19 August 2017

Prelude to Mars Sample Return: The Mars 1984 Mission (1977)

The Viking 2 rover on the frosty plain at Utopia. Image credit: Pat Rawlings/NASA
Even before Viking 1 landed on Mars (20 July 1976), NASA and its contractors studied post-Viking robotic Mars missions. Prominent among them was Mars Sample Return (MSR), considered by many to be the most scientifically significant robotic Mars mission.

The Viking missions reinforced this view of MSR, and also revealed the perils of making too many assumptions when planning costly and complex Mars exploration missions. The centerpiece of the $1-billion Viking mission, a briefcase-sized package of three biology experiments, yielded more questions than answers. Most scientists interpreted their data as evidence of previously unsuspected reactive soil chemistry, not biology. The truth, however, was that no one could be certain what the Viking biology experiment results meant.

With that unsatisfying experience in mind, A. G. W. Cameron, chair of the National Academy of Sciences Space Science Board, wrote in a 23 November 1976 letter to NASA Administrator James Fletcher that
[to] better define the nature and state of Martian materials for intelligent selection for sample return, it is essential that precursor investigations explore the diversity of Martian terrains that are apparent on both global and local scales. To this end, measurements at single points. . .should be carried out as well as intensive local investigations of areas 10-100 [kilometers] in extent.
Soon after Cameron wrote his letter, NASA Headquarters asked the Jet Propulsion Laboratory (JPL) to study a 1984 MSR precursor mission. The JPL study, results of which were due by July 1977, was meant to prepare NASA to request "new start" funds for the 1984 mission in Fiscal Year 1979. NASA also created the Mars Science Working Group (MSWG) to advise JPL on the mission's science requirements. The MSWG, chaired by Brown University's Thomas Mutch, included planetary scientists from several NASA centers, the U.S. Geological Survey (USGS) Astrogeology Branch, and Viking contractor TRW.

The MSWG's July 1977 report called the Mars 1984 mission the "next logical step" in "a continuing saga" of Mars exploration and a "required precursor" for an MSR mission, which it targeted for 1990. Mars 1984 would, it explained, provide new insights into the planet's internal structure and magnetic field, surface and sub-surface chemistry and mineralogy ("especially as related to the reactive surface chemistry observed by Viking"), atmosphere dynamics, water distribution and state, and geology of major landforms.

The Mars 1984 mission would also seek answers to "The Biology Question." The MSWG declared that
on-going exploration of Mars must address the issue of biology. Although there does not appear to be active biology at the two Viking landing sites, there may be other localities with special environments conducive to life. Life-supportive aspects of the Martian environment must be defined in greater detail. The characterization of former environments [and] a search for fossil life. . .should be conducted.
Mars 1984 would begin in December 1983-January 1984 with two Space Shuttle launches no less than seven days apart. The piloted, reusable Space Shuttle Orbiters would each place into low-Earth orbit a Mars 1984 spacecraft comprising one 3683-kilogram orbiter based on the Viking Orbiter design, three penetrators with a combined mass of 214 kilograms, and one 1210-kilogram lander/rover combination housed in an extended Viking bioshield/aeroshell. Together with an adapter linking it to a two-stage Intermediate Upper Stage (IUS), each Mars 1984 spacecraft would weigh a total of 5195 kilograms.

A Viking orbiter releases an aeroshell containing a Viking Mars lander. The Mars 1984 orbiter would have a similar design; the aeroshell, however, would stand taller to provide sufficient room for the lander/rover combination within it.

Viking aeroshell (left) and Mars 1984 aeroshell. Image credit: Martin Marietta
The Shuttle Orbiters would each deploy a spacecraft/IUS combination from its payload bay, then would maneuver away before IUS first-stage ignition. The MSWG calculated that the IUS would be capable of placing 5385 kilograms on course for Mars on 2 January 1984, near the middle of a launch opportunity spanning 28 days.

The twin Mars 1984 spacecraft would reach Mars from 14 to 26 days apart between 25 September and 18 October 1984, after voyages lasting a little more than nine months. Each would perform a final course-correction rocket burn using attitude control thrusters a few days before planned Mars Orbit Insertion (MOI). Their penetrators would separate two days before MOI and fire small solid-propellant rocket motors to steer toward their target impact sites on Mars. The motors would then separate from the penetrators.

