Because of the many delays in the development of the Centaur upper stage, that used cryogenic propellants, starting from the winter of 1961 NASA began thinking about scrubbing the launch of two Mariner A probes to Venus in 1962 and instead launching in 1964 two more capable Mariner B. In any case, the launch window of 1962 would go unused, with the risk that the Soviets set some new, humiliating, first.
JPL then proposed the Mariner R project: launching two simplified Ranger Block I experimental lunar probes to Venus using an Atlas-Agena B rocket. Stopping Mariner A and starting Mariner R was estimated to cost some three to four million dollars, but the image return was well worth it. On August 31, 1961, NASA approved the JPL idea and canceled Mariner A altogether, some components of which was already being built. In hindsight, NASA's decision was a very risky one, for Ranger succeeded only at the seventh try, in 1964.
Two research groups were built: the first to determine the modifications to be made to the lunar Ranger probe, the other working with Lockheed on the Agena B.

The probe was based on an hexagonal base of 102 cm diameter to which were attached two rectangular solar panels capable of generating a maximum of 222 W of power. One of the panels carried 5,810 solar cells, the other 4,900 and was tipped by a small "solar sail" able to generate some thrust for attitude control. The total span of the probe, with the solar panels unfolded was 503 cm. Inside the base was accommodated the attitude control system and a communication system requiring only 3.5 W of power (compare this figure with Pioneer-5!). A parabolic high gain antenna of 48.9 cm radius was hinged at one base of the probe while on the other side was mounted a pyramidal truss tower which carried the scientific instruments and the omnidirectional antenna. At the center of the base was mounted the trajectory correction system, a monopropellant (hydrazine) engine of 225 N thrust. Mariner R weighted 203.6 kg, 18.5 of which was accounted by the seven scientific instruments.
The most important scientific instruments was the microwave radiometer, a tilting parabolic antenna of 48.5 cm diameter and 19.3 cm of focal length which measured the heat irradiated by the planet in two different wavelengths. The instrument was derived from a three channel prototype built for Mariner A. A second radiometer, mounted on the same tilting platform, measured the irradiated heat in the infrared part of the spectrum.
The other instruments were a micrometeorite sensor, a solar plasma sensor, a three axis flux gate magnetometer, an ion chamber and a charged particle sensor. The main aim of the scientific payload was to measure the temperature range on the planet disc, possibly including both the sunlit and the night hemisphere and to measure any "brightness" variation  in the microwave part of the spectrum across the disc. In fact, if the planet was found to be as "bright" at the limb as at the center of the disc, the ground measured temperature of 600 K may have been caused only to the Venusian ionosphere. If instead the brightness varied across the disk, this very high temperature was the result of some atmospheric phenomena.

Two twin Mariner R probes were readied in less than a year and carried from Pasadena (California) to Cape Canaveral for launch. The first, Mariner-1 was launched on July 22, 1962. Less than five minutes after lift off and a few seconds before Agena staging the rocket was destroyed upon ground command for getting out of the launch corridor and into commercial shipping lines. The telemetry of the probe was received for one minute more as it plummeted into the Ocean. In the following weeks, it was determined that the tracking station had lost contact with the probe four times and that a "bug" into the FORTRAN program controlling the rocket's attitude (a missing - ) enabled it to interpret the requests of communication coming from the ground as commands, sending the rocket out of its planned trajectory.
The second probe, whose Atlas launcher incorporated also the software bug was launched on August 27. This time neither the launch was not nominal and the roll control of the Atlas was close to its limits when staging occurred. As luck wanted, 26 minutes after lift off, Mariner-2 was on a Type I solar orbit, heading for Venus. The minimum approach distance was 375,900 km, well superior to the optimum 29,000. A course correction was made eight days after launch which moved the closest approach to 34,773 km (six times the Venus diameter).
The 109 days of cruise were not too easy: the probe lost attitude for three minutes probably because of a micrometeorite impact, the Earth sensor, controlling the high gain antenna position risked loosing visual contact with our planet and finally one of the solar panels ceased producing energy, shutting down all of the functioning scientific instruments (all except the two radiometers). The panel then recovered and was lost again, for good, fifteen days later. When this happened, however, the distance had decreased so that one solar panel was enough for sustaining all the fly-by operations.
During the cruise the probe made some detailed studies of the solar wind and of the interplanetary magnetic fields. It was discovered that the field intensity, near our planet is 60 millionth of gauss and that its direction describes a "lawn sprinkler" figure: a spiral with the Sun at the center. The micrometeorite sensor detected many impacts close to the Earth but just two thereafter. This possibly indicated the lunar origin of micrometeorites.
The morning of December 14, 1962, the radiometers were switched on and at 19.59 GMT the probe reached minimum distance, while collecting 35 minutes of scientific data. The radiometers, whose scans were programmed to account for the low planetary ephemeris accuracy, collected 18 temperature measures: first were collected five readings on the night hemisphere (490 K mean temperature), then eight across the terminator (595 K) and finally five on the sunlit hemisphere (511 K), measuring a non uniform "brightness" across the disk. The derived atmospheric models indicated a ground temperature of 700 K, well above the one expected for a "twin planet" of the Earth. Some fluctuation of up to 11 K on the southern side of the terminator hinted at the existence of a possible mountain range (or a lake!). While the probe did not investigate the composition of the atmosphere, it was possible to make some hypothesis on it: the emerging model depicted the Venusian atmosphere ad being almost totally deprived of water and very rich of carbon dioxide and of sulfuric acid fog. According to this model the ground pressure should have been 20,000 hPa (20 atm). The on board magnetometer did not discover any magnetic field greater than one tenth of the terrestrial one, so that Venus does not have anything similar to the Van Allen Belts, but is just creating a cavity in the solar wind. Finally, the probe was used for measuring the mass of Venus, very close to 81.485 % of the Earth's mass.
After the fly-by the probe reached perihelion (105,464,560 km) and communicated for some day, before falling silent on January 3, 1963, 86.7 million km from Earth and 129 days after launch.
Despite the very low scientific productivity, Mariner-2 demonstrated how to prepare and develop a successful interplanetary mission.
A third Mariner R, scheduled to be launched in 1964 and carrying an improved microwave radiometer was not launched after being judged superfluous.

A diagram showing the scanning pattern of the Mariner-2  microwave radiometer.  (Author's image adapted from Schneiderman's article)

Bibliography
Barath F. T., et al: Mariner 2 Microwave Radiometer Experiment and Results, The Astronomical Journal, vol. 69, No. 1, pp. 49-58 (this article is available on line on NASA Astrophysic Data System)
Corliss, W. R.: Space Probes and Planetary Exploration, Princeton, Van Nostrand
Forney R. G.: Mariner II - Attitude Control System. Paper presented at the XIV IAF Congress, Paris, 1963
Pollack, J. B., Sagan, C.: An Analysis of the Mariner 2 Microwave Observations of Venus, The Astronomical Journal, vol. 150, October 1967, pp. 327-344 (this article is available on line on NASA Astrophysic Data System)
Schneiderman, D.: Mariner II - An Example of an Attitude-Stabilized Space Vehicle. Paper presented at the XIV IAF Congress, Paris, 1963
Wilson, A.: Solar System Log, London, Jane's
Mariner Reveals 800 F Venus Temperature, Aviation Week and Space Technology, March 4, 1963, pp. 30-31
 

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