Selenian Boondocks

A few years ago I did a post suggesting that Starship skipped a base or two in development. Going for the biggest launcher in history with new engine types, burning a new propellant combination, novel landing techniques, and going for complete reusability all in one shot on vehicle one being an extreme gamble. One of my points being that there was no market appropriate to the projected flight rate and fleet size of Starship not to mention the huge cargo capacity. Second Guessing Starship | Selenian Boondocks

Some of my points have been answered, though not in the manner I expected. The record-breaking IPO was heavily influenced by the projected capabilities of Starship and orbital data centers. Million sat orbital data centers never crossed my mind, or that of anyone else that I know of. That potential market makes more sense than Mars colonization from a business standpoint. Though I know nothing about data centers or their potential profitability, many investors apparently do.

In the Second-Guessing post, I suggested that SpaceX would have been better served by going with a smaller vehicle building on the well-known methods of the Falcon9. Falcon Raptor for that post. The water tower was flying in 2019, and the flip flights by 2021. I suggested then that if they had gone with a Falcon9 layout and operational technique, that vehicle would have been operational and matching Falcon9 tempo by 2022. Falcon9 has flown several hundred times since then.

In 2022, I mentioned that some people were certain that Starship would be operational in a year (2023), while some were saying end of decade. Not to mention those saying never. (Oler, Church) or those projecting a massive ramp up to thousands (Eagleson, David, etc) annually by now. Those of us saying somewhere between the extremes didn’t have much of a following.

With almost 4 years since that post, and with Starship flight 13 scheduled in a few days, I’ll try to make the case for a smaller and simpler precursor vehicle as it relates to Starship and the IPO.

The Raptor engine is the key to Starship success. It has considerably more Isp that Merlin, a thrust/weight at least as good, and burns a cleaner fuel that is also much cheaper than RP1 kerosene. The cleaner is the key point as it should lead to far less maintenance between flights. Cheaper is important, but less so. The higher Isp is good on the vacuum engines compared to Merlin, but really shines on the sea level units with a 50 point jump.

A Falcon9 layout with Raptor engines would necessarily be wider, probably 5-7 meters in diameter. That would allow the height to match some of the Falcon9 access points on the launch towers so that total rebuilds wouldn’t be required. New Glenn class except with an organization that MOVES. With over double the thrust of the Merlins and higher Isp, payloads should be well over double that of the kerosene Falcon9.

The critical thing though is the cleaner operation of the engines. A Raptor Falcon in either RTLS mode or returning on a barge should have turnaround times a fraction that of Falcon9 due to not having soot clean up. Also Raptor supposed to have been designed and built with low maintenance and fast turnaround. Falcon9 has a cadence of over 150 flights a year for the last three years. If Raptor is as good as most of us think, Raptor Falcons would be at double or more of that flight rate.

How does this relate to Starship, data centers and the record-breaking IPO? If the Raptor Falcon (or whatever a proper name would be) were flying hundreds of times per year for the last few years, there would be an experienced team on the ground that knew fast paced methane vehicle operations inside and out. That experience would have been feeding into the development of Starship for the last several years. It seems very likely that Starship would not only be further along but also would have left far less hardware in the deeps of the Gulf and the Indian Ocean. Excess capacity to date could have already tested in orbit refueling, prototype data centers, and a Mars flight or two.

So if you were an investor, wouldn’t you be more impressed with a high flight rate vehicle delivering up to 40 tons a shot on a daily basis and a Starship entering operability already with flight 5 or 6? Or with flight 14 hoping to be the first orbital? And the same company retiring the most successful launch vehicle in history (Falcon9) because it was so much more expensive to operate than the Raptor Falcon?

Last time I talked about how successful human colonies would likely need to begin with an existing culture. This time I’d like to talk about one of the places that humans might wish to colonize, and it is the first post in a series about Venus that I have wanted to write for a long time. What would it take to make the surface Earth-like, what would it take to live there before we make it Earth-like? I think Venus deserves more attention than it gets for three simple reasons, gravity, pressure, and temperature.

The series will get to the terraforming engineering eventually, including things like energy budgets, propulsion problems, orbital mechanics, parasols and solettas and electromagnetic spin-up motors. All of that good stuff eventually. But first I want to talk about where humans can actually live, right now, with the engineering capabilities we have or could plausibly develop in the next century.

That place is Venus. Not the hellish surface of course—which is at a temperature of 737K/464C/867F under 95 bars of carbon dioxide—but the cloud layer, at 50 km altitude.

This was an extremely fringe view for a long time, but less and less as people seriously think about it. NASA Langley spent two years on the High Altitude Venus Operational Concept (HAVOC), a phased mission architecture built around floating habitats. Geoffrey Landis published the modern technical case in a 2003 NASA paper that is still the canonical reference. The Soviets were working on atmospheric Venus habitation concepts in the 1970s. Paul Birch’s 1991 paper, which we will spend several posts on later in this series, treats floating colonies as the primary human habitat throughout the centuries-long terraforming process. The case has been in the literature for half a century. The popular conversation just has not caught up to it, so let’s plunge in.

The 50 km altitude

Venus’s atmosphere falls off exponentially with altitude. The scale height depends on local temperature, varying from about 16 km at the surface (where T = 737 K) to about 6 km at the cloud deck (where T ≈ 290 K). The 95 bar surface pressure drops by a factor of about 95 over the first 50 km of altitude, working out to a column-averaged scale height of around 11 km. At 50 km altitude you reach 1 bar of pressure, which is almost exactly Earth sea level, and temperatures between 0 and 50 degrees Celsius depending on local time and latitude. Climb another 5 km and the temperature drops into the 20s. A spring day, in other words.

Gravity at this altitude is essentially Venus’s surface gravity, 8.87 m/s², or 0.904 g. That is meaningfully lower than Earth gravity — about 10% less, which is much larger than the 0.5% variation humans experience between the equator and the poles, or between sea level and the top of Everest. We do not know exactly what 0.904 g does to the human body over decades and generations, because we have no data points between 1 g and zero. But the deficit is small. Mars at 0.379 g and the Moon at 0.166 g are dramatically further from Earth gravity than Venus is. If there exists a habitable gravity range for long-term human life with some lower bound, Venus’s 0.904 g is almost certainly inside it, and Mars and the Moon are almost certainly outside it.

The atmosphere above 50 km provides about 1 kg per square centimeter of mass shielding against cosmic radiation, which is approximately the same column density as Earth’s atmosphere. Dale Arney and Chris Jones, in the HAVOC study, put it bluntly: radiation exposure at the 50 km altitude on Venus is about the same as you would experience in Canada. Call it 0.01 mSv per day above Earth-surface background. Comparable to a high-altitude airline pilot.

Compare those numbers to the alternatives:

Mars surface: 0.64 to 0.7 mSv per day, measured directly by the Curiosity rover’s RAD instrument. About 230 mSv per year.

Lunar surface: roughly 0.4 mSv per day. No atmospheric shielding at all. Habitats require buried construction or regolith berms or water tanks for radiation protection.

Free space, International Space Station, O’Neill habitat: 0.4 to 1.0 mSv per day depending on the solar cycle.

Mars transit: 1.8 mSv per day for the duration of the cruise.

Venus 50 km altitude: approximately equivalent to Earth’s surface.

The cloud layer of Venus is, in radiation terms, the most Earth-like environment in the solar system outside Earth itself. By a factor of 50 to 100 over the Martian surface. By larger factors over the Moon or free space.

That single fact is almost enough to make the case. But there is more.

Your air is your envelope

Here is the design feature that makes everything else work. The first time you hear it, it sounds like science fiction. It turns out to be elementary atmospheric chemistry.