During MOI, each spacecraft would fire a solid-propellant braking rocket motor, then the orbiter's liquid-propellant maneuvering engine would ignite to place it into a 500-by-112,000-kilometer "holding" orbit with a five-day period. Spacecraft #1's orbit would be near-polar, while spacecraft #2 would enter an orbit tilted from 30° to 50° relative to the martian equator. MOI completed, flight controllers would turn the orbiter's cameras toward Mars to assess weather conditions ahead of lander separation.

The Bendix Mars penetrator was designed to enter Mars's atmosphere directly from an interplanetary trajectory and embed itself in solid rock. A = radio antenna; B = meteorology package and magnetometer; C = isotope heater; D = aft body electronics; E = Aft body/fore body separation plane; F = cable linking aft body and fore body; G = accelerometer and neutron detector; H = fore body electronics; I = drill assembly; J = sampling drill bit; K = geochemical analysis package; L = seismometer; M = batteries; N = radioisotope thermal generator. Image credit: Bendix Corporation 
At about the time the twin spacecraft entered their respective holding orbits, the six penetrators would impact at widely scattered points. Each would split at impact into two parts linked by a cable. The aft body, which would include a weather station and an antenna for transmitting data to the orbiters, would protrude from the martian surface after impact. The fore body would include a drill for sampling beneath Mars's surface and a seismometer. According to the MSWG, penetrators were "the only economic means" of establishing a Mars-wide sensor network. Establishing a network of widely scattered seismometers was considered vital for charting the planet's interior structure.

After several months in holding orbit, spacecraft #2 would move to a 300-by-33,700-kilometer "magneto orbit," where it would explore Mars's magnetospheric bow wave and tail. It would then maneuver to a 500-by-33,500-kilometer "landing orbit" with a period of one martian day (24.6 hours). During a one-month landing site certification period, scientists and engineers would closely inspect orbiter images of the candidate landing site. Spacecraft #1, meanwhile, would proceed directly from holding orbit to landing orbit.
The Mars 1984 landing system for delivering the Mars 1984 rover to the surface would include five main parts. 1= top bioshield for protecting the sterilized lander and rover from contamination; 2 =  top aeroshell for protecting the lander from reentry heating; 3 = folded lander (rover not displayed); 4 = bottom aeroshell with attitude control/deorbit thrusters and propellant tanks; 5 = bottom bioshield/heat shield. Landing would occur as follows: the top bioshield would be left behind on the Mars 1984 orbiter as the rest of the lander moved away; motors on the bottom aeroshell would ignite to deorbit the lander; following reentry, the top aeroshell would deploy a single large parachute; the bottom aeroshell/heat shield would fall away; and the lander would fall free of the top aeroshell and ignite its landing motors for final descent. Image credit: Martin Marietta
The Mars 1984 landers would have one purpose: to deliver the Mars 1984 rovers to Mars's surface. Lander #2 would set down first at about 6° south latitude and lander #1 would land at about 44° north latitude at least 30 days later. JPL estimated that imaging data from the Viking orbiters would enable each Mars 1984 lander to set down safely within a "error ellipse" 40 kilometers wide by 65 kilometers long (for comparison, Viking's landing ellipse measured 100 kilometers wide by 300 kilometers long).

The Mars 1984 landers, based on a Martin Marietta design, would each include a "terminal site selection system." This would steer them away from boulders and other hazards as they descended the final kilometer to the martian surface. In other respects, their deorbit and landing systems would closely resemble those of the Vikings.

After lander separation, orbiter #1 would maneuver to a 500-kilometer near-polar circular orbit and orbiter #2 would move to a 1000-kilometer near-equatorial circular orbit. Orbiter #1's low near-polar orbit would permit global mapping at 10-meter resolution, while orbiter #2's more lofty near-equatorial orbit would enable it to map the equatorial region at 70-meter resolution. Low-flying Orbiter #1 would serve as the radio relay for the six penetrators, which would transmit relatively weak signals, while orbiter #2 would relay signals to and from the twin rovers.

The MSWG expected that most orbiter science operations would require minimal planning, since they would "be highly repetitive with most instruments acquiring data continuously and sending it to Earth in real time without tape recording." The exception would be imaging operations, since imaging data would be "acquired at a rate many times too great for real-time transmission." The MSWG suggested that the orbiters relay to Earth about 80 images of Mars per day.

