After the launch of the first Soviet Sputnik satellites, the US found itself scrambling to get its first satellites into orbit. But in order to repair American prestige and recover from this international public relations disaster, it was also necessary to anticipate the Soviet Union’s next move and beat them to the next goal in space. One of the next targets was obviously the Moon. In March of 1958, President Eisenhower approved the start of Operation Mona – a two-part plan to launch the first probes to the Moon under the sponsorship of the Defense Department’s new Advanced Research Projects Agency (ARPA).

The first phase of Operation Mona called for the USAF to launch three small lunar orbiters using their Thor-Able rocket – a lash up of the Thor IRBM with upper stages from the Vanguard satellite launch vehicle which originally had been developed to support high-speed reentry tests of ICBM warhead designs (see “Pioneer 1: NASA’s First Space Mission“). The second part of Operation Mona was a proposal from a team at the Army Ballistic Missile Agency led by Wernher von Braun to use a modified Jupiter IRBM to launch a small probe towards the Moon (see “Vintage Micro: The Pioneer 4 Lunar Probe”). But even as these ARPA-sponsored projects raced to beat the Soviet Union to the Moon, there were also plans in the works to beat them to the next obvious goal of the Space Age: the planets.

America’s First Venus Probe – Almost

In 1958, the USAF under the aegis of ARPA started working with the builder of its first lunar probes, STL (Space Technology Laboratory – a subsidiary of TRW), to study a probe to reach Venus. With a mass of about 170 kilograms, these spin-stabilized, one-meter in diameter spherical probes would carry a suite of instruments powered by four solar panels to study the conditions near Venus during a quick flyby. The Thor-Able launch vehicle used by the first USAF lunar orbiters for Operation Mona (today known as Pioneers 0, 1 and 2) was too small to lift these new “paddle-wheel” probes, as they were nicknamed because of their quartet of prominent solar paddles. Instead, modified versions of the Able upper stages would be married to the larger Atlas D to create much more capable launch vehicle. Not only would the Atlas-Able be used to launch the new Venus probe, it would also carry a planned follow-on paddle wheel lunar orbiter of similar size. All involved were confident that the new probe and its launch vehicle would be ready in time for the June 1959 Venus launch window.

But just as ARPA was about to proceed with the USAF Venus probe plans, a radical change in United States space policy took place. When NASA was founded on October 1, 1958, all space science programs run by ARPA were transferred to the new civilian space agency. This included not only the remaining lunar probes as part of what was now being called the Pioneer program, but also the follow-on paddle-wheel probes the USAF was planning. In November of 1958 NASA essentially adopted the existing USAF Pioneer program and started work to send a probe to Venus during the upcoming launch window.

But these plans were changed almost as soon as they were approved. After the successful launch of Soviet Luna 1 towards the Moon in January 1959 and the failure of the first four American lunar probes, the near term goals of NASA’s Pioneer program were redirected: instead of going to Venus, the first of the new Atlas-Able-launched Pioneers would be sent to the Moon in hopes of beating the Soviet Union into lunar orbit (see “NASA’s Forgotten Lunar Program“). Insufficient funding as well as a tight supply of Atlas rockets meant that the new mission to Venus would have to be deferred.

This delay meant that the launch of the first American Venus probes would have to wait until the next launch window in the beginning of 1961. The change in NASA’s plans gave the Soviet Union a chance to beat the Americans once again. Still there was an option available to salvage something for the 1959 launch opportunity. The USAF already had a plan to send a much lighter and simpler probe to flyby Venus using the less powerful but more readily available Thor-Able space carrier rocket. With the promise of low mission costs and quick development, this plan was adopted by NASA with the hope that a smaller Pioneer could be launched to Venus in 1959. Like the heavier Pioneers, the new probe would be a spin stabilized aluminum alloy spheroid but now only 66 centimeters in diameter. It would still use four 36-by-46 centimeter paddles covered with 4,800 solar cells to recharge its batteries and power its systems. Because of mass constraints for this mission, only minimal instrumentation could be carried by this 36-kilogram probe.

