|Robotic flyby: encapsulated in a streamlined shroud, the Mars-bound Mariner IV spacecraft awaits launch from Florida atop an Atlas-Agena rocket. Image credit: NASA.|
The piloted Mars/Venus flyby concept — the hallmark of which was a Sun-centered trajectory intersecting one or more planets and beginning and ending at Earth — was first proposed by Gaetano Crocco in 1956, at a time when robotic probes to other worlds were scarcely mentioned by champions of spaceflight. A decade later, when NASA performed its first high-level NASA-wide study of the piloted flyby concept, that situation had changed markedly — thanks largely to a 261-kilogram (575-pound) spacecraft called Mariner IV.
Launched on 28 November 1964, Mariner IV demonstrated for the first time that a robotic spacecraft could return useful data from the vicinity of Mars. The success of Mariner IV — it captured 21 images of the planet as it flew past on 15 July 1965, and slowly transmitted them to Earth over the following month — gave ammunition to those who called into question the utility of piloted flybys.
The Mariner IV images revealed a cratered, arid surface, and its radio occultation experiment found an atmosphere only 1% as dense as Earth's. Mars, seemingly so promising for the development of life, suddenly appeared almost as inhospitable as Earth's barren Moon.
Supporters of Mars exploration were quick to point out, however, that the little spacecraft had imaged only about 1% of Mars at a resolution so low that, had it flown past Earth, it would not have found life. NASA and the planetary science community persisted with plans to launch new scientific exploration missions to Mars in the Mariner and Voyager programs (the latter not to be confused with the late 1970s Voyager originally called Mariner Jupiter-Saturn — please see "More Information," below).
|Cratered planet: One of the sharpest Mariner IV images of Mars. Image credit: NASA.|
In like fashion, piloted flyby proponents — such as the participants in the Planetary Joint Action Group (JAG), led by Edward Z. Gray of the NASA Headquarters Office Manned Space Flight (OMSF) — proceeded with their planning meetings in spite of Mariner IV's achievement. The first Planetary JAG meeting, held at the NASA Manned Spacecraft Center (MSC) in Houston, Texas, on 4 May 1966, gave roughly equal priority to piloted Mars/Venus flybys and a piloted Mars landing mission.
By the Planetary JAG's second meeting, held at NASA Headquarters in Washington on 9 June 1966, piloted flybys had assumed primacy. "The object of the effort," the Group declared, "is a Mars/Venus Fly-by capability by 75 with a manned Mars landing in view."
Ironically, given the negative effect of Mariner IV, a new class of robotic probe was largely responsible for the positive change in the piloted flyby concept's fortunes. The Mars Surface Sample Returner (MSSR), first discussed at the NASA Headquarters meeting, would borrow technology and experience from Mariner and Voyager. It would ride to Mars on board the piloted flyby spacecraft. After carefully checking out and servicing its systems, the crew would release it several days before the flyby. It would reach Mars hours ahead of the flyby spacecraft, land, collect samples of martian air and dirt, and launch them into space.
The astronauts would capture the sample container minutes after their closest approach to Mars. They would analyze the sample in a lab on board the piloted flyby spacecraft within an hour of its launch from Mars, before any living things it contained could perish.
More than 50 engineers from across NASA attended the Planetary JAG's third meeting at NASA Kennedy Space Center (KSC) on 29-30 June 1966. By then, the Group had begun work on a pair of documents: a briefing to the high-level managers of the NASA Management Council and a preliminary Program Development Plan (PDP). Bellcomm, NASA's Washington, DC-based planning contractor, provided the Group with technical assistance.
The briefing took place a month later — the Council's response is not recorded, but it was apparently favorable enough for work on the PDP to continue. The completed PDP, labelled "For Internal Use Only," bore the date 3 October 1966.
The Planetary JAG envisioned that the 1975 Mars flyby, launched out of low-Earth orbit between 5 September and 3 October of that year, would be the first in a series of four Mars and Venus flyby missions. It would last 667 days. The other three were a 715-day Venus/Mars/Venus flyby mission launched in February 1977, a 625-day Venus/Mars flyby mission launched in December 1978, and a 686-day Mars flyby mission launched in November 1979. Regardless of the number of flybys it contained, no piloted flyby mission would require more than modest course-correction propulsion after departure from low-Earth orbit.
Only the 1975 mission was described in detail in the PDP. It would follow a low-energy twilight trajectory that would take it past Mars and would reach aphelion — its greatest distance from the Sun — on the inner edge of the Asteroid Belt, at 2.2 times the Earth-Sun distance. The term "twilight" referred to the Mars pass geometry; flyby spacecraft Mars periapsis (closest approach) would occur over the line separating the dayside from the nightside.