Mars 1984 rover. A = antenna for signal relay through orbiter #2; B = antenna for direct transmission to and from Deep Space Network antennas on Earth; C = optics port cluster and strobe light (1 of 2); D = imaging/laser rangefinder mast (1 of 2); E = selenide radioisotope thermal generator (cover removed to display cooling vanes); F = rover chassis; G = manipulator arm with sampling drill (folded in travel position); H = sample-analysis inlet port; I = hazard detectors; J = loopwheel mobility system (1 of 4).

Mars 1984 rover and lander folded within their aeroshell and bioshield. A = folded landing leg (1 of 3); B = Viking-type landing footpad (1 of 3); C = lander body; D = Viking-type terminal descent engine (1 of 3); E = Viking-type parachute canister with deployment mortar; F = terminal site selection system sensors; G = folded rover ramp (1 of 2); H = folded loopwheel mobility system (2 of 4); I = stowed imaging/laser rangefinder mast (1 of 2); J = folded antenna for direct communication with Earth; K = rover chassis; L = radioisotope thermal generator; M = outer surface of aeroshell (tanks and thrusters not shown); N = outer surface of bioshield (heat shield not shown); O = attachment point linking bioshield to Mars 1984 orbiter. Image credit: Martin Marietta
The MSWG envisioned that the Mars 1984 rovers would be "substantial vehicles" capable of traveling up to 150 kilometers in two years at a rate of 300 meters per day. They based their rover concept on a Jet Propulsion Laboratory (JPL) design. Each would include four "loop-wheel" treads on articulated legs, a radioisotope thermal generator providing heat and electricity, laser range-finders for hazard avoidance, an "improved Viking-type manipulator" arm, twin cameras for stereo imaging, a microscope, a percussion drill for sampling rocks to a depth of 25 centimeters, and a sample processor for distributing martian materials to an on-board automated laboratory for analysis.

The MSWG acknowledged that a costly automated lab on an MSR precursor mission might be hard to justify, given that the MSR mission meant to follow it was intended to return samples to well-equipped labs on Earth for detailed analysis. The group argued, however, that clues to the nature of the reactive soil chemistry found by the Vikings might "reside in loosely bound complexes or interstitial gases" that "would be extraordinarily difficult to preserve in a returned sample." The scientists might also have worried that the planned MSR mission would be postponed or cancelled, leading them to attempt to exploit every opportunity to acquire new data.

The rovers would store particularly interesting samples for collection during the MSR mission and test the effects of Mars's reactive soil chemistry on MSR sample container materials. They would also each drop off three seismometer/weather stations as they moved over the surface to create a pair of 20-kilometer-wide regional sensor networks.

The rovers would employ three Mars surface operation modes. The first, Site Investigation Mode, would enable "intensive investigation of a scientifically interesting site." The rover would be fully controlled from Earth.

In Survey Traverse Mode, the second mode, the rover would operate nearly autonomously in a "halt-sense-think-travel-halt" cycle. Each survey/traverse cycle would last about 50 minutes and move the rover forward from 30 to 40 meters. Science operations would occur during the "halt" portion and while the rover was parked at night. Flight controllers would update rover commands once per day. The rover would cease autonomous operations and alert Earth when it encountered a hazard or a feature of scientific interest.

The third mode, Reconnaissance Traverse Mode, would occur when the terrain was sufficiently smooth (and scientifically dull) to allow the rover to move at its top speed of 93 meters per hour. The rover would make few science stops and would travel both by day and by night.

Valles Marineris with Mars 1984 landing ellipses marked in red and labeled. Image credit: NASA
To conclude its report, the MSWG drew on USGS studies based on Mariner 9 and Viking orbiter data to offer two candidate near-equatorial landing sites for lander #2. Capri Chasma, at the eastern end of Valles Marineris, included heavily cratered (thus ancient) highlands terrain, lava flows of different ages, lava channels, and possible water-related channels and deposits. Candor Chasma, a north-central branch of Valles Marineris, included at least two rock types in its four-kilometer-high canyon walls. The group expected that a Mars 1984 rover might find ancient crystalline rocks on the canyon floor.