Venus’s atmosphere is 96.5% carbon dioxide. The molar mass of CO₂ is 44 grams per mole. A breathable nitrogen-oxygen mix — 78% N₂ at 28 grams per mole, 21% O₂ at 32 grams per mole — has an average molar mass of about 29 grams per mole. Gas density scales linearly with molar mass at the same temperature and pressure, so breathable air is about two-thirds the density of the surrounding Venusian atmosphere. The buoyant lift is the difference, about 0.63 kg per cubic meter at cloud-deck conditions.

That means breathable air is a lifting gas on Venus. About 0.6 kilograms of lift per cubic meter, which is around 60% of what helium gives you on Earth.

Think about what this means. On Earth, a balloon requires helium or hot air — something different from the surrounding atmosphere. You have to manufacture or import the lifting gas, contain it in a sealed envelope, and keep it separate from the air your passengers breathe. On Venus, the air your colonists breathe is itself the lifting gas. There is no separate lift envelope. There is no pressure differential to maintain. The inside of your habitat is 1 bar of N₂/O₂, the outside is 1 bar of CO₂, and the structure sits in mechanical equilibrium.

This is the dream of every pressure-vessel engineer. Pressure vessels are hard! They want to burst. They fatigue under thermal cycling. They require thick walls, leak monitoring, double-hull architectures. Every other off-Earth habitat anyone has proposed — Mars surface, lunar surface, O’Neill cylinder, space station — is a pressure vessel holding 1 bar against vacuum. A Venus cloud city is not a pressure vessel at all. It is a containment envelope at zero pressure differential. The walls only need to keep the two atmospheres from mixing across the boundary. They do not need to contain pressure.

The structural mass implications are staggering. Birch estimated his floating colonies at 1000 kg per square meter total areal density, with 200 kg/m² for the base structure and 50 kg/m² for the roof. The rest is soil, water, agriculture, and habitation. By comparison, an O’Neill cylinder requires meters of bulk shielding, plus a pressure-rated hull, plus structural accommodation for rotational stress, and even the lightest designs are orders of magnitude heavier per unit habitable area.

Versus Mars

Mars is the obvious comparison.

Where Mars wins: Mars has water ice in known accessible quantities. Mars has a 24.6-hour day, well within human circadian tolerance. Mars has surface chemistry — Sabatier reactions, perchlorate processing — that produces oxygen and propellant from in-situ resources. Mars is energetically cheap to reach from Earth, with delta-V budgets we know how to handle. Rovers have been operating on the surface for decades and we have a deep operational understanding of the environment.

These are real advantages.

But here is what you trade for those advantages: surface gravity of 0.379 g, which we have no biological evidence is safe for long-term human habitation; atmospheric pressure of 0.006 bar, which means full pressure suits required any time anyone steps outside; mean surface temperature of −63 °C, with excursions to −140 °C at the poles; radiation exposure 50 to 100 times higher than at Earth’s surface; solar power density at 590 W/m² compared to Earth’s 1361 and Venus’s 2620; and global dust storms that bury solar panels and grind into every seal and joint in your equipment.

The dust point is underappreciated. Martian regolith is electrostatically charged, mildly toxic from perchlorates, and gets into everything. Apollo astronauts complained about lunar dust within days of their first surface operations. Mars colonists will be dealing with dust intrusion problems for decades, in habitats, in equipment, in their lungs. Venus cloud habitats face sulfuric acid, which is corrosive but contained. Acid is a chemical engineering problem. Dust is a mechanical infiltration problem. The first is easier.

What about day length? Venus rotates once every 243 days retrograde — pathologically slow, and we will spend three posts on that problem later in this series. But at the cloud altitude, the super-rotating atmosphere circles the planet much faster than the surface. A floating colony drifting with the cloud layer experiences a solar day of about 4 Earth days at the equator. Not 24 hours. But not 117 days either, which is what surface settlements would face. And cloud cities can choose their drift rate to some extent by altitude selection. Mars wins on day length. But the gap is much smaller than people usually assume.

Mars has water and propellant and a Sun-friendly day. Venus cloud altitude has nearly every other habitability advantage that matters for human bodies and human structures. We have been investing in the harder planet.

Versus the Moon

The Moon is the closest body and the cheapest in delta-V. Three-day transit each way. No atmosphere to complicate landing. Known ice deposits in permanently shadowed polar craters. Working ISRU concepts for regolith oxygen extraction.

The Moon makes sense as an industrial outpost, a science platform, a stepping stone for cislunar infrastructure. It does not make sense as a place where humans live for generations.

Lunar gravity is 0.166 g. That is even further from Earth gravity than Mars is — about 44% of Martian gravity. Whatever speciation pressure 0.379 g produces on Mars, 0.166 g produces faster and more severely on the Moon. The lunar surface is a vacuum environment with temperature swings of 300 °C across the 14-day lunar day. Radiation exposure is in free-space range, with no atmospheric shielding at all. Habitats have to be buried under regolith, built underground, or shielded with water tanks. The whole architecture is constrained by these requirements.

A lunar base is a good idea. A lunar civilization, in the sense of millions of people living their lives there for generations, is a more troubling proposal. The biology does not work, and we have not been honest about that.

Versus O’Neill cylinders

The free-space habitat advocates — Gerard O’Neill’s intellectual descendants in the National Space Society and adjacent groups — have a real case I want to take seriously. O’Neill habitats can be built anywhere there is solar power and raw material. They can be sized for any population. They can rotate to provide any artificial gravity, including exactly 1 g, which puts them in a different biological category from Mars or Moon surfaces. They scale with industrial capacity in ways that planetary surfaces do not.

The problem is the construction cost.

A Stanford torus or Bernal sphere is millions to tens of millions of tonnes of structure. The radiation shielding alone is meters of bulk regolith or water, which is kilotonnes to megatonnes of mass per habitat. The pressure containment is a serious materials problem. The rotational stability and station-keeping require active control. Every single habitat has to be built before anyone can move in. There is no incremental occupation of a half-finished O’Neill colony.

Compare that to a Venus cloud habitat, which can be a single inflated structure deployed from a single lander mission, lifted by the breathable air inside it. Birch’s floating colonies at 1000 kg/m² scale linearly with envelope area. A square-kilometer colony at one million kilograms is comparable in mass to the International Space Station and hosts hundreds of people. Scaling up means inflating more envelope.

O’Neill cylinders are more flexible. You can put them anywhere, size them however you want, give them whatever rotational gravity you choose. Venus cloud cities are more immediate. They can be built with substantially less industrial infrastructure than a free-space colony requires. For the early decades and centuries of off-Earth settlement, cloud cities arrive first.

There is also a deeper distinction. An O’Neill cylinder is a constructed environment. Every cubic meter of air, every kilogram of water, every photon of light is provided by engineering. A Venus cloud city sits inside an environment. The air outside is at breathing pressure (if not breathable composition), the temperature is shirt-sleeve, the gravity is set by the planet. The cloud city is the more forgiving habitat. A pressure leak in an O’Neill cylinder is a hull breach. A pressure leak in a Venus cloud city is a slow exchange between two atmospheres at the same pressure. The failure modes are categorically different.

What a cloud city actually looks like

Let me describe a plausible early Venus cloud habitat, drawing on Birch, Landis, and HAVOC.

You arrive in Venus orbit and aerocapture into an entry trajectory aimed at the equatorial day side. Your habitat is packed inside an aeroshell similar in scale to a Mars EDL system — a few tonnes of payload behind a heat shield. At about 70 km altitude you deploy a parachute. At 60 km you jettison the aeroshell and begin inflating the habitat envelope. The same envelope serves as parachute (during inflation) and as the buoyant structure once full. By 55 km altitude the envelope is inflated and your lift is sufficient to halt descent.

