In 1960, the Pasadena, California-based Jet Propulsion Laboratory (JPL), a spaceflight engineering laboratory managed by California Institute of Technology on contract to NASA, commenced study of Voyager, a robotic spacecraft program for exploring Mars and Venus in the late 1960s and 1970s. NASA Headquarters formally approved Voyager in 1964. Cuts in NASA's space science budget, debate over how Voyager should be managed and launched, and new Mars atmosphere data from the Mariner IV flyby (July 1965) delayed NASA's push for formal Voyager start-up until January 1967, when President Lyndon Johnson's Fiscal Year (FY) 1968 NASA budget called for $71.5 million for the new program.
In January 1967, NASA's Office of Space Science and Applications published a 26-page brochure as part of its efforts to move Voyager from planning to development. It constituted an introduction (and sales pitch) aimed at members of Congress and other individuals who would need to support Voyager if it would become part of NASA's approved program for the 1970s.
In the brochure's foreword, Homer Newell, NASA Associate Administrator for Space Science and Applications, explained that Voyager's chosen launch vehicle was the "awe-inspiring" Saturn V. One three-stage Saturn V rocket would launch two 12-ton Voyager spacecraft toward Mars. For comparison, Mariner IV, launched on an Atlas-Agena D rocket in November 1964, had had a mass of only 574 pounds. Newell wrote that
NASA, the booklet explained, favored Mars over Venus as Voyager's first target because "the high surface temperatures on Venus make the existence of extraterrestrial life less likely than on Mars" and because "the thin, normally transparent Martian atmosphere is conducive to detailed scanning of its surface features from orbit." In addition, "manned landings on Mars will someday be possible. . .[but] they may not be possible on Venus."
The brochure placed Voyager within an evolutionary Mars exploration program designed to take advantage of low-energy Earth-Mars transfer opportunities that occur every 26 months. It retroactively included in its program 573.5-pound Mariner IV, which had returned 21 close-up images of about 1% of Mars's surface after flying past the planet on July 14-15, 1965. A radio-occultation experiment using Mariner IV had revealed that Mars's atmosphere is less than 1% as dense as Earth's. The brochure acknowledged that the new atmosphere data had forced a redesign of the Voyager landing system, which had been designed for a martian atmosphere with 10% of Earth's density. The new design would replace lightweight parachutes with heavier landing rockets. According to historians Edward Clinton Ezell and Linda Neumann Ezell, writing in their book On Mars (NASA SP-4212, 1984), the redesign bumped Voyager's projected cost above $1 billion.
The brochure called for new Mariner Mars flybys in 1969 and 1971. In 1969, the spacecraft would photograph the entire visible disk of Mars during approach and return detailed images of 10% of the planet. During the 1971 flyby, the Mariner spacecraft would drop a small sterilized probe into Mars's atmosphere to measure pressure, density, temperature, and composition as it plummeted toward surface impact and destruction. The flyby spacecraft would act as a relay for probe signals and would image 10% of Mars at high resolution.
The first Voyager missions would take place in 1973. A battery-powered Voyager lander with a mass of up to 860 pounds would seek life and observe changes at the landing site over several days, and a solar-powered Voyager orbiter would observe seasonal changes on a planet-wide scale for months.
The Voyager 1975 orbiters and landers would all rely on Radioisotope Thermoelectric Generators (RTGs) for electricity. This would allow the landers to survive for one martian year (about two Earth years); that is, long enough for them to observe seasonal changes at their landing sites. Voyager could land up to 1100 pounds on Mars in the 1975 opportunity. The 1977 and 1979 Voyager missions would see introduction of a lander-deployed Mars surface rover and biological experiments specially designed to study any life found in 1973 and 1975. A Voyager lander could deliver up to 1500 pounds to Mars's surface in 1977 and 1979.
The brochure then detailed the 1973 Voyager Mars mission, which it described as typical. Voyagers would lift off from the Kennedy Space Center Complex 39 launch pads built for the Apollo Saturn V launches. The 1970s Mars launch windows would last at least 25 days and would include daily one-hour launch opportunities. Voyager Saturn V rockets would be identical to Apollo lunar Saturn Vs; that is, each would consist of an S-IC first stage with five F-1 engines, an S-II second stage with five J-2 engines, and an S-IVB third stage with one J-2.
The twin Voyager lander/orbiter combinations would be stacked atop the S-IVB third stage within a protective launch shroud. The first stage would burn for 2.5 minutes and fall away at an altitude of 39 miles, then the second stage would burn for 6.5 minutes and fall away at an altitude of 114 miles. The third stage would fire briefly to place itself, the twin Voyagers, and their launch shroud into Earth parking orbit.
Voyager's launch shroud would measure 22 feet in diameter - the same diameter as the S-IVB stage - and would have a mass of 4.7 tons. Once in Earth orbit, the shroud's top section would jettison, exposing the upper Voyager to space. The S-IVB stage would then ignite a second time to push the Voyagers out of Earth orbit toward Mars. After S-IVB shutdown, the upper Voyager would separate. The shroud's cylindrical central portion would then jettison to expose the lower Voyager, which would separate from the S-IVB a short time later. In the 1973 opportunity, each Voyager would have a mass of 10.25 tons after separation.
