Apollo to Mars & Venus: North American Aviation's 1965 Plan for Piloted Planetary Flybys in the 1970s

U.S. President Lyndon Baines Johnson (right) welcomes home astronauts Gus Grissom (center) and John Young (left) after their March 1965 Gemini III test flight. Earth-orbital Gemini was conceived as a means of bridging the yawning gap between "simple" one-man Mercury flights and complex three-man Apollo lunar-orbital and landing missions. Piloted Mars and Venus flybys based on Apollo technology might have played a Gemini-like role in the 1970s NASA program. Image credit: NASA.

A flyby is the simplest planetary exploration mission. We are accustomed today to seeing a flyby as a mission strictly reserved for automated spacecraft. In the early-to-mid 1960s, however, many within the NASA advance planning community believed that piloted flybys based on technology and techniques developed for the Apollo Moon program could enable U.S. astronauts to carry out effective exploration of Mars and Venus as early as the 1970s.

Much like robotic flybys, piloted flybys would limit themselves to small course-correction maneuvers after departing Earth. Robotic flyby spacecraft have no cause to return to Earth after passing their target or targets. Piloted flyby spacecraft, on the other hand, would swing past their target world or worlds on Sun-centered orbital paths that would intersect Earth, enabling their crews to return home.

The piloted flyby concept is usually attributed to Italian aeronautical engineer Gaetano Crocco, who in 1956 presented a paper describing a piloted Mars/Venus flyby. NASA-funded contractor work on piloted flybys began in 1962 with the Early Manned Planetary-Interplanetary Roundtrip Expeditions (EMPIRE) studies at NASA Marshall Space Flight Center (MSFC) in Huntsville, Alabama (see "More Information," below). EMPIRE also tasked its contractors with looking at piloted Mars and Venus orbiters. 

Crocco and NASA MSFC targeted their first missions for 1971, a date chosen with an eye toward limiting the time and propulsive energy required to reach Mars. In that year, Mars would be close to the Sun as Earth, nearer the Sun and moving faster, passed it. Because Mars has an eccentric orbit, this Earth-Mars geometry recurs only every 15 years or so. An opportunity for an Earth-Mars transfer as favorable would not occur again until 1988. 

The piloted flyby mission concept became increasingly attractive during 1964 and early 1965, when U.S. President Lyndon Baines Johnson made clear his vision of NASA's future after the Apollo Program. At that time, Apollo was expected to accomplish its first lunar landing during 1968. 

Johnson wanted Apollo lunar exploration to continue after the first successful landing, but mainly he wanted to see astronauts working on board Earth-orbiting laboratories derived from the Apollo spacecraft and Saturn rockets developed for the Moon program. These laboratories would, it was hoped, provide low-cost tangible benefits to American taxpayers through research in the fields of medicine, manufacturing processes, Earth resources discovery, agricultural monitoring, and advanced technology development. 

LBJ's vision of NASA's future made no mention of piloted Mars/Venus flybys based on Apollo's technological legacy. On the other hand, neither did it specifically forbid them.

A 15-month NASA MSFC in-house study begun in August 1963, shortly after EMPIRE ended, became, by its end, the first to examine the possibility of adapting Saturn rockets and Apollo spacecraft to piloted Mars and Venus flybys (see "More Information," below). Even before the NASA MSFC engineers completed their study in November 1964, NASA Headquarters and other NASA centers launched their own studies of Apollo-derived piloted flybys. The NASA Manned Spacecraft Center (MSC) in Houston, Texas, for example, completed an in-house study in February 1965. 

NASA MSC, which managed the Apollo Command and Service Module (CSM) spacecraft, contracted with North American Aviation (NAA), the CSM prime contractor, for a seven-month piloted flyby study that began on 1 October 1964. MSC then tacked on a two-month extension to NAA's contract so that the company could focus on enhancement of piloted flyby spacecraft long-term reliability through use of in-flight maintenance. NAA briefed MSC management on the results of its study in Houston on 18 June 1965.

The NAA study was significant in the evolution of piloted flyby planning because it was the first conducted by a major manufacturer of prospective piloted flyby hardware. In addition to the CSM, NAA was responsible for other Apollo hardware that might be adapted to a piloted flyby mission; specifically the Spacecraft-Launch Adapter (SLA) and the Saturn V rocket S-II second stage. 

This striking view of the Apollo 15 Command and Service Module (CSM) Endeavor in lunar orbit displays distinctive Apollo CSM features. These include the large Service Propulsion System (SPS) engine bell and the four-dish high-gain antenna (left), the slightly discolored housing for umbilicals linking the barrel-shaped Service Module (SM) with the silvery conical Command Module (CM), and the extended probe docking unit on the CSM's nose (right). Image credit: NASA.

This image displays the SLA shroud, the structural basis for NAA's piloted flyby Mission Module (MM), and, above that, the Apollo 11 CSM Columbia. The SLA, which linked the bottom of the CSM's silver-and-white SM to the top of the three-stage Saturn V S-IVB third stage, protected the Lunar Module Moon lander and the CSM's SPS engine bell during ascent through Earth's atmosphere. Please note launch gantry workers for scale. Image credit: NASA.

An NAA-built S-II Saturn V stage, with five J-2 engines, is slowly lowered into place atop an S-IC Saturn V first stage inside the immense Vehicle Assembly Building (VAB) at Kennedy Space Center, Florida. Please note workers for scale. Image credit: NASA

A brief aside is justified at this point: EMPIRE, the NASA MSFC and NASA MSC in-house studies, and the NAA study took place against the backdrop of Project Gemini. The two-seater Gemini spacecraft, the advanced cousin of NASA's first piloted spacecraft, one-man Mercury, was conceived as a training and biomedical research tool; it would provide astronauts, engineers, and flight controllers with experience in rendezvous, docking, and spacewalks, and would enable NASA doctors to certify that astronaut bodies could withstand roughly two-week Apollo lunar landing flights. 

In December 1961, NASA awarded McDonnell Aircraft Company, the Mercury prime contractor, the contract to build Gemini. Gemini I carried out a test flight without a crew a little more than two years later (8 April 1964). The first piloted Gemini, Gemini III, reached orbit with Gus Grissom and John Young on board on 23 March 1965. Project Gemini ended with its tenth piloted mission (Gemini XII) in November 1966.

Gemini served admirably as a bridge between Mercury and Apollo. Piloted flybys based on Apollo could, some felt, serve as a bridge between Apollo-derived Earth-orbiting space stations in the late 1960s/early 1970s and Mars/Venus orbiters and Mars landers in the late 1970s and 1980s. 

The early piloted flyby studies also took place against a backdrop of Mariner IV, which left Earth atop an Atlas-Agena rocket on 28 November 1964, nearly two months after the NAA study for NASA MSC began. When NAA briefed MSC managers, Mariner IV's planned 15 July 1965 Mars flyby was still a month away.

Few today would argue that robot probes need humans in close proximity to be able to accomplish their missions, but in the first years of the Space Age, it was different. Most robot probes failed. Those that succeeded delivered low-quality (though often tantalizing) data because they included relatively crude instruments and returned data at an agonizingly slow rate (Mariner IV was expected to return data at 8.3 bits per second; at that rate, 20 black-and-white images of the surface of Mars recorded on tape during its flyby would need a month to play back to Earth). 

Piloted flyby planners argued that a piloted flyby mission would be ideal for improving robot probe success rate and data quality. Astronauts could act as caretakers for a varied flock of probes they would release during the flyby. The probes would reach their target planet in tip-top condition. The piloted flyby spacecraft could act as a data relay, improving data rate. 

NAA added another argument: that robot probes released during a piloted flyby could be more sophisticated than those launched from Earth. Instruments and experiments could be made more complex (hence more prone to malfunction). Hitching a ride on a piloted spacecraft could also enhance probe flexibility; astronauts could, for example, direct an automated Mars lander to an intriguing site they had discovered through successive observations of increasing resolution made using a telescope on the piloted flyby spacecraft during approach to the planet.

NAA proposed a two-phase piloted flyby program. Phase I would see a piloted Venus flyby spacecraft launched in 1973 on a 415-day mission. During Phase II, which the company emphasized, a piloted Mars flyby spacecraft launched in 1975 would carry out a 700-day mission that would take it past Mars to the inner edge of the Asteroid Belt. In both phases, the piloted flyby spacecraft would comprise a modified CSM, a three-deck Mission Module (MM) containing living and working space for the crew, and a pressurized Probe Compartment (PC) bearing a cargo of automated probes tailored to their destination.

NAA envisioned that NASA would begin work on a formal piloted flyby Program Development Plan in mid-1966 and would award contracts to build the flyby spacecraft, robot probes, Saturn V rockets, and ground facilities with a "go-ahead" date of 1 July 1967. "Any slippage" in the go-ahead date, a "key milestone" in the piloted flyby program, would, NAA declared, "jeopardize the [19]73 and [19]75 launch window opportunities."

The company proposed a complex development, manufacture, and test program modeled on the one it was at the time following to build and test the CSM for Apollo lunar missions. Major Phase I milestones would include a test of a Command Module (CM) with a heat shield upgraded for high-speed reentry following a Venus flyby (June 1972), an Earth-orbital test of the complete Venus flyby spacecraft (August 1972), and a test of the Venus flyby spacecraft in solar orbit (December 1972-January 1973). 