You float at 50 to 55 km altitude in equilibrium with the surrounding atmosphere. The envelope is filled with breathable air at 1 bar — the same pressure as outside but about half the density. Your habitat hangs below the envelope, or is integrated into it, shielded from cloud-layer sulfuric acid by an outer skin of fluoropolymer or coated polymer. Your structure is light because there is no pressure differential to contain. Your radiation environment is Earth-equivalent. Your gravity is 0.904 g. Your power comes from solar panels on the upper envelope surface, which receive nearly twice the irradiance of an Earth-surface panel plus significant reflected light from the cloud layer below.

You move horizontally by riding the super-rotation winds, about 95 m/s eastward at cloud tops, slower below. You move vertically by adjusting envelope volume through compression or venting. The super-rotation is a feature, not a problem. It gives you a 4-day day-night cycle and continuously carries you across the planet, exposing you to varying weather and giving you global access without translational propulsion.

You grow food in pressurized greenhouses or in the main envelope itself, since the air is already there. You collect water from cloud humidity or extract it from sulfuric acid, which is, after all, mostly water by molar count. You scrub CO₂ leakage and replenish O₂ through electrolysis or photosynthesis. You communicate with Earth through a relay satellite in Venus orbit, with round-trip light times of 4 to 28 minutes depending on planetary geometry.

This is mission-architecture-compatible with present technology. The HAVOC study laid out a phased approach — robotic precursors, then short-duration crewed flybys, then longer crewed stays in orbit, then atmospheric habitats — using technology that is at TRL 5 or higher today. The hardest unsolved problems are sulfuric acid resistance for long-duration envelope materials, large-scale envelope deployment, and propulsive return from the atmosphere to orbit. None of these requires breakthrough physics. All of them are tractable engineering.

Why this is not the dominant proposal

Why does Mars dominate the human-spaceflight conversation rather than Venus? Several reasons.

First, cultural inertia. Venus became unfashionable after Mariner 2 confirmed the surface temperature in 1962. The popular space-advocacy community moved its attention to Mars and never came back. The cloud city case requires explaining a counterintuitive idea — that you can live in a planet’s atmosphere without being on its surface — which is a conceptual move that surface-bound Mars colonization does not require. Familiar wins over unfamiliar.

Second, surface bias. “Colonize Venus” sounds, to most people, like “colonize the Venusian surface,” which is obviously preposterous given that the surface is 737 K. The cloud city case requires a conceptual move that popular discussions do not make. Mars surface settlement is at least visualizable. Venus cloud settlement requires explanation.

Third, economic geography. Mars has been pitched as a stepping stone outward to the asteroid belt, the outer planets, eventually the stars. Venus is on the inside of Earth’s orbit and leads outward only to Mercury and the Sun. For a settlement strategy organized around outward expansion, Mars is in the right place geographically and Venus is not.

Fourth, ISRU. Mars has accessible water ice, mineral resources, and surface materials usable for construction. Venus has none of those in easily extractable form. Atmospheric mining is harder than regolith mining, and the surface is inaccessible. For mission concepts emphasizing in-situ resource utilization, Mars has the obvious edge.

Fifth, robotic exploration. We have continuous robotic presence on Mars and have for decades. Our intuitions about Mars are rich and our institutional momentum on Mars exploration is enormous. Venus has been visited briefly by Soviet landers (lasting about two hours each) and by atmospheric probes. The robotic exploration gap reinforces the cultural and political gap.

These are real reasons. They are not, however, engineering reasons. They are reasons about which planet we are familiar with, which planet fits an outward-expansion narrative, and which planet has better local resources for an isolated outpost. None of them are reasons that the habitats themselves would be harder to build on Venus than on Mars.

The engineering case is straightforward. Venus has a layer where humans can live in shirt sleeves at 1 g with Earth-equivalent radiation shielding. Mars does not. The Moon does not. Free space does not. The settlement strategy that takes biology seriously settles where biology wants to live.

What this means

Venus is the easiest place humans can live off Earth. Not the easiest place to land. Not the easiest place to extract resources. Not the easiest place to reach. But the easiest place where, once you have a habitat in place, humans actually live in a way that does not constantly fight their biology and their engineering.

Habitability is not a surface property. Earth’s surface is not habitable for most of life that we care about. Most of Earth’s biosphere lives in oceans, not on land. Our own habitable zone is a thin layer at the bottom of an atmosphere. The lesson of Venus is that habitability is about finding the right layer in a planetary system, not about finding the right planet.

That lesson generalizes. Jupiter has a 1-bar layer. So does Saturn, Uranus, Neptune. The radiation environments at most gas giants are hostile, especially Jupiter’s, but the atmospheric pressure and temperature of the right layer of any gas giant is closer to Earth-like than the surface of any rocky world except Venus and possibly an early-period Mars. Cloud-layer habitation may be the normal way humans will eventually live on most worlds in the solar system. Rocky-surface settlement of the Earth kind may turn out to be the exception, obtaining only on the small minority of bodies where the geology cooperates.

That is a different vision of human expansion than the one we have inherited. I think it is the one the engineering favors.

The next post in this series will make the case that beginning a serious cloud-city presence on Venus makes terraforming the surface inevitable. Not as an aspiration. As a consequence of biology, economics, and what people want from a planet they have committed to live on. The argument turns on something I have come to think is the deepest constraint on off-world human settlement, which has nothing to do with engineering and everything to do with what we are.

I hope these thoughts help you see why Venus deserves more serious attention than it has been getting, and why the conversation we have been having about Mars for the last forty years has been, in important ways, the wrong conversation.

News broke recently of rumors of a space company looking to buy up like a couple hundred square miles of land in Louisiana, possibly to set up a launch site.

Why Louisiana?

Two reasons:

1. Launch site advantages
Louisiana is one of the very few pro-industrial-development locations with a coastline suitable for polar orbit launches. This is required for SpaceX’s AI satellite constellation in sun-synchronous orbit. A viable launch path for polar orbits exists from Louisiana.

2. Proximity to cheap energy
It is located right next to the Henry Hub, the primary U.S. benchmark for natural gas pricing.
Current price: under $3/MMBTU
→ under $3/Gigajoule
→ under 1 cent/kWh-thermal

Direct connection to this pipeline network would allow SpaceX to:

• Undercut the cost of trucking LNG
• Beat typical delivered industrial rates for U.S. electrical power plants
• Achieve energy cheaper per unit than Appalachian coal

For all intents and purposes, it is the cheapest source of energy humanity has access to — other than starting a coal mine on fire.

Starship v4 propellant requirements (approximate)

• First stage: 4050 t of propellant
• Second stage: 2300 t of propellant

Total propellant: ~6350 t
→ to deliver >200 t of payload to orbit

Based on these figures for Starship V4:

Propellant-to-payload ratio
Ratio of propellant mass to LEO payload mass = 31.75

Raptor engine mixture ratio
Oxidizer-to-Fuel (O:F) ratio = 3.6 by mass
(Design goal was 3.8; future operations may approach this.)

Propellant breakdown (per 1 t LEO payload)

• Methane (CH₄): 6.9 t
• Oxygen (O₂): 24.85 t

Total propellant: 6.9 t + 24.85 t = 31.75 t

Energy calculation
High-Heating-Value (HHV) of methane = 55.5 MJ/kg

Energy content of the methane used per kg to orbit:

Cost implications
This energy figure alone suggests raw fuel costs on the order of ~$1/kg to orbit.