New Mars missions stood little chance of acceptance in the late 1970s, when NASA's limited resources were largely devoted to Space Shuttle development and public enthusiasm for the Red Planet was (thanks the equivocal Viking biology results) at a nadir. Though MSR remained a high scientific priority (as it does today), the planetary science community opted to seek support for missions to other destinations: for example, the Jupiter Orbiter and Probe mission, later renamed Galileo, got its start in NASA's Fiscal Year 1978 budget.

NASA's next Mars spacecraft, the Mars Observer orbiter, was approved in 1985 for a 1990 launch; launch was subsequently postponed until September 1992, then the spacecraft failed during Mars orbit insertion in August 1993. NASA would return successfully to Mars for the first time since Viking in July 1997, when the 264-kilogram Mars Pathfinder spacecraft landed in Ares Valles bearing the 10.6-kilogram rover Sojourner.


Post-Viking Biological Investigations of Mars, Committee on Planetary Biology and Chemical Evolution, Space Science Board, National Academy of Sciences, 1977

Mars '84 Landing System Definition: Final Report, "Technical Report," Martin Marietta, April 1977

A Mars 1984 Mission, NASA TM-78419, "Report of the Mars Science Working Group," July 1977

More Information

Robot Rendezvous at Hadley Rille (1968)

The Russians are Roving! The Russian are Roving! A 1970 JPL Plan for the 1979 Mars Rover

Safeguarding the Earth from Martians: The Antaeus Report (1978-1981)

12 August 2017

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

Cutaway of a reentering Apollo Command Module showing the position of its crew. Image credit: NASA
On 14 April 1964, a NASA Atlas-D rocket lifted off from Cape Kennedy, Florida, bearing the first Flight Investigation Reentry Environment (FIRE) payload. Project FIRE aimed to gather data on atmosphere reentry at lunar-return speed - about 36,000 feet per second (fps) - to enable Apollo engineers to develop the heat shield for the conical Apollo Command Module (CM).

Initiated in 1961 and managed by NASA's Langley Research Center (LaRC) under direction of the NASA Headquarters Office of Advanced Research and Technology, FIRE focused mainly on testing instrumented sub-scale model CM capsules in wind tunnels and thermal chambers at LaRC. Engineers realized, however, that there could be no substitute for data gathered in the actual spaceflight environment.

NASA rolls back the gantry structure surrounding the Atlas-D rocket bearing the first Project FIRE spacecraft, April 1964. Image credit: NASA
The Atlas-D rocket lobbed the Project FIRE payload, the 14-foot-long, 4150-pound Velocity Package (VP), onto an arcing course toward remote Ascension Island in the South Atlantic Ocean, a British possession that since 1957 had been home to U.S. missile tracking facilities. The VP cast off its two-part aerodynamic shroud and separated from the spent Atlas-D a little more than five minutes after liftoff. Attitude control motors mounted in its roughly cylindrical support shell then ignited to adjust its pitch so that it pointed its nose at Earth at a shallow angle.

About 21 minutes after separation from the Atlas-D and 800 kilometers above Earth, three rockets on the support shell ignited to spin the VP, giving it gyroscopic stability. Three seconds later, the VP cast off the support shell, revealing the engine bell of its solid-propellant Antares II-A5 rocket motor. Three seconds after support shell separation, the 24,000-pound-thrust motor ignited, driving the VP toward Earth's atmosphere.

The Antares motor burned out 33 seconds later, with the VP moving at nearly 37,000 fps. About 26 seconds later, the Apollo CM-shaped Reentry Rackage (RP) separated. Seven seconds after that, the 200-pound capsule fell past 400,000 feet, where the aerodynamic effects of reentry began to become obvious.

Image credit: NASA
Project FIRE Reentry Package. Image credit: NASA
The FIRE RP's heat shield heated rapidly as the falling capsule compressed and heated the atmosphere in its path. More than 300 sensors gathered data on the high-speed reentry environment. As the RP achieved a maximum speed of about 38,000 fps, the shockwave in front of the heat shield reached about 20,000° Fahrenheit (that is, about twice as hot as the Sun's surface).

Reentry heating formed a sheath of ionized gas around the FIRE RP, blocking radio signals. During the "blackout" period, which lasted for about 40 seconds, the RP stored data on magnetic tape. It transmitted the data after blackout ended.