Phase II milestones would include a test of the more robust Mars flyby CM heat shield and an Earth-orbital test of the Mars flyby spacecraft (December 1973-January 1974). Results from the Venus solar-orbital test could be extrapolated to the Mars flyby spacecraft, so no Mars solar-orbital test would be necessary.

NAA explained that the piloted Venus flyby would require at most two Saturn V launches, so could get by with the twin Complex 39 Saturn V launch pads (Pad 39A and Pad 39B) built for the Apollo lunar program at Kennedy Space Center, Florida. The piloted Mars flyby, on the other hand, might require as many as four Saturn V launches in rapid succession, so NASA would need to build two new Saturn V pads. Pad 39C and Pad 39D would be ready in August 1974.

The Phase I Venus flyby mission would leave Earth orbit on 30 October 1973. The Phase II Mars flyby would depart on 5 September 1975. Only the Mars flyby will be described in detail here, in keeping with NAA's emphasis on Phase II. 

Piloted flyby spacecraft in Earth-orbital configuration. A = piloted flyby Command Module (CM); B = piloted flyby Service Module (SM); Mission Module (MM); D = Probe Compartment (PC); E = docking adapter linking PC to S-IIB orbital launch stage; F = S-IIB orbital launch stage; G = adapter for linking S-IIB stage to two-stage Saturn V launch vehicle (discarded before launch from Earth orbit). Image credit: North American Aviation/NASA.

NAA assumed that its flyby missions would be boosted from Earth orbit by an S-IIB orbital launch stage (see "More Information," below), a modified version of NAA-built S-II, the second stage of the Apollo Saturn V. The piloted flyby spacecraft and the S-IIB would launch separately on two-stage Saturn V rockets and dock in a 262-nautical-mile-high (485-kilometer-high) assembly orbit. 

The Mars flyby S-IIB stage would, if loaded with the necessary propellants, be too heavy for the two-stage Saturn V to deliver to assembly orbit. NAA proposed that the S-IIB be launched with a full load of liquid hydrogen fuel and an empty liquid oxygen tank. One or two Saturn V-launched liquid oxygen tankers would then dock with the S-IIB to fill the oxygen tank in orbit. After the oxygen tank was full, the tanker would withdraw and deorbit itself over a remote ocean area. The Mars flyby spacecraft and S-IIB stage would dock, then the latter would ignite its J-2 engines to begin the journey to Mars.

Cutaway of four-person flyby Command Module (CM). A = heat shield; B1 = forward middle crew couch; B2 = right crew couch; B3 = after middle crew couch; C = Apollo-type probe docking unit; D = housing for life support and data umbilicals linking CM to Service Module (SM); E = mercury-rankine isotopic power system; F = housing for isotopic power system cooling and electricity umbilicals linking CM to SM. Image credit; North American Aviation/NASA.

Citing the many responsibilities of the crew during Mars close passage, NAA argued for a four-person Mars flyby crew. To make room for a fourth astronaut in the Mars flyby mission CM, the center launch-and-reentry couch would be relocated forward of its Apollo CM position, placing it closer to the main display and control console. A new fourth couch would be mounted on the aft interior bulkhead about two feet behind the relocated center couch. The left-hand couch and the right-hand couch would remain in their Apollo CM positions. 

NAA reminded its MSC audience that the Mars flyby CSM would be called upon to support its crew for a much shorter period of time than would the Apollo CSM. The flyby crew would reach and depart Earth-orbit on board the flyby CSM, return to Earth in the flyby CM in the event of an abort during the hour immediately after Earth-orbit departure, briefly power up the flyby CSM and fire its center engine during the mission's anticipated eight modest course corrections, and return to Earth's surface in the flyby CM at the end of their mission. The company estimated that the flyby astronauts would inhabit the flyby CM cabin for no more than 72 hours at a stretch, not the 10 or more days of a lunar mission.

Image credit: NASA/DSFPortree.

The most obvious external modification to the CSM for NAA's piloted Mars/Venus flyby missions was replacement of the Apollo CSM's single Service Propulsion System (SPS) main engine with three modified Lunar Module (LM) descent engines, each with independent propellant tanks and plumbing. Two half-cone housings added to the sides of the Service Module (SM) would provide room for the two outboard engines. 

Any single flyby CSM engine could perform all necessary flyby mission maneuvers, NAA declared. If all three rocket engines remained functional throughout the flyby mission, however, the middle engine would be used to perform course corrections and the two outboard engines would together carry out the retro burn at the end of the Mars or Venus flyby mission. 

An abort at the start of the Earth-Mars transfer, during the hour following Earth-orbit departure, would burn propellants which would, in a successful mission, be used to perform course corrections and to slow the Mars flyby CSM ahead of Earth-atmosphere reentry. The abort burn would expend nearly all of the Mars flyby CSM's propellants.

Assuming that no abort were necessary, the flyby astronauts would cast off a cylindrical two-part adapter linking their CSM to the top of the MM. They would then move the CSM away using reaction control thrusters, turn the CSM end for end to face the MM, and dock with an Apollo-type drogue docking unit on top of the MM. The crew would then shut down the CSM and transfer to the 5600-cubic-foot MM, their home for the next 700 days.

The MM drogue unit would be inset within a housing that would, after docking, encase the conical CM. If NASA opted for a weightless environment for its piloted flyby crews, the housing would shield the CM from meteoroid damage. If, on the other hand, NASA opted for an artificial-gravity environment, the housing would be split into two parts. The upper part would latch onto the sides of the CM below its windows; the lower part, firmly attached to the MM, would contain cable reels. 

Transition from zero-gravity configuration to artificial-gravity configuration. Image credit: North American Aviation/NASA.

The crew in the MM would spin up the piloted flyby spacecraft using thrusters in the PC. After a gentle nudge from the thrusters, they would unlatch connectors linking the upper and lower parts of the housing and begin to pay out the cables. The CSM, linked to the cables by the "collar" formed by the upper housing, would move away from the MM/PC combination. This would slow the rate of spin about the flyby spacecraft center of gravity, which would in turn reduce tension in the cables, raising the possibility of tangling. 

The crew would, however, continue to fire the thrusters in brief bursts, slowly increasing the spin rate and keeping cable tension constant. When the cables reached full extension, the CSM and MM/PC would be 158 feet (48.1 meters) apart, completing four rotations per minute. This would provide the crew in the MM with acceleration that they would feel as gravity roughly equal to the pull of gravity on Mars (0.4 G). Providing the crew with Mars-level gravity complemented the flyby mission biomedical research program; data on human response to Mars-level gravity would clear the way for long stays on the surface of Mars in the 1980s.

The piloted Mars flyby spacecraft would spin for 660 days of its 700-day voyage. The 40-day non-spinning period would include course-correction rocket burns using the center CSM engine at 73 days, 139 days, 260 days, 472 days, and 550 days, plus an unspecified period surrounding the Mars flyby on 2 February 1976, 150 days into the mission, during which spin would be stopped to facilitate Mars observations and release of robot probes. Spin-down would require a reversal of the spin-up process; the crew would activate the cable reels to slowly retract the CSM while burst-firing thrusters in the PC to decrease the spin rate gradually.

After spin-down and spin-up, the flyby crew would need to reorient their main communications link with Earth, the 13.1-foot-diameter (4-meter-diameter) high-gain dish antenna mounted on a boom on the PC. The high-gain was designed to spin at four rotations per minute in the direction opposite the piloted flyby spacecraft's spin, enabling it to maintain a constant lock on Earth. During periods when the flyby spacecraft did not spin, the high-gain rotation motors would make slight adjustments to its orientation to maintain a steady lock on Earth.

A simplified view of the major components of the flyby CSM's electrical power system. A = the mercury-rankine isotopic power system; B = umbilicals for circulating cooling fluid from the power system in the CM to the radiator panels on the SM and back again; C = redundant radiator panels for cooling the mercury-rankine isotopic power system. Image credit: North American Aviation/NASA.

NAA determined that using the CSM as an artificial-gravity counterweight created an opportunity. The company proposed that the CM include a compact 1370-pound (620-kilogram) plutonium-fueled mercury-rankine isotopic power system capable of generating four kilowatts of continuous electricity for the flyby CSM, the MM, and the PC. If it was to be ready in time for the 1975 Mars flyby mission, NAA estimated, development of the isotopic system would need to start in July 1965 — that is, less than two weeks after the company briefed NASA MSC.

Putting the isotopic system in the Mars flyby CM would place it at a distance from the crew throughout most of the mission, so would expose them to a negligible radiation dose. Special-purpose shielding and water for evaporative cooling of the isotopic system after CM separation from the SM just before Earth atmosphere reentry would shield the flyby astronauts from radiation while they were inside the CM. NAA was confident that the Mars flyby crew would receive an acceptably low cumulative radiation dose from the isotopic system during the brief time they rode in the Mars flyby CM. 

Umbilical hoses would link the isotopic system in the flyby CM to redundant radiator panels on the SM's hull. Liquid metal (potassium-sodium) coolant would flow through the isotopic system, hoses, and radiator panels in a continuous loop. NAA envisioned using the same cooling loops for CSM life support system cooling.

NAA's chief justification for reliance on an isotopic source had to do with the Mars flyby mission's maximum distance from the Sun. The spacecraft would race past Mars on a low-energy path that would take it to the inner edge of the Asteroid Belt, more than 200 million miles (320 million kilometers) from the Sun. It would then fall back toward the Sun and intersect Earth. The solar arrays required to generate four kilowatts of electricity continuously at that distance would be prohibitively large and heavy. Their extent would make them prime targets for marauding meteoroids, which were expected to become a significant hazard as the spacecraft skirted the Asteroid Belt.

