|Mariner II. Image credit: NASA|
Odorless and colorless hydrogen becomes liquid, and thus dense enough to pour into rocket fuel tanks, only by cooling it to a temperature of minus 253° Celsius (C). At that temperature, hydrogen causes microscopic cracks to form in almost any material, which means that rocket engine components turn brittle. Hydrogen also gradually escapes in the form of gas from almost any tank, creating a potential explosion hazard.
In the late 1950s, the U.S. Air Force and NASA contracted with the Convair Division of General Dynamics, makers of the Atlas missile, to build the Centaur, the world's first liquid hydrogen/liquid oxygen rocket stage. By 1960, the dual-engine Centaur had been made an essential element of planned U.S. robotic lunar and planetary programs such as Mariner and Surveyor. It also became a propulsion development test-bed for large hydrogen-fueled Saturn and Nova rockets. After President John F. Kennedy's 25 May 1961 "moon speech" before a joint session of Congress, Centaur development became a pacing item in the U.S. piloted lunar program.
Development of the Centaur did not, however, proceed smoothly. In January 1961, Pratt & Whitney, developers of the Centaur's twin RL-10 engines, halted testing on test-stands after a series of engine explosions. In early February 1961, NASA and contractor representatives met at NASA Headquarters in an urgent effort to sort out Centaur's problems.
Bracketing the Centaur meeting were a pair of rocket launches from Baikonur Cosmodrome in Central Asia. The first, on 4 February, launched a Venus flyby probe. The Soviet Union designated it "Heavy Sputnik" to hide its true nature after it failed to leave low-Earth orbit. The second, on 12 February, launched an identical probe. The Soviets named it Venera ("Venus") shortly after it departed Earth orbit.
|Venera 1. Image credit: TASS|
Opportunities for low-energy Earth-Venus transfers occur every 19 months. The U.S. and Soviet Union both meant to launch Venus probes during the next Earth-Venus opportunity, which spanned from mid-July to mid-September 1962. In the U.S., the Pasadena, California-based Jet Propulsion Laboratory (JPL), operated on contract to NASA by the California Institute of Technology, continued work on a roughly 500-kilogram Mariner-A flyby probe that it hoped NASA could dispatch to Venus on an Atlas-Centaur rocket.
Centaur, meanwhile, suffered new setbacks. Because of these, in August 1961, NASA asked JPL to examine how it might launch a Venus flyby probe on an Atlas-Agena rocket during the July-September 1962 transfer opportunity. The Agena upper stage, which burned hypergolic ("ignite-on-contact") propellants, had first flown successfully in May 1960. NASA tapped Atlas-Agena to launch the JPL-designed Ranger lunar probes, the first of which - Ranger 1, a Block I spacecraft intended to gather data on conditions in cislunar space - lifted off on 23 August 1961.
|Block I Ranger. Image credit: NASA|
After a study lasting just three weeks, JPL engineers told NASA that they could indeed ready a Venus flyby probe in time for the July-September 1962 opportunity. To save time, they based their probe design on Block I Ranger. JPL dubbed its makeshift interplanetary spacecraft Mariner-R. NASA approved JPL's plan and, on 28 September 1961, told the world that it would launch a flyby Mariner to Venus during the July-September 1962 period atop an Atlas-Agena rocket.
The major science goal of the Mariner-Venus 1962 mission was to scan the planet with a pair of radiometers to try to settle the contentious debate surrounding Venus's surface and atmospheric temperatures. The spacecraft would include no camera: most scientists thought it unlikely that it would glimpse the planet's surface through the thick Venusian clouds. At its extremely low transmission rate - just 8.3 bits per second - the spacecraft would need weeks to stream even a few fuzzy images to Earth.
JPL built the Mariner-R spacecraft against a backdrop of continued high-profile failures in the Ranger Program. On 20 November 1961, in a near-repeat of Ranger I, Ranger II's Agena stage stranded it in low-Earth orbit. On 31 December 1961, NASA announced that it would launch twin Mariners to Venus in July-September 1962 to help to ensure mission success.
On 26 January 1962, Ranger 3, a Block II spacecraft intended to deposit a spherical balsa wood-cushioned capsule on the lunar surface, missed the moon by about 37,000 kilometers after a cascading series of malfunctions and errors during launch and low-Earth orbit departure. Ranger IV, another Block II spacecraft, struck the moon on 26 April 1962, becoming the first U.S. spacecraft to touch another world; it returned no data, however, because it had failed to deploy its electricity-generating solar arrays and ran down its batteries soon after launch.
|Mariner I/Mariner II. Image credit: NASA|
Mariner I emerged from the fireball intact and transmitted until it splashed into the Atlantic Ocean 64 seconds later. The Atlas malfunction was quickly traced to a single missed symbol in the rocket's guidance computer code and a poor radio link between the rocket and the ground.
