Human Exploration Using Real-Time Robotic Operations (HERRO) (2009-2011)

26 November 2011: Mars Science Laboratory rover Curiosity lifts off from Space Launch Complex 41 at Cape Canaveral Air Force Station, Florida, atop an Atlas V 541 medium-lift expendable rocket. The first launches of the HERRO mission in late 2030-early 2031 would see three Atlas V or equivalent rockets each launch a Truck rover bearing two Rockhound rovers to Mars. The three Truck/Rockhound combinations would land in widely separated 100-kilometer-diameter exploration regions. A six-person HERRO crew would arrive in Mars orbit in 2033 to begin a 549-day stay, during which they would remotely operate (that is, teleoperate) the rovers to explore the three regions. Image credit: NASA.

This (long) post outlines the most recent (2009) high-level NASA plan for landing humans on Mars. Called Design Reference Architecture (DRA) 5.0, it embraces long-held expectations regarding the role of astronauts in Mars exploration; the most significant of these is that when astronauts are dispatched to Mars for the first time they will land on its surface. 

The post then describes an alternative piloted Mars mission concept, called Human Exploration Using Real-Time Robotic Operations (HERRO), which could serve as an interim step designed to place on firmer ground planning for a follow-on piloted Mars landing mission. The HERRO concept could in fact provide data that would enable us to make an essential determination; that is, whether the traditional goal of humans on Mars remains a desirable one. 

In January 2007, NASA began efforts to update Design Reference Mission (DRM) 4.0, its plan for a piloted Mars landing mission. DRM 4.0 was based mostly on work performed in the 1998-2001 period. The updated DRM, the aforementioned DRA 5.0, was developed by the Mars Architecture Working Group (MAWG), which drew its more than 100 members from across NASA. The MAWG worked under the guidance of the Mars Architecture Steering Group (MASG). DRA 5.0 was formally published in July 2009. 

The term "Architecture" replaced the term "Mission" when referring to DRA 5.0 in part to signal a shift in NASA's plans for an evolutionary program of scientific space exploration. In addition to multiple piloted Mars landing missions, DRA 5.0 sought to include International Space Station (ISS) missions, piloted lunar exploration in the Constellation Program, and robotic Mars missions including Mars Sample Return (MSR) missions. 

DRA 5.0 scheduled its first piloted Mars landing for the late 2030s. Taking into account plans to cancel the Space Shuttle in 2010 that were hatched in the aftermath of the February 2003 Columbia accident, it replaced NASA's venerable semi-reusable winged crew and cargo spacecraft with the Ares I rocket for launching Orion crew vehicles and the Ares V heavy-lift rocket for launching cargo. The rockets, both under development in 2007, were to be derived from Shuttle hardware. Though named for the Greek god the Romans renamed Mars, the Ares rockets were primarily designed with Constellation Moon missions in mind. 

In documents used as sources for this post there occur discrepancies in spacecraft designs, weights, sizes, and other particulars. For example, a document will on one page display a spacecraft assembly sequence that requires three Ares V launches; on another page of the same document, the text will then describe an assembly sequence that requires four Ares Vs. 

These discrepancies are not explained, though they might reflect changing energy requirements for Earth-Mars and Mars-Earth minimum-energy transfers. Mars has a decidedly elliptical orbit about the Sun, so the energy required to visit and return from it changes markedly from one minimum-energy Earth-Mars transfer opportunity to the next; such opportunities occur every 26 months. To avoid confusion, in most cases in this post the DRA 5.0 and HERRO spacecraft appear in the forms that occur most often in their respective study documents.

The Shuttle-derived Ares I crew rocket (left) and Ares V cargo rocket as envisioned in 2009. Mars DRA 5.0 would require a minimum of three Ares I rockets and 21 Ares V rockets launched over a period of about eight years to explore three 100-kilometer-diameter exploration regions on Mars. In addition to the three medium-lift rockets described in the image at the top of this post, the HERRO mission would need one Ares I rocket and four Ares Vs to explore three 100-kilometer-diameter regions. Twenty-six months would separate the medium-lift rocket launches from the HERRO Ares I and Ares V launches; the Ares Vs would be launched and their payloads joined in Earth orbit over a period of about five days. Image credit: NASA.

