Mariner 2

• 1. Introduction

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  Mariner 2 left Cape Canaveral on the 27th of August 1962, and flew for 110 days before passing within 34,773 kilometers of Venus. When it arrived on the 14th of December 1962, it became the first robotic spacecraft in history to successfully report back from a planetary encounter. No mission had ever done this before. What did it find there? And how did a probe cobbled together in an 11-month crash program manage to succeed where so many others had failed?

• 2. A Race to the Planets

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  Sputnik 1 launched on the 4th of October 1957, and the United States answered with Explorer 1 on the 1st of February 1958. After both superpowers had reached Earth orbit, attention shifted outward. The Moon came first, then Venus.

  Venus presents itself as a launch target roughly every 19 months, when its position relative to Earth minimizes the fuel required for the crossing via what engineers call a Hohmann Transfer Orbit. The first such window of the Space Race opened in late 1957, before either nation had the capability to use it. The second, around June 1959, was close enough to grasp. U.S. Air Force contractor Space Technology Laboratory drafted a plan in January 1959, but could not complete the probes in time. One of those spacecraft was repurposed as Pioneer 5, launched on the 11th of March 1960, which was designed to maintain contact with Earth out to a distance of 20 million kilometers.

  The Soviet Union did not miss its next chance. On the 12th of February 1961, Venera 1 launched toward Venus. It became the first probe to fly by the planet on May 19-20 of that year, though it had stopped transmitting on February 26 and returned no data. The Americans sent nothing during that same window. The summer 1962 opportunity would be the one that mattered.

• 3. The Crash Program

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  In July 1960, NASA contracted the Jet Propulsion Laboratory to build a 1,250-pound spacecraft called Mariner A, intended for launch on the Atlas-Centaur rocket. By August 1961, it was clear the Centaur would not be ready in time. JPL proposed a lighter vehicle using the smaller Atlas-Agena instead.

  The Atlas-Agena could lift only about half as much as the Soviet 8K78 booster. This meant the American probe would carry far less instrumentation than the Soviet Venera spacecraft of the same era, including no television camera. What emerged was a hybrid of Mariner A and JPL's Block 1 Ranger lunar probe, already under development. NASA accepted the plan, and JPL launched an 11-month effort to build what they called Mariner R, with R standing for its Ranger-derived lineage.

  Three Mariner R vehicles were built: two for launch and one for testing and spare use. All three came in within 3 pounds of the design weight of 447 pounds. Of that total, 406 pounds were devoted to non-experimental systems: fuel, maneuvering hardware, and communications equipment. Only 40 pounds could be allocated to science.

• 4. The Machine Itself

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  Fully deployed, with its two solar-panel wings extended, Mariner R stood 12 feet tall and measured 16.5 feet across. The main body was a hexagonal bus, roughly 100 centimeters in diameter, housing six separate cases of electronics.

  Two cases formed the power system. The 9,800 solar cells fed a 1,000-watt silver-zinc battery weighing 33.3 pounds. Two more cases held the radio receiver, a three-watt transmitter, and experiment control systems. A fifth case handled analog-to-digital conversion for transmitting data. The sixth contained three gyroscopes and the central computer and sequencer, the onboard brain that operated the spacecraft according to a schedule synchronized with an electronic clock tied to equipment on Earth.

  At the rear sat a 225-newton rocket motor burning anhydrous hydrazine for course corrections. Orientation was maintained by ten nitrogen-gas jet nozzles responding to signals from the gyroscopes, Sun sensors, and Earth sensors. Temperature control was both passive, using insulated and highly reflective components, and active, using louvers to shield the computer case. No test chamber existed at the time that could simulate the solar environment near Venus, so the cooling system's effectiveness would only be proven in flight.

  Carl Sagan, one of the Mariner R scientists, argued that at least a camera should be included. He was overruled, but noted that cameras could also answer questions that scientists were, in his words, "way too dumb to even pose."

• 5. Getting Off the Ground

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  Mariner 1 launched first, on the 22nd of July 1962, but veered off course when a defective signal from the Atlas combined with a software bug in the ground-based guidance computer. The Range Safety Officer destroyed the spacecraft. The software flaw that caused the loss had not been identified two days later when Mariner 2 rolled out to Launch Complex 12 on its Atlas booster, designated vehicle 179D.

  Preparations were difficult. Multiple serious problems with the autopilot surfaced, including full replacement of a servoamplifier that had suffered component damage from shorted transistors. At 1:53 AM EST on August 27, Mariner 2 lifted off from Cape Canaveral Air Force Station at 06:53:14 UTC.