Mariner II lifted off on 27 August 1962. Its Atlas-Agena rocket performed as planned. Immediately after the Agena stage shut down for the second time, 211-kilogram Mariner II separated, hinged open its twin solar arrays, and, using 10 cold-gas attitude-control thrusters, gradually turned itself to point the arrays at the Sun. The Agena stage, meanwhile, turned 140° and slowed itself by venting unused propellants so that it would not follow Mariner II to Venus. As planned, a week later, Mariner II pointed its dish-shaped high-gain antenna at Earth.
Meanwhile, in Central Asia, Soviet engineers readied three Venus probes. The first, launched 25 August, and the second, launched 1 September, were designed to release spherical capsules that would enter the Venusian atmosphere and, it was hoped, survive to reach the planet's surface. Upper-stage failures meant that neither left low-Earth orbit. The first burned up in Earth's atmosphere the day after Mariner II left Earth; the second met the same fate on 5 September, two days after Mariner II's high-gain antenna locked on Earth.
The third Soviet probe, a flyby Venera, was launched on 12 September, at the end of the 1962 Earth-Venus transfer opportunity. Its launch vehicle exploded before it reached Earth orbit.
Mariner II thus became the only survivor of the five Venus mission attempts of 1962. As the 4.9-meter-wide, 3.7-meter-tall spacecraft fell Sunward toward its rendezvous with Venus, it alternated between sending to Earth engineering data describing its state of health and science data describing conditions in the interplanetary void. For example, a dust detector registered a single impact during the Mariner II mission; engineers planning future interplanetary missions breathed a tentative sigh of relief, for this indicated that spacecraft could expect fewer micrometeoroid strikes than had been previously supposed.
Mariner II withstood a solar flare starting on 23 October 1962. An ionization chamber and a Geiger counter monitored the flare's onset, peak, and gradual decline over a period of days. Though scientists admitted that they would need much more data before they could adequately assess the effects of solar flares on piloted interplanetary vessels, their initial finding was that solar activity was less of a threat than had been anticipated.
With the voyage of Mariner II well under way, the scientific debate over what it might find at Venus reached a new intensity. Measurements made from Earth indicated that the planet's surface temperature was at least 342° C, yet the temperature of the upper atmosphere was a frosty -39° C. The Venusian atmosphere almost certainly contained carbon dioxide; the jury was still out as far as oxygen and water vapor were concerned.
Young professor Carl Sagan, a member of the atmosphere radiometer instrument team, contended that Venus was an inferno, with carbon dioxide gas in its atmosphere acting like the glass in a greenhouse. Others explained away the temperature measurements by invoking a Venusian ionosphere dense with electrons.
Some scientists expected that Mariner II would find that Venus's clouds were made of tawny dust tossed aloft by violent winds; friction between dust grains, they maintained, accounted for the planet's high temperature. Others opted for older hypotheses: that Venus was covered by a carbonated water ocean; that it was a swamp-world resembling Earth in the Carboniferous era; or that it had seas of bubbling petroleum.
Mariner II was a very short mission by modern standards; the spacecraft encountered Venus after a flight of only 109 days. That 109-day flight was, however, replete with heart-stopping glitches.
For example, a mid-course correction on 4 September, when Mariner II was 1.5 million kilometers from Earth, increased its velocity by about four kilometers per hour more than planned. That meant that its closest approach to Venus would occur at a distance of nearly 33,800 kilometers from the planet, or more than twice the planned 14,500 kilometers. This raised fears that the spacecraft might pass too far from Venus to accomplish useful science. The mid-course correction was, incidentally, the first-ever rocket maneuver in interplanetary space.
One of Mariner II's solar arrays short-circuited and stopped producing electricity, mysteriously cured itself, then failed for good. Fortunately, the arrays produced more electricity than expected and Mariner II was near enough to the Sun by the time of the final failure that a single array was enough to power it.
Despite automatic cooling louvers and careful placement of heat-reflective coatings and insulation, Mariner II soon began to overheat. Many of its systems were rated for a temperature no greater than 54° C; by mid-November, however, most had exceeded that maximum by up to 22° C. The spacecraft's simple computer remained mostly reliable despite the heat until five days before the planned flyby, when it failed to transmit an expected "update pulse." Controllers on Earth opted to send Mariner II the command that would begin its Venus flyby science program rather than rely on the computer.
|Image credit: NASA|
Mariner II's closest approach to Venus - at a distance of 34,773 kilometers - took place over its dayside. The spacecraft then moved away from the planet, still transmitting.
Mariner II reached perihelion - the point in its orbit where it was closest to the Sun - just inside Venus's orbit on 27 December. A week later, on 3 January 1963, Mariner II fell silent, never to be heard from again.