A DRA 5.0 mission would begin with four Ares V launches from NASA Kennedy Space Center (KSC) in Florida. Two Ares Vs would each boost into low-Earth orbit a 28.8-meter-long, 96.6-ton Nuclear-Thermal Rocket (NTR) core stage. The NTR stage would contain liquid hydrogen actively cooled to prevent boiloff. Two more Ares Vs would each place into orbit an approximately 64-ton payload packaged inside a 30-meter-long Mars aeroshell with a mass of about 43 tons. In addition to enabling aerocapture and atmosphere entry at Mars, the bullet-shaped "triconic" aeroshells would serve as streamlined Earth launch shrouds. 

The stages and aeroshells would rendezvous and dock autonomously in low-Earth orbit to form an outwardly identical pair of cargo spacecraft. Neither spacecraft would carry a crew as it ignited its three 25,000-pound-thrust NTR engines to begin a minimum-energy Earth-Mars transfer lasting about 350 days.

Near Mars, the two cargo spacecraft would cast off their NTR core stages. One aeroshell, containing a 63.7-ton Mars Descent/Ascent Vehicle (MDAV) lander, would perform an aerocapture maneuver in the thin martian atmosphere to slow itself so that the planet's gravity could capture it using minimal propellants; then, after rising to apoapsis (orbit high point), it would reenter the atmosphere.

After any aeromaneuvers required to reach its preselected landing site, the MDAV would discard its two-part aeroshell, ignite descent rockets, and land in a level area near the center of a 100-kilometer-diameter exploration region. Orbital mechanics and aeromaneuvering limitations would mean that DRA 5.0 landers could not land at high-latitude and polar sites.

Following touchdown, the MDAV would deploy a nuclear power system on a robotic cart. Trailing cables, it would move a safe distance away from the landing site before power system activation. The MDAV would then begin splitting carbon dioxide drawn from the atmosphere to fill its empty oxidizer tanks with liquid oxygen. The first Mars mission crew would not leave Earth if the MDAV failed to fill its tanks.

The other aeroshell, containing a 64.3-ton Surface Habitat (SHAB) lander, would not land immediately. Instead, it would ignite rocket motors at apoapsis to raise its periapsis (orbit low point) out of the martian atmosphere and achieve a stable orbit. There it would remain awaiting arrival of the first six-person DRA 5.0 Mars landing crew in about 18 months.

The spacecraft at the top in the illustration above is the DRA 5.0 Mars Transfer Vehicle (MTV) for carrying the crew to and from Mars orbit; the spacecraft at the bottom could be either of the two cargo spacecraft. They are shown as they would appear in Earth orbit just before departure for Mars. Image credit: NASA.

The Mars Transfer Vehicle (MTV) bearing the first crew would leave Earth orbit 26 months after the first two cargo spacecraft. In the months prior to departure, three Ares V rockets would place into low-Earth orbit an NTR core stage, a "saddle-shaped" truss structure containing a drop tank, and a "supporting payload." They would rendezvous and dock autonomously. The completed MTV would measure 96.7 meters in length and have a mass of 356.4 tons. 

The supporting payload, located at the front of the newly assembled MTV, would include four solar arrays and an inflatable Transhab crew module with a docking port at its front. Within a short saddle truss mounted behind the Transhab the supporting payload would include a cylindrical contingency food container and a backup docking port. 

The food container would be called into play if the astronauts could not land on Mars; in that event, they would be required to remain on board the MTV in Mars orbit long enough for Earth and Mars to align to permit a minimum-energy Mars-Earth transfer, a period of about 500 days. Its contents might also be put to use if, after landing successfully on Mars, the crew had to evacuate the surface early.

The food container was not the only contingency hardware included in DRA 5.0. At the time NASA launched the first MTV, it would also launch four more Ares V rockets carrying two NTR core stages and two aeroshells containing MDAV and SHAB landers. The latter would be virtually identical to the first pair launched 26 months earlier. The NTR stages and aeroshell/lander combinations would autonomously dock in orbit to form two more cargo spacecraft. 

The second set of cargo spacecraft would be intended to serve the crew of the second Mars landing mission, which would leave Earth orbit in the second MTV 26 months after the first. They could, however, also serve as backups for the first Mars landing crew's MDAV and SHAB landers. They would reach Mars about 170 days after the first crew. If the first crew were required to use them, the second crew's departure from Earth would be delayed until a third set of cargo spacecraft could reach Mars.