But even the development of a smaller Venus probe proved to take far longer than the six-months available. In the end, delays meant that the June 1959 launch opportunity was missed. Fortunately for America’s already bruised prestige, the Soviet Union did not attempt a launch of a Venus probe during this window either. Unknown to the West at the time, the Soviet Union had planned to launch a pair of Venus probes designated Object 1V during June. But delays in building the complex probes and, more importantly, the continued lack of a suitable launch vehicle with the needed performance forced the Soviet planners to wait for 19 months along with the Americans until the next Venus window (see “The First Mars Mission Attempts“).

Change of Mission

Despite the fact that the chance to reach Venus had been missed, the former Venus probe, now designated P-2 by NASA, could still serve as an important pathfinder for future planetary missions. Instead of being sent to Venus, P-2 would simply be launched into a 295-day solar orbit with a perihelion near the orbit of Venus. The now slightly fattened 43-kilogram P-2 probe would carry 18 kilograms of instrumentation on a mission planned to last one month. While this was insufficient time to reach the orbit of Venus, the mission would still answer some basic questions about the largely unexplored interplanetary environment as well as collect data vital to the success of the next Pioneer missions to Venus.

The original plan called for the launch of a STL-built paddle-wheel prototype designated S-2 by NASA to precede the P-2 and the new lunar orbiter missions. This 64-kilogram satellite was eventually launched into an extended 12.6-hour Earth orbit on August 7, 1959 by a Thor-Able rocket to become Explorer 6. With a roughly spherical shape about 66 centimeters across, Explorer 6 tested many of the systems and instruments that were to be flown on P-2 and the subsequent P-3 Pioneer lunar orbiters. During its two months of operation, the innovative suite of instruments carried by Explorer 6 also gathered much new data on magnetic fields and radiation in space as well as secured the first crude image of the Earth from orbit.

Among the scientific goals of the P-2 mission was to resolve the cause of what was known as the “Forbush Effect”. In 1937 American physicist Scott E. Forbush with the Department of Terrestrial Magnetism at the Carnegie Institution of Washington observed that the intensity of galactic cosmic rays as measured from the Earth decreased soon after a solar flare. Forbush and others conjectured that there were one of two possible causes: Either the flare material itself was somehow blocking the cosmic rays or it triggered some effect in the Earth’s magnetic field that blocked them. Both of these possibilities were debated at a scientific symposium in Washington, DC held in April of 1959. In order to sort things out, the scientists proposed measuring the cosmic ray intensity from a vantage point far removed from the influence of the Earth’s magnetic field to see what would happen during a solar flare. The scientists at the time knew that the Forbush Effect was one of many pieces of evidence for the Sun’s possible influence on the Earth and interplanetary space. NASA’s P-2 probe operating in solar orbit offered the ideal platform for such an investigation.

To address this objective, the suite of instruments carried by P-2 included a proportional counter telescope built by a team at the Laboratory for Applied Science at the University of Chicago under John A. Simpson. This omni-directional sensor would detect protons and electrons with energies exceeding 75 and 13 MeV (mega-electron volt) respectively. Also carried were an ion chamber and an Anton 302 Geiger tube with quasi-omnidirectional sensitivity which would also detect protons and electrons at thresholds as low as 25 and 1.6 MeV respectively. This pair of detectors, supplied by a team at the University of Minnesota, would be able to monitor not only the flux of galactic cosmic rays but also high energy solar and terrestrial radiation. The Geiger tube was similar to those carried by earlier Explorer satellites and Pioneer probes allowing easy comparison of the data sets.

Another key instrument carried by P-2 was a search-coil magnetometer built by a team at STL headed by Charles P. Sonett. This instrument would provide data on the variations in the local magnetic field strength in the one microgauss to 12 milligauss range. This instrument was similar to one flown on the STL-built Pioneer 1 and Explorer 6. The last experiment was micrometeorite spectrometer supplied by the Air Force Cambridge Research Laboratory designed to measure the numbers and momentum of interplanetary dust particles between the orbits of Earth and Venus. This instrument consisted of a metal diaphragm on the probe exterior with an area of 0.04 square meters attached to a microphone to detect and measure the vibrations caused by a micrometeorite strike.