Seven years of development and testing would precede launch of the 1975 mission. The flyby program nested within a series of extant and anticipated NASA programs and missions, which the Planetary JAG stated provided "a sound base for the development of a manned flyby system." The piloted flyby spacecraft, for example, would be based on hardware and experience from Apollo and its planned follow-on, the Apollo Applications Program (AAP). Set to begin as early as 1968, AAP was envisioned primarily as an Earth-orbital space station program, but was also expected to include "limited lunar exploration" beyond the Apollo baseline.
AAP astronauts would carry out progressively longer missions on board Orbital Workshops, achieving a one-year stay in weightlessness in 1970 or 1971. The AAP Orbital Workshop would take the form of a converted S-IVB stage, which formed the second stage of the Saturn IB rocket and the third stage of the Saturn V. A third Orbital Workshop launched in 1972-1973 would test prototype piloted flyby spacecraft subsystems.
In the same timeframe, robotic precursor probes would provide important data to engineers designing piloted flyby mission systems. A Mariner spacecraft would release a probe to plumb the martian atmosphere in 1971 and possibly again in 1973 and probes would assess the meteoroid population between the orbit of Venus and the inner edge of the Asteroid Belt in 1970 or 1971. The former would aid in piloted flyby probe design and the latter might permit reduced piloted flyby spacecraft meteoroid shielding. Weight saved by reducing shielding might permit a heavier scientific payload on board the piloted flyby spacecraft.
Late 1972 would see a test of a Earth Entry Module (EEM) derived from the Apollo Command Module design but capable of withstanding Earth-atmosphere reentry at the end of the piloted flyby mission at up to 50,000 feet per second — that is, 14,000 feet per second more than the maximum planned Apollo lunar-return speed. In 1973-1974, four astronauts — the number planned for the piloted flyby spacecraft crew — would rehearse the piloted flyby mission for a year in low-Earth orbit on board a prototype 180,000-pound (81,650-kilogram) piloted flyby spacecraft. They would ride to Earth orbit and return to Earth in a 51,500-pound (23,350-kilogram) CSM launched with the flyby spacecraft.
The piloted flyby spacecraft and CSM would reach Earth orbit atop a two-stage "Improved" Saturn V rocket, the workhorse launch vehicle of the piloted flyby program. The Improved Saturn V would comprise an S-IC first stage stretched 20 feet (6.6 meters) to hold additional propellants for its beefed-up F-1 engines. In addition to launching the piloted flyby spacecraft and CSM, it would be used to rapidly launch a series of three 231,400-pound (105,000-kilogram) Modified S-IVB (MS-IVB) stages which would link up with the piloted flyby spacecraft in 485-kilometer (300-mile) assembly orbit to form an Orbital Launch Vehicle (OLV).
The MS-IVB stages would each contain 195,800 pounds (88,800 kilograms) of cryogenic liquid hydrogen fuel and liquid oxygen oxidizer. Even with added insulation, the liquid hydrogen would boil and escape, so the stages would have to be used within about 50 hours of launch from NASA KSC to ensure that they would contain sufficient fuel to launch the piloted flyby spacecraft out of Earth orbit on flyby course past Mars.
The Planetary JAG envisioned launching the three MS-IVB stages 12 hours apart. To make possible this "salvo" launch campaign, a third Launch Complex 39 Saturn V launch pad would need to be built beside the two NASA had already built for the Apollo Program.
|Orbital Launch Vehicle (OLV) assembly configurations. Image credit: NASA/DSFPortree.|
OLV assembly came in for special consideration in the Planetary JAG PDP. In 1973, a crew in a CSM launched on a Saturn I-B rocket would dock with the prototype piloted flyby spacecraft after the one-year test crew returned to Earth. Shortly thereafter, NASA would launch two MS-IVB stages in rapid succession. The CSM, docked with a special docking collar at the front of the flyby spacecraft, would act as a space tug to push the piloted flyby spacecraft to a rendezvous and docking with the first MS-IVB in assembly orbit, then would push the piloted flyby spacecraft/MS-IVB combination to a rendezvous and docking with the second MS-IVB. The stages would then separate from the prototype flyby spacecraft and ignite in succession to carry out an MS-IVB flight test. The CSM crew would undock from the docking collar and return to Earth.
A full-up OLV assembly rehearsal involving a crew in a CSM, the piloted flyby spacecraft prototype, and three MS-IVB stages would follow in mid-1974. The CSM would dock with the docking collar on the front of the prototype, push it to a docking with the first MS-IVB, push the prototype and MS-IVB to a docking with the second MS-IVB, and finally push the prototype and the two MS-IVBs to a docking with the third MS-IVB.