With the MTV assembled in Earth orbit, the first Mars landing crew would lift off in an uprated Orion spacecraft atop an Ares I rocket. They would dock their nine-meter-long crew transport, developed for the Constellation lunar program, with the MTV front docking port on the inflated Transhab, then would enter the MTV and check out its systems. A standard Orion atop an Ares I and a repair crew would stand by at NASA KSC, ready to provide assistance if the MTV failed its orbital checkout. 

An alternate launch configuration would see the uprated long-lived Orion boosted to Earth orbit on an Ares V rocket as part of the supporting payload. In that case, the crew, launched separately in a standard Orion on an Ares I, would dock with the MTV's backup docking port. The standard Orion used for crew delivery would be cast off before the MTV left Earth orbit for Mars.

Assuming the MTV passed orbital checkout, its three NTR engines would ignite to perform a 57.8-minute Trans-Mars Injection (TMI) burn. The crew would then settle in for a weightless Earth-Mars transfer lasting about 180 days.

As Mars loomed large ahead, the MTV's NTR engines would fire for 16 minutes to slow it so the planet's gravity could capture it into orbit. Following the Mars Orbit Insertion (MOI) burn, the crew would mothball the MTV and transfer in the Orion spacecraft to the orbiting SHAB. After a successful SHAB checkout, the astronauts would command the Orion to undock, then would fire the SHAB's deorbit engines. Following reentry, aeromaneuvers, aeroshell separation, and descent, the SHAB bearing the crew would land near the MDAV. The Orion, meanwhile, would return to the MTV, dock, and shut itself down.

After six weightless months in interplanetary space, the crew would need to adapt to life in Mars surface gravity, which is a little more than a third as strong as Earth surface gravity. Drawing on advice from a 13-member Human Health and Performance Team, the MAWG/MASG opted for a one-month acclimatization period after landing, during which the astronauts would inflate the SHAB habitat and perform other "initial setup" activities, such as deploying crew rovers. The MAWG/MASG assumed that Mars gravity — possibly in combination with an exercise regimen — would be sufficient to maintain astronaut health. This would not, however, have been shown to be correct before the first Mars landing mission.

A Mars DRA 5.0 landing site. At left, in the middle distance, stands the MDAV beside an unpressurized crew rover. In the foreground at right, a small pressurized crew rover stirs up dust as it crawls over the surface. Behind it stands the SHAB lander, the six-person crew's home on Mars for about 500 days, with a second pressurized crew rover to its right and an unpressurized (possibly robotic) rover nearby. Image credit: NASA.

The overriding objective of the DRA 5.0 surface missions would be acquisition of scientific knowledge. This would take in goals which have historically been among the most significant justifications of space exploration. All would be related through the science of geology since rocks can serve as recording devices for those who can read them. Goal I would focus on whether life ever arose on Mars, while Goal II would emphasize Mars climate history and processes. The most overtly geological would be Goal III, which would focus on the evolution of the surface and interior of Mars. 

The MAWG/MASG examined three candidate Mars surface mission strategies. The Mobile Home strategy assumed the presence of large robust pressurized crew rovers towing trains of trailers bearing exploration equipment, including a drill rig for sampling the subsurface to a depth of hundreds of meters. The astronauts in the rovers would range for hundreds of kilometers over the martian surface. 

The less-ambitious Commuter strategy — the strategy the MAWG/MASG selected for DRA 5.0 Mars surface missions — would see reliance on a pair of "modest" pressurized rovers, two unpressurized rovers, and a trailer-mounted drill rig for sampling tens of meters beneath the martian surface. Prior to carrying out a series of monthly 100-kilometer traverses, each lasting up to two weeks, supply caches would be placed along prospective exploration traverse routes. How this would be accomplished was not explained in detail.

The Commuter pressurized rovers would be designed to carry two astronauts under normal operating conditions or four astronauts if a rover broke down far from the SHAB and its crew needed rescue. The "nimble" rovers would place astronauts in bulky Mars surface space suits within reach of diverse surface features of geologic interest, thus satisfying science requirements while limiting risk to Mars-walking astronauts.  

Both the Mobile Home and Commuter strategies would collect large quantities of samples which would be returned to the SHAB for analysis. About 250 kilograms of the most scientifically important samples would be retained for return to Earth; the remainder would be discarded. The MAWG/MASG expressed concern that 250 kilograms of samples might not fit readily into the MDAV ascent stage or the Orion capsule used to reenter Earth's atmosphere at the end of the mission.