The Venus flyby spacecraft, by contrast, could rely on an ample solar energy supply and, it was expected, would contend with a meteoroid population less dense than found at Earth. NAA assumed that a 525-pound (240-kilogram) solar-cell power system would be adequate to power the Venus flyby spacecraft.

The Mission Module (MM) with major components indicated by letters. A = crew transfer tunnel leading to the Probe Compartment (PC); B = hatch and retractable ladder; C = Probe Compartment; D = shelter/control center (lower deck); E = centrifuge; F = middle deck (main living and working area); G = sleep area; H = crew transfer tunnel linking the drogue docking unit to the top and middle decks; I = gaseous oxygen tank and life support equipment; J = liquid nitrogen tank; K = liquid oxygen tank; L = docking collar; M = drogue docking unit; N = two-part adapter linking flyby CSM and MM. Image credit: North American Aviation/NASA.

The four-segment SLA, the NAA-built adapter that linked the Apollo CSM to the top of the Apollo Saturn V S-IVB third stage, would form the structural basis for the piloted flyby MM, the crew's home and workplace during interplanetary travel. NAA did not design an MM specifically for its piloted flyby study. Instead, it tapped the Apollo Orbital Research Laboratory, a 1962-1963 NAA concept for a small space station based on the SLA structure. 

The tapered MM would include three decks with a total of 800 cubic feet (22.65 cubic meters) of open space per crewmember. The top deck, smallest of the three, would include at its center a crew transfer tunnel, which would lead from the drogue docking unit atop the MM to the ceiling of the middle deck. Liquid oxygen and liquid nitrogen tanks would surround the upper part of the transfer tunnel just above the top deck ceiling. The top deck, accessible through an opening in the side of the tunnel, would contain sleeping, medical, and hygiene facilities, as well as life support equipment and a large tank of high-pressure gaseous oxygen.

NAA described its MM life support system in some detail. During the first year of the 700-day Mars flyby mission, the crew would breath oxygen and nitrogen stored in dense, super-cold liquid form; they would then switch to oxygen stored as gas. The large tank on the top level of the MM could completely pressurize the module six times over the course of the mission; this might become necessary to flush out built-up trace gases outgassed from furnishings and produced during soldering, food preparation, and other processes.

The crew would take in oxygen and exhale carbon dioxide. NAA proposed to split the carbon dioxide to recover oxygen using the Bosch reaction, which uses hydrogen and produces carbon and water. The water would then be electrolyzed to yield hydrogen and oxygen. NAA calculated that, assuming 10 pounds of air leakage per day, the piloted Mars flyby mission would need to carry a total of 11,035 pounds (5005 kilograms) of oxygen. 

Ladder rungs in the transfer tunnel would continue as a ladder on the middle deck, the crew's main living area. The middle deck would include the galley, multipurpose table, equipment for making repairs and performing data reduction, and portholes with provisions for securely mounting cameras and other instruments. 

The bottom deck, widest of the three, was probably the most interesting. It would contain a centrifuge for subjecting astronauts to acceleration equal to the pull of gravity on Earth. The centrifuge would include two seats and two storage cabinets which between them would hold more than 900 pounds of Mars flyby spacecraft spare parts. The cabinets would serve as counterweights, stabilizing the centrifuge.

The centrifuge would spin around the "storm cellar" shelter/control center, a 600-cubic-foot (17-cubic-meter) bell-shaped compartment that could be sealed off from the rest of the flyby spacecraft. It could support the four-person crew for up to four days without resupply, allowing them to safely ride out solar flares. To save weight, the shelter/control center would contain little special-purpose radiation shielding, relying instead on its central location on the flyby spacecraft's widest deck and the bulk of equipment — centrifuge, spare parts cabinets, and control consoles for operating MM/PC systems — surrounding it.

NAA described a regular day in the Mars flyby crew's voyage. The crew would sleep for six hours, work for eight hours, grab a 1.5-hour nap, then work again for 8.5 hours. Work periods would be interspersed with four 20-minute periods set aside for eating and 50 minutes total for personal hygiene. The company expected that on average each crewmember would spend about 6.5 hours per day on MM and probe maintenance, and 2.5 hours per day advancing the flyby mission science program.

Exercise would count toward work time: in the hope of counteracting the effects of life in Mars gravity, NAA scheduled 1 hour of light exercise, 30 minutes of medium exercise for biomedical monitoring, and 30 minutes of heavy exercise. Crewmembers would spend one hour per day riding the centrifuge. 

Probe Compartment (PC) exterior. A1 = side view of high-gain antenna; A2 = partial front/rear view of high-gain antenna; A3 = high-gain antenna dish folded prior to deployment; B = magnetometer boom (side and aft views); C = 40-inch (one-meter) telescope (side and aft views); D = cutaway of PC showing interior structure; E = PC aft pressure hull; F1 = deployment panel for Soft-Lander Probe (SLP) 2; F2 = deployment panel for SLP 1; F3 = deployment panel for Orbiting Environment Monitor (OEM) and Orbiting Astronomical (OAT) probe; F4 = deployment panel for Hard-Landing Probes (HLPs); F5 = deployment panel for Parachuted Atmosphere Probes (PAPs) 1, 2, and 3; F6 = deployment panel for PAPs 4, 5, and 6. Image credit: North American Aviation/NASA.

Cutaway of Probe Compartment showing probes. A = spin-up/de-spin motors; B = spin-up/de-spin propellant tank; C = probe propellant tanks; D = Soft-Lander Probe (SLP) 2; E = Parachuted Atmosphere Probes (PAPs) 4, 5, and 6; F =  Hard-Landing Probes; G = SLP 2; H = Orbiting Astronomical (OAT) probe; I = Orbiting Environment Monitor (OEM) (PAPs 1, 2, and 3 behind). Image credit: North American Aviation/NASA.

A hatch in the middle of the shelter/control center floor would lead to a crew transfer tunnel. The tunnel would in turn lead to the PC, which would contain 15 probes of six types with a combined weight of between 6000 and 12,000 pounds (2720 and 5440 kilograms), telescoping launchers, and tanks of "sterilization gas" of unspecified composition for killing Earth microbes ahead of probe launch.

 

The PC would include a pair of probe maintenance stations which would between them feature a folding work bench, displays for monitoring probe health, and 94 cubic feet of storage including 65 cubic feet of probe spare parts storage. In addition, it would carry spherical tanks containing unspecified propellants for the two Mars orbiters.

The orbiters, designated the Orbiting Environment Monitor (OEM) and the Orbiting Astronomical (OAT) probe, would be the first of the carefully tended probes to be launched. Each would include a two-stage propulsion system. The first stage was intended to deliver the probe to Mars ahead of the piloted flyby spacecraft; the second would slow it so that the planet's gravity could capture it into Mars orbit. 

Minus their two rocket stages, they would take the form of 60-inch-by-125-inch (152-centimeter-by-318-centimeter) drums. The OEM would weigh 3900 pounds (1770 kilograms) and the OAT, 4390 pounds (1990 kilograms). Each would include an extendable solar array/instrument platform mounted on pivoting arms. They were expected to operate independent of the piloted flyby spacecraft for up to 180 days. In addition to gathering data using their own instruments, they would relay data from two Soft-Lander Probes (SLPs) on Mars.

Probes meant to enter the martian atmosphere would all have "blunt" shapes; NAA hoped that this would cause them to decelerate rapidly in the upper martian atmosphere, allowing them to descend slowly toward the surface, gathering data for as long as they could. Most would be shaped like the Apollo CM. Five Parachuted Atmosphere Probes (PAPs) were the exception; each would take the form of a 24-inch (61-centimeter), 160-pound (72.6-kilogram) sphere. 

The PAPs were intended to operate for just 200 seconds before they crashed into the surface of Mars. Only the six Hard-Landing Probes (HLPs) had shorter planned useful lives; each 47-inch (120-centimeter), 150-pound (68-kilogram) HLP would return data for just 100 seconds.

SLP 1 was the largest lander; it was it would measure 12.8 feet (3.9 meters) in diameter and would weigh 1870 pounds (848 kilograms). Meant to operate for 180 days, it would carry a variety of scientific instruments, including an Automated Biological Laboratory (ABL). The ABL would, as its name implies, gather samples of its surroundings to seek out biology. 

In 1964-1965, many scientists expected to find microbial life on Mars; not a few anticipated that higher forms, perhaps resembling moss, lichen, or even lithops ("living stones") or cacti, might occur. A few scientists — possibly not the greatest logicians in the scientific community — expected that plants naturally meant that animals should exist to eat them. The ABL, which was proposed in many forms in the early 1960s, would carry a complex payload of life-detection instruments intended to anticipate all of these possibilities.

SLP 2 would be less that half as heavy as SLP 1 (just 840 pounds/381 kilograms), yet would encompass within its 9.3-foot (2.85-meter) diameter a variety of intriguing (and poorly described) subprobes. These would include three "projectile" probes, three balloon probes, and a "TV probe."  

In addition to the probes, the PC would carry mounted on its exterior a 40-inch (one-meter) telescope and a rear-pointing magnetometer boom. The telescope, which would be used for many planetary science and astronomy objectives during the 700-day mission, could be steered and slewed to track on Mars during the flyby. This would avoid photographic image smearing. NAA envisioned equipping the telescope with folding, steerable mirrors to expand its field of view during the flyby, enabling it to track on the surface below the speeding spacecraft and on the horizon simultaneously.