After docking with the piloted flyby spacecraft prototype, the crew in the CSM would have difficulty seeing the MS-IVB stages during docking maneuvers. The Planetary JAG proposed that strategically placed TV cameras and a viewscreen in the CSM could enhance visibility. In addition, an astronaut with an Astronaut Maneuvering Unit backpack might position himself where he could see the docking operation and call out directions to the crew in the CSM.
The Planetary JAG suggested that, following CSM separation, the three-stage OLV might launch the piloted flyby spacecraft prototype on an interplanetary trajectory without a crew. Alternately, the OLV might boost it into a high Earth orbit where it could serve as a space station.
The Planetary JAG assumed that the OLV for the 1975 piloted flyby mission would be ready for Earth departure on 5 September 1975, at the opening of its month-long launch window. After OLV assembly and prior to Earth departure, the flyby crew in the CSM would undock from the docking collar on the front of the piloted flyby spacecraft and redock at an airlock port on its side. After entering the flyby spacecraft's drum-shaped, two-deck Mission Module (MM), their home for the next 22 months, they would cast off the CSM and the docking collar. The MS-IVB stages would then ignite, burn to depletion, and separate in succession to commence a 130-day Earth-to-Mars transfer.
During flight to Mars, the flyby crew would operate a 40-inch astronomical telescope almost continuously. The Planetary JAG took pains to stress the value of their research program, which would include multispectral solar, stellar, planetary, and asteroid observations. Solar images were expected to contain details as small as 95 miles (150 kilometers) across. The telescope would be housed when not in use within the flyby spacecraft's tightly packed Experiment Module (EM).
In total, at launch from Earth the EM would contain 30,190 pounds (13,690 kilograms) of experimental apparatus, including six robotic probes and the biology lab for analyzing the samples collected by the MSSR probe. In addition to the 11,692-pound (5300-kilogram) MSSR, probes would include a 1290-pound (585-kilogram) Lander for surface photography, geology/geophysics, and atmosphere studies, three 100-pound (45-kilogram) Aerodynamic Drag/Impacter probes, and a 10,130-pound (4600-kilogram) Orbiter for planet-wide mapping photography.
The piloted flyby spacecraft would pass Mars between 23 January and 4 February 1976, the exact date and time depending on the date of Earth departure and magnitude of course corrections required. As the flyby spacecraft approached Mars, the crew would spend an increasing amount of time imaging it using the telescope and relaying the high-resolution images to Earth using the 19-foot-diameter (5.8-meter-diameter) radio dish antenna mounted on a stalk attached to the EM. The images, which would rapidly reveal new details as the piloted flyby spacecraft closed in on Mars, would be of profound scientific interest, but would also serve an operational purpose: they would be used to select targets for the robotic lander probes, which would be released between five and 10 days before piloted flyby spacecraft Mars periapsis (closest approach to the planet).
The MSSR probe would be the first of the six probes to be released. After release, the MSSR would cast off a two-part biological shield. A rocket motor would then ignite to increase the probe's speed by about 500 feet (150 meters) per second. The crew would begin careful tracking the MSSR's course. Twelve hours before landing and about 19 hours ahead of piloted flyby spacecraft periapsis, they would command the motor to perform a final course correction, after which it would be ejected.
The MSSR would enter the martian atmosphere and land in pre-dawn darkness about seven hours ahead of flyby spacecraft periapsis. Immediately after landing it would open like a flower with four fat triangular petals. The petals would contain most of the MSSR's scientific equipment, including two identical sets of four sample collectors on opposite sides of the lander. The MSSR would image the landing site using a panoramic camera and transmit the images immediately to the piloted flyby spacecraft, where the astronauts would use them to select sampling locations. The crew would seek to deploy the collectors outside the zone contaminated by the MSSR's descent rocket engines.
The Planetary JAG proposed four sample collectors of different designs in the hope that at least one would successfully sample the unknown surface materials of Mars, plus a single rock drill for collecting subsurface samples and a filter for sampling airborne dust. Of the four collectors, three — two with rotating cylinders intended to scrape the surface and a "vacuum cleaner" — assumed a dry and dusty Mars, while a fourth — the "sticky string" collector — would serve well if Mars turned out to be "tacky or viscous." About two pounds (0.9 kilograms) of material would be collected. A color film camera would automatically photograph the sampling sites then would transfer its film to the MSSR sample container.