The Mobile Home and Commuter strategies would face other operational challenges. Crews would not be able to rove far afield at times of heightened solar activity lest they suffer excessive radiation exposure. Even assuming advanced technology for generating and storing electricity, roving was not likely to occur every day during a traverse. The MAWG/MASG determined that, even if the pressurized rover moved as slowly as half a kilometer per hour, it might need to park every other day and deploy solar arrays with 400 square meters of area — which would extend for 40 meters in all directions from the rover — to make enough electricity to recharge its batteries after a day of driving. 

Possibly the most significant challenge from a scientific standpoint for DRA 5.0 astronauts would be observance of planetary protection protocols. The MAWG/MASG assumed that "Special Regions" on Mars where organisms might reside could be identified in advance of the piloted Mars landing missions based on data from robotic precursor missions. Such regions would only be explored using sterilized rovers remotely operated in real time (that is, teleoperated) by astronauts in the SHAB.

The third candidate surface exploration strategy, dubbed Telecommuter, received the least attention of the three described in the DRA 5.0 report. It would see astronauts in shirtsleeves in the SHAB rely mostly on teleoperated rovers to explore Mars; in-person astronaut exploration would be limited to places accessible from the SHAB on foot or using unpressurized rovers that might travel at most 20 kilometers during a traverse. 

The MAWG/MASG expected that deep drilling and extensive surface sampling would be extremely difficult within the confines of the Telecommuter strategy. On the plus side, the sterilized teleoperated rovers could venture with impunity anywhere on the surface of Mars without violating planetary protection rules.

Regardless of which surface strategy was used, after about 500 days on Mars the first DRA 5.0 astronauts would enter the MDAV ascent stage and ignite its engines to begin ascent to the MTV waiting in Mars orbit. The ascent engines would use the oxygen the MDAV collected from the martian atmosphere to burn methane it had brought from Earth. After docking and transfer to the MTV, the astronauts would cast off the MDAV ascent stage. They would also discard the contingency food canister after filling it with waste. 

The crew would then fire the NTR core stage engines for 10.7 minutes to perform Trans-Earth Injection (TEI). Following a six-month Mars-Earth transfer, the astronauts would board the uprated Orion, undock, and perform a burn that would bend its course to intersect Earth's atmosphere. They would then cast off the Orion service module. As the crew reentered the atmosphere and descended to a landing, the vacant MTV would swing past Earth and enter a graveyard orbit about the Sun.

Before the first DRA 5.0 crew returned to Earth, the second crew would set out for Mars. They would land in a new 100-kilometer-diameter exploration region in the equatorial or mid-latitudes. The third crew would depart Earth to visit a third such region before the second crew left Mars. The third crew's return to Earth, about eight years after the first DRA 5.0 mission began, would complete the initial scientific exploration of Mars by astronauts. 

The MAWG/MASG anticipated that the first three DRA 5.0 missions would together serve as a prudent stepping stone to Mars missions four through 10. These follow-on missions would, the DRA 5.0 report explained, see a "sustained human presence" on Mars, but they were otherwise undefined.

While the MAWG/MASG put the finishing touches on DRA 5.0, engineers and scientists at NASA Glenn Research Center (GRC), Case Western Reserve University, and Carnegie Mellon University prepared a preliminary plan for a piloted Mars mission that would carry the Telecommuter strategy to its logical conclusion. HERRO — which, it will be recalled, stands for Human Exploration Using Real-Time Robotic Operations — would see six astronauts on board a Crew Telerobotic Control Vehicle (CTCV) in elliptical Mars orbit explore three widely separated 100-kilometer-diameter regions on Mars using teleoperated Truck and Rockhound rovers. The CTCV would operate much as does an ocean research ship on Earth and the Trucks and Rockhounds would explore Mars's surface much as Remotely Operated Vehicles deployed from a research ship explore the ocean depths.

HERRO — it might be called "DRA 5.0 Telecommuter on steroids" — was intended as a prudent step toward Mars DRA 5.0, not a substitute. The HERRO study team argued that NASA needed to add interim steps to gain experience and knowledge ahead of piloted Mars landings. They noted, for example, that in 2009 no data existed on whether humans could remain healthy in Mars gravity for 500 days (this remains true at this writing). Assuming that they could without supporting data could make investment in piloted Mars landers, crew rovers, and surface suits a costly gamble.