NAA listed 28 Mars flyby mission primary science and engineering objectives, most of which aimed to prepare the way for more advanced piloted Mars missions in the 1980s. Scientific exploration was, of course, not to be neglected during the flyby mission, but the company took pains to stress that science would not become the chief mission emphasis until NASA conducted orbiter and landing missions. 

On 2 February 1976, 150 days after Earth departure, NAA's piloted Mars flyby spacecraft would reach its target. The company's representatives told MSC managers that the 32 hours surrounding "periplanet" — as it called closest approach to the surface of Mars — would be the mission's "pay-off phase." 

The top-priority objective would be to collect photographic data required to make detailed Mars maps. The crew would observe and photograph Mars using the telescope and a 35-mm film camera with a "turret" of different lenses mounted on a flyby spacecraft porthole. 

Mars maps in 1965 included few surface features beyond the largest light and dark areas. They were largely based on photographic plates taken from Earth using large telescopes and sketches made by astronomers peering through telescope eyepieces. Most still contained at least a hint of the "canals" first noted by Giovanni Schiaparelli in 1877 and popularized by Percival Lowell beginning in the 1890s. 

The crew would monitor and take data from the robot probes, which they would release at carefully determined times to ensure that they would reach targets selected on the basis of telescope observations made during approach to the planet. Radio signals from the probes would be received through an antenna attached to the flyby CSM in place of the Apollo CSM's four-dish high-gain antenna. 

The crew would, as might be expected, alter their regular daily schedule during the flyby. Sleep would be reduced by 1.5 hours per crewmember per day, eating time would be cut in half, and exercise and biomedical monitoring would be eliminated. NAA allotted 4.5 hours per crewmember for probe monitoring, two hours for non-probe science using the telescope and 35-mm camera, and three hours for "unscheduled" observations (The company suggested, for example, that the astronauts might wish to sketch what they saw on Mars).

NAA plotted the ground track the flyby spacecraft would follow from 24.8 hours (one martian day) before periplanet to 24.8 hours after periplanet. At the start of that period, an entire martian hemisphere would be in view centered on the nondescript light-colored region Aethiopis, about 10° north of the equator. The ground track would then run westward, passing over dark-colored Syrtis Major and light-colored Aeria. 

At 18.6 hours before periplanet, the flyby spacecraft would enter the martian "sphere of influence" and would begin to accelerate under the pull of martian gravity. Between that time and 12 hours before periplanet, it would pass over the light-colored regions Eden, Chryse, and Xanthe, north of dark-hued Sabaeus Sinus, Meridiani Sinus, and Margaritifer Sinus. 

Twelve hours before periplanet, the ground track would pass through little-hued Candor. By that time, the flyby spacecraft would be close enough to Mars that the field of view outside the portholes would take in a region between about 55° north and 35° south latitude and from 30° west to 140° west longitude. 

With the flyby spacecraft moving ever faster, the ground track would sweep west over light-hued Tharsis and Amazonis south of the enigmatic bright spot Nix Olympica. Six hours ahead of periplanet, the field of view would take in Amazonis between 30° north and 10° south latitude. Features a kilometer across would become readily visible through the telescope. 

In the last three hours before periplanet, the ground track would sweep south of mysterious Elysium. On Mars maps available in 1965, Elysium was a light-hued pentagon bounded by five diffuse canals. 

Finally, the track would turn northwest and sweep across light-colored Arabia. Elevated features on Mars would by then show west-pointing shadows; the piloted flyby spacecraft would race toward night, and behind it the Sun would sink rapidly toward the planet's limb. 

Minutes before periplanet, with the ground track passing just south of Cydonia, the Sun would set. Periplanet would take place in faint twilight, with the surface cloaked in blackness, at an altitude of 189 miles (305 kilometers). The piloted flyby spacecraft would then begin to move away from Mars. 

The crew would use the flyby CSM center engine to perform a small course correction immediately after periplanet. The maneuver would compensate for the effects on the spacecraft's course of any irregularities in the martian gravitational field. Performing the correction close to Mars would reduce the quantity of propellants required to carry it out.

Some of the Mars feature names mentioned above will sound familiar, for many were preserved, sometimes in slightly altered form, after U.S. robotic spacecraft mapped Mars. Candor, for example, lent its name to a section of Valles Marineris, the great equatorial rift and canyon system imaged by the Mariner 9 orbiter in 1971-1972. Meridiani Sinus was renamed Terra Meridiani; it became the landing area for the Opportunity rover in 2004. Chryse is now Chryse Planitia; the Viking 1 lander performed the first successful Mars soft-landing there on 20 July 1976. The name Tharsis was applied to a vast volcanic bulge atop which rise four shield volcanoes; one of these, Olympus Mons, is the largest volcano known in the Solar System. 

The Syrtis Major hemisphere of Mars. Syrtis Major Planum is the dark feature at the center of the image; the light area to its left is Arabia Terra and the dark area on the limb at left is Meridiani Terra. Image credit: NASA.

The Tharsis hemisphere of Mars. Patches of cloud mark the four great shield volcanoes; Olympus Mons is above and to the left of center. Western Valles Marineris is on the limb at center right. Image credit: NASA.

All of these surface features would be visible to the four astronauts during the Mars flyby. NAA assumed that only robotic precursors of minimal capability would precede them to Mars, so they would become the first humans to glimpse its wonders.

NAA compared its piloted flyby with planned robotic Mars missions. The company told NASA MSC managers that the Voyager probe proposed for launch in 1975 (not to be confused with Voyager outer planets probes launched in 1978-1979) would transmit data at a rate of between 100 and 350 bits per second. The piloted flyby mission, in stark contrast, would return 2000 bits per second and would deliver rolls and cassettes of high-resolution film to cartographers and researchers on Earth. NAA declared that its analysis had shown that "types, ranges, accuracies, and quantities of data obtained [by a piloted Mars flyby mission] should exceed (by orders of magnitude in some cases) that which could be returned to Earth with equivalent instruments on unmanned systems."

With Mars flyby activities tapering off, the crew would return to their regular daily schedule and commence the 550-day voyage home. They would begin Mars data analysis and, as they skirted the Asteroid Belt, observe any nearby asteroids using their telescope. 

The crew would also pay close attention to the health of their spacecraft's systems during the long trip home. They would have at hand tools and carefully selected spares to perform repairs. These would be available in part as a result of the two-month study of piloted Mars flyby spacecraft systems reliability NASA MSC added to NAA's original study task. 

The company estimated that 57% of piloted Mars flyby spacecraft subsystems — which included life support, power, propulsion, guidance, communications, and data handling — could be provided by 164 hardware "assemblies" designed for Apollo lunar missions. Another 22% (63 assemblies) could take the form of modified Apollo hardware, and 15% (44 assembles) could comprise hardware borrowed from other programs, such as the U.S. Air Force Manned Orbiting Laboratory. 

This meant that 94% of piloted Mars flyby hardware would have a test record and failure history by the time the piloted Mars flyby mission left Earth in 1975 even if the Phase I Venus flyby did not fly in 1973. The remaining 6% (just 17 assemblies) would require new development and testing.

Based on existing Apollo reliability predictions, NAA calculated that from six to 85 failures would occur during the 1975 piloted Mars flyby mission. Most would occur in subsystems that could be repaired or replaced by the crew. Those assemblies that could not be repaired or replaced — for example, the large thermal radiator on the outside of the MM — could be modified during the design phase to avoid failure or backed up by a redundant system.

NAA became concerned that some subsystems would take so long to repair that the crew could be harmed by the malfunction before they could finish. Analysis showed, however, that no repair time would exceed allowed downtime. A failed cabin heat control system, for example, could be repaired in an hour but would need from eight to 24 hrs to create a problem sufficiently serious that it would harm the crew. 

The company found that up to 185 spares weighing about 900 pounds would be required as insurance against all possible failures. Of course, very few were likely to be used. Repair time spread over the mission would amount to only about 15 minutes per day. 

Return to Earth would occur on 5 August 1977. As Earth grew large outside the portholes, the flyby spacecraft crew would prepare to abandon their home of 700 days. They would reel in the CSM for the last time and load it with film and other data. About two hours before planned reentry they would separate the CSM from the drogue docking unit and the artificial-gravity collar on the MM and back away. 

The crew would orient the Mars flyby CSM so its three engines pointed in its direction of travel and, 30 minutes before planned reentry, would ignite the two outboard engines. Flyby mission Earth-return speed would depend on many factors: for example, a close Mars flyby typically would mean a fast Earth-atmosphere reentry.

The Apollo CM was designed to reentry Earth's atmosphere at 36,000 feet (10,970 meters) per second. NAA told MSC that the CM's bowl-shaped heat shield could, in theory, be beefed up to withstand reentry at 52,000 feet (15,850 meters) per second. The company argued, however, that "engineering conservatism" made such high-speed reentries unattractive. Hence the retro burn, which would slash reentry velocity to no more than 45,000 (13,715 meters) feet per second. NAA told NASA MSC that the Apollo CM heat shield would need only modest modifications to withstand reentry at that velocity.

