television as the first object to be successfully recovered from space.
The unused Jupiter C missiles, like Round 29 shown here in its satellite launch configuration, were placed into storage in hopes of using them in the near future. (MSFC//NASA)
With this successful third flight, the Jupiter C program had met all of its objectives and the program was declared completed. Future test flights of warhead RV designs would use purpose-built rockets like the USAF X-17 which had already started flights. The remaining three Jupiter C rockets, which could be converted into satellite launchers with only a couple of months notice, were carefully tucked away ostensibly to test the effects of long-term storage on rocket hardware. Of course the hope was that the Eisenhower Administration and the Department of Defense would change their minds and authorize a satellite launch using this hardware.
Fate Intervenes
After the completion of the Jupiter C program, von Braun and Medaris continued to press for permission to launch a satellite as part of a proposed six-vehicle program that would serve as a backup for Vanguard. The proposal would have probably languished indefinitely had fate not intervened. As luck would have it, von Braun and Medaris were attending a cocktail party with Secretary of the Army Wilbur Brucker and the new Secretary of Defense, Neil McElroy, in the officer’s club at the Redstone Arsenal on the evening of October 4, 1957. In the midst of the festivities the announcement of the launch of Sputnik was made (see “Sputnik: The Launch of the Space Age”). One can only imagine von Braun’s frustration knowing he could have placed a satellite into orbit a year earlier if he had only received permission. Von Braun immediately made his now famous pitch to the gathered officials to launch a satellite using a surplus Jupiter C within 60 days of authorization. Medaris suggested instead that 90 days would be a more realistic goal.
Secretary of Defense Neil McElroy (on the left in the front) getting a tour of the Redstone Arsenal in Huntsville, Alabama. (US Army)
A series of meetings ensued culminating in a three-day long conference starting on October 23 at Fort Bliss, Texas where von Braun and Medaris presented their proposal to the Army Scientific Advisory Panel. As a result of this meeting, ABMA’s confidence in their hardware, and the public’s strong reaction to the Soviet satellite launch, Brucker was now much more supportive of the project. On October 27 the Advisory Group on Special Capabilities, now under Homer J. Stewart, approved the launching of two satellites using a four-stage version of the Jupiter C. To further distance the project from military programs, this satellite launch vehicle was now designated Juno I (although the “Jupiter-C” label was frequently used interchangeably by all at the time and the years since). On November 8, just five days after the launch of Sputnik 2 (see “Sputnik 2: The First Animal in Orbit”), Secretary of Defense McElroy finally authorized the two-satellite program with the first launch to take place in March of 1958. Within a month $3.5 million had been earmarked for the launches and, as result of the advance state of preparation, the date of the first flight was pushed up to the end of January 1958. With the failure of the Vanguard TV-3 flight on December 6, 1957, von Braun and the ABMA would finally get their chance to launch America’s first Earth satellite (see “Vanguard TV-3: America’s First Satellite Launch Attempt”).
The disastrous launch attempt of Vanguard TV-3 on December 6, 1957. (NASA)
The satellite that would be lofted by ABMA’s Juno I launch vehicle (which would finally receive the name “Explorer 1” on launch day) was the responsibility of JPL then directed by William H. Pickering. Development work on the satellite actually began back in December of 1954 after the Project Orbiter study had been submitted. The payload compartment, which remained attached to the last stage of the Juno I carrier rocket once in orbit, had a mass of only 8.23 kilograms of which 5 kilograms was actual instrumentation. The stainless steel tube that would carry the instruments was 15 centimeters in diameter and 81 centimeters long including the aerodynamic cone at the top. The total length of the satellite, with the spent fourth stage motor casing, was 2.03 meters and it had a mass of 14.0 kilograms. Despite its diminutive size, especially compared to the first Soviet Sputniks, the satellite was able to carry a respectable array of scientific instruments due to America’s lead in miniaturized electronics.
Members of the ABMA satellite team (with Gen. Medaris and Dr. von Braun seated in center) with a model of the Explorer 1 satellite. (US Army)
The satellite was equipped with a pair of mercury battery-powered, phase-modulated telemetry and tracking transmitters operating at a frequency of about 108 MHz like the Navy’s Vanguard satellite. Each “microlock” transmitter had eight telemetry channels to relay data back to the ground. The primary transmitter, with a power of 10 milliwatts, used the satellite casing, with the aid of a dipole antenna gap toward the top of the satellite, as an antenna to transmit data to large military receivers. Its battery was expected to allow continuous transmissions for about two months. The backup 60 milliwatt transmitter used a four-wire turnstile antenna that could be detected using more modest amateur radio equipment. It was expected that this transmitter would operate for two or three weeks before its batteries were depleted. A prototype of this satellite successfully tested an early version of the microlock transmitter on the maiden launch of the Jupiter C in September 1956.
