Voyager

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In contrast to the Pioneers, which were commanded from Earth, the Voyagers would have sophisticated computers that could be uploaded periodically with sequences of activities that they would execute autonomously, and they would have a high degree of fault tolerance to enable them to look after themselves. While they were to study particles and fields, they were also to be capable of remotely sensing a planet or a moon, and so could stabilise themselves. On the interplanetary cruise, and while studying a planetary magnetosphere, they would slowly rotate in order to maximise their perception of the surrounding space, but they would stabilise themselves and aim a scan platform carrying a battery of co-aligned optical instruments in order to investigate a specific target, slowly slewing the platform to compensate for relative motion. The spin-scan imagery had been enlightening, but everyone was eager to see what a 'real' camera would reveal. Voyager had two Attitude and Articulation Control Subsystem (AACS) computers, one of which would be in control at any given time while the other stood by to intervene. This would orient the spacecraft to undertake scientific activities, aim the scan platform and maintain the antenna facing the Earth. A pair of Flight Data Subsystem (FDS) computers, again one serving and the other standing by, would run the science instruments and format their data for transmission to Earth.

A diagram of the Voyager spacecraft.

Instrumentation

The Voyager instrument suite comprehensively addressed two primary themes. The aimed imaging systems mounted on the scan platform were boresighted so that they could observe a given object at a range of wavelengths simultaneously, and the other instruments continued the Pioneer investigation of the particles and fields in space.

Aimed instruments

The Imaging Science System (ISS) incorporated a 200-millimetre-focal-length f/3 wide-angle lens boresighted with a 1,500-millimetre-focal-length f/8.5 narrow-angle telescope. The field of view of the 'wide' subsystem was actually only 3 degrees across, comparable to using a 500-millimetre 'telephoto' on a 35-millimetre film camera, and the narrow-angle subsystem's field of view was one-tenth the width, so these were both really telescopic subsystems. Each subsystem had its own camera. Unfortunately, CCD solid-state technology was developed just too late to be exploited by Voyager, and a refined version of the vidicon system developed for the later Mariner spacecraft was used. It had an 11 -millimetre-square selenium-sulphur television tube which was 'read out' by a slow-scan process which took 48 seconds to produce an image comprising 800 lines, each with 800 picture elements (pixels), encoding the brightness on a 256-point scale.1 The images were monochrome, but each camera had an 8-option filter wheel so that colour images could be made (on Earth) from a succession of frames exposed through three filters. The system was considerably more capable than the spin-scan technique of the Pioneers. In addition to general imaging and specialised observations using narrow-pass filters in the visual and ultraviolet range, it would provide 'context' to assist in interpreting the results from the other optical instruments. The principal investigator of the imaging science team was B.A. Smith of the University of Arizona.

The Photopolarimeter System (PPS) utilised a 200-millimetre-focal-length f/1.4 Cassegrain telescope and a photoelectric photometer. Three wheels intersected the optical path, one providing four aperture settings ranging from 2.1 to 61 milliradians, one with filters for 8 narrow wavelength bands in the range 0.235 to 0.750 micron, and the other with 8 polarisation options, so that the size of the field, passband and polariser orientation could be set independently for any particular observation. It was to investigate particles in interplanetary space, cloud particles and gases in planetary atmospheres, and the chemical composition and texture of the surfaces of satellites.2 C.F. Lillie of the University of Colorado's Laboratory for

A diagram of the optical configuration of the wide-angle camera for Voyager.

Corrector plates

Vidicon faceplate (image plane)

Filter

Vidicon faceplate (image plane)

Filter

Shutter

A diagram of the optical configuration of the narrow-angle camera for Voyager.

Shutter

A diagram of the optical configuration of the narrow-angle camera for Voyager.

Atmospheric and Space Physics was the principal investigator until A.L. Lane of JPL took over in 1978.

The Ultraviolet Spectrometer (UVS) did not have a lens, it had a series of linear apertures set in line which served as a collimator to produce a field of view 2 by 15 milliradians, then a diffraction grating illuminated a linear array of 128 detectors, each of which measured the brightness on a 1,024-point scale to measure the range 50 to 170 nanometres with a spectral resolution of 1 nanometre (10 Angstroms). It was to investigate ultraviolet 'glows' in interplanetary space and in ionospheres, and use 'limb-sounding' measurements of the extent to which insolation was absorbed during solar occultations to profile the chemical composition of the upper regions of planetary atmospheres.3 A.L. Broadfoot of the University of Arizona was the principal investigator.

The Infrared Radiometer, Interferometer and Spectrometer (IRIS) incorporated a wide-field 'sighting scope' and a 500-millimetre-diameter Cassegrain telescope with a 4.4-milliradian (quarter-degree) field of view. A Michelson interferometer served two channels, one in the range 0.33 to 2.0 microns to measure reflected insolation and the other sensitive out to 50 microns to measure thermal emission. The IRIS was primarily to determine the chemical composition of clouds in planetary atmospheres and investigate the temperatures and pressures as functions of depth.4,5 R.A. Hanel of the Goddard Space Flight Center in Greenbank, Maryland, was the principal investigator.

Particles and fields sensors

Some of the particles and fields instruments addressed the same themes as those investigated by the Pioneers, but others were new.