NAA reported that, at launch from Earth, the Apollo CSM would have a mass of 57,690 pounds (26,170 kilograms). Hypergolic (ignite-on-contact) Hydrazine/nitrogen tetroxide propellants would account for 37,360 pounds (16,950 kilograms) of that total. The hefty Mars flyby CSM would have a mass of 73,080 pounds (33,150 kilograms) of which 44,770 pounds (20,310 kilograms) would constitute propellants for course corrections and the reentry retro burn. 

During the retro burn, the outboard engines would fire for up to 29 minutes to slow the flyby CSM. The flyby SM would then separate, exposing the CM's modestly uprated heat shield and depriving the isotopic power system of its heat radiators (it would switch to its temporary water boil-off cooling system). During passage through Earth's atmosphere, the heat shield might attain a temperature of 5000° F (2760° C). 

NAA anticipated that the Mars flyby CM would parachute to a land landing. Modifications to the shock absorbers in the crew couches would protect the astronauts from injury as the CM bumped to a stop on Earth. Soon after landing, the isotopic power system would boil off the last of its cooling water; hence, linking it to an ground-supplied auxiliary cooling system would be assigned nearly as high a priority as removing the astronauts from the CM.

Direct Venus flyby and "in transit" assembly Mars flyby Saturn V launch configurations. A = Launch Escape System tower; B = piloted flyby Command and Service Module (CSM); C = Mission Module (MM); D = Probe Compartment (PC); E = Saturn V S-IVB stage; F = Saturn V S-II stage; G = Saturn V S-IC stage; H = Spacecraft-Launch Adapter (SLA); I = aerodynamic shroud. Image credit: North American Aviation/NASA.

Near the end of its study, as it firmed up its spacecraft weight estimates, NAA determined that a single three-stage Saturn V, virtually identical to that used to launch Apollo lunar missions, could launch its Venus flyby spacecraft directly to Venus. The Saturn V S-IVB third stage would do the job of the S-IIB orbital launch stage. This led the company to reexamine its Mars flyby Earth-orbital launch scheme.

The company found that the heavier Mars flyby spacecraft could not launch directly onto its Mars flyby path if it were launched on a single three-stage Saturn V. It proposed instead that the Mars flyby spacecraft be split into two payloads — the CSM bearing the crew and the MC/PC combination — and that they be launched on a pair of three-stage Saturn Vs. CSM and MC/PC would then rendezvous and dock "in transit" soon after their S-IVBs placed them on course for Mars. 

The CSM would play the active role in the in-transit rendezvous. As Earth shrank behind it, its crew would separate the spacecraft from the S-IVB third stage that boosted it toward Mars, rendezvous and dock the MM/PC combination, and detach it from its S-IVB. After it moved a safe distance away, piloted flyby spacecraft instrument and antenna deployment and artificial-gravity spin-up could begin.

NAA provided a cost estimate for its 1973 Phase I Venus and 1975 Phase II Mars piloted flyby missions. The Venus mission would cost $2,301,700,000 between the 1 July 1967 contract go-ahead date and return to Earth on 19 December 1974. The Mars flyby without the Venus flyby would cost $3,439,500,000.

NAA representatives told MSC managers that its study had demonstrated that "existing and currently programmed hardware and facilities and systems contemplated for other NASA space flight programs can be used to achieve early Mars and/or Venus flyby missions." The company declared that "[f]ailure to take timely advantage of this opportunity could result in a delay in the achievement of advanced [orbital] and/or landing missions to Mars until the next century."

Sources

"One-Year Exploration Trip Earth-Mars-Venus-Earth," G. Crocco; paper presented at the 7th International Astronautical Federation Congress in Rome, Italy, 1-7 September 1956.

"Laboratory in Space," M. Yarymovych, NASA Headquarters; paper presented at the First Space Congress in Cocoa Beach, Florida, 20-22 April 1964.

"Future Effort to Stress Apollo Hardware," Aviation Week & Space Technology, 16 November 1964, pp. 48-51.

"An Evolutionary Program for Manned Interplanetary Exploration," M. W. Jack Bell; paper presented at the AIAA/AAS Stepping Stones to Mars Meeting in Baltimore, Maryland, 28-30 March 1966. 

Manned Mars and/or Venus Flyby Vehicles Systems Study Final Briefing Brochure, SID 65-761-6, North American Aviation, Inc., 18 June 1965. 

More Information

EMPIRE Building: Ford Aeronutronic's 1962 Plan for Piloted Mars/Venus Flybys

After EMPIRE: Using Apollo Technology to Explore Mars and Venus (1965)

The First Voyager (1967)

Flyby's Last Gasp: North American Rockwell's S-IIB Interplanetary Booster (1968)

Revival: A Piloted Mars Flyby in the 1990s (1985)

An Orbital Transfer Vehicle (OTV) carrying a drum-shaped Command Module aerobrakes in Earth's atmosphere in this NASA painting by Pat Rawlings. 

In the 1960s, NASA expended nearly as much study money and effort on piloted Mars and Venus flyby mission planning as it did on its more widely known plans for piloted Mars landings. Italian aviation and rocketry pioneer Gaetano Crocco had described a free-return piloted Mars/Venus flyby mission in 1956. Piloted flyby studies within NASA began with the EMPIRE study the Marshall Space Flight Center (MSFC) Future Projects Office initiated in 1962 and culminated in the NASA-wide Planetary Joint Action Group (JAG) piloted flyby study of 1966-1967.

The Planetary JAG, led by the NASA Headquarters Office of Manned Space Flight, brought together engineers from MSFC, Kennedy Space Center, the Manned Spacecraft Center (MSC), and Washington, DC-based planning contractor Bellcomm. It issued a Phase I report in October 1966 and continued Phase II study work in Fiscal Year (FY) 1967. The Phase I report emphasized a piloted Mars flyby mission in 1975, but included Mars and Venus flyby missions tailored to low-energy mission opportunities through 1981. All would be based on hardware developed for the Apollo Program and its planned successor, the Apollo Applications Program (AAP).

The piloted flyby spacecraft would carry automated probes, including one that would land on Mars, collect a sample of surface material and launch it back to the flyby spacecraft for immediate analysis. A leading point in favor of the piloted flyby mission was, in fact, the ability of the flyby crew to examine a Mars sample for signs of life less than an an hour after it left the martian surface.

Red planet off the port bow: a piloted flyby spacecraft based on Apollo spacecraft hardware releases probes as it passes Mars. Image credit: Douglas Aircraft Company.

According to Edward Clinton Ezell and Linda Neumann Ezell, writing in their 1984 NASA-published history On Mars: Exploration of the Red Planet, 1958-1978, NASA MSC was largely responsible for the demise of 1960s piloted flyby mission planning. On 3 August 1967, the Houston, Texas-based center, home of the astronaut corps and Mission Control, distributed to 28 aerospace companies a Request for Proposal (RFP) for a piloted Mars flyby spacecraft sample-returner design study. By doing this, MSC appeared to disregard warnings from Congress that no new NASA program starts would be tolerated.

In the summer of 1967, NASA was preoccupied with recovery from the 27 January 1967 Apollo 1 fire, which had killed astronauts Virgil Grissom, Roger Chaffee, and Ed White. Many in Congress felt that NASA had been lax in enforcing quality and safety standards at North American Aviation, the Apollo Command and Service Module spacecraft prime contractor, so deserved to be "punished" for the accident. Other members of Congress were angered by NASA's apparent failure to share its concerns regarding NAA's performance so they could exercise Congressional oversight. They did not, however, wish to cut Apollo funding and endanger accomplishment of Apollo's very public goal of a man on the Moon by 1970.

In addition, by August 1967, the Federal budget deficit for FY 1967 had reached $30 billion. Though negligible by modern standards, this was a shocking sum in 1967. The deficit was driven in large part by the cost of fighting in Indochina, which had reached more than $2 billion a month, or the entire Apollo Program budget of $25 billion every 10 months.

After learning of the MSC RFP, long-time House Space Committee Chair and NASA supporter Joseph Karth declared angrily that "a manned mission to Mars or Venus by 1975 or 1977 is now and always has been out of the question — and anyone who persists in this kind of misallocation of resources. . .is going to be stopped." On 16 August, the House cut all funding for advanced planning from NASA's FY 1968 budget bill and slashed the budget for AAP from $455 million to $122 million. Total cuts to President Lyndon Baines Johnson's January 1967 FY 1968 NASA budget request amounted to more than $500 million, or about 10% of NASA's FY 1967 budget total.

Though he opposed the cuts, President Johnson bowed to the inevitable and signed the budget into law. The Planetary JAG and Bellcomm tied up loose ends of the piloted flyby study during FY 1968, but most work on the concept ended within a few months of the Houston center's ill-timed RFP.

It is ironic, then, that NASA's next piloted Mars flyby study took place in Houston, at Johnson Space Center (JSC), as MSC had been re-christened following President Johnson's death in January 1973. Barney Roberts, an engineer in the JSC Engineering Directorate, reported on the study to the joint NASA-Los Alamos National Laboratory (LANL) Manned Mars Missions workshop in June 1985.

The workshop, held at NASA MSFC, was a significant step in the revival of piloted Mars exploration planning within NASA after the long drought of the 1970s. Unfortunately, in their plan for a piloted Mars flyby in the 1990s, NASA JSC engineers demonstrated little sign of awareness of the 1960s piloted flyby studies. As a result, their proposed mission was less credible than it might have been.