  The same uncorrected software bug rode along in the rocket's code, but it resided in a section that only activated when the ground data-feed was interrupted. No such interruption occurred, and the bug caused no trouble. A different problem appeared instead: after the Agena engine cut off, the V-2 vernier engine lost pitch and yaw control and began oscillating against its stops, triggering a rapid roll of the launch vehicle. At 189 seconds after launch, the rolling stopped. A loose electrical connection in the vernier feedback transducer had been pushed back into place by the centrifugal force of the roll itself. By coincidence, the roll left the Atlas only a few degrees off its intended orientation, within range of the Agena's horizontal sensor.

  The Johannesburg tracking station acquired the spacecraft about 31 minutes after launch. Solar panel deployment completed at roughly 44 minutes, and Sun lock was established 18 minutes after that. The solar panels were generating slightly above their predicted output.

• 6. The Long Crossing

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  Cruise science experiments were switched on August 29, after all subsystems showed normal function and the battery was fully charged. A mid-course correction was needed on September 4, because the Atlas-Agena had placed Mariner slightly off target. Commands were sent at 21:30 UTC, and the maneuver sequence began at 22:49:42 UTC. The entire correction took approximately 34 minutes, after which the spacecraft lost and then re-acquired lock on both the Sun and Earth.

  The first attitude anomaly struck on the 8th of September at 12:50 UTC. The spacecraft automatically engaged its gyroscopes and shut down the cruise science instruments. The exact cause was never determined; sensors returned to normal before telemetry could sample the failure. A similar episode occurred on September 29 at 14:34 UTC. On October 31, one solar panel suffered an abrupt partial short circuit. As a precaution, cruise science was again suspended. A week later the panel resumed normal function. It failed permanently on November 15, but by that point Mariner 2 was close enough to the Sun that one panel provided sufficient power.

  Along the way, Mariner 2 was measuring things no spacecraft had ever measured. It confirmed the existence of the solar wind, a continuous stream of charged particles flowing outward from the Sun, building on the Luna 1 observations of 1959. It detected several brief solar flares and recorded cosmic rays from beyond the Solar System. Interplanetary dust turned out to be far scarcer than scientists had predicted.

• 7. Venus Revealed

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  On the 14th of December 1962, the microwave radiometer made three scans of Venus in 35 minutes, beginning at 18:59 UTC. The first scan crossed the dark side, the second near the terminator, and the third on the sunlit hemisphere. Peak temperatures in the 19-millimeter band reached 595 plus or minus 12 Kelvin near the terminator and 511 plus or minus 14 Kelvin on the light side. There was no meaningful temperature difference between the day and night sides of the planet.

  Both the microwave and infrared radiometers detected limb darkening, a pattern of cooler temperatures near the edge of the planetary disk and warmer readings near the center. The infrared data at 8.4 and 10.4 micrometers agreed with prior Earth-based measurements. The phase effect visible in the 8.4-micrometer channel indicated that heat was being transported efficiently from the sunlit side to the dark side, consistent with a greenhouse effect. Because both infrared channels showed equal radiation temperatures, scientists concluded the limb darkening was coming from a cloud layer rather than from the atmosphere alone.

  The magnetometer found no detectable magnetic field near Venus. What it measured in interplanetary space was a persistent field ranging between 2 and 10 nanotesla, consistent with observations made by Pioneer 5 in 1960. The particle detector, the Anton type 213 Geiger-Muller tube provided by the University of Iowa, logged an average counting rate of 0.6 counts per second. It detected seven small solar radiation bursts in September and October and two more in November and December. No radiation belt like Earth's Van Allen Belts was found at Venus.

  The data also yielded improved estimates of Venus's mass and the value of the Astronomical Unit. Research from the encounter later confirmed by Earth-based radar showed that Venus rotates very slowly and in a direction opposite to that of Earth.

• 8. After the Encounter

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  Following the Venus flyby, Mariner 2 resumed cruise mode. Its closest approach to the Sun after the encounter came on December 27, at a distance of 105,464,560 kilometers. The last signal received from the spacecraft arrived on the 3rd of January 1963, at 07:00 UTC, marking 129 days from launch to the end of the mission.

  Mariner 2 then entered a permanent orbit around the Sun. In 1980, the Pioneer 12 mission would confirm what Mariner 2 had suggested: Venus does possess a magnetic field, but it is at least ten times weaker than Earth's. Eugene Parker's prediction that the solar wind streams continuously through interplanetary space, a finding that Mariner 2 helped validate, had gone unconfirmed until Mariner 2 made those measurements during its crossing.