The data were transmitted back to Earth in analog or digital form using a five-watt transmitter. Weight limitations did not allow the transmitter enough power to be used continuously. Instead about four 25-minute long communication sessions would take place each day when the probe was in view of a tracking station in Hawaii or the huge 76-meter radio telescope at Jodrell Bank in England today known as the Lovell Telescope. More sessions could be occasionally held during times of special interest such as after a solar flare occurred. An experimental 150-watt transmitter was also carried to conduct long range communication tests in support of the still-planned Pioneer missions to Venus. In addition to the scientific instruments, a range of engineering measurements were also made. The tiny P-2 promised to return much new information about the little explored interplanetary realm.

The Mission

The Thor-Able 4 launch vehicle using Thor number 219 as its first stage was erected at the pad at Launch Complex 17A in October 1959. After numerous delays, two attempts to launch P-2 were made in mid-December with the last coming within 36 hours of launch when a failure of the probe’s voltage converters forced a scrub to remove the spacecraft from the pad for repair. A tentative January 28, 1960 launch date was pushed out to March 1 so that P-2 could undergo additional testing on the advice of NASA engineers. As STL engineers struggled to resolve last minute issues with P-2, by late February the target launch date had slipped to the March 4 to 8 time frame.

Issues with the third stage of Thor-Able 4 pushed the first launch attempt out to March 8 which was scrubbed at T-38 seconds because of problems with the ground LOX supply. Finally with all problems resolved, at 8:00:07 AM EST on March 11, 1960, Thor-Able 4 lifted off smoothly from the pad at LC-17A carrying P-2 on a direct ascent escape trajectory. For only the second time in its career as a satellite launch vehicle, all three stages of the Thor-Able fired essentially as intended boosting the tiny probe, now renamed Pioneer 5, to a speed of 11.12 kilometers per second after 322 seconds of powered flight (the first Thor-Able satellite launch success was the Explorer 6 paddle-wheel prototype lofted seven months earlier). Although this speed was about 150 meters per second slower than intended, it still exceeded Earth’s escape velocity making Pioneer 5 only the third spacecraft to enter solar orbit after the Soviet Luna 1 and American Pioneer 4 probes in January and March of 1959, respectively.

Once free of Earth’s gravitational influence, Pioneer 5 assumed a heliocentric orbit with an aphelion of 148.5 million kilometers and a perihelion of 120.5 million kilometers inclined 3.35 degrees to the ecliptic. Although the velocity shortfall resulted in a longer than intended 311.6-day orbit that would pass no closer than 12 million kilometers outside the orbit of Venus, the healthy Pioneer 5 was on its way. The only noteworthy loss early in the mission was the micrometeorite spectrometer whose data system became saturated resulting in no usable data being returned.

On March 13, Pioneer 5 passed the 658,000 kilometer mark breaking the communication record set a year earlier by Pioneer 4 (see “Vintage Micro: The Pioneer 4 Lunar Probe”). The small probe continued on its journey day after day setting new records and collecting new data on the interplanetary environment. Finally on March 30, Pioneer 5 had a chance to prove itself. Solar astronomers noted the eruption of a huge solar flare at 14:55 UT followed by a magnetic storm on the Earth 21 hours later that produced auroral displays and knocked out trans-Atlantic radio communications.

Just as predicted, the Forbush effect was observed at a monitoring station at Deep River, Canada. At the same time, Pioneer 5 also noted a decrease in cosmic rays from its vantage point about 5 million kilometers from the Earth. Simultaneously the probe’s magnetometer noted an order of magnitude increase in interplanetary magnetic field strength. A mass of plasma from the solar flare was traveling along a spiral path from the Sun at about 1000 kilometers per second. This solar storm produced a magnetic bottle that blocked galactic cosmic rays after it engulfed the Earth.