Several of the NASA GRC engineers who participated in the MAWG also took part in the three-month HERRO study, which was performed under the auspices NASA's 2009 Innovative Partnerships Program. In fact, their participation in the DRA 5.0 and HERRO studies overlapped in time. They were thus well positioned to integrate HERRO and DRA 5.0. 

As described in the picture caption at the top of this post, a HERRO Mars mission would begin with launch of three medium-lift rockets. Each would carry an aeroshell containing a Truck rover bearing two Rockhound rovers. The three launches might occur in late 2030.

The HERRO team sought to employ technology under development for NASA robotic Mars missions in their mission design. The Truck/Rockhound combinations would each ride to Mars in an aeroshell similar to that designed for Mars Science Laboratory Curiosity, which at the time of the HERRO study in 2009 was scheduled to launch to Mars in 2011. They would land using skycrane systems similar to the one expected to gently lower Curiosity to Mars's surface in 2012. 

Two Truck/Rockhound combinations would land on opposite sides of Mars at sites near the center of their assigned exploration regions. The third would land in a  south polar region. 

Truck rover with Rockhound rovers packed within an aeroshell derived from that of the Mars Science Laboratory Curiosity rover. The skycrane landing system, which would ride atop the Truck, is not shown. Image credit: NASA.

If the three Truck/Rockhound combinations left Earth in 2030, the CTCV bearing the crew would depart for Mars in early 2033. The HERRO study team described a CTCV resembling the DRA 5.0 MTV that would require four Ares V launches. The first would place into low-Earth orbit a 24.8-meter-long, 51.6-ton supporting payload outwardly similar to that of the DRA 5.0 MTV apart from the addition of two large dish antennas for transmitting large quantities of data to Earth. The antennas would need to be deployed before the Transhab could be inflated. 

The second Ares V would place in orbit a 26.1-meter-long, 135.8-ton saddle truss and drop tank. Again, it would differ from its DRA 5.0 counterpart mainly by the presence of two antennas, this time for transmitting to and receiving from the Trucks and Rockhounds on Mars large quantities of real-time data. 

The third Ares V rocket would launch into Earth orbit a 26.6-meter-long, 136-ton in-line propellant tank and the fourth a 29.9-meter-long, 132-ton NTR core stage outwardly identical to the one on the DRA 5.0 MTV. Supporting payload, saddle truss and drop tank, in-line tank, and NTR core stage would then rendezvous and dock autonomously in low-Earth orbit to form the 106.9-meter-long, 455.4-ton CTCV.

CTCV supporting payload. The deflated Transhab with docking port and large antennas is at right; the small saddle truss with food canister, docking port, and twin solar arrays (shown here folded) is at left. Image credit: NASA.

Saddle truss with drop tank. The twin dish antennas for transmitting to and from the Truck and Rockhound rovers on Mars are stowed on the disc-shaped docking structure at left. Image credit: NASA.

CTCV in-line propellant tank. Image credit: NASA.

Nuclear-Thermal Rocket core stage with three engines. The CTCV could complete its mission with only two functioning engines. Image credit: NASA.

Fully assembled HERRO CTCV with deployed antennas, inflated Transhab, and docked long-lived Orion (right). Image credit: NASA.

The HERRO crew, which would comprise four geologists and two pilots, would lift off from Earth atop an Ares I rocket in an uprated, long-lived Orion spacecraft and rendezvous and dock with the CTCV front docking port on the Transhab. After CTCV checkout they would ignite the three NTR core stage engines to begin the TMI maneuver; unlike DRA 5.0, the HERRO TMI burn would be split into two parts to reduce propellant consumption. TMI Burn 1, which would place the CTCV into an elliptical orbit about Earth, would expend all of the liquid hydrogen in the saddle truss drop tank; the tank would be discarded before TMI Burn 2, which would occur at next perigee. 

Shortly after TMI Burn 2, the crew would set the CTCV spinning end-over-end at a rate of 2.7 rotations per minute. This would produce acceleration in the Transhab equal to the pull of gravity on Mars's surface, thus providing an opportunity to confirm that Mars gravity would be sufficient to keep future landing crews healthy. The astronauts would then settle in for a six-month Earth-Mars transfer. 

The HERRO team proposed that the crew sleep and work in the Transhab's central core, which would be surrounded by tanks holding about 14 tons of water. By spending 16 hours of every day in the water-lined Transhab core, the astronauts would greatly reduce their exposure to Galactic Cosmic Radiation. 