Roberts explained that the NASA JSC flyby plan aimed to counter a possible Soviet piloted Mars flyby. He cited a 1984 Central Intelligence Agency (CIA) memorandum that suggested (without citing much in the way of evidence) that the Soviet Union might attempt such a mission in the 1990s — possibly as early as the 75th anniversary of the Bolshevik Revolution in 1992 — in order to garner international prestige. The CIA study had been performed at the request of Apollo 17 moonwalker Harrison Schmitt, whose chief spaceflight interest in the early-to-mid 1980s was a piloted Mars mission.

NASA's piloted Mars flyby would be based on space hardware expected to be operational and readily available in the late 1990s. Space Shuttle Orbiters would deliver to NASA's Space Station an 18-ton Mission Module (MM) and a pair of expendable propellant tanks with an empty mass of 11.6 tons each. The MM, derived from a Space Station module, would carry a 3000-pound solar-flare radiation shelter, 7000 pounds of science equipment, and 2300 pounds of food and water.

Going for a ride: a piloted Mars flyby spacecraft prepares for launch from Earth orbit in the late 1990s. A = twin Orbital Transfer Vehicles (OTVs); B = twin strap-on propellant tanks; C = Command Module; D = Mission Module. Image credit: NASA/David S. F. Portree.

The MM would be docked to a six-ton Command Module (CM) and two 5.75-ton Orbital Transfer Vehicles (OTVs). The OTVs would each include an aerobrake heat shield and two rocket engines derived from the Space Shuttle Main Engine. The JSC engineers had assumed that the CM and OTVs would be in space already as part of a late 1990s NASA Lunar Base Program. The strap-on tanks would be joined to the MM/CM stack by trunnion pins similar to those used to anchor payloads in the Space Shuttle Orbiter payload bay, then Space Station astronauts would perform spacewalks to link propellant pipes and electrical and control cables.

Shuttle-derived heavy-lift rockets would then deliver a total of 221 tons of cryogenic liquid hydrogen and liquid oxygen propellants to the Space Station to fill the piloted flyby spacecraft's twin tanks. The propellants would be pumped aboard just prior to departure from Earth orbit to prevent liquid hydrogen loss through boil off. Mass of the piloted flyby spacecraft at the start of its Earth-departure maneuver would total 358 tons.

As the launch window for the Mars flyby opportunity opened, the piloted flyby spacecraft would move away from the Space Station using small thrusters on retractable arms, then the four OTV engines would ignite and burn for about one hour to put it on course for Mars. The only propulsive maneuver of the baseline mission, the burn would empty the OTV and strap-on propellant tanks. Roberts advised retaining the spent tanks to serve as shielding against meteoroids and radiation for the MM and CM during the year-long flight.

Roberts told the workshop that the flyby spacecraft would spend two-and-a-half hours within about 20,000 miles of Mars. Closest approach would bring it to within 160 miles of Mars. At closest approach, the spacecraft would be moving at about 5 miles per second.

The spacecraft would then begin its long return to Earth. Roberts provided few details of the interplanetary phases of his piloted Mars flyby mission.

As Earth grew from a bright star to a distant disk, the Mars flyby astronauts would discard the twin strap-on tanks. They would then undock one OTV by remote control and re-dock it to the front of the CM. After entering the CM and sealing the hatch leading to the MM, they would discard the MM and second OTV, then would then strap into their couches to prepare for aerobraking in Earth's upper atmosphere and capture into Earth orbit. After the OTV/CM combination completed the aerobraking maneuver, the crew would pilot it to a docking with the Space Station.

Almost home: the piloted Mars spacecraft prepares for the aerobraking maneuver in Earth's atmosphere at the end of its epic year-long interplanetary voyage. A = OTVs; C = Command Module bearing crew; D = discarded Mission Module (attached to discarded OTV). Image credit: NASA/David S. F. Portree.

Roberts told the NASA/LANL workshop that Earth return would be the most challenging phase of the piloted Mars flyby mission. The OTV/CM combination would encounter Earth's upper atmosphere at a speed of 55,000 feet (10.4 miles) per second, producing reentry heating well beyond the planned capacity of the OTV's heat shield. In addition, the crew would suffer "exorbitant" deceleration after living for a year in weightlessness.

Roberts proposed a "brute-force" solution to these problems: use the OTV's twin rocket motors to slow the OTV/CM to lunar-return speed of 35,000 feet (6.6 miles) per second. The braking burn would, however, increase the Mars flyby spacecraft's total required propellant load to nearly 500 tons. He calculated that, assuming that a Shuttle-derived heavy-lift rocket could be designed to deliver cargo to LEO at a cost of $500 per pound (an optimistic assumption, as it would turn out), then Earth-braking propellant would add $250 million to his mission's cost.

Roberts briefly considered partially compensating for the large mass of braking propellants by substituting an MM derived from a five-ton Space Station logistics module for the 18-ton MM. This would mean, however, that the crew would have to spend a year in cramped quarters with no exercise or science equipment.

Planners in the 1960s had wrestled with and prevailed over the same problems of propellant mass and Earth-return speed that NASA JSC engineers faced in their 1985 study. Bellcomm had, for example, proposed in June 1967 that the Planetary JAG's piloted Mars flyby mission conserve propellants through assembly of the flyby spacecraft in an elliptical orbit, not circular Space Station orbit. The elliptical assembly orbit would mean, in effect, that the flyby spacecraft would begin Earth-orbit departure even as it was being assembled.

In addition, returning the crew directly to Earth's surface in a small Apollo-type capsule with a beefed-up heat shield would greatly reduce the quantity of braking propellants required; it could eliminate the braking maneuver entirely. It would also enable an aerodynamic "skip" maneuver that would reduce deceleration stress on the astronauts.

TRW Space Technology Laboratory had proposed as early as 1964, during the EMPIRE follow-on studies, that NASA use a Venus flyby to slow spacecraft returning from Mars. Crocco had described the concept in 1956, in fact, though in a form that turned out to be unworkable because of errors he made when he calculated his flyby spacecraft's orbit about the Sun.

Exploiting a Venus flyby to reduce speed would, of course, limit Earth-Mars-Earth transfer opportunities to those that would intersect Venus on the return leg, but would also eliminate the costly end-of-mission braking burn and enable Venus exploration as a bonus. The Planetary JAG's October 1966 report described Mars-Venus and Venus-Mars-Venus flyby missions in the late 1970s. Bellcomm determined in late 1966 and 1967 that Mars/Venus flyby opportunities are not rare.

Sources

"Soviet Plans for a Manned Flight to Mars," C. Cravotta and M. DeForth, Office of Scientific and Weapons Research, Central Intelligence Agency, 2 April 1985.

"Concept for a Manned Mars Flyby," Barney B. Roberts, Manned Mars Missions: Working Group Papers, Volume 1, NASA M002, NASA/LANL, June 1986, pp. 203-218; proceedings of a workshop held at NASA Marshall Space Flight Center, Huntsville, Alabama, 10-14 June 1985.

On Mars: Exploration of the Red Planet, 1958-1978, NASA SP-4212, Edward Clinton Ezell & Linda Neuman Ezell, NASA History Office, 1984, pp. 117-118.

Humans to Mars: Fifty Years of Mission Planning, 1950-2000, Monographs in Aerospace History #21, NASA SP-2001-4521, David S. F. Portree, NASA History Division, February 2001, pp. 11-12, 15, 60-62.

More Information

After EMPIRE: Using Apollo Technology to Explore Mars and Venus (1965)

Relighting the FIRE: A 1966 Proposal for Piloted Interplanetary Mission Reentry Tests

Apollo Ends at Venus: A 1967 Proposal for Single-Launch Piloted Venus Flybys in 1972, 1973, and 1975

Triple Flyby: Venus-Mars-Venus Piloted Flyby Missions in the Late 1970s/Early 1980s (1967)

A New Step in Spaceflight Evolution: To Mars by Flyby-Landing Excursion Mode (1966)

The 12 images and captions that accompany this post are a post-within-a-post. R. R. Titus, author of the study discussed in the main post text, did not describe his Flyby-Landing Excursion Mode (FLEM) spacecraft, so these images constitute a plausible guess. They draw inspiration from the 1966 NASA Planetary JAG piloted Mars/Venus flyby design; the SIM Bays used to explore the Moon from lunar orbit during Apollos 15, 16, and 17; Orbital Workshop concepts; the informed imagination of their creator, artist William Black; and sundry inputs from the post author. In the image above, a stack assembled in Earth orbit from three modified Saturn S-IVB rocket stages and the FLEM spacecraft undergoes final checks before beginning Earth-departure maneuvers.

This dramatic image is 21st-century artist William Black's tribute to a very similar hand-painted NASA image of the 1960s. Earth-orbit departure would require three S-IVB stage burns at perigee over about two days. Because of this, the first and second S-IVBs (center left and lower left) would not be in view when the third S-IVB ignited its J-2 engine. After engine shutdown, the third S-IVB would detach and vent leftover propellants to nudge its course so that it would not follow the FLEM spacecraft to Mars.

During its first dozen years, NASA piloted spaceflight followed an evolutionary course, with simple missions and spacecraft leading to more complex and capable ones. Single-man Mercury suborbital missions led to Mercury orbital missions of increasing duration; then, in 1965-1966, two-man Gemini missions progressively added maneuverability, the ability to rendezvous and dock, spacewalk capability, and flight durations of up to 14 days.

Next came Apollo. NASA conducted four piloted preparatory missions in 1968-1969 ahead of the first lunar landing attempt. Apollo 7 (October 1968), launched on a two-stage Saturn IB rocket, tested the Command and Service Module (CSM) in Earth orbit. The CSM comprised a drum-shaped Service Module (SM) and the conical Command Module (CM) bearing its three-man crew.