However, real-world propellant costs include substantial overhead:

• Purification of natural gas (byproducts such as butane and propane are often more valuable than methane, which can offset some costs)
• Liquefaction of methane
• Liquefaction of oxygen
• Subcooling of both propellants for rocket loading
• Chill-down losses for the vehicle and ground support equipment

Realistic target
If liquefaction equipment is built on-site (as SpaceX is now explicitly doing per environmental permits), total propellant costs to orbit could reach the range of ~$2–3/kg.

Launch vehicles are just elaborate machines for turning chemical energy into payload kinetic energy. The best launch vehicles are on the order of 10% efficient at doing this. Just like a power plant, an aluminum smelter, or a data center, it makes sense to put such a machine as close as possible to the cheapest source of energy.

In February 1967, five Bellcomm engineers completed a 177-page study showing that a manned Venus flyby was feasible with a single Saturn V and Apollo hardware. Nobody flew it. But every engineering conclusion they reached applies, with interest, to the 2034 inner solar system circumnavigation.

The previous post described the 2034 Earth-Venus-Mars-Venus-Earth circumnavigation trajectory — the mission that departs August 4th, flies past Venus, then Mars, then Venus again, and returns to Earth 750 days later, all without a single deterministic propulsive maneuver after departure. I traced the trajectory family back to A. A. VanderVeen’s 1969 Journal of Spacecraft and Rockets paper, and to the internal Bellcomm memo that preceded it in April 1968.

But VanderVeen’s work was trajectory analysis. The question his colleagues were asking simultaneously — and answering in parallel — was the engineering question: what would you actually need to build, and could you do it with hardware that existed or was nearly in hand?

That question was answered in February 1967 in Bellcomm Technical Report TR-67-600-I-I, “Manned Venus Flyby,” authored by M. S. Feldman, L. A. Ferrara, F. L. Havenstein, J. E. Volonte, and P. H. Whipple. It is 177 pages long. It is thorough, sober, and technically careful. And it sits in a NASA archive largely unread, having described a mission that was never flown despite being demonstrated feasible fifty-seven years ago.

Reading it now is enlightening.

What the Study Was Trying to Do

The Bellcomm Venus flyby study was not primarily about flying to Venus. It was conducted under NASA contract NASw-417 as guidance for the Apollo Applications Program — the post-Apollo program that was supposed to leverage Saturn and Apollo hardware for longer-duration missions. The AAP had no single clear goal of its own, and it needed to understand what future planetary missions would require in order to make sensible technology investments in the present.

The study’s authors put it plainly: “It is clearly not the intent of this study to recommend that NASA undertake a Venus flyby mission in 1973 or at any time; but it is the intent to show that such a mission is feasible under the above ground rules and, therefore, provides a reasonable basis for choosing long duration system characteristics.”

In other words, the Venus flyby was a requirements generator. By designing a complete spacecraft for a one-year interplanetary mission, the study team could identify which subsystem technologies needed development — which existing Apollo systems could be extended and which required entirely new approaches. The answers to those questions would then inform what AAP should be building toward.

The mission they analyzed was a three-man Venus flyby departing October 31, 1973, passing Venus on March 3, 1974 at an altitude of approximately one Venus radius (~6,052 km above the surface), and returning to Earth on December 1, 1974. Total mission duration: 396 days. The entire mission would be accomplished on a single standard Saturn V, with a total injected mass of 48,430 kg — within the 50,350 kg capability available for a 30-day launch window.

What followed was one of the most detailed spacecraft systems analyses ever performed for a manned interplanetary mission that was never flown.

The Vehicle Configuration

The flight configuration had three major elements. The Command and Service Module was essentially Apollo, modified for long-duration storage and high-speed Earth return. The Environmental Support Module (ESM) occupied the adapter area where the Lunar Module would normally sit — it was a new module, not derived from any existing Apollo hardware, providing environmental control, additional propulsion, experiments, and communications. And the spent S-IVB upper stage — the third stage of the Saturn V, which would normally be discarded after trans-lunar injection — was retained and converted into habitable volume for the crew during the interplanetary cruise.

The S-IVB habitability concept is obviously one that had been considered several ways. The hydrogen tank of the S-IVB had an internal volume large enough for three crew members to live and work comfortably for a year. The study team proposed equipping it with curtains and cord webbing for compartmentation, installing approximately 680 kg of fittings and equipment stored in the ESM until after injection, and using the solar panels mounted on the outside of the S-IVB as a meteoroid bumper shield — serving double duty as protection and power generation. The result was a vehicle with generous habitable volume, more than enough for the mission if zero-gravity conditions proved physiologically acceptable.

From Venus Flyby to Skylab: The Same Idea, Grounded

The Bellcomm team’s S-IVB habitability concept was not original to them — it was the central idea of the entire Apollo Applications Program, and it was being developed in parallel by multiple groups within NASA at the same time the Venus flyby study was being written. Understanding what happened to that concept, and how it was eventually realized in a very different form, illuminates exactly what was lost when the planetary mission program collapsed.

The original AAP concept for an orbital workshop was a “wet workshop”: a Saturn IB would launch an S-IVB stage that had to fire its engine to reach orbit. Once there, the crew would arrive on a second Saturn IB, enter the hydrogen tank through an airlock, pressurize it with oxygen and nitrogen from tanks in a docking module, and then — working in zero gravity — install furnishings, fabric floors and walls, lighting, and experimental equipment transferred from the Command Module. The equipment had to be stored outside the tank during launch because the hydrogen fuel occupied the interior. The crew would be doing construction work in orbit, in spacesuits, in a freshly emptied cryogenic tank.

This is essentially what the Bellcomm Venus flyby study proposed for the interplanetary mission — with the difference that the Bellcomm concept was somewhat more elegant. Rather than requiring the crew to outfit the tank in zero gravity after arrival, Bellcomm proposed storing approximately 680 kg of fittings, curtains, cord webbing, and equipment in the Environmental Support Module during launch, then transferring and installing them in the S-IVB hydrogen tank after injection into the interplanetary trajectory. It was still a wet workshop concept in spirit, but with the installation work occurring in the less time-pressured environment of interplanetary cruise rather than in Earth orbit immediately after rendezvous.

The wet workshop concept was abandoned in July 1969, when NASA Administrator Thomas Paine approved the switch to a “dry workshop” — a fully ground-outfitted S-IVB launched atop a Saturn V without needing to fire its own engine. The cancellation of Apollo missions 18, 19, and 20 had freed up Saturn V rockets, and their much greater payload capacity meant the S-IVB could be converted into a complete space station on the ground, lifted into orbit already equipped, and entered by the crew immediately on arrival. This became Skylab, launched in May 1973.

Skylab’s Orbital Workshop was exactly what the Bellcomm Venus flyby study had proposed using as a cruise habitat — the liquid hydrogen tank of an S-IVB stage, converted into habitable volume. The S-IVB’s 73,280-litre liquid oxygen tank below the Orbital Workshop was used to store trash and wastewater. The liquid hydrogen tank — the large upper volume, 6.6 meters in diameter — became the main living and working space, with two floors installed on the ground, an exercise area, galley, waste management system, and a zero-gravity shower. Solar panels extended from the exterior. The total pressurized volume was approximately 320 cubic meters, more than enough for three crew members for months at a time.

The Skylab crews set long-duration spaceflight records that stood for years. The third and final crew spent 84 days aboard — far longer than any previous mission, and sufficient to demonstrate that humans could survive and work productively in extended weightlessness with appropriate countermeasures. The medical data gathered on all three Skylab crews became the foundation for understanding long-duration spaceflight physiology.