The crew would despin the CTCV as they approached Mars, then would ignite the NTR core stage engines to perform MOI. The planet's gravity would capture the spacecraft into an elliptical, steeply inclined orbit with a period of 12.3 hours, or half a 24.6-hour martian day (known as a sol). The CTCV would reach apoapsis over the sunlit hemisphere of Mars twice per sol, with periapsis occurring twice per sol low over the nightside equator. The crew would then spin up the CTCV to restore Mars-level artificial gravity in the Transhab. 

The CTCV in artificial-gravity configuration. Red arrows indicate the spacecraft's end-over-end rotation. During flight to and from Mars and in Mars orbit the twin solar arrays and large antennas would respectively point continuously toward Sun and Earth. The small antennas would point toward the Truck and Rockhound rovers on Mars. Image credit: NASA.

As might be expected, the CTCV's orbital parameters were chosen to place it in line-of-sight radio contact at apoapsis with the Trucks and Rockhounds at their landing sites on opposite sides of Mars. This would enable two two-person geologist teams to take turns teleoperating the Trucks and Rockhounds in two work shifts, each lasting up to eight hours per sol, yielding a total teleoperations time per sol of up to 16 hours. 

Over the course of the 549-day CTCV stay in Mars orbit, lighting conditions at the Truck/Rockhound sites during the teleoperations shifts would change. At the non-polar sites shifts would start in early morning during the first third of the mission, around noon during the middle third, and in early evening during the final third. The south polar site would be in radio contact for two periods per sol totaling up to 10.6 hours during the first two-thirds of the mission.

The exploration regions centered on the Truck/Rockhound landing sites would each contain several one-kilometer-diameter areas of interest up to 20 kilometers apart. Within these the teleoperators would seek to identify 10-meter-diameter science sites. The rovers would spend up to two weeks within an area of interest and about a sol at each science site. 

Truck rover bearing two Rockhound rovers. The image shows the articulated control arms attached to each wheel, twin stowed Rockhounds, the 10-sided solar array, and twin boom-mounted high-gain antennas. Image credit: NASA.

Rockhound rover. The light blue boxes at the corners of its two-part body are navigation cameras and laser terrain mappers. Note the six "whegs," the mid-body hinge, hands with fingers and thumbs, and a very normal-appearing geology hammer. Image credit: NASA.

The HERRO team described their teleoperated Trucks and Rockhounds in considerable detail. The 800-kilogram Trucks, which could travel at up to 3.6 kilometers per hour, would each have four large independently motorized wheels mounted on "articulated control arms." The arms would permit the Truck to lower its two-meter-square chassis to the surface. This would enable two Rockhounds to disembark for exploration or board for long-range transport or battery charging. 

A box on the Truck located behind the Rockhound charging stations would support two low-gain antennas for relaying transmissions to and from the Rockhounds, a mast bearing a vertical four-meter-diameter solar array, and two boom-mounted high-gain antennas for relaying transmissions to and from the CTCV. In addition to batteries, the box would house a drill for sampling tens of meters below the surface and a lab for analyzing samples the drill and Rockhounds collected. 

The 145-kilogram Rockhound rover would resemble a mythical centaur. At the front of its horizontal aluminum-frame body would be mounted a vertical robotic torso with shoulders, two arms with elbows and hands, and three cameras in place of a head. It would stand a little more than a meter tall on a level surface. 

A motorized "hinge" would divide the rectangular Rockhound body into two 0.5-meter-square parts and a single motor drawing power from batteries would drive six wheel-legs ("whegs") arrayed along its sides. A low-gain antenna for transmissions to and from the Truck, navigation cameras, a small "arsenal" of science instruments, and a rack of tools including a geology hammer would round out its description.

The HERRO team explained that the Rockhound mobility system was based on a "biologically inspired" design developed, built, and tested by Case Western Reserve University. Its movement scheme was modeled on that of the lowly cockroach, which can flex its body and alter its six-legged gait to climb over obstacles taller than it is. 

The agile little rover would move at a top speed of about 10 centimeters per second and would be able to climb and descend 45° slopes of loose rocky material. By raising the front half of its body and tilting forward its humanoid torso, it would be able to "rear up" against rock walls to examine and sample features more than two meters above the ground. It would turn its torso 180° to drive backwards and to reach the tools stored on its aft section. 