During the months-long interplanetary cruise, the twin Voyagers would turn their ring-shaped body-mounted solar arrays toward the Sun. They would use course-correction engines based on the Minuteman missile second-stage engine to place themselves on precise paths to Mars. The S-IVB trailing them would make no course adjustments, so would miss the planet by a wide margin. Because the Voyagers would perform course corrections at different times, they would arrive at Mars up to 10 days apart.
As the Voyagers neared Mars, each would fire its main rocket engine to slow down so that Mars's gravity could capture it into an elliptical orbit. Initial orbit periapsis (low point) would be about 700 miles above the planet, while apoapsis (high point) would occur beyond the orbit of Deimos, Mars's outer moon, which on average orbits 14,080 miles above the planet. The brochure noted that the leading Voyager main engine candidate was a modified Apollo Lunar Module descent engine. The complete Voyager propulsion system with propellants would weigh 6.5 tons. After orbit insertion, the Orbiter's instruments would turn toward Mars to image candidate target landing sites for the landing capsule.
The 2.5-ton Voyager landing capsule would eject its sterilization canister, separate from the orbiter beyond Deimos, and fire a 415-pound solid-propellant deorbit rocket to change its path so that at periapsis it would intersect the martian atmosphere. The deorbit rocket would then eject.
The Voyager landing capsule would enter Mars's atmosphere moving at between two and three miles per second. Aerodynamic braking using the 20-foot-diameter conical heatshield would cut speed to between 400 and 1000 feet per second by the time the capsule fell to within 15,000 feet of the surface. The heatshield would eject, then the capsule would fire its descent engines and deploy a supplemental parachute.
During descent, the capsule would image the surface and collect atmospheric data. It would release the parachute, then slow to a hover 10 feet above Mars. Its descent engines would then shut off, allowing it to drop to a gentle touchdown on three legs.
The 1973 capsule would include 300 pounds of science gear. Over several days, it would search for water and life, measure cosmic and solar radiation, and study the atmosphere - it would, for example, measure the amount of dust in the atmosphere. The 1973 orbiter, for its part, would include 400 pounds of scientific instrumentation, which it would use to map Mars's surface in detail and determine its composition, search for surface changes, and measure solar and cosmic radiation. The orbiter would also act as a martian weather satellite. It would use its main engine to change orbit several times during its two-year operational lifetime, allowing detailed study of much of Mars.
Congress refused to fund Voyager in FY 1968, in part because it had come to be seen as a lead-in to a costly post-Apollo manned Mars/Venus flyby program, and also because the Apollo 1 fire (Jan. 27, 1967) undermined confidence in NASA. The U.S. civilian space agency formally abandoned its Voyager plans in September 1967.
In 1968, however, Congress agreed to fund the Viking program in FY 1969. Like Voyager, Viking would emphasize the search for life and would use twin spacecraft, each including a lander and an orbiter. Unlike its ill-starred progenitor, however, Viking made no claim to be a precursor for a piloted Mars mission. In addition, Viking would be managed by NASA's Langley Research Center, not JPL, though the later would build the Viking orbiters. Many interpreted assignment of Viking management to Langley as a congressional rebuke to JPL for its independent mindset; efforts to preserve NASA centers as Apollo spending began to wind down may also have played a role.
Twin flyby Mariners 6 and 7 flew by Mars in 1969, and Mariner 9 orbited the planet in 1971-1972. After skipping the 1973 Mars launch opportunity, NASA launched Viking 1 on a Titan-IIIE rocket with a Centaur upper stage on Aug. 20, 1975. Viking 1's Mariner-based, solar-powered orbiter and RTG-powered lander together had a launch mass of 2.56 tons. After deploying the lander in Mars orbit, the Viking 1 orbiter had a mass of about 1980 pounds.
The Viking 1 lander became the first spacecraft to land successfully on Mars on July 20, 1976, seven years to the day after Apollo 11 became the first manned moon lander. The lander had a mass of about 1320 pounds after touchdown; of this, about 93 pounds comprised scientific instrumentation. Viking 2 launched from Earth on Sept. 9, 1975, and its lander touched down on Sept. 3, 1976.
NASA and JPL recycled the Voyager name in 1977, applying it to twin Mariner-based Jupiter-Saturn spacecraft. Voyager 2 left Earth first, on Aug. 20, 1977, on a Titan III-E/Centaur, and Voyager 1 launched 16 days later, on Sept. 5. Voyager 1 passed Voyager 2 on Dec. 15, and flew closest by Jupiter on Mar. 5, 1979. Voyager 2 flew by Jupiter on July 9, 1979. Voyager 1 flew by Saturn, its last planetary target, on Nov. 12, 1980; Voyager 2 passed Saturn and bent its course toward Uranus on Aug. 25, 1981. It passed Uranus on Jan. 24, 1986, and continued on to Neptune, passing that distant world on Aug. 25, 1989.
Both Voyagers remain operational, and are more than 22 years into their "Interstellar Mission." At this writing, Voyager 1 is 119.9 times Earth's distance from the Sun; radio signals traveling at the speed of light need more than 16 hours to reach it. Voyager 2, which dove below the plane of the Solar System after departing Neptune, is 98.6 times Earth's distance from the Sun; radio signals need almost 14 hours to reach it.
Reference:
Summary of the Voyager Program, NASA Office of Space Science and Applications, January 1967.
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