The Pioneers heading for Jupiter coordinated with their predecessors in the inner Solar System to study the solar wind.

the distance from the Sun, this would not be practicable in the outer Solar System. The only option was to use Radioisotope Thermo-electric Generator (RTG) power cells that produce power by the natural radioactive decay of non-weapons-grade plutonium dioxide, transforming it into electricity using solid-state thermoelectric converters. It had two, mounted on short booms, splayed at 120 degrees, providing a total of 140 watts. Because the transmitter would radiate only 8 watts of power, it would need a large narrow-beam antenna to communicate from so far away, even at a 2,048-bit-per-second data rate. The overall design therefore assumed the form of a cluster of instruments mounted at the rear of a large dish antenna. As the Pioneer family were not fitted with computers capable of extended autonomous operations, they had to be commanded from Earth, so as the new spacecraft slowly rotated to monitor the particles and fields in its environment its antenna would be constantly aimed at the Earth to enable its commands to be received and its data to be reported. As far as possible, off-the-shelf systems were used to minimise development costs.

The configuration of the Pioneer spacecraft dispatched to the outer Solar System. Instrumentation

The 260-kilogram spacecraft had a 28-kilogram payload of scientific instruments. The Magnetometer had an optical sensor which monitored a cell filled with helium in which magnetic fields passing through the instrument induced electrical discharges. A lightweight boom would project it 6.6 metres from the spacecraft's axis, away from the body of the spacecraft and the 'noisy' RTGs. It was to measure the strength and direction of the magnetic field washing over the vehicle, both in interplanetary space and within the Jovian magnetosphere, and note the transition from one environment to the other.2 E.J. Smith of the Jet Propulsion Laboratory - operated for NASA by the California Institute of Technology - was the principal investigator.

The Plasma Analyser sampled through an aperture in the high-gain antenna. Solar wind particles were to be passed through a pair of plates, across which a sequence of voltages would be applied to derive the energy spectrum. The principal investigator was J.H. Wolfe of Ames.

The Charged Particle Detector comprised two pairs of 'particle telescopes', one pair to study the solar wind, the other pair for the harsher Jovian environment. During the interplanetary cruise, one was to measure the composition of cosmic rays in the energy range 1 to 500 MeV and the other was to measure protons and 'alpha particles' (helium nuclei) with energies from 400 keV to 10 MeV to distinguish ions of light elements in the solar wind from those in cosmic rays. In Jovian space, fission induced in a foil of thorium struck by protons exceeding 30 MeV would measure the flux of protons, and a solid-state electron current detector would measure the flux of electrons exceeding 3 MeV that were believed to be responsible for the decimetric radio emissions. J.A. Simpson of the University of Chicago was the principal investigator.

The Cosmic Ray Telescope employed three solid-state detectors: one to measure the fluxes of electrons in the range 50 keV to 1 MeV and protons in the range 50 keV to 20 MeV; a second to measure the flux of protons in the range 56 to 800 MeV; and a third to measure the fluxes of nuclei of the ten lightest elements (up to neon). The principal investigator was F.B. McDonald of the Goddard Space Flight Center in Greenbelt, Maryland.

The Geiger Tube Telescope was to use a set of Geiger-Muller tubes to measure the intensity, energy spectra and angular distribution of the electrons and protons as the spacecraft flew through the Jovian radiation belts. It was a much improved form of the instrument on Explorer 1. The Geiger Tube Telescope was supplemented by a Trapped Radiation Detector which had five detectors: two scintillation counters to detect the ionisation trails of electrons less than 5 keV and protons less than 50 keV passing through them; an electron-scatter detector to count electrons in the range 100 to 400 keV; a solid-state diode to detect protons in the range 50 to 350 keV; and a Cerenkov counter to detect the flashes as electrons in the range 500 keV to 12 MeV passed through it. C.E. Mcllwain of the University of California at San Diego was the principal investigator.

The distribution of dust in space was to be investigated by two instruments. The Meteoroid Detector employed 13 sensor panels arranged in a circle on the rear of the high-gain antenna dish, each of which contained 18 sealed cells pressurised with gas. The rate at which a cell depressurised upon being punctured would be proportional to the size of the hole. It would be able to detect strikes by particles with masses as small as one-billionth of a gram. The principal investigator was W.H. Kinard of the Langley Research Center in Hampton, Virginia. The other instrument employed a remote-sensing technique. The Meteoroid-Asteroid Detector had a cluster of four 20-centimetre-diameter reflecting telescopes illuminating photomultiplier tubes. The 8-degree fields of view of the telescopes overlapped slightly, so if any three reported a near-simultaneous increase in brightness this would be interpreted as a reflection, and the range, path and velocity of the source would be computed. It was hoped that as it passed through the asteroid belt this instrument might measure the distribution of larger bodies as well as dust. The principal investigator was R.K. Soberman of the General Electric Company in Philadelphia.

The Ultraviolet Photometer had photocathodes to detect neutral hydrogen and helium by their 1,216-Angstrom and 584-Angstrom emissions, respectively. The instrument was to measure the distribution of these gases in interplanetary space, and measure how Jupiter's upper atmosphere scattered solar ultraviolet to compute the amounts of atomic hydrogen and helium in order to determine the 'mixing rate' of the atmosphere. The observations would test fundamental theories, because the

'Big Bang' theory of the origin of the Universe made a specific prediction about the overall ratio of hydrogen to helium, and theories of how Jupiter formed from the solar nebula and evolved had been based on assumptions that could not easily be tested by terrestrial observations. D.L. Judge of the University of California at Los Angeles was the principal investigator.

The Infrared Radiometer utilised a 76-millimetre-diameter Cassegrain telescope to illuminate an array of thin-film bimetallic thermocouples. It was to determine the temperature across Jupiter's cloud tops in the 14- to 56-micron wavelength range. It had long been known that Jupiter radiates more energy than it receives from the Sun. Although its infrared emissions are strongest between 20 and 40 microns, the Earth's atmosphere is opaque. Studies at shorter wavelengths had indicated the distinctive latitudinal structure of the atmosphere to be rather more pronounced than at visual wavelengths, so close-up observations during a fly-by would not only measure the overall 'energy budget' but also yield clues as to the thermal structure and chemical composition of the upper atmosphere. Guido Munch of the California Institute of Technology was the principal investigator.

As with their predecessors, the new spacecraft were 'spinners' which rotated for stability and to optimise the spatial resolution of their sensors. This would enable the

A diagram showing how the narrow-angle photopolarimeter served as a 'spin-scan' imager as the Pioneer spacecraft rotated. An image was built up a line at a time by tilting the instrument's field of view and (on Earth) joining adjacent scan strips.

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