Now consider the counterfactual the Bellcomm study implicitly describes. The S-IVB habitability work that eventually produced Skylab was proceeding in 1967 — and simultaneously, the Venus flyby mission study was showing that the same vehicle configuration, deployed on an interplanetary rather than Earth-orbital trajectory, could carry three humans to Venus and back in 396 days. The departure energy was the same as a planetary mission. The S-IVB hydrogen tank provided the same habitable volume. The modifications required were similar in kind if not in detail.

What happened instead was that the wet workshop became the dry workshop became Skylab — an Earth-orbiting station, firmly tethered to the resupply and rescue capabilities that only low Earth orbit provides. The planetary mission that motivated the entire long-duration technology development program was never flown. The AAP, which had been conceived in part as a pathway toward interplanetary capability, ended with three crews in Earth orbit. The Venus flyby windows of 1973 and 1977 opened and closed with no one aboard.

The irony is sharpest when you look at the timing. Skylab’s final crew returned to Earth in February 1974. Mariner 10 flew past Venus on February 5, 1974. The Bellcomm reference Venus flyby mission would have passed Venus on March 3, 1974. Three things were happening in the same month that could have been one thing: the end of a long-duration orbital mission that proved humans could survive in space for months, the first operational gravity assist demonstrating the trajectory mechanism, and the opening of the planetary window that had been planned for in 1967. The convergence was exact. The mission was not flown.

We now know the answer. Valery Polyakov demonstrated in 1994–95 that a human being can survive 437 days in zero gravity and eventually recover, but the recovery was grueling and the long-term bone and cardiovascular effects were severe. The 750-day circumnavigation would push far beyond what any human has experienced in microgravity, and the crew would need to survive atmospheric entry at the end of it. The Bellcomm team’s “if” has been answered: artificial gravity is not optional for a two-year mission. The tethered rotation architecture described in the previous post addresses this, using the spent Earth departure stage as a counterweight exactly as the Bellcomm team used the S-IVB — the same concept, applied to the same problem, six decades later.

The Trajectory

The 1973 Venus flyby trajectory had an injection velocity of 3,932 m/s from a 185 km circular parking orbit, and an Earth return velocity of 13,655 m/s. These numbers frame an important comparison with the 2034 EVMVE mission.

The 2034 circumnavigation departs at approximately 4.23 km/s from a 200 km parking orbit — essentially the same departure energy as the 1973 Venus flyby. But the Earth return velocity is dramatically lower: 11.5 km/s versus 13.7 km/s. The circumnavigation returns more gently than the simple Venus flyby, despite having traveled immeasurably farther. This is a consequence of the triple-planet trajectory’s geometry — the two Venus gravity assists on the outbound and return legs, combined with the nearly tangential Mars encounter, leave the spacecraft on a much more benign Earth approach than a direct Venus round-trip would produce.

The Bellcomm study confirmed that the Apollo Command Module heat shield could be modified to survive 13,655 m/s entry without retropropulsion, with an entry corridor of 34 km between the overshoot and undershoot limits — manageable, but demanding. At 11.5 km/s, the 2034 return is substantially more forgiving. If the Bellcomm team showed that an Apollo CM could survive the harder problem, the easier problem is clearly within the performance envelope of modern heat shield technology.

It is also worth noting that VanderVeen had already mapped this entire flyby landscape before discovering the low-energy 1977 triple-planet trajectory. In a May 1966 Bellcomm survey memo — written nearly two years before he found the 1977 conversion opportunity — VanderVeen catalogued the various classes of manned flyby missions available in the 1970s: high-energy lightside Mars flybys, high-energy darkside flybys, low-energy “twilight” Mars flybys, dual-planet flybys (Earth-Venus-Mars-Earth or Earth-Mars-Venus-Earth), and simple Venus flybys. He compared them all on a common mass basis, normalizing to a 213,800 kg 1979 Mars twilight flyby reference. The 1973 Venus flyby came in at a mass ratio of 0.70 — about 150,000 kg of mass-in-Earth-orbit, the cheapest of any class. The 1972 dual-planet Earth-Venus-Mars-Earth mission came in at 1.18, only modestly more expensive than the reference, with a 464-day duration. The 1979 Venus-swingby Mars orbiter mission with an 8-day Mars stopover required 4.32 km/s of total ΔV and would have placed 1,060,000 kg in Earth orbit. The trajectory landscape was understood. The cheapest entry point was a Venus flyby — exactly the mission Bellcomm was sized for in 1967.

What VanderVeen discovered the following year was not a new class of trajectory but a remarkable coincidence: in 1977 and again in 1983, the departure conditions for a low-energy triple-planet flyby and a simple Venus flyby were nearly identical, allowing one to be converted into the other at Venus with a small midcourse burn. This converted the simple Venus flyby — already the cheapest crewed planetary mission — into a circumnavigation of the inner solar system at essentially no additional cost, with a four-month abort option built in. The 1977 window was the first such opportunity identified, but they recur every 6.4 years. The 2034 window which I identified is the best one in decades.

Exactly how much harder is the Venus flyby return problem relative to the EVMVE? The answer comes from a companion 1965 Lockheed study — NASA Contractor Report CR-308, “Study of Heat Shielding Requirements for Manned Mars Landing and Return Missions” — which examined this question in quantitative detail across a wide range of entry velocities and vehicle configurations. Its results are directly applicable here, since the one-year Venus flyby missions have essentially the same Earth entry conditions as the lower end of what CR-308 analyzed.

The Heat Shield: What the Numbers Actually Say

The Lockheed CR-308 report examined Apollo-configuration heat shielding requirements for Earth entry velocities ranging from 11.0 to 22.9 km/s, covering everything from lunar return through interplanetary return velocities. Some key numbers from their analysis:

The Venus flyby return at 13.7 km/s requires a heat shield representing about 15% of the vehicle’s total mass. This is real — it is substantially heavier than the Apollo lunar heat shield — but it is not catastrophic. A 4,536 kg capsule returning from a Venus flyby needs about 680 kg of heat shield material. For comparison, the Apollo CM heat shield weighed approximately 385 kg returning from the Moon; the Venus flyby shield is roughly twice that, which is the penalty for the additional 2.7 km/s of entry velocity.

The 2034 EVMVE, returning at 11.5 km/s — closer to the Apollo lunar return than to the Venus flyby return — would require a shield mass fraction well below 15%, likely in the 8–10% range based on the CR-308 curves. This is consistent with a straightforward upgrade of an existing Apollo-class or Dragon-class heat shield, not a materials breakthrough.

What makes the Lockheed study particularly reassuring is a counterintuitive finding buried in their analysis of charring ablator performance. They found that heat shield mass does not scale linearly with entry velocity — and in fact, at higher heating rates, charring ablators become more efficient. A sixfold increase in total heat load increased required shield thickness by less than 30%. The reason is that at intense heating rates, the ablation products vaporizing off the shield surface blow outward through the boundary layer, partially blocking incoming heat from reaching the shield surface. At the most extreme heating rates they examined, this mass injection effect becomes so strong that it effectively blows off the thermal boundary layer entirely, stopping convective heating altogether. The ablator shields itself. This is why the Apollo shape — highly blunt, allowing the bow shock to do most of the work — remains the most mass-efficient configuration all the way up to about 15.2 km/s. Above that, slender high-lift configurations like the M2 can reach higher velocities with a manageable corridor, but require significantly more shield mass.

The practical implication for our mission: the 2034 EVMVE’s 11.5 km/s return sits in the most benign portion of the interplanetary entry regime. The heat shield is a well-understood engineering problem, solved in principle by Apollo, improved substantially by SpaceX’s PICA-X development, and not close to any material performance limit. The Lockheed team in 1965 was examining entry velocities nearly twice as high as what the 2034 circumnavigation requires. The heat shield is not the mission’s hard problem.