As in biological evolution, contingency played a role in spaceflight evolution: for example, Apollo 8, intended originally as a Saturn V rocket-launched high-Earth-orbit test of the CSM and the bug-like Lunar Module (LM) moon lander, became a CSM-only lunar-orbital mission after the LM was delayed and the Soviet Union appeared close to launching a cosmonaut around the moon.

The Apollo 8 astronauts reached lunar orbit on Christmas Eve 1968. In addition to forestalling Soviet attempts to upstage the first Apollo lunar landing mission, Apollo 8's 10 lunar orbits tested upgrades made to the Manned Space Flight Network, NASA's world-wide radio communications and tracking system, and gave console operators in Mission Control early experience in supporting a piloted lunar mission. The Apollo 8 CSM left lunar orbit on Christmas Day 1968, and splashed down in the Pacific Ocean on 27 December 1968.

Apollo 9 (March 1969) saw the first Earth-orbital test of the LM and CSM together. Apollo 10 (May 1969) was a dress-rehearsal in low-lunar orbit for Apollo 11 (July 1969), the first piloted lunar landing.

Apollo 11 is perhaps best understood as an engineering mission; it was a cautious end-to-end test of the Apollo system with a single two-and-a-half-hour moonwalk and only limited science objectives. Apollo 12 (November 1969) demonstrated the pin-point landing capability required for pre-mission geologic traverse planning by setting down near a known point on the moon: specifically, the Surveyor III automated soft-lander, which had landed in April 1967. It also saw a pair of moonwalks lasting nearly four hours each and deployment of the first Apollo Lunar Scientific Experiment Package (ALSEP).

Apollo 13 (April 1970), the first science-focused mission, suffered a crippling explosion midway to the Moon, scrubbing its lunar landing, but its crew's safe return to Earth demonstrated the Apollo system's maturity and the Apollo team's experience. Apollo 14 (January-February 1971) included two moonwalks, each lasting more than four-and-a-half hours. They included a strenuous 1.3-kilometer trek through the hummocky ejecta blanket surrounding 300-meter-wide Cone Crater, a natural drill hole in the scientifically important Fra Mauro Formation.

Apollo 15 (July-August 1971), Apollo 16 (April 1972), and Apollo 17 (December 1972), designated "J" missions, featured a host of evolutionary improvements. Beefed-up LMs permitted surface stay times of up to three days at complex and challenging landing sites, heavier returned lunar samples, and more complex ALSEPs. Space suit improvements and Boeing's Lunar Roving Vehicle enabled geologic traverses ranging over kilometers of the lunar surface. Each "J" mission CSM included a suite of sensors which its pilot could turn toward the moon. Apollo 15 visited the Hadley Rille/Apennine Mountains area; Apollo 16 the central Nearside Lunar Highlands; and Apollo 17, Taurus-Littrow, on the edge of Mare Serenitatis.

Conceptual FLEM spacecraft. A black-and-white band midway along its hull marks where it will split in two as it nears Mars. To the left of the band is the Mars Orbiter (R. R. Titus called it the "excursion module"); to the right, the Flyby Spacecraft (the "parent" spacecraft). The Orbiter mission module has a single deck with living and working space for two astronauts; the Flyby Spacecraft, two decks with room for four crew.

As early as 1962, engineers foresaw two evolutionary paths for Saturn rockets and Apollo spacecraft hardware after they accomplished President John F. Kennedy's goal of a man on the Moon by 1970. The engineers were guided by President Lyndon Baines Johnson's 1964 decision that, to contain cost, NASA's piloted space program after the Moon landing should be based on hardware developed for Apollo. This marked the advent of the Apollo Applications Program (AAP).

One path would see Moon missions continue more or less indefinitely, growing ever more capable and culminating in a permanent lunar base in the 1980s. Alternately, NASA might repurpose Apollo hardware to build, launch, and maintain an evolutionary series of space stations in Earth orbit.

The space station path appeared pedestrian compared to the lunar base path, yet it offered great potential for long-term future exploration. This was because it promised to prepare astronauts and spacecraft for long-duration missions in interplanetary space. In 1965-1966, NASA advance planners envisioned a series of Earth-orbiting space workshops based on the Apollo LM and the Saturn IB rocket S-IVB stage. Apollo CSMs would ferry up to six astronauts at a time to the workshops for progressively longer stays.

Some planners thought that NASA should jump straight from the early space workshops to piloted Mars landing missions using nuclear-thermal propulsion, but others called for a conservative continuation of the evolutionary approach. If the latter had won the day, the mid-1970s might have seen a new-design space station climb to Earth orbit atop an improved Saturn V rocket. Derived from Apollo hardware and new technology tested on board the orbiting workshops, the station would in fact have constituted a prototype interplanetary Mission Module. A crew might have lived on board without resupply or visitors for almost two years to help prepare NASA for its first piloted Mars voyage.

In keeping with the evolutionary approach, the first piloted voyage beyond the Moon might have been a Mars flyby with no piloted Mars landing. The piloted Mars flyby spacecraft, which would have carried a cargo of robotic Mars probes, would have been built around the Mission Module tested in Earth orbit. The mission might have commenced as early as late 1975, when an opportunity to launch a minimum-energy Mars flyby was due to occur.

As they raced past Mars in early 1976, the four flyby astronauts would have released automated probes and turned a suite of sensors mounted on their spacecraft toward Mars and its irregularly shaped moons Phobos and Deimos. They would have reached their greatest distance from the Sun in the Asteroid Belt, so asteroid encounters would have been a possibility. As their Sun-centered elliptical orbit brought them back to Earth's vicinity in 1977, they would have separated in an Apollo CM-derived Earth-return capsule and reentered Earth's atmosphere.

In addition to observing Mars close up, the astronauts would have continued the effort, begun in earnest during Gemini flights and continued on board the Earth-orbiting workshops and prototype interplanetary Mission Module, to determine whether extended piloted missions were medically feasible. The flyby crew might have confirmed, for example, that artificial gravity is a must during years-long interplanetary voyages. Their results would have shaped the next interplanetary mission, which might have taken the form of a piloted Mars orbiter in the spirit of Apollo 8 and Apollo 10, or, if the space agency felt sufficiently confident in its abilities, a Mars orbital mission with a short piloted excursion to the martian surface in the spirit of Apollo 11.

Sixty days from Mars: two of the FLEM mission's astronauts transfer to the Orbiter. After exhaustive systems checks, they fire explosive bolts to cast off the two-part "spacer" linking their spacecraft and the drum-shaped Flyby Spacecraft. A docking mechanism retracts, then small thrusters push the two spacecraft apart.

The Orbiter backs away from the Flyby Spacecraft. The two astronauts on the Flyby Spacecraft inspect the Orbiter's exterior and transmit television of its departure to Mission Control on Earth.

Using its Apollo-type thruster quads, the Orbiter turns away from the Flyby Spacecraft, positioning itself for the separation burn that will cause it to reach Mars 16 days ahead of the Flyby Spacecraft. Visible on both are engines based on the Apollo Lunar Module descent engine design. The four engines on the Flyby Spacecraft are part of the Earth Return Module (ERM) braking stage. The ERM, an Apollo CM with seating for four astronauts, is linked by its nose-mounted docking unit to the aft deck of the Flyby Spacecraft Mission Module. Life support gas and liquid tanks supplying the Mission Module surround the CM; these act as radiation shielding, allowing the CM to serve double-duty as a solar storm shelter.

In January 1966, United Aircraft Research Laboratories engineer R. R. Titus unveiled a proposal for a new intermediate step in spaceflight evolution. He dubbed it FLEM, which stood for "Flyby-Landing Excursion Mode." FLEM missions would, Titus wrote, have a natural place in the evolutionary sequence between piloted Mars flybys and piloted Mars orbiters. A FLEM mission might even have become the basis for an early, very brief, piloted Mars landing.

Titus explained that, in the "standard stopover mode," a label that encompassed Mars orbital and landing missions, all major maneuvers would involve the entire Mars spacecraft. This meant that the Mars spacecraft would need a large mass of propellants, which in turn meant that many expensive heavy-lift rockets would be required to launch the spacecraft, its propellants, and its Earth-orbit departure stages into Earth orbit for assembly.

Propellant mass required would vary greatly from one Earth-Mars transfer opportunity to the next over a roughly 15-year cycle because Mars has a decidedly elliptical orbit. Because of this, the Mars spacecraft and the sequence of launches needed to boost its components and propellants into Earth orbit would have to be redesigned for each standard stopover mode Mars mission.

The United Aircraft engineer added that errors or malfunctions during standard stopover mode "high-risk" Mars capture and escape maneuvers could result in "complete mission failure" because the entire spacecraft would be involved. Because the Mars spacecraft would be very massive already, it would be difficult and costly to include extra propellants that would enable a mission abort that could save the crew.

Titus noted that required propellant mass might be reduced and made to vary less over multiple Earth-Mars transfer opportunities if the spacecraft skimmed through the martian upper atmosphere to slow down and capture into Mars orbit (that is, if it performed aerocapture). If, however, artificial gravity were found to be necessary for crew health, then stowing a spinning artificial-gravity system of sufficient radius behind an aerocapture heat shield would probably prove infeasible. The mission would then have to rely entirely on propulsive braking.