Observers on Ascension Island - where the Sun had set - were able to track the FIRE RP visually as it automatically threw off two layers of heat shield material. They also observed the destructive reentry of the spent Antares II-A5 motor.

Thirty-two minutes after launch, the RP splashed into the Atlantic southeast of Ascension, about 5200 miles from Cape Kennedy. It was not designed for recovery.

NASA carried out the Project FIRE II test 13 months later, on 22 May 1965. The FIRE II RP was nicknamed the "flying thermometer" because it transmitted more than 100,000 temperature readings before ocean impact 5130 miles from Cape Kennedy. After FIRE II, engineers felt confident that they understood the atmosphere reentry effects the Apollo CM would experience as it returned from the moon.

The unmanned Apollo 4 (November 1967) and Apollo 6 (April 1968) Saturn V test missions carried out full-scale Apollo CM reentry tests. Astronauts first put the CM heat shield to the test at lunar-return speed during the Apollo 8 mission, which saw the second manned Apollo Command and Service Module (CSM) spacecraft orbit the moon 10 times on Christmas Eve 1968. Frank Borman, James Lovell, and William Anders reentered Earth's atmosphere in the Apollo 8 CM at nearly 36,000 fps on 27 December and splashed down safely in the Pacific southwest of Hawaii.

The FIRE flight tests were fresh in the minds of D. Cassidy, H. London, and R. Sehgal, engineers with Bellcomm, when they wrote a 14 April 1966 memorandum that proposed heat shield tests ahead of piloted Mars and Venus missions. Bellcomm was formed in 1962 to serve as the NASA Headquarters Apollo planning contractor, but almost immediately had extended its bailiwick to include planning beyond Apollo.

A piloted flyby spacecraft of the 1970s dispenses automated probes near Mars while a radar dish and a telescopic camera scrutinize the planet. Image credit: NASA
The three engineers wrote that Mars has a noticeably elliptical orbit around the Sun. Because of this, a piloted Mars flyby mission with a duration of 1.5 years would return to Earth at speeds ranging between 45,000 and 60,000 fps depending on where Mars was in its orbit when the flyby took place. A two-year Mars flyby mission would reenter Earth's atmosphere at between 45,000 and 52,000 fps. An opposition-class (short-stay) Mars "stopover" (orbiter or landing) mission would reenter at between 50,000 and 70,000 fps.

Venus, by contrast, has a nearly circular orbit around the Sun, so all flyby missions would return to Earth moving at about 45,000 fps. All Venus stopovers would reach Earth moving at between 45,000 and 50,000 fps. An opposition-class Mars stopover mission that flew past Venus before reaching Mars to speed up so that it could use a slow Earth-return path or flew past Venus during return from Mars to slow its approach to Earth would also reenter at between 45,000 and 50,000 fps.

Cassidy, London, and Sehgal wrote that, at speeds beyond 50,000 fps, reentry data gathered through testing for Apollo lunar missions no longer applied. Reentry heating would occur through different mechanisms and encompass a broader swath of the electromagnetic spectrum. This would increase turbulence and decrease the effectiveness of Apollo-type ablative heat shields (that is, heat shields designed to char and erode to dissipate reentry heat). In fact, at speeds beyond 50,000 fps, shield fragments detached by ablation could contribute to turbulence and heating.

The Bellcomm engineers acknowledged that braking propulsion might be used to slow a crew capsule to a better-understood Earth-atmosphere reentry velocity. They calculated, however, that slowing a piloted crew capsule derived from the Apollo CM from 70,000 fps to 50,000 fps would double the Earth-departure mass of the entire Mars stopover spacecraft. This would occur because extra propellants would be needed to launch the Earth-reentry braking propellants from Earth orbit to Mars and back again. Doubling the mass of the Mars spacecraft would in turn double the number of expensive heavy-lift rockets required to launch its components and propellants from Earth's surface to assembly orbit about the Earth.

They acknowledged that ground tests had provided some data on the interplanetary reentry velocity regime, but warned that the problem of aerodynamic surface heating involved "a complex interaction of vehicle size, shape[,] and heat protection characteristics." There would be, they added, “no substitute for testing specific configurations and materials in the actual environment of interest."