What is the hard problem, returning from the harder one-year Venus flyby, is the entry corridor. The Bellcomm study found a 34 km corridor between overshoot and undershoot for the 13.7 km/s Venus flyby return — narrower than the lunar return corridor, requiring more precise navigation and guidance in the final approach. For the EVMVE at 11.5 km/s, the corridor is wider and the navigation problem is correspondingly easier. But in either case the Lockheed analysis confirms: “Entry into the earth atmosphere upon return from a Mars mission can be accomplished by relatively simple maneuvers using the trimmed-lift, roll-control mode.” The same applies to Venus flyby returns. Roll-controlled lifting entry — the same mode used by Apollo — handles it.

The 1973 mission also had a far more brutal abort window than the EVMVE. The Bellcomm study analyzed post-injection abort capability and found that it existed for only the first 65 minutes after injection. After that, the crew’s propulsion was insufficient to return them to Earth within the CSM’s operational lifetime, and they had to accept the full 396-day mission. The 2034 EVMVE provides an abort window of 132 days — the entire outbound leg to the first Venus encounter. After Venus, the crew is committed, but for more than four months they have the option to apply a small burn at Venus and return home on the free-return trajectory. This represents an enormous improvement in crew safety architecture, entirely attributable to the fortuitous properties of VanderVeen’s trajectory class.

Mariner 10: The Robotic Proof of Concept

There is a remarkable coincidence embedded in this history that deserves explicit attention.

The Bellcomm reference mission departed Earth on October 31, 1973, and flew past Venus on March 3, 1974. NASA’s Mariner 10 launched on November 3, 1973 — just three days later — and reached Venus on February 5, 1974. While the Bellcomm team was writing the engineering study for a crewed Venus flyby in that exact launch window, NASA was simultaneously flying an unmanned spacecraft through the same interplanetary corridor to validate, for the first time in history, the very mechanism that makes multi-planet flyby trajectories work.

Mariner 10 was the first spacecraft to use the gravitational pull of one planet to reach another, and the first probe to visit two planets. It flew past Venus, used the gravity assist to redirect toward Mercury, and made three encounters with the innermost planet before its fuel ran out. During the Venus gravity assist, Mariner 10’s heliocentric velocity dropped from 37.008 km/s to 32.283 km/s in just four hours — a velocity change of nearly 5 km/s accomplished entirely by Venus’s gravity, at zero propellant cost. This is the same physical mechanism — a close planetary passage bending the spacecraft’s trajectory and exchanging momentum with the planet — that the EVMVE trajectory relies on at both Venus encounters.

The differences between Mariner 10’s trajectory and the EVMVE are instructive. Mariner 10 went inward from Venus toward Mercury. The EVMVE goes outward from Venus toward Mars, then bends back inward past Venus again to return to Earth. But the Venus gravity assist that does the bending is structurally identical. Mariner 10 demonstrated in 1974 that a spacecraft could swing past Venus and emerge on a precise new trajectory, hitting a target — Mercury — months later. The two-Venus-one-Mars trajectory is a more elaborate version of exactly what Mariner 10 proved.

The total mission cost was budget-capped at $98 million — in 1973 dollars, roughly $700 million today. For that sum, NASA obtained the first proof that gravity assists work as advertised, the first close images of both Venus and Mercury, and a trajectory that permitted two additional Mercury flybys at six-month intervals because the spacecraft’s orbit was resonant with Mercury’s. The mission was, in every relevant sense, a proof of concept for multi-planet ballistic flyby trajectories at zero additional propellant cost.

The irony is layered. The Bellcomm engineers were designing a crewed mission that would depart in the same window as Mariner 10, using the same Venus gravity assist mechanism. VanderVeen had already shown — in the 1968 memo and the 1969 journal paper — that a triple-planet ballistic flyby was possible at essentially the same departure energy. And NASA, that same year, flew a robotic spacecraft that validated the gravity assist technique upon which the entire mission concept depends. The three pieces of work — the crewed mission design, the triple-planet trajectory analysis, and the robotic gravity-assist demonstration — were all occurring simultaneously, in the same launch window, and they were never connected into a program.

Mariner 10 proved the physics. Bellcomm proved the spacecraft engineering. VanderVeen proved the trajectory. Nobody put the three together and flew the mission.

What the Crew Was Supposed to Do

The 1967 Bellcomm vehicle study answered the question of whether the crew could survive the trip. A second Bellcomm document, completed in November 1967 by D. E. Cassidy, C. L. Davis, and M. H. Skeer — TR-67-730-1, “Preliminary Considerations of Venus Exploration via Manned Flyby” — answered a different question: assuming they got there, what would they actually do?

The answer was an ambitious planetary science campaign organized around three Venus encounters: the 1977 triple-planet mission’s two Venus passes, and a 1978 dual-planet Venus flyby that would build on the data returned from the first two. Total Venus probe complement across both crewed missions was 12,300 kg in 1977 and 8,600 kg in 1978 — more than 21 metric tons of robotic spacecraft launched together with the crew, deployed under their direction during close approach, and operated through orbital relays after the spacecraft departed.

The 1977 first-pass complement was designed to characterize the atmosphere and begin surface exploration: six drop-sondes targeting different solar geometries (sub-solar, anti-solar, terminator, mid-light side, mid-dark side); four meteorological balloon probes deployed at varied altitudes for circulation studies; two small landers, one near the north pole and one on the mid-light side; two photo-RF probes returning images during atmospheric descent; and a polar orbiter for global radar mapping. The second pass, fourteen months later, was reserved for follow-up: five additional landers and five additional photo-RF probes, deployed to specific surface targets identified from the first-pass data.

The 1978 mission was the most scientifically interesting. Two high-altitude buoyant Venus devices — 25-meter hydrogen-filled super-pressure balloons designed to float in the temperate region between 40 and 60 km altitude for one to six months — would carry electron microscopes, mass spectrometers, gas chromatographs, and culture growth experiments specifically designed to detect aerosol-borne life forms in the Venusian cloud layer. Two near-surface floaters — 9-meter steel-fiber-weave balloons with liquid-hydrogen-cooled instrument capsules rated for 540°C surface temperatures — would descend on 600-meter tethers, anchor near the surface, and acquire samples with clam-shell devices for on-board analysis.

Reading this document fifty-eight years after it was written, the historical irony is overwhelming. Almost everything in this campaign has now been done — but as separate uncrewed missions, conducted over decades, often with more thoroughness than the crewed mission could have managed.

The atmospheric drop-sonde campaign was effectively flown by NASA’s Pioneer Venus Multiprobe in December 1978 — four probes targeting different latitudes and longitudes, returning atmospheric profiles down to the surface. The results validated and dramatically refined the atmospheric models the Bellcomm team had been working from. The same mission’s orbiter conducted radar mapping of approximately 93% of the surface at coarse resolution from 1978 through 1992. The polar orbiter that the Bellcomm team had specified, with its X-band mapping radar, was effectively flown by NASA’s Magellan mission from 1990 through 1994 — at vastly higher resolution than the 1977 plan would have achieved, mapping 98% of the surface at 100-meter resolution and producing the topographic dataset that remains the foundation of Venus geology to this day.

The meteorological balloon concept was partially realized by the Soviet VeGa mission in 1985, which deployed two small helium-filled balloons into the Venusian cloud layer during gravity-assist flybys en route to Halley’s Comet. Each balloon operated for about 46 hours, drifting roughly 11,000 km across the planet and confirming the atmospheric circulation patterns the Bellcomm team had hoped to characterize. The Soviet Venera lander program — Venera 7 through 14, plus the VeGa landers — accomplished what the Bellcomm landers were designed to do, multiple times over, between 1970 and 1985: landing on the surface, returning images and soil composition data, surviving for between 23 minutes (Venera 7) and 127 minutes (Venera 13) in the brutal surface environment.