The Explorer 1 satellite. (JPL)
The scientific payload of the satellite consisted of three instruments. The first, supplied by JPL engineers, was a set of four thermistors to measure spacecraft temperatures. One of the objectives of this mission was to test passive thermal control techniques so that the payload could withstand the temperature extremes of space. First the stainless steel exterior of the payload was sandblasted so that micrometeorite impacts would not substantially change its surface properties. Eight white “Rokide-A” aluminum oxide stripes were painted down the length of the payload section to reflect sunlight while still allowing heat to be efficiently radiated away. The thermistors, which were placed throughout the satellite, would allow the engineers to assess their efforts to control the satellite’s temperature. The frequency drift of the transmitters subcarrier oscillator could also be used to independently check on the temperature. This data would then be used to improve the thermal design of future spacecraft
The components of the Explorer 1 satellite – (l to r) the nose cap, the internal systems and the outer casing. (Van Allen/University of Iowa Libraries)
The remaining pair of instruments were concerned with characterizing the environment in orbit. First was a cosmic ray experiment designed and built by a University of Iowa team led by James A. Van Allen. Originally Dr. Van Allen had hoped to launch his instrument on a Vanguard satellite but it exceeded the satellite’s limited payload capacity. This instrument consisted of a commercially-available Anton 314 Geiger-Muller tube that was 10.2 centimeters long and 2.0 centimeters in diameter. The instrument was similar to those Van Allen and others had flown during the previous decade on sounding rockets and high altitude balloons. The electronics of this Geiger counter were designed to handle counting rates five times higher than predictions based extrapolations of rocket and balloon data. Van Allen and his team hoped to determine if cosmic ray intensities continued to increase with altitude as had been observed in earlier experiments.
Cutaway diagram showing Explorer’s arrangement of internal equipment. (MSFC/NASA)
Next was a trio of detectors supplied by the Air Force Cambridge Research Center to detect micrometeorites. One sensor was a lead zirconate piezoelectric crystal microphone designed to detect the impact of micrometeorites against the satellite’s metallic casing. This detector had an effective area of 0.23 square meters and could detect a micrometeorite as small as two nanograms travelling at 12 kilometers per second. The other pair made use of electrical resistance measurements to detect the effects of micrometeorite strikes. One consisted of a group of a dozen fine wire gauges, mounted on the fourth stage motor casing, that were electrically connected in parallel. The total resistance of this array, which had a total effective area of about 12 square centimeters, would change when a wire broke from a the impact of a micrometeorite larger than ten microns. The last micrometeorite sensor consisted of metallic film deposited on a substrate whose resistance would increase as its surface eroded.
Preparing for Launch
The rocket that would attempt to launch America’s first satellite was designated Round 29 by ABMA. This rocket was the backup for Jupiter C Round 27 which flew the design’s maiden flight in 1956. Since this earlier test flight employed the satellite launch vehicle configuration (save for a dummy fourth stage and some other instrumentation), Round 29 was the easiest to reconfigure for a satellite launch. The modified Redstone first stage was flown to Cape Canaveral from the Redstone Arsenal in Alabama in a C-124 Globemaster cargo aircraft on December 20, 1957 only six weeks after von Braun received authorization to proceed. After its arrival, the first stage was moved to the US Army missile firing laboratory’s Hangar D where it was checked out and integrated with the upper stage rocket cluster which had been checked out earlier at JPL’s spin-test facility on the Cape. By January 13, 1958 the Rocketdyne A-7 powerplant had been checked and final preparations were begun.
The Explorer 1 satellite and the fourth stage of the Juno 1 shown during weight and balancing tests before launch. (MSFC/NASA)
The USAF, which operated the Cape Canaveral’s test range, assigned January 29, 1958 as the beginning of a three day period for the ABMA satellite launch attempt. If the launch of the Juno I was delayed beyond this, it would have to wait until after the Vanguard TV-3BU satellite launch attempt which was being prepared only 1½ kilometers away in Hangar S. To help even out the satellite’s exposure to sunlight during the first days in orbit, the launch window was set to extend from 10:30 PM to 2:30 AM EST.