The Rockhound would employ four teleoperational modes. Mode 1, called Traverse to New Location, would see it leave the Truck and move over easy terrain for about an hour at a time. No science would be performed and the rover and its teleoperator on board the CTCV would rely on low-resolution navigation cameras and laser terrain mappers to avoid obstacles. Mode 2, Visual Imagery, would see the Rockhound park in one location while its teleoperator put to use hand tools and microscopic, multispectral, high-resolution visual, and other imaging and sensing systems. 

Quiescent/Operator Off Duty, the third teleoperational mode, would see the Rockhound resting in its charging station on board the parked Truck. Charging the Rockhound's batteries would require about 16 hours using an induction charging system that would need no physical contact. The HERRO study team expected that induction might avoid problems created by ever-present airborne Mars dust that could plague a system reliant on a plug and socket. Mode 4, Rockhound Scout Mode, would see the rover move over the surface under teleoperator control for up to eight hours at a time in search of scientifically interesting sites. 

In some HERRO documents, the study team suggested that the HERRO mission might include an MSR option. This could take either of two forms: an independent robotic MSR mission or an MSR mission that would rely on the Truck/Rockhound rovers for sample collection. 

In the second instance, three MSR lander/ascent vehicles would launch to Mars at the same time as the Truck/Rockhounds. This would add three medium-lift rocket launches to the three slated to occur 26 months ahead of the CTCV launch in the baseline HERRO mission. In both cases, the MSR vehicles would land in the same three regions as the Truck/Rockhound combinations.

In both HERRO MSR scenarios, an independently launched teleoperated sample retrieval vehicle, based possibly on the Orion service module, would collect the sample canisters launched to Mars orbit by the three MSR ascent vehicles and deliver them to the CTCV. Sample retrieval in Mars orbit would, the HERRO team estimated, require retrieval vehicle maneuvers spanning about four months.

As their mission in Mars orbit reached its end, the HERRO mission crew would stop the CTCV's spin, recover the MSR sample canisters, discard the waste-filled food canister, and ignite the three NTR engines to perform TEI. As Mars shrank behind them, they would spin up the CTCV again. About six months later, they would stop the CTCV's spin for the final time, take their places in the uprated Orion, and undock. A short Orion burn would place them on course for Earth-atmosphere reentry. 

Meanwhile, the CTCV would swing past Earth. The HERRO study team suggested that it might adjust its course so that it would travel to the Earth-Moon L1 point, where it would park pending possible refurbishment and reuse.

The DRA 5.0 study was completed in 2009 in part to support the activities of the Review of U.S. Human Spaceflight Plans Committee chaired by former aerospace executive Norman Augustine. The Augustine Committee was appointed to advise the new Administration of President Barack Obama concerning NASA's path forward in the 21st century. 

The Augustine Committee requested a briefing on the HERRO study. The briefing helped to inform an approach to NASA's future that the Augustine Committee dubbed the "Flexible Path."

 

As stated at the beginning of this post, the MAWG/MASG envisioned that DRA 5.0 missions would be reached through interim missions — specifically, astronaut stays on board ISS and robotic Mars and piloted Moon missions. The Flexible Path called for new interim missions to be added to this sequence. Although HERRO is not referred to by name in the Augustine Committee's October 2009 final report, among the missions on the Flexible Path was a Mars orbital mission including "joint robotic/human exploration and surface operations [with] sample return."

Adding interim steps to existing U.S. space programs is nothing new. The most obvious example is the addition of Gemini to the NASA piloted program in 1962, a step made necessary when Apollo, which had been conceived initially as mainly an Earth-orbital program, became the U.S. lunar program. Gemini provided opportunities for astronauts, flight controllers, and others to develop new spaceflight skills and for life scientists to determine whether humans could survive in space long enough to reach and return from the Moon.

View of the Gemini VII spacecraft from the cockpit of Gemini VI-A in Earth orbit, 15 December 1965. Gemini VII and Gemini VI-A performed the first close rendezvous between two piloted spacecraft. Image credit: NASA.

Thumbs up: Robonaut II, a humanoid robotic torso developed by NASA Johnson Space Center (JSC) and General Motors, participates in a 2011 field exercise among the volcanic landscapes near Flagstaff, Arizona. Robonaut II's predecessor, the NASA JSC/Defense Advanced Research Projects Agency Robonaut robotic torso, was the inspiration for the HERRO Rockhound torso. Robonaut II is shown here attached to the front of a small remotely operated rover. Image credit: NASA.