The high-altitude biological experiment — the search for aerosol life in the Venusian cloud layer — has not been flown as a dedicated mission, but the question itself remains live. The 2020 announcement (later disputed) of phosphine in the Venus atmosphere, and ongoing analysis of cloud-layer chemistry from Venus Express (ESA, 2006–2014) and Akatsuki (JAXA, 2015–present), continue to grapple with whether the temperate cloud-deck altitudes the Bellcomm team identified might in fact harbor microbial life. The DAVINCI mission, currently under development, will conduct in-situ atmospheric chemistry analysis during descent in the early 2030s. The biological question is being addressed; the equipment is more sophisticated than what fit on a 1967 balloon; the answer, when it comes, will not require a crewed observer.

The near-surface floater — a tethered balloon descending to within hundreds of meters of the surface to acquire samples — has not been built, and remains genuinely difficult. The closest analog, the Soviet VeGa balloons, operated only at high altitude. The 9-meter steel-fiber-weave balloon with active liquid-hydrogen cooling that Cassidy, Davis, and Skeer described in 1967 was a remarkable piece of engineering; nothing like it has flown since, primarily because no Venus mission has needed that capability. Future Venus aerial platforms — the proposed Venus Aerobot concepts at JPL, balloon designs in development at various academic groups — still struggle with the same constraints the Bellcomm team identified, and have not yet matured to flight.

The point of this comparison is not that the Bellcomm team got it wrong. They got the science exactly right. The campaign they designed for crewed flyby in 1977–1978 anticipated, in essentially every particular, the robotic exploration of Venus that took place over the following half-century. Their drop sondes were Pioneer Venus. Their orbiter was Magellan. Their landers were Venera. Their balloons were VeGa. Their long-duration aerosol biology experiment is what DAVINCI and follow-on missions are still trying to do. The list is almost complete.

What the crewed flyby would have added, beyond robotic spacecraft, was real-time human supervision during deployment — the ability to retarget probes based on what was being seen, to reprogram experiments mid-mission, to make judgment calls about where to send the next lander. This is genuinely valuable, and there are scenarios where a human supervisor a few light-seconds away can outperform a ground operator twenty light-minutes away. But it is a marginal advantage relative to what the Bellcomm team’s robotic campaign was already designed to accomplish autonomously, and it does not justify the cost differential between a crewed and uncrewed mission. The science case for crewed Venus flyby was overtaken by robotic capabilities almost immediately after the 1967 study was written. The Mariner 10 robotic flyby in 1974 — the same launch window the Bellcomm crewed mission was sized for — accomplished more first-of-kind planetary science, at $98 million in 1973 dollars, than any crewed mission could have done at any imaginable cost.

This is part of why the Venus flyby was never flown, and why the 2034 EVMVE circumnavigation should not be sold on its scientific merit. The science has been done, or is being done, by spacecraft that do not require return trajectories or radiation shelters or 750-day life support systems. What the crewed mission offers is something different: the experience of being there, the human achievement of crossing the inner solar system, and the records that come with it. The Bellcomm planetary science team, working in 1967, did not have to make this argument because the robotic campaign they were anticipating had not yet flown. We do. The honest framing of the 2034 mission is not as a science mission but as an exploration mission — closer in spirit to Lindbergh than to Mariner 10 — and that framing is more defensible, not less, because the robotic alternative is so capable.

Radiation Shielding

The Bellcomm study identified solar cosmic radiation as the dominant hazard of the mission. Galactic cosmic ray background was acknowledged but less threatening than the acute dose from a major solar particle event. The study’s design criterion was to protect the crew against the worst year measured to that date — 1959, an unusually active solar year — which required shielding of approximately 49 kg/m² of crew volume.

The solution was a storm shelter: a reduced, heavily shielded volume within the spacecraft where the crew would retreat during a solar particle event. Non-sensitive equipment — food, water, waste water — was placed around the periphery of this shelter, contributing to the shielding mass without adding dedicated radiation shielding weight. The overall structure of the S-IVB hydrogen tank, with its large radius and the mass of systems distributed around its interior, provided partial shielding during normal cruise operations.

This is precisely the approach described in the previous post for the 2034 circumnavigation — a storm shelter lined with consumables, providing the crew a protected volume during SPE events. The concept has not changed in fifty-seven years because the physics has not changed. What has improved is our understanding of the radiation environment, our ability to predict solar events with some lead time, and our knowledge of the long-term biological effects of galactic cosmic ray exposure.

The GCR dose for a 750-day mission is approximately 750–1,500 mSv — meaningfully above Earth-surface background, enough to increase long-term cancer risk, but not acutely life-threatening. The Bellcomm team was working with 1960s radiobiology; their conservative shielding estimates may actually be more than sufficient by modern standards for the GCR component, while remaining appropriate for SPE protection. The storm shelter concept is sound. The uncertainty is not in the engineering but in the individual crew member’s willingness to accept an elevated long-term cancer risk — which is ultimately a personal decision, not an engineering constraint.

Life Support and Consumables

The study selected a semi-closed ecological system that recycled water but not carbon dioxide. Oxygen was stored cryogenically. The authors noted that even with modest leakage rates, the system was heavier than ideal, and that additional recycling would complicate an already difficult technology without proportional mass savings. Their conclusion: reduce leakage rates by minimizing airlocks and hatches, rather than trying to close the ecological loop more completely.

For the 2034 mission, the calculus has shifted somewhat. Water recycling technology has advanced enormously since 1967 — the ISS water recovery system processes approximately 90% of cabin humidity and urine back to potable water, and further improvements are in development. CO₂ removal and oxygen recovery systems have similarly improved. But the Bellcomm team’s fundamental insight remains correct: for a mission of this duration, reliability of the life support system matters more than its mass efficiency. A slightly heavier system that is robust and redundant is preferable to a lighter system with more failure modes.

The Bellcomm study allocated consumables for three crew members for approximately 400 days, within the mass budget of a single Saturn V. For the 2034 mission, the crew is one or two people for 750 days, and the departure vehicle is far more capable than a Saturn V in terms of mass to orbit per dollar. The consumables budget is not the constraining problem.

Power: Solar Cells, Then and Now

On electrical power, the Bellcomm study’s conclusion was unambiguous: “Solar cell electrical power is a clear choice for both flyby and long duration systems.” Fuel cells — the Apollo standard — were rejected for long-duration missions because of their consumable hydrogen and oxygen requirements. Nuclear power was not considered mature enough. Solar cells, particularly fixed non-articulating arrays mounted on the S-IVB exterior, provided adequate power throughout the mission’s range of solar distances (0.7 to 1.2 AU from the Sun) with significant redundancy.

The 2034 EVMVE traverses similar solar distance ranges. At Mars (approximately 1.44 AU), solar flux falls to about 48% of Earth-normal — lower than the Bellcomm mission’s aphelion, but still workable for high-efficiency modern solar cells. At Venus (approximately 0.72 AU on the inbound leg), flux reaches about 192% of Earth-normal, well within the thermal management capability of modern spacecraft. The Bellcomm team’s solar cell conclusion holds fifty-seven years later, with the additional advantage that modern multijunction cells are roughly three times more efficient than the silicon cells available in 1967.

Communications: The Unchanging Problem

The Bellcomm study confirmed that the sun-spacecraft-Earth angle never drops below 27° throughout the Venus flyby mission, meaning solar noise would not interrupt the communications link. The study required continuous deep-space network tracking, with the spacecraft transmitting all scientific data to Earth rather than relying on tape return.