Juno I Round 29 shown enclosed in its gantry at LC-26A during preparations for launch from Cape Canaveral. (NASA)
In order to avoid alerting the press to the impending launch, the Juno I was erected under cover of darkness on Pad A of Launch Complex 26 (LC-26A) which had been completed in May 1957 to support Redstone and Jupiter launches. By dawn, the gantry was in position so that the upper stages were covered. To observers on the beach, Round 29 looked like just another Redstone missile test. On January 24, 1958 the press finally started receiving daily briefings on the upcoming launch on the condition that no information would be released until after liftoff. Amazingly, both the US Army and press corps kept the agreement. With the successful completion of a flight simulation test on January 28, America’s second attempt to send a satellite into orbit was set to go.
The upper stages of the Juno I topped with Explorer 1 are revealed as the gantry is rolled back. (MSFC/NASA)
While all the hardware was ready, Mother Nature refused to cooperate. Weather reports on January 29 were not promising and indicated a wind speed of 270 kilometers per hour at altitudes of 11 to 12 kilometers. The threat of lightning, which could prematurely set off the upper stage rocket motor igniters on the pad, combined with the buffeting from the upper level winds made a launch impossible. The next day the winds had hardly died down and it was a cool and cloudy day on the ground. Hoping for improved conditions later in the day, the 11-hour countdown started at 11:30 AM EST and the Redstone was loaded with fuel. With wind speeds reported to be 349 kilometers per hour at 12 kilometers, the countdown was finally halted at 9:00 PM. As the night wore on the winds slackened somewhat but not enough for a launch. Although there was some concern that the modified Redstone’s corrosive Hydyne fuel might degrade the seals in the launch vehicle’s plumbing, the rocket was left fueled for one last launch attempt on January 31 when conditions were predicted to be better.
The blockhouse launch crew at LC-26A during the countdown for Explorer 1. (MSFC/NASA)
Exploring a New Frontier
As hoped, conditions on the January 31, 1958 were good enough for a launch attempt. That night about one hundred reporters had gathered at a hastily assembled grandstand 2.3 kilometers from LC-26A near Hangar D. Range safety officers would not allow them any closer. With 15 minutes to go before launch, the pad area was cleared. Three minutes later the upper stage cluster was set spinning at 560 RPM as programmed. After a rapid paced series of events, the count reached “Zero” and the firing command was given. This was followed by a series of events culminating with the ignition of Redstone’s main engine 14 seconds later. Finally 15.7 seconds after the T-0 mark at 10:47:56 PM EST on January 31, 1958 (03:47:56 GMT on February 1), the first Juno I leapt from the launch pad and into the night sky.
The launch of Explorer 1 on January 31, 1958. (MSFC/NASA)
Seventy seconds after launch the upper stage cluster’s spin rate was automatically increased to 650 RPM to avoid resonant vibrations that could shake the ascending rocket to pieces as it burned through its propellant. After another 45 seconds the spin rate was increased again to its final value of 750 RPM. Finally 2 minutes and 36.7 seconds after launch the Redstone shut down just a fraction of a second early but travelling about 112 meters per second faster than planned. Six seconds later the instrument section with the spinning upper stage cluster separated from the now-spent Redstone and continued to coast towards the apex of its trajectory for another four minutes. Six minutes 43 seconds after launch, at an altitude of 362 kilometers and 685 kilometers down range, the second stage rocket cluster was ignited on ground command. Because of the slight excess velocity from the Redstone, the ignition occurred about eight seconds before the optimum moment when apogee was hit. Still, at this time the high speed assembly was travelling 130 meters per second faster and was 11 kilometers higher than planned. After 6.5 seconds the third stage cluster ignited followed 6.5 seconds later by the final stage.
This diagram shows the major milestones of Explorer’s ascent trajectory. Click on image to enlarge. (JPL)
Telemetry received on the ground indicated that the satellite had probably reached orbit about 428.6 seconds after launch but they would have to wait until 12:41 AM EST when the satellite made its first pass over JPL’s tracking station in Earthquake Valley, California for confirmation. Although the signal was received an agonizing eight minutes after the predicted time, it was finally detected confirming the success of the launch. America’s first satellite, now called Explorer 1, was in a 359 by 2,542 kilometer orbit inclined 33.2° to the equator. America was finally in the Space Race and von Braun and his team managed to do it not only before Vanguard but six days before their 90-day deadline was reached.