The mission was judged to be an outstanding success. It went a long way toward settling debates about Venusian surface and atmosphere conditions; data it returned hinted that Venus was harsher than anyone had guessed. Some scientists remained reluctant to embrace the new data; in the Soviet Union in particular the concept of a more or less clement Venus held on for several more years.
|JPL's float in the 1963 Tournament of Roses Parade, Image credit: JPL|
Unfortunately, JPL - and, in particular, its director, William Pickering - misjudged Mariner II's significance in the NASA program. Mariner II's interplanetary voyage was the first major U.S. "first" in the space race with the Soviet Union. With that prestige goal checked off the list, the U.S. civilian space agency's attention shifted back to the troublesome Ranger and Centaur programs. Persistent calls for new funding for advanced robotic planetary missions were at best seen as a distraction; at worst, they were seen as baiting an impatient U.S. Congress.
On 18 October 1962, as Mariner II traveled toward its target, Ranger V, the third Block II spacecraft, failed mysteriously after an apparently flawless launch toward the moon. Congress soon grilled NASA officials over the failures in the Ranger Program.
In response, NASA postponed funding for Mariner-B and Voyager and tasked JPL with preparing Block III Rangers. The new moon probes would include a completely redesigned electronics system. They amounted to a climb-down; instead of delivering an intact science package to the moon's surface, they would map the equatorial Apollo landing zone close-up using six television cameras as they plummeted toward intentional destruction.
Coming so soon after Mariner II's success, the Voyager and Mariner-B postponements fueled growing tension between JPL and NASA. It was, however, a feud that JPL could not hope to win.
After publicly announcing that it was considering moving Mariner-B and Voyager to NASA Goddard Space Flight Center or NASA Ames Research Center, NASA tasked a somewhat chastened JPL with building two Mariner-R Mars flyby spacecraft. They lifted off three weeks apart in November 1964. Mariner III became entangled in its streamlined launch shroud and ran down its battery soon after separating from its Agena, but Mariner IV survived a 7.5-month interplanetary voyage and carried out the first successful Mars flyby on 15 July 1965.
When NASA decided to explore Venus again, the Centaur upper stage was available, so an advanced Mariner might have flown for the first time. NASA judged, however, that a Mariner-R derivative launched on an Atlas-Agena could do the job of taking some of the shine off the Soviet Union's expected Venus landing attempt. Mariner V flew past Venus on 14 June 1967.
Two months after the successful Mariner V flyby, Congress cancelled Voyager. Planetary exploration using robots would not come into its own until Apollo achieved its goal of beating the Soviets to the moon.
Aeronautical and Astronautical Events of 1961, Report of the National Aeronautics and Space Administration to the Committee on Science and Astronautics, U.S. House of Representatives, 87th Congress, 7 June 1962, pp. 4-6, 9-10, 26, and 50
"Centaur Slippage Drag on Apollo," H. Taylor, Missiles & Rockets, 12 February 1962, pp. 13-14
"Centaur Faces Probe by House Group," H. Taylor, Missiles & Rockets, 14 May 1962, pp. 13, 38
"Centaur Troubles Explained," Missiles & Rockets, 21 May 1962, p. 7
Press release, "Mariner Spacecraft," JPL, 19 July 1962
Press release, "Mariner Scientific Experiments," JPL, 19 July 1962
"Mariner 1 Poised for Venus Shot," Missiles & Rockets, 26 November 1962, pp. 32-33
"Mercury, Jupiter Probes Planned if Funds Permit," Missiles & Rockets, 26 November 1962, pp. 122-129
Press release, "Venus Encounter," JPL, 14 December 1962
Press release, "Mariner Radiation Experiments," JPL, 28 December 1962
"JPL Faces Mission Curtailment," H. Taylor, Missiles & Rockets, 7 January 1963, p. 14
"Mariner Unlocks Venusian Mysteries," Missiles & Rockets, 7 January 1963, p. 16
"After Venus Report. . .New Mariner Planning Pushed," H. Taylor, Missiles & Rockets, 4 March 1963, pp. 12-13
Aeronautical and Astronautical Events of 1962, Report of the National Aeronautics and Space Administration to the Committee on Science and Astronautics, U.S. House of Representatives, 88th Congress, pp. 78, 108
Venus Space Probes, Novosti Press Agency Publishing House, 1979, pp. 1-12
"Essential Data": A 1963 Pitch to Expand NASA's Robotic Exploration Programs
Multiple Asteroid Flyby Missions (1971)
The Challenge of the Planets, Part One: Ports of Call
The Challenge of the Planets, Part Two: High Energy
The Challenge of the Planets, Part Three: Gravity