Spaceflight planners suggested interim steps toward humans on Mars long before HERRO. In the 1960s, for example, they proposed piloted Mars and Venus flyby and orbiter missions. In 1993, in the waning days of the abortive Space Exploration Initiative, NASA GRC's Geoffrey Landis, a HERRO study participant, proposed a scenario he dubbed "Footsteps to Mars." These and other proposed interim missions leading toward humans on Mars can be explored by following the links in the "More Information" section below.

In the years since the 2009 DRA 5.0 and HERRO studies, NASA robotic Mars missions have displayed both the capabilities and limitations of robotic landers and rovers on Mars that are remotely operated from distant Earth. The Curiosity lander, which reached Mars on 6 August 2012, proved the capabilities of its aeroshell and skycrane systems. As of July 2022, after nearly a decade on Mars, the Curiosity rover had traversed only 28.15 kilometers. 

Lunar and planetary surface teleoperations remain of interest both inside and outside NASA. In the years since the HERRO study, astronauts on board the ISS in Earth orbit have teleoperated robots on Earth. The lunar-orbiting Gateway station, now under development in NASA's Artemis lunar program, is intended to support teleoperation of exploring robots on the Moon. 

Sources

Human Exploration of Mars Design Reference Architecture 5.0, NASA-SP-2009-556, Mars Architecture Steering Group, B. Drake, editor, July 2009.

COMPASS Final Report: Human Exploration Using Real-Time Robotic Operations (HERRO) — Rockhound Design, CD-2009-34, NASA Glenn Research Center/Case Western Reserve University/Carnegie Mellon University, August 2009.

COMPASS Final Report: Human Exploration Using Real-Time Robotic Operations (HERRO) — Truck Design, CD-2009-35, NASA Glenn Research Center/Case Western Reserve University/Carnegie Mellon University, August 2009.

COMPASS Final Report: Human Exploration Using Real-Time Robotic Operations (HERRO) — Crew Telerobotic Control Vehicle (CTCV) Design, CD-2009-36, NASA Glenn Research Center/Case Western Reserve University, September 2009.

Seeking a Human Spaceflight Program Worthy of a Great Nation, Review of U.S. Human Spaceflight Plans Committee, October 2009.

"HERRO (Human Exploration Using Real-Time Robotic Operations): A Robotically Intensive Strategy for Human Exploration," G. Schmidt and Steve Oleson, NASA Glenn Research Center, presentation materials, 28 October 2009.

"HERRO: A Science-Oriented Strategy for Crewed Missions Beyond LEO," AIAA-2010-69, G. Schmidt, G. Landis, S. Oleson, S. Borowski, and M. Krasowski; paper presented at the 48th AIAA Aerospace Sciences Meeting in Orlando, Florida, 4-7 January 2010.

"Human Exploration of Mars Design Reference Architecture 5.0," JSC-CN-19120, B. Drake, S. Hoffman, and D. Beaty; paper presented at the IEEE Aerospace Conference in Big Sky, Montana, 6-13 March 2010. 

"Human Exploration Using Real-Time Robotic Operations (HERRO) — Crew Control Vehicle (CTCV) Design," AIAA-2010-6817, S. Oleson, M. McGuire, L. Burke, D. Chato, J. Fincannon, G. Landis, C. Sandifer, J. Warner, G. Williams, T. Colozza, J. Fittje, M. Martini, T. Packard, D. McCurdy, and J. Gyekenyesi; paper presented at the 46th AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exposition in Nashville, Tennessee, 25-28 July 2010.

"HERRO Missions to Mars and Venus using Telerobotic Surface Exploration from Orbit," G. Schmidt, G. Landis, and S. Oleson; paper presented at the AIAA Space 2011 Conference & Exposition in Long Beach, California, 26-29 September 2011.

More Information

NASA's Planetary Joint Action Group Piloted Mars Flyby Study (1966)

A Bridge from Skylab to Station/Shuttle: Interim Space Station Program (1971)

Bridging the Gap Between Space Station and Mars: The IMUSE Strategy (1985)

Footsteps to Mars (1993)

Astronaut Telescope Servicing at Earth-Sun L2 (1999)

NASA Glenn Research Center's 2001 Plan to Land Humans on Mars Three Years Ago

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)