The Magnetic Fields (MAG) experiment incorporated four magnetometers, two high-field instruments on the body of the spacecraft and two low-field instruments mounted mid-way, and at the far end of a 13-metre boom in order to be clear of the spacecraft's own magnetic field. Each magnetometer sensed the ambient field in three orthogonal axes so that the strength and direction of the field could be measured. The instrument was sufficiently sensitive to measure a magnetic field one-millionth of the strength of that at the Earth's surface. It was to study the magnetic fields present in interplanetary space and planetary magnetospheres. The principal investigator was N.F. Ness of the Goddard Space Flight Center.

In the presence of a magnetic field, the charged particles (electrons and ionised atomic nuclei) of a plasma are subjected to a force, and phenomena arise which have no counterpart in a purely gaseous state. In particular, oscillations in the density of the plasma as the charged particles are herded by the magnetic field produce waves, with resultant fluctuations in the strength of the electric field. The Plasma Wave System (PWS) used two 10-metre-long antennas projecting from the body of the spacecraft in a 'V' shape. To determine the direction of the flows resulting from the passage of plasma waves, a wideband waveform receiver and a spectrum analyser measured the variations in the electric field component over the 10 Hz to 56 kHz range.6 The principal investigator was F.L. Scarf of Thompson Ramo & Wooldridge (TRW) in Redondo Beach, California.

In 1955, Jupiter was serendipitously discovered to be a strong and highly variable radio source in the decimetric and decametric bands, prompting the inference that the planet had a strong magnetic field, and this had been confirmed by Pioneer 10. The Planetary Radio Astronomy (PRA) experiment used the same antennas as the PWS to detect radio emissions in two bands, one from 1.2 kHz to 1.2 MHz at intervals of 19.2 kHz and a bandwidth of 1 kHz, and the other from 1.2 to about 40 MHz at intervals of 307 kHz and a bandwidth of 200 kHz. It operated by cycling through the 198 discrete frequencies in six seconds and reporting their intensities on a 256-point logarithmic scale. Measurements of the polarisation and spectral characteristics of emissions from planets would enable the plasma densities in the generation regions to be inferred, so in contrast to the other instruments which sensed the spacecraft's immediate environment, the PRA was the remote-sensing member of the particles and fields suite.7 The principal investigator was J.W. Warwick of Radiophysics Incorporated in Colorado.

Whereas PWS studied the flows of charged particles indirectly by their electric fields, the Plasma Science (PLS) experiment collected them. It had a pair of Faraday cup plasma detectors: one was aimed in the general direction of the Earth and the other was mounted orthogonally. The Earth-facing detector had three apertures which, between them, were open to that entire hemisphere. The side-looking detector had a narrower field of view. They were sensitive to particles with energies ranging from 10 eV to 5.95 keV - the latter moving at about 1 per cent of the speed of light. It was to determine the composition and energy of the particles, as well as their direction of flow in interplanetary space and in planetary magnetospheres.8 The principal investigator was H.S. Bridge of the Massachusetts Institute of Technology.

The Low-Energy Charged Particle (LECP) experiment had two instruments on a rotating mount. The low-energy magnetospheric particle analyser incorporated eight solid-state detectors that could discriminate electrons from ions and, between them, were sensitive to charged particles with energies from 10 eV to 15 keV. It was to investigate the composition of the plasmas in interplanetary space and in planetary magnetospheres.9 Its name notwithstanding, the Low-Energy Particle Telescope was to investigate the solar wind by extending the energy range to several millions of electron volts (the top end of the PLS range was several thousands of electron volts).

S.M. Krimigis of the Applied Physics Laboratory of the Johns Hopkins University in Maryland was the principal investigator.

The Cosmic Ray Science (CRS) instrument utilised two particle telescopes, one to measure the energy spectra of heavy nuclei (those with atomic masses ranging up to that of iron, and for some elements discriminating between isotopes) across a broad energy range, and one to measure the energy spectrum of electrons in the 5 to 110 MeV range, in order to determine the cosmic ray component of the plasmas found in interplanetary space and in planetary magnetospheres.10 The principal investigator was R.E. Vogt of the California Institute of Technology in Pasadena.

The PLS, LECP and CRS instruments characterised the charged particles striking their detectors and, between them, characterised the fluxes of electrons and nuclei in the space through which the spacecraft flew.

In addition, the spacecraft's X-Band and S-Band radio transmitters provided the basis for a number of Radio Science Subsystem (RSS) experiments.11 The principal investigator was G.L. Tyler of Stanford University's Center for Radar Astronomy. During an occultation, the manner in which the signal was refracted would serve to characterise the occulting object's atmosphere, if it possessed one, and the Doppler effect enabled the object's gravitational field to be charted, its moment of inertia to be calculated, and the state of its interior to be inferred by the manner in which the spacecraft's trajectory was deflected. In a sense, this was 'free' science as it required no specific instrumentation.

Irrespective of whether the Voyagers were rotating or 3-axis stabilised, the 3.66-metre-diameter dish of the high-gain antenna was always aimed at the Earth. The communications system was rated at 115 kilobits per second from Jupiter to handle the output of imagery - a rate almost 100 times that of the Pioneers. At Saturn, the data rate would be 45 kilobits. For redundancy, there were two 23-watt transmitters. A 500-megabit reel-to-reel tape recorder was included to act as a buffer during times when the spacecraft was occulted, or when it was securing data faster than it could transmit it. A trio of RTGs mounted in line on a single boom provided 400 watts of power for the spacecraft's systems and instruments. The instruments accounted for 118 kilograms of the launch mass of 815 kilograms - a rather better ratio than on the outer Solar System Pioneers.

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