Pioneer 5 was not instrumented to detect this plasma directly but properly equipped Soviet Luna probes and high flying American Explorer satellites had detected hints of this “solar wind” earlier when they penetrated Earth’s magnetopause and entered interplanetary space. Two days later a second big solar flare occurred and its effects were observed by Pioneer 5. Unlike the first flare, a burst of 100 MeV protons were detected by the probe’s instruments indicating that the fast-moving protons were traveling along the curved field lines inside the magnetic bottle still remaining from the first flare. The Forbush effect was explained but scientists were now beginning to get a taste of the true complexity of the interplanetary particle and fields environment.

Pioneer 5 continued returning data to Earth throughout April surpassing its 30-day design life. Tracking of the receding probe allowed its orbit to be determined which in turn yielded an independent measure of the astronomical unit or AU. Although today the AU is officially defined by the International Astronomical Union to be 149,597,870.7 kilometers, back in 1960 the AU was still a measured quantity equal to the average size of the Earth’s orbit with somewhat conflicting values based on decades of measurements made using different techniques. In order to improve the accuracy of navigating future probes to various planetary targets, a more precise value of the AU was required. Based on the Pioneer 5 tracking results, STL engineers were able to derive an AU value of 149,516,000±14,000 kilometers. Although this differs by about 80,000 kilometers from today’s defined value, it was the first step in using tracking data from interplanetary spacecraft to refine the derived value of the AU.

As the distance of Pioneer 5 from the Earth continued to increase, its radio signal weakened making it more difficult to extract usable data. By April 30, 1960 the signal had weakened so much that Pioneer 5 ceased normal operations and afterwards infrequently returned data from its instruments. On May 8, Pioneer 5 successfully performed a communication test using its 150-watt transmitter from a range of 12.3 million kilometers. Pioneer 5 made its last report back to Earth via the 76-meter dish at Jodrell Bank on June 26, 1960 from a range of 36.2 million kilometers – a communication record that would stand for two years. On August 10, Pioneer 5 reached its closest point from the Sun for the first time some 225 million kilometers from Venus – the original target when the mission was conceived about two years earlier which was now on the other side of the Sun.

By almost any measure, the Pioneer 5 mission was an outstanding success having lasted for 106 days – over three times its design life. It returned over three million bits of data during a total of 139 hours of operation. But the follow-on Pioneer probes to fly to Venus never materialized. By the summer of 1960, NASA had cancelled these probes and adopted a new plan to build much larger and more capable three-axis stabilized spacecraft to be launched on the Atlas-Centaur then under development. The first of this new series of probes, Mariner A, would be launched to Venus in the summer of 1962. It would be several years before the Pioneer-series was revived and new probes launched into interplanetary space.

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Related Reading

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

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

“NASA’s Forgotten Lunar Program”, Drew Ex Machina, September 27, 2015 [Post]

General References

R.L. Arnoldy, R.A. Hoffman and J.R. Winckler, “Solar Cosmic Rays and Soft Radiation Observed”, NASA Press Release 60-185, April 29, 1960

Gideon Marcus, “Earthbound Pioneer (Explorer 6)”, Quest, Vol. 19, No. 1, pp. 38-49, 2012

Joel W. Powell, “The Forgotten Mission of Pioneer 5”, Spaceflight, Vol. 47, No.5, pp. 188-191, May 2005

Robert Reeves, Superpower Space Race, Plenum Press, 1994

J.A. Simpson, C.Y. Fan and P. Meyer, “Preliminary Results from the Space Probe Pioneer V University of Chicago Experiments”, NASA Press Release 60-179, April 29, 1960

NASA Pioneer 5 press package (parts 1 to 5), March 8, 1960

“Pioneer V Progress Report”, NASA Press Release No. 60-145, March 18, 1960

“Project Thor Able-4 Final Mission Report”, STL/TR-60-V001-02092, May 25, 1960

“Results of Space Research: The Solar Wind”, STL Space Log, Vol. 3, No. 1, pp. 32-38, March 1963