The successful launch of Explorer 1 made headlines around the globe and finally put the US in the space race. (MSFC/NASA)
While the high power transmitter on Explorer 1 depleted its battery after only a dozen days, the low-power unit continued to transmit data until May 23, 1958 when its batteries were finally exhausted. During its 112 day active lifetime in orbit, its instruments returned some interesting data. Variations in the signal strength suggested that Explorer 1 was tumbling – not unexpected since this was dynamically more stable mode than spinning about its long axis as it did during ascent. Temperature measurements also showed that the satellite was cooler than expected confirming the effectiveness of the passive thermal control method developed by JPL engineers. The microphone used to detect the small micrometeorites recorded 153 impacts during a total observation time of almost 22 hours. While a slight change in resistance was noted, none of the wire gauge detectors were unambiguously broken by a micrometeorite.
Left to right, William Pickering (JPL), James Van Allen (University of Iowa) and Wernher von Braun (ABMA) shown holding a model of Explorer 1 at a post-launch press conference. (JPL/NASA)
Most interesting of all were the results from Explorer 1 was from Dr. Van Allen’s cosmic ray experiment. Starting at the perigee altitude of 360 kilometers, the cosmic ray counts increased with altitude about as expected. Above altitudes of around 1000 kilometers, however, the Geiger counter mysteriously fell silent. Once the satellite fell below 600 kilometers altitude as it returned towards perigee, the instrument started returning data once more. Since the other instruments continued to transmit normally, it was suspected that the cosmic ray experiment’s behavior was a symptom of a problem in the instrument or its telemetry channel. It was only with data from subsequent Explorer satellites which included more shielding on their radiation detectors that it was discovered that the cosmic ray experiment on Explorer 1 was operating as designed after all. The Geiger counter on Explorer 1 fell silent because it had been saturated by an unexpectedly high flux of radiation trapped in Earth’s magnetosphere. What would become known as the Van Allen Belts had been discovered.
Although it took time to understand the data from Explorer 1, the satellite was the first to return data about the Van Allen Belts of trapped radiation as shown in this modern depiction. (JHUAPL/LASP)
While a veritable armada of satellites would be launched in the years and decades to come to build on the work started by Explorer 1, the silent satellite continued to orbit the Earth far longer than the originally predicted six years. It would be March 31, 1970 before the orbit of Explorer 1 had finally decayed ending in the fiery reentry of the small satellite. While this left Vanguard 1 (which was successfully launched 45 days after Explorer 1) as the oldest satellite still in orbit around the Earth, the legacy of Explorer 1 lives on to this day.
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Related Video
Here is an excellent JPL/ABMA documentary film from 1958 on the events leading up to the launching of Explorer 1.
Related Reading
“America’s First Satellite… Almost”, Drew Ex Machina, October 4, 2015 [Post]
“Redstone: The Missile That Launched America Into Space”, Drew Ex Machina, April 26, 2016 [Post]
General References
David Baker, The Rocket, Crown Publishers, 1978
Josef Boehm, Hans J. Fichtner, and Otto A. Hoberg, “Explorer Satellites Launched by the Juno 1 and Juno 2 Carrier Vehicles”, in Astronautical Engineering and Science, edited by Ernst Stuhlinger, Frederick I. Ordway III, Jerry C. McCall, and George C. Bucher, McGraw-Hill, pp 215-239, 1963
Dwayne A. Day, “New Revelations About the American Satellite Programme Before Sputnik”, Spaceflight, Vol. 36, No. 11, pp 372-373, November 1994
Charles A. Lundquist, “Progress in Design and Implementation of Scientific Spacecraft”, in Space Research: Proceedings of the First International Space Science Symposium, edited by Hilde Kallmann Bijl, Interscience Publishers, pp 540-562, 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
Wernher von Braun, “The Redstone, Jupiter, and Juno”, in The History of Rocket Technology, edited by Eugene M. Emme, Wayne State University Press, pp. 107-121, 1964
Allen E. Wolfe and William J. Truscott, Juno Final Report Volume I – Juno I: Re-entry Test Vehicles and Explorer Satellites, JPL Technical Report 32-31, September 6, 1960