Meanwhile, the other probes would arrive at Mars. The solar-powered Orbiter, with a 200-pound (90-kilogram) camera, would use a three-stage liquid-propellant propulsion system to slow down and capture into a 185-mile (300-kilometer) near-polar orbit four hours before flyby spacecraft periapsis. The Lander would reach Mars two hours before periapsis. An hour after landing it would launch a 50-pound (23-kilogram) solid-propellant sounding rocket to an altitude of about 45 miles (70 kilometers). The three Aerodynamic Drag/Impacters would enter the martian atmosphere six minutes before periapsis. Their missions would end when they struck the martian surface.
The three-stage MSSR ascent vehicle would lift off in daylight 11.5 minutes before flyby spacecraft periapsis. Stage 1 would burn out and separate 5.5 minutes later, when the sample container was about 1540 miles (2480 kilometers) behind the flyby spacecraft. Stage 2 would then burn for 4.5 minutes, closing the distance to about 540 miles (870 kilometers). After a pause, Stage 3 would burn for about a minute, placing the sample container very near the piloted flyby spacecraft over the night side of Mars five minutes after periapsis. The astronauts would extend an arm-mounted docking ring, capture the sample container and third stage, and swing them to a linkup with a port on the biology lab located inside the EM.
Flyby spacecraft periapsis would occur about 125 miles (200 kilometers) above Mars, at which time the spacecraft would be moving at about 30,000 feet per second (9140 meters per second). At that altitude, the spacecraft's telescope would, with motion-compensation slewing, in theory be capable of discerning surface features 1.5 feet (0.5 meters) across. During the flyby, Mars's gravity would bend the spacecraft's course by only 17°.
The MSSR, Lander, and Orbiter probes would continue to explore Mars as the piloted flyby spacecraft moved outward past the planet, at first relaying data at a high rate via the flyby spacecraft and then transmitting directly to Earth at a lower rate. The authors of the PDP hoped that they could continue to return data from Mars for several years.
Assuming Earth departure on 5 September 1975, the flyby crew would need a further 537 days to return to Earth. Early in that period, they would spend much of their time examining the Mars samples. Later, they would return to their wide-ranging astronomy studies. The Planetary JAG suggested that they could study 12-mile-wide (19.75-kilometer-wide) asteroid 149 Medusa at a distance of about 20 million miles (32 million kilometers) 170 days after Mars periapsis and 75-mile-wide (120-kilometer-wide) asteroid 156 Xanthippe at a distance of about 14 million miles (22.5 million kilometers) 150 days after that. They might also discover new asteroids and comets.
The flyby spacecraft would be on the opposite side of the Sun from the Earth when it reached aphelion at 2.2 times the Earth-Sun distance. The Planetary JAG anticipated that data the astronauts collected from their unique vantage point could be combined with data collected simultaneously on Earth to generate a full Sun portrait for the first time.
During the long flight, the crew could expect to observe many solar flares. Some would be directed toward the piloted flyby spacecraft. At such times, the crew would shelter in the thick-skinned EEM. Toroidal tanks containing PM course-correction propellants and spherical tanks containing MM life support gases and liquids would surround and provide additional radiation shielding for the EEM.
The piloted flyby spacecraft would return to Earth between 18 and 26 July 1977. The four astronauts would enter the EEM with the Mars samples and separate from the piloted flyby spacecraft. They would use the attached PM to nudge their course toward Earth, then would cast it off. The abandoned flyby spacecraft would swing past Earth and enter solar orbit; the EEM, meanwhile, would enter Earth's atmosphere at a speed of 49,100 feet per second, decelerate, and descend on parachutes to a land landing. Solid-propellant rocket motors would soften touchdown.
The Planetary JAG envisioned that its series of four piloted flybys would pave the way for piloted Mars landing and Venus orbital missions using nuclear-thermal rockets. These could begin as early as 1980 and might continue into the 1990s, when a Mars outpost might be established.
With the PDP in circulation, planning began for a new phase of Planetary JAG activity. In the minutes of a meeting held on 12 October 1966 at NASA Headquarters, Edward Z. Gray proposed spending $1.7 million of NASA's $8.45-million advance planning budget on the piloted flyby concept in Fiscal Year (FY) 1967. Of this, $250,000 would be spent to prepare for release to U.S. industry of contracts for an MSSR study in FY 1968.
The pace of piloted flyby planning picked up in November and December 1966. On 17 November 1966, Gray called an advance planning meeting at NASA Headquarters for 6 December 1966. In a telex message dated 2 December, he explained that "the purpose of continuing activity in the manned planetary area is to be in a position to initiate a flyby project in FY 1969." He wrote that "to accomplish this end, we need to prepare a project proposal by mid-April 1967, in time for consideration in the FY 1969 budget cycle."