For the 2034 mission, the geometry is similar. The spacecraft spends most of its time at solar elongation angles well clear of solar conjunction, with communications interrupted only during brief periods if any. Maximum one-way light travel time at Mars closest approach is approximately 12 minutes. This is not a communications problem in the engineering sense — high-gain antennas of modest aperture can close the link budget at Mars distance with modern transmitter and ground station technology. It is a human factors problem: the crew has a 24-minute round-trip delay for any Earth conversation, and mission control cannot intervene in real time during emergencies.

The Bellcomm team’s communications conclusions were sound in 1967. The 2034 challenge is not link budget but latency, and latency cannot be solved by engineering — only by crew training, autonomy, and psychological preparation.

Development Cost: Apollo vs. Commercial

The Bellcomm study was written in the context of an agency with an annual budget measured in the tens of billions of dollars (in today’s money) and a mature heavy-lift vehicle in production. The Saturn V cost approximately $185 million per flight in 1967 dollars — roughly $1.7 billion in 2026 dollars. Three of them were proposed for the development flight test program, plus the operational mission. Development of the ESM module and modifications to the CSM and S-IVB would have required additional investment. Total program cost was not estimated in the document, but it would have been substantial by any measure.

The 2034 circumnavigation operates in a fundamentally different cost environment. A Falcon Heavy launch currently costs approximately $150 million — roughly one-tenth of a Saturn V in real terms. Starship, when operational at scale, aims for launch costs measured in the tens of millions per flight. The commercial habitat and crew capsule market, which barely existed when I opened the NSF thread in 2009, now has multiple serious players. The hardware that Bellcomm’s team had to design from scratch — the environmental support module, the modified command module, the long-duration life support system — has near-equivalents available commercially or in advanced development.

The Bellcomm study demonstrated that a one-year manned interplanetary mission was feasible within the constraints of a government program in 1967. The same conclusion, applied to today’s commercial capabilities and cost structure, suggests that the 2034 circumnavigation is feasible within the constraints of a well-funded private effort — at a cost perhaps two orders of magnitude lower than the Apollo-era equivalent.

What Bellcomm Got Right and What Has Changed

Reading the 1967 Bellcomm report and the 1965 Lockheed heat shield study alongside the 2034 mission analysis, the continuities are more striking than the differences. The storm shelter approach to radiation protection: unchanged. The use of solar cells for power: unchanged. The semi-closed life support architecture: refined but structurally similar. The identification of long-duration reliability as the dominant design driver: more true today than ever. The recognition that zero-gravity physiology was an open question: answered, and the answer requires artificial gravity. The confirmation that atmospheric entry from interplanetary velocity is manageable with a blunt ballistic capsule and an ablative heat shield: confirmed repeatedly in the decades since, and now quantified with the precision the Lockheed study provided.

On that last point the Lockheed CR-308 study is worth quoting directly: “Atmospheric braking of hyperbolic entry vehicles appears thermally feasible provided that efficient configurations and heat shield materials are selected.” This was written in 1965 for entry velocities ranging up to 22.9 km/s — nearly twice what the 2034 EVMVE requires. The conclusion was not hedged. The blunt Apollo shape is thermally efficient precisely because it generates a strong detached bow shock that converts most of the kinetic energy to radiation well ahead of the vehicle surface, rather than transferring it conductively through a thin boundary layer. The ablator then handles what gets through, becoming more efficient as the heating rate increases. The physics works in favor of getting home.

What has fundamentally changed is the cost structure. The Bellcomm team was designing within a government program with government budget constraints and government risk tolerance. The 2034 circumnavigation is a different kind of mission: private, voluntary, and accepting of risk levels that no government program could countenance. This is not a weakness of the mission concept — it is its defining feature. The historical parallels are not Apollo but Lindbergh, not the space shuttle but the Voyager aircraft that completed the first nonstop around-the-world flight in 1986.

The Bellcomm report concluded in 1967 that a manned Venus flyby was feasible and that its technical conclusions were applicable to guiding long-duration system development. The Lockheed study, two years earlier, confirmed that getting home from interplanetary velocity was a solvable heat shield problem, not a materials miracle. Nobody followed that guidance toward a flight. The AAP shrank and eventually became Skylab — three missions to a single Earth-orbiting station, remarkable achievements in their own right but a long way from interplanetary space. The Venus flyby window opened in 1973 with no one ready to fly it.

The Fifty-Seven Year Lag

There is something worth sitting with in the chronology.

In 1967, Bellcomm engineers demonstrated that a manned Venus flyby was feasible with existing hardware. In 1968, VanderVeen discovered that the triple-planet trajectory made a circumnavigation of the entire inner solar system possible at essentially the same energy cost, with a four-month abort option built in. In 1969, that result was published in the peer-reviewed literature. The 1973 window opened and closed. The 1977 window — the one with the most favorable geometry VanderVeen had identified — opened and closed. The opportunities have continued to come, approximately every six years, for more than fifty years.

In 2009, I opened a forum thread arguing that the mission was within reach of a private effort using commercial hardware. The Augustine panel was considering something similar. SpaceX was flying its first Falcon 1. Bigelow was building habitat modules. Nothing came of it.

The 2034 window opens on August 4th. Between now and that date, approximately eight years remain. That is not a long time to develop and test life support systems for a 750-day mission, commission and qualify a habitat module, design and build an Earth departure stage, certify a heat shield for hyperbolic entry, and train a crew for the most demanding and isolated human spaceflight ever attempted.

But it is not an impossible time. The Bellcomm team showed in 177 pages that the engineering was tractable in 1967. The commercial space industry has made most of the individual pieces available or nearly so. The remaining work is integration, testing, and will.

The 1967 report sits in the NASA Technical Reports Server at accession number 19790072165. It is freely available to anyone who wants to read it. Its authors demonstrated, more than half a century ago, that humans could travel to another planet and return safely on a single launch vehicle with Apollo-derived hardware. They were right. The mission they designed was never flown.

The 2034 window will not wait for committees or consensus. The trajectory opens when the planets are in the right positions, and closes again regardless of what we decide to do about it. VanderVeen’s trajectory family has already cycled through more opportunities than most space programs have had missions. Sooner or later, someone will fly it. The question is only whether that happens in 2034 or in some future cycle when perhaps the will finally matches the capability that has existed, in principle, since before the first Moon landing.

The Bellcomm vehicle study discussed in this post — “Manned Venus Flyby,” TR-67-600-I-I, February 1, 1967, by M. S. Feldman, L. A. Ferrara, F. L. Havenstein, J. E. Volonte, and P. H. Whipple — is available from the NASA Technical Reports Server at accession number 19790072165. The companion Venus science study is D. E. Cassidy, C. L. Davis, and M. H. Skeer, “Preliminary Considerations of Venus Exploration via Manned Flyby,” Bellcomm TR-67-730-1, November 30, 1967. The earlier flyby trajectory survey is A. A. VanderVeen, “A Survey of Manned Mars and Venus Flyby Missions in the 1970’s,” Bellcomm Memorandum for File, May 17, 1966, Case 103-2 (NASA CR-152882). The heat shield analysis is drawn from Lockheed Missiles and Space Company, “Study of Heat Shielding Requirements for Manned Mars Landing and Return Missions,” NASA Contractor Report CR-308, October 1965, prepared under Contract NAS 2-1798. The triple-planet flyby trajectory analysis is from A. A. VanderVeen, “Triple-Planet Ballistic Flybys of Mars and Venus,” Journal of Spacecraft and Rockets, Vol. 6, No. 4, April 1969, and VanderVeen’s preceding internal Bellcomm memo TM-68-1013-2, April 1968.