In 1958, while the US were still working on the ill-fated Able lunar exploration project, they began studying the flight of a real interplanetary probe.
The problems of such an endeavor were overwhelming: to that date no space vehicle had ever transmitted form distances of tens on millions of kilometers, and no satellite had ever endured months of endless solar irradiation.
At the beginning, consideration was given to the launch of a probe to an high visibility target, possibly Venus, "easily" reachable in about 150 days, departing in June 1959. A trip to Mars would have taken 250 days and would have taken place in October 1960.
With time, however, the launch window approached and then went, as the development of the probe and its scientific instruments was running late.
The launch window finally closed and the mission was redesigned: the space vehicle would be launched on a fake transfer orbit to Venus, fake because when the probe reached the orbit of Venus, the planet was elsewhere. The probe would then test long range transmissions and transmit precious data on the interplanetary environment.
Pioneer-5, as the probe was named, was managed by NASA's Ames Research Center and was built by STL (Space Technology Laboratories). Its design was based on the Explorer-6 satellite, launched in August 1959. The probe was a 66 cm diameter spin stabilized sphere that contained the radio system and the scientific experiments. To this sphere were attached four solar panels, mounted at the tip of short booms, each of which carried 1,200 solar cells. On board, on the instrument platform, were two radio transmitters, one of 5 W power, the other of 150 W power, that could be activated on command, able to transmit data at three different speeds so as to adapt to the increasing distance from Earth. The transmission of data used an omnidirectional antenna, which explains the huge power of the radio transmitter.
18.1 kg of the 43 kg probe mass consisted of five scientific instruments: a radiation proportional counter, an ion chamber, a Geiger-Muller tube, a single coil magnetometer for measuring interplanetary magnetic fields and a micrometeorite counter.
The launcher was to be the same Thor-Able as the Able lunar probes. This time, the launch was perfect and Pioneer-5 left pad 17A of Cape Canaveral on March 11, 1960. A few minutes later, the third solid stage ignited and gave Pioneer-5 the speed of 11.119 km/s, enough to make it leave the Earth and enter an orbit that would have taken it from 148.4 to 120.6 millions of kilometers form the Sun every 312 days. The orbit was inclined 3.35 degrees on the ecliptic, the plane of Earth's orbit and reached to within 11 millions kilometers to the orbit of Venus.
Pioneer-5 was contacted four times every days on 25 minutes long communication passes on the very first days of its flight. During those days the most interesting data collected were those of the micrometeorite detector, which recorded 87 impacts in a week, during which the probe traveled some 1.6 million kilometers. The terrestrial magnetosphere was discovered as being much  wider than thought (14 Earth radii instead of 6). As luck wanted, at the end of March the Sun erupted with a flare, which was observed both on board the probe and on the Earth 20 minutes later. During one flare, the passage was recorded of protons having energies above 75 MeV and electrons above 13 MeV. At the same time, the flux of galactic cosmic rays markedly decreased. During the periods of quiet sun, Pioneer-5 discovered the existence of an interplanetary magnetic field having a very low intensity.
Pioneer-5 transmitted useful data with the 5 W radio until April 30, 1960. When it was switched to the 150 W radio, because the huge British Jordell Bank radiotelescope was also losing the signal, it was discovered that the 28 batteries on board, vital for the functioning of the 150 W transmitter were slowly venting gas into space. The probe was followed sporadically until June 26, when it was 36.4 million kilometers from Earth.
On August 10, Pioneer-5 reached perihelion and in 1963 again approached our planet. It was initially thought possible to re-establish contact, but without the batteries the probe was considered dead.
Pioneer-5 collected 3 megabit of data for a total of 138 hours, 54 minutes of transmission. The measurement of the radial velocity of the probe enabled scientists to measure the value of the Astronomical Unit (AU), whose non rigorous definition is the mean distance between the Earth and the Sun. The previous measurements, obtained from twenty years of minor planet Eros observations, yielded a value of 149,527,000 +/- 10,000 km, while Pioneer-5 yielded a value of 149,545,000 +/- 20,000 km. Both values are respectively 70,000 and 50,000 km less than the present day accepted value which is based on forty years of Venus radar observations. For Pioneer-5 measurements, the effects of perturbations due to light pressure were probably underestimated.
Finally, it is worth noting that a Pioneer-5 backup vehicle is currently stored at NASM Garber Restoration Facility but there is no plan to put it on show any time soon. (Anyone has a picture of it?)

Pioneer-5 heliocentric orbital elements (reference frame unknown, possibly 1960)

Semiaxis (AU)	Inclination	eccentricity	Ascending node	Perihelion longitude	Date of perihelion
0.89958	3.351 deg.	0.10396	349.712 deg.	357.715 deg.	1960 Aug. 10

Bibliography
Corliss, W. R.: Space Probes and Planetary Exploration, Princeton, Van Nostrand
Greenstadt, E. W.: Final Estimate of the Interplanetary Magnetic Field at 1 A.U. from Measurements made by Pioneer V in March and April, 1960; Astrophysical Journal, Vol. 145, 1966, pp. 270-295 (this article is available on line on NASA Astrophysic Data System)
Kotenikov, V. A., et al.: Radar Observation of Planets in the USSR. In: Kotelnikov, V. A. (editor): Problems of Modern Radio Engineering and Electronics; Moscow, Nauka
Powell, J. W.: Thor-Able and Atlas-Able; JBIS, vol. 37, pp. 219-225
Robotto, A.: Missilistica e Astronautica; Torino, UTET (in Italian)
Wilson, A.: The Eagle has Wings; London, British Interplanetary Society
Pioneer V Transmits Deep Space Data; Aviation Week, March 21, 1960, pp. 28-30
Pioneer V Deep Space Reports Parallel Earlier Radiation Data; Aviation Week, March 28, 1960, p. 32
Pioneer 5 And Its Orbit; Flight, April 1, 1960, p. 437
Pioneer Signals May Be Received form 75-Million mi. Distances; Aviation Week, April 11, 1960, p. 33
Pioneer V Provides New Scientific Data; Aviation Week, May 9, 1960, pp. 32-33
Pioneer V Switched to 150-Watt Unit; Aviation Week, May 16, 1960, p. 34

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