The next Planetary JAG meeting was held at NASA MSC on 17 January 1967. In the early afternoon on 27 January, in response to issues raised during that meeting, Gray dispatched a telex in which he called on participants in the Planetary JAG to address "soft areas" in the 3 October 1966 report by mid-April. He called "experiment return from a flyby mission. . .one of its major attractions and an area which has received many searching questions." A little more than five hours after Gray sent out his message, fire raged through the AS-204/Apollo 1 spacecraft at Cape Kennedy, Florida, killing astronauts Virgil "Gus" Grissom, Edward White, and Roger Chaffee.
Piloted flyby planning became a casualty of Congressional backlash from the fire, which generated searching questions for NASA more pressing and immediate than any associated with the piloted flyby mission. Planetary JAG work did not, however, end immediately. In fact, in May 1967, Edward Z. Gray and his deputy Franklin Dixon felt confident enough to go public with the piloted flyby mission at the 5th Goddard Memorial Symposium in Washington, DC. They called for the 1975 piloted Mars flyby to be made a formal new start program in NASA's FY 1969 budget.
Gray and Dixon eschewed the term "flyby," which had become closely associated with robotic probes after Mariner IV, in favor of calling the proposed mission a "retriever" or an "encounter." Whatever it was called, the piloted flyby concept — and, indeed, all NASA planning designed to give the space agency a future beyond Apollo — was in deep trouble by the summer of 1967. In September 1967, goaded by a Request For Proposals (RFP) NASA MSC distributed to industry aimed at selecting contractors for the MSSR study, Congress zeroed out all funding for NASA advance planning in FY 1968. NASA MSC collected RFP responses from industry but awarded no contracts.
Meanwhile, the Jet Propulsion Laboratory (JPL), with assistance from the Illinois Institute of Technology Research Institute (IITRI), completed a study of an all-robotic Automated Mars Surface Sample Return (AMSSR) mission. The small study team had begun work toward their 15 March 1967 report, a direct response to the Planetary JAG's 3 October 1966 PDP, on 26 October 1966. The team argued that an AMSSR mission based on Voyager technology and requiring but a single Saturn V launch could be much cheaper than a piloted flyby with MSSR. It was the first U.S. study of an robotic Mars Sample Return (MSR) mission.
If an AMSSR probe ever flew, however, it would not be derived from Voyager, for that program had come to be seen as an expensive foot in the door leading to an even more expensive piloted Mars mission. Congress cancelled Voyager in August 1967, just before it slashed NASA advance planning funding.
In large part because the Soviet Union had declared that it would explore the Solar System with robots, U.S. robotic Mars exploration fared better than did piloted flybys. Negotiations with Congress in September 1967 led to a promise of funding in FY 1969 for a pair of Mariner Mars orbiter missions in 1971 and a pair of reduced-cost Mars lander/orbiter missions in 1973 in a new program dubbed Viking.
Memorandum, J. West, AD/Chief, Advanced Spacecraft Planning to Distribution, "Planetary Exploration Program Study — request for review and comments on systems parameters," NASA Manned Spacecraft Center, 9 May 1966.
Memorandum, R. Hock, DP/Chief, Advanced Programs Office (PPR-2) to Distribution, "Minutes of Joint Action Group Meeting on June 29 and 30, 1966," NASA Kennedy Space Center, 8 July 1966.
Planetary Exploration Utilizing a Manned Flight System, NASA Office of Manned Space Flight, 3 October 1966.
Memorandum, MT/Director, Advanced Manned Missions Program to NASA George C. Marshall Space Center, NASA Manned Spacecraft Center, and NASA John F. Kennedy Space Center, "FY 1967 Advanced Studies Planning," 27 October 1966.
Telex, Edward Z. Gray, Dir, Advanced Manned Missions Program, NASA Office of Manned Space Flight, to NASA MSFC Huntsville, NASA MSC Houston, and NASA KSC FLA, 2 December 1966.
Telex [Priority], E. Z. Gray, Director Advanced Manned Missions Program to NASA MSFC Huntsville ALA, Kennedy Space Center FLA, and MSC Houston TEX, 27 January 1967.
Humans to Mars: Fifty Years of Mission Planning,1950-2000, NASA SP-2001-4521, Monographs in Aerospace History #21, David S. F. Portree, NASA Headquarters History Office, February 2001, pp. 23-32.
Apollo to Mars and Venus: North American Aviation's 1965 Plan for Piloted Planetary Flybys in the 1970s