Titus explained that his FLEM concept, in addition to forming a natural evolutionary extension of piloted Mars flybys, would address many inherent problems of the standard stopover mode. One part of the FLEM spacecraft, the "parent" spacecraft, would not capture into Mars orbit. It could include a spinning artificial-gravity system. The other part, the "excursion module," would capture into Mars orbit using chemical rockets or, perhaps, by skimming through the martian atmosphere behind an aerocapture heat shield.

Orbiter alone: immediately following the separation burn, the two astronauts aboard the Orbiter deploy and orient its solar array and high-gain dish antenna; they are identical to those on the Flyby Spacecraft. The array points at the Sun, out of view beyond the upper left corner of the image, while the antenna aims at the bright blue-white dot that is Earth. Just above the Orbiter, half-lit Mars is visible. Also in view are the Orbiter's four science pallets (covered by white rectangular panels), gray airlock hatch, and yellow spacewalk handholds. The airlock, partly surrounded by propellant tanks, serves as the Orbiter's solar storm shelter.

He noted that Earth-Mars transfer opportunities that need less propellant for Earth departure tend to arrive at Mars moving quickly, while opportunities that require more propellant for Earth departure arrive at Mars moving slowly. In the former instance, the excursion module would need a large quantity of propellants to slow down enough for martian gravity to capture it into orbit, so would be the most massive of the two FLEM modules. Because of this, the lower-mass parent spacecraft would ignite its rocket motors to slow down so that the excursion module could reach Mars first.

In the latter case, the excursion module would not need a large mass of propellants to capture into Mars orbit, making it the least massive of the two FLEM spacecraft. It would thus fire rockets to speed up and reach Mars ahead of the parent spacecraft.

Titus calculated that separation 60 days ahead of the Mars flyby would enable the excursion module to reach the planet 16 days ahead of the parent spacecraft; separation 30 days before flyby would enable it to reach Mars when the parent spacecraft was nine days behind it. While it awaited the arrival of its parent, the excursion module might remain in Mars orbit or all or part of it might land on Mars for a stay of several days.

In Mars orbit: during the Mars orbit capture burn, the Orbiter crew stowed the solar array and high-gain antenna to protect them from deceleration damage. Soon after achieving Mars orbit, they re-deployed both, then ejected the panels covering the Orbiter's instrument pallets. The two forward pallets include identical suites of high-resolution film cameras; the aft pair, identical suites of spectrometers. Identical pallets provide redundancy if instruments fail. As the Orbiter circles Mars, passing in and out of daylight, its solar array can temporarily lose lock on the Sun; in the image above, this has occurred, so the array reflects a crater under scrutiny by the Orbiter's instruments.

The Orbiter crew is not inactive as the Mars-facing instrument pallets record images and other data. In addition to maintaining Orbiter systems, they use handheld and porthole-mounted cameras to capture images of "variable phenomena." These include clouds, dust storms, morning fogs and frosts, and the retreating or expanding edges of the seasonally variable polar ice caps. The Orbiter's single-deck Mission Module has eight portholes.

FLEM, Titus noted, offered a "partial success capability" which, he opined, "may be very attractive." If the excursion module were lost, then the astronauts remaining on board the parent spacecraft could still return safely to Earth. If the excursion module were found during pre-separation checkout to be incapable of accomplishing its mission, it would not undock from the parent spacecraft, and the FLEM mission could still achieve some of its goals while settling for a Mars flyby.

Assuming that the mission took place as planned, the excursion module would ignite its rocket motors as the parent spacecraft passed Mars to depart Mars orbit and catch up with it. Following rendezvous, docking, and crew transfer, the excursion module would be cast off.

Sixteen days after Orbiter capture into Mars orbit, the Flyby Spacecraft raced past Mars. The Orbiter crew ejected their spacecraft's solar array and ignited its engines to catch up with their ride home. In this image, the two astronauts on board the Flyby Spacecraft (left) have turned on rendezvous lights. The Orbiter engines glow red after the "chase" maneuver. Its antenna is positioned so it can serve as a handrail should an emergency spacewalk to the Flyby Spacecraft be needed. The Orbiter halts near the Flyby Spacecraft, then its crew steps outside to collect film cassettes from the four pallets. The Flyby Spacecraft crew records the spacewalk and inspects the Orbiter exterior for damage that might interfere with docking. The Orbiter astronauts stow the recovered cassettes inside their spacecraft, then dock with the Flyby Spacecraft. The reunited FLEM astronauts then transfer film cassettes to the Flyby Spacecraft and discard the Orbiter.

To squeeze even more benefit from FLEM, Titus proposed a variant of the standard ballistic flyby (that is, one in which the only major propulsive maneuver would occur at the start of the mission, when the spacecraft departed Earth orbit). His "powered flyby" would include an optional propulsive maneuver near Mars that would dramatically reduce FLEM spacecraft mass during unfavorable Earth-Mars transfer opportunities, limit the wide swings in propellant mass required from one Earth-Mars transfer opportunity to the next, and slash total trip time.

The maneuver would be optional in the sense that, if it could not occur, the FLEM spacecraft's Sun-centered orbit would return it to Earth, though after a longer than expected trip. During return to Earth after a powered flyby, the FLEM spacecraft would pass as close to the Sun as orbits the planet Mercury.

Titus determined that a powered-flyby maneuver in 1971 would have almost no effect on spacecraft mass at Earth-orbit departure — both the standard ballistic and powered-flyby FLEM spacecraft would have a mass of about 400,000 pounds — but would slash trip time from 510 to 430 days. The most dramatic improvement would occur in 1978, when the ballistic-flyby FLEM spacecraft's mass would total nearly two million pounds and its mission would last 540 days. The powered-flyby FLEM spacecraft would have a mass of just 800,000 pounds at the start of Earth-orbit departure and its mission would last only 455 days.

The FLEM concept apparently influenced NASA piloted flyby studies that occurred under the auspices of the Planetary Joint Action Group (JAG). The NASA Headquarters-led Planetary JAG, which met between 1965 and 1968, included representatives from NASA Marshall Space Flight Center, NASA Kennedy Space Center, and the NASA Manned Spacecraft Center, as well as Washington, DC-based planning contractor Bellcomm. The Planetary JAG's work will be described in detail in subsequent posts.

FLEM was a mission mode without a spacecraft for accomplishing it — but no longer. For this post, artist William Black worked with the author to create a plausible FLEM spacecraft based on flown and conceptual 1960s space hardware. In keeping with the post's overriding theme, we put FLEM into an evolutionary program. We assumed at first that NASA would launch a piloted Mars flyby in 1975 and follow it in 1978 with a FLEM mission that would include a powered Mars flyby. As explained in the post text, a powered flyby would reduce mission duration and, in 1978, cut by more than half the quantity of propellants needed to reach Mars. It would, however, also cause the FLEM Flyby Spacecraft to pass as near the Sun as the orbit of Mercury. Closest approach to the Sun would occur in 1979, during a period of maximum solar activity. Because of this, we rejected both 1978 flyby opportunity and the powered flyby. We chose instead a Mars flyby launched in 1981 for our FLEM mission, when solar activity would be in decline. Earth-return speed would be high (about 50,000 feet per second), so we decided to employ a maneuver pioneered during the 1975 flyby: a braking burn during Earth approach to reduce reentry speed to Apollo lunar-return speed (36,000 feet per second).

The two images above show the Apollo CM-based Earth-Return Module (ERM) and its braking stage during final Earth approach. The ERM/braking stage would have backed out of its "hangar" in the Flyby Spacecraft two days before the events portrayed here. In the top image, the braking stage engines have just shut down after the braking burn; their engine bells still glow red. In the bottom image, the spent braking stage has been cast off. Below the ERM, city lights outline the Indian subcontinent; in the background, over the western Pacific, dawn glows. The ERM will soon streak through the atmosphere over China and Japan, deploy four parachutes, and descend to a mid-morning splashdown southwest of Hawaii, completing the first FLEM voyage and a new evolutionary step toward humans on Mars.

All images in this post are Copyright © 2017 by William Black (http://william-black.deviantart.com/) and are used by special arrangement with the artist.

Sources

Manned Mars and.or Venus Flyby Vehicle Systems Study — Final Briefing Brochure, SID 65-761-6, North American Aviation, 18 June 1965.

"FLEM - Flyby-Landing Excursion Mode," AIAA Paper 66-36, R. R. Titus; paper presented at the 3rd AIAA Aerospace Sciences Meeting in New York, New York, 24-26 January 1966.

Planetary Exploration Utilizing a Manned Flight System, Planetary Joint Action Group, NASA Office of Manned Space Flight, 3 October 1966.

"Manned Expeditions to Mars and Venus," E. Z. Gray and F. Dixon, Voyage to the Planets, Proceedings of the Fifth Goddard Memorial Symposium, 14-15 March 1967, pp. 107-135.

Wonderful Life: The Burgess Shale and the Nature of History, Stephen Jay Gould, W. W. Norton & Co., 1990.

More Information

After EMPIRE: Using Apollo Technology to Explore Mars and Venus (1965)

Relighting the FIRE: A 1966 Proposal for Piloted Interplanetary Mission Reentry Tests

To Mars by Way of Eros (1966)

Apollo Ends at Venus: A 1967 Proposal for Single-Launch Piloted Venus Flybys in 1972, 1973, and 1975

Triple-Flyby: Venus-Mars-Venus Piloted Missions in the Late 1970s/Early 1980 (1967)