The first JPL Solar System exploration program was Vega, begun and ended in 1959.
After Vega collapsed, JPL began studying a new project, called project Mariner, which initially comprised two different probes: Mariner A and Mariner B.
Mariner A was a 487-686 kg mass space probe that was to fly within 27.000 km of planet Venus.
The launch was initially scheduled for 1962 on an Atlas-Centaur rocket. The probe was built around an hexagonal central unit which contained the on-board avionics. To this unit were hinged a parabolic high gain antenna to communicate with mother Earth, two wings of solar panels of 1.8 sq. meters total area, able to provide 300 W of power in the vicinity of Earth and 600 W in the vicinity of Venus. Three booms carried the scientific instruments and an omnidirectional antenna was mounted on top of the probe.
For the flight to Venus Mariner A was to carry at least six scientific instruments: a microwave radiometer to measure the temperature of the planet, an ultraviolet spectrometer to determine the composition of the atmosphere, a magnetometer to investigate the possible existence of a venusian magnetic field and to improve the knowledge of the interplanetary magnetic field, a plasma sensor, a radiation counter and a micrometeorite sensor.
Some of the instruments, for which orientation with respect to the planet was not important, were mounted on separate booms, while the ultraviolet spectrometer and microwave radiometer were on a two degree of freedom mobile platform, able to be pointed both in azimuth and elevation, which was kept pointed at Venus by a planetary sensor.
The probe was three axis stabilized and carried a motor for trajectory correction. It was planned to test the motor to depletion after the fly-by.
If the fist flight of the probe was successful, other versions could have carried a mapping radar (sic!) and a TV camera.
It was even proposed to fly a Mariner A probe, equipped with a vidicon telescope, to Mars.

Click here to see an image of the Mariner A probe modified to fly-by Mars

What effectively killed Mariner A was its launcher Centaur, whose tests began in 1961. Centaur was not operational until 1966, four years later than planned!
So, after Mariner A was delayed from 1962 to 1964, JPL came up with another idea: launching a modified version of its Ranger Block I experimental lunar probe (which had failed on two previous occasion) during the 1962 window to Venus. Thus, Mariner A was canceled on August 30, 1961 and Mariner R was born.

BIBLIOGRAPHY
"Mariner to Scan Venus' Surface on Flyby", Aviation Week, June 12, 1961, pp. 52-57.
Ezell, E. C., Ezell. L. N. "On Mars", NASA SP-4212, available on-line.
"Space: the new frontier", NASA publication, 1962.

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