Cassidy, London, and Sehgal proposed that up to eight reentry capsules with attached solid-propellant motors be added to an Apollo Applications Program (AAP) Saturn V flight. AAP was NASA's planned post-Apollo program of Earth-orbital and lunar missions. The program aimed to use Apollo lunar mission vehicles in new ways. In addition to keeping the Apollo industrial team intact, AAP would see astronauts perform pioneering space biomedical and technology testing in Earth and lunar orbit, paving the way for piloted interplanetary voyages in the mid-to-late 1970s and the 1980s.

Image credit: NASA
Saturn V S-IVB third stage with cutaway and section showing spin tables and reentry capsules within the aft adapter that would link the stage to the Saturn V S-II second stage. Also shown is an Apollo Lunar Excursion Module (LEM)-derived lunar laboratory within the forward adapter that would link the top of the S-IVB to the bottom of the Apollo CSM. Image credit: Bellcomm/NASA
The Bellcomm trio proposed an interplanetary reentry test during a piloted lunar-orbital mission. The eight reentry capsules, each with a solid-propellant motor, might be housed in the adapter linking the bottom of the Saturn V S-IVB third stage with the top of the S-II second stage. Normally S-IVB separation would see the adapter left behind on the S-II, but for this mission it would remain attached to the S-IVB. Each reentry capsule-motor combination would be mounted on an individual spin table to spin it about its long axis for gyroscopic stability before release.

The AAP mission Cassidy, London, and Sehgal envisioned would include an Apollo CSM and a small lunar-orbital laboratory derived from the Apollo Lunar Module (LM) lander. The S-IVB's single J-2 engine would accelerate the S-IVB stage, the S-II/S-IVB adapter, the eight reentry capsules and their associated hardware, the LM Lab, and the CSM out of Earth parking orbit into a high elliptical Earth orbit.

After S-IVB shutdown, the crew in the CSM would detach their spacecraft from the stage, turn it end for end, and dock it with the LM Lab. They would extract the LM Lab from the front end of the S-IVB stage, then ignite the CSM's Service Propulsion System (SPS) main engine to place the CSM/LM Lab combination on course for the moon. A few days later they would fire the SPS again to enter orbit around the moon.

The S-IVB stage would retain about 30,000 pounds of liquid hydrogen/liquid oxygen propellants after the CSM and LM Lab went on their way. About 12 hours after departure from parking orbit, the S-IVB, with its cargo of reentry capsules and solid-propellant motors, would reach its maximum altitude above the Earth. The stage would aim at Earth, restart, and burn all of its remaining propellants, attaining a velocity of about 41,100 fps.

After J-2 engine shutdown, the spin tables would spin up the eight reentry capsules and their motors, then springs would push them out of the S-II/S-IVB adapter. Once clear of the S-IVB stage, the motors would ignite to further accelerate the reentry capsules.

Cassidy, London, and Sehgal calculated that Project FIRE's Antares II-A5 motor could increase a 10-pound reentry capsule's speed to 56,100 fps after release from the S-IVB stage. It could boost a 200-pound capsule to 48,500 fps. A TE-364 solid-propellant motor of the type used to brake unmanned Surveyor landers during descent to the lunar surface could accelerate a 10-pound capsule to nearly 60,000 fps. A 200-pound capsule with a TE-364 motor could attain 53,500 fps.


"NASA Schedules Project FIRE Launch," NASA News Release No. 64-69, April 14, 1964

Astronautics & Aeronautics, 1964: Chronology on Science, Technology, and Policy, NASA SP-4005, NASA Historical Staff, Office of Policy Planning, 1965, pp. 135, 350

"Reentry Heating Experiment on Saturn V AAP Flights or Unmanned Saturn IB Flights - Case 218," D. Cassidy, H. London, and R. Sehgal, Bellcomm, 14 April 1966

Astronautics & Aeronautics, 1965: Chronology on Science, Technology, and Policy, NASA SP-4006, NASA Historical Staff, Office of Policy Analysis, 1966, pp. 244

Project FIRE in Langley Researcher - https://crgis.ndc.nasa.gov/crgis/images/2/26/Project_Fire_Newsletters.pdf

More Information

Starfish and Apollo (1962) (Bellcomm)

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

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

"Assuming that Everything Goes Perfectly Well in the Apollo Program. . ." (1967) (AAP)

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