Manned Circumnavigation of the Inner Solar System

In 1969, a Bellcomm engineer named A. A. VanderVeen published a paper describing a family of trajectories so elegant they seem almost accidental. Fifty-five years later, the most attractive instance of that trajectory family departs Earth in August 2034. This post is about what it would mean to fly it.

Back in February 2009, I opened a thread on the NASASpaceFlight.com forum with what I thought was a simple thought experiment: what interplanetary mission might be within reach of an extremely wealthy private individual — not for scientific merit, but purely for the historical record? Something that would put a human being somewhere no human had ever been, set records that might stand for generations, and require the kind of courage that makes a person famous forever?

I had been reading an old paper, published in April 1969 in the Journal of Spacecraft and Rockets, authored by A. A. VanderVeen of Bellcomm Inc. — the systems engineering firm that supported NASA during Apollo. The paper was titled “Triple-Planet Ballistic Flybys of Mars and Venus,” and it described something that had been discovered almost by accident: a family of round-trip trajectories from Earth that would fly past Venus, then Mars, then Venus again, and return — all without any propulsive maneuver after the initial departure burn. The planets themselves would do the work.

The NSF thread ran for over a hundred pages and several years. It attracted serious engineers and enthusiastic amateurs, but none of it came to anything then. But the trajectory doesn’t care about programmatic timelines. It comes around again whether we’re ready or not.

The most attractive upcoming instance of this trajectory departs Earth on August 4, 2034. I’ve run the numbers with JPL’s MIDAS trajectory optimization code, and the results are striking. This post is my attempt to lay out the full case for what I’ve been calling the “inner solar system circumnavigation” — the trajectory, the history, the mission design implications, and why I think it deserves serious attention from whoever has the resources and the nerve to fly it.

The VanderVeen Discovery To understand the 2034 opportunity, you need to understand what VanderVeen actually found — because it’s considerably more subtle than a simple gravity-assist tour.

By the mid-1960s, NASA contractors had catalogued most of the obvious interplanetary trajectory opportunities. Single-planet flybys were well understood. Mars stopover missions via Venus swingby had been studied extensively. What had not been found was a clean, low-energy, fully ballistic trajectory that would fly past Venus, fly past Mars, fly past Venus again, and return to Earth — all as a single connected trajectory with no deterministic maneuvers after departure.

VanderVeen found this trajectory class, and he found it in a characteristic way: by accident. He was investigating whether a spacecraft could depart on the easy trajectory of a simple Venus flyby mission and then, at Venus, fire a small rocket to redirect itself onto a triple-planet profile. During the investigation, something surprising happened. The required maneuver at Venus didn’t just become small — it approached zero. He had accidentally discovered a new family of ballistic triple-planet flyby trajectories whose energy requirements were lower than any previously identified, and whose first leg was identical to a simple roundtrip Venus flyby.

The structure of these trajectories is elegant. The spacecraft departs Earth, swings past Venus on the way inward, continues outward to a nearly tangential encounter with Mars, then swings past Venus again on the way back, and finally returns to Earth. The profile is nearly symmetric — the outbound and inbound halves mirror each other — and the planetary encounters are all relatively gentle. The Mars flyby in particular is nearly tangential, which is what keeps the encounter velocity low and the trajectory energy moderate.

VanderVeen identified three distinct classes of this trajectory, characterized by which type of Venus swingby geometry they employ. The most attractive class — the one he called #5/#5 — occurs when the outbound and inbound Venus swingbys are both of the “type 5” variety, which produces the symmetric profile and the lowest energy requirements. This class recurs approximately every 6.4 years, though the 6.4-year repeatability is only qualitative. The actual availability of a clean ballistic solution depends on Mars’s heliocentric distance at the time of its encounter with the spacecraft, and not every 6.4-year cycle produces a usable trajectory.

The 1977 opportunity — the specific case VanderVeen analyzed in detail — had a nominal mission duration of 720 days and required a departure delta-V of approximately 4.24 km/s from a 200 km circular Earth orbit. The Earth return velocity was about 11.95 km/s — high, but survivable with a robust heat shield. VanderVeen noted that the 1977 opportunity coincided almost perfectly with a simple Venus flyby mission window, which meant that the crew could fly the Venus flyby trajectory until they reached Venus, and then, if all was well, apply a burn of less than 150 m/s to redirect onto the triple-planet profile. If anything had gone wrong, they simply continued on the Venus free-return trajectory and came home — saving almost a year compared to completing the full mission. This abort option is one of the trajectory’s most attractive features.

The 1977 window opened. No one flew it. The 1983 window opened. No one flew it. The opportunities have kept coming, approximately every six years, through 1989, 1996, 2002, 2009, 2015, 2021, 2028. Now 2034 is approaching, and it may be the most attractive opportunity of the modern era.

The 2034 Opportunity: What the Numbers Say I ran the 2034 trajectory through JPL’s MIDAS optimization code, searching for the minimum delta-V Earth-Venus-Mars-Venus-Earth trajectory in the 2034–2036 timeframe. The code converged in 27 iterations to a clean solution. Here is what it found.

Event Date Elapsed days Notes Earth departure August 4, 2034 0 Departure burn from LEO Venus flyby #1 December 14, 2034 132 Abort window closes here Mars flyby May 16, 2035 286 Very distant, nearly tangential Venus flyby #2 March 14, 2036 589 Final gravity assist for Earth return Earth arrival August 23, 2036 750 Atmospheric entry at ~11.5 km/s Total mission duration: 750 days — just over two years, and almost exactly the duration VanderVeen calculated for the analogous 1977 mission. The near-identical duration is not a coincidence; it’s a consequence of the same underlying planetary geometry repeating itself.

Departure: The minimum delta-V from LEO to start this trajectory is 3.986 km/s, corresponding to a hyperbolic excess velocity of 4.186 km/s and a C3 of 17.52 km²/s². From a 200 km circular parking orbit, the actual departure burn is approximately 4.23 km/s. The departure asymptote has a declination of +35.8°, which means the trajectory leaves Earth on a fairly steep angle above the ecliptic plane — a factor to consider in launch site and inclination planning, but not a prohibitive constraint.

Venus flyby #1 (Day 132): The spacecraft passes Venus at a closest approach distance of 6,584 km from Venus’s center — approximately 532 km above the surface, well clear of the atmosphere. The flyby velocity relative to Venus is 6.53 km/s, and the gravity assist bends the trajectory by 64.9°. This is a substantial turn — Venus is doing real work here, redirecting the spacecraft from its inbound trajectory toward Mars. The flyby geometry is nearly polar (inclination 96.7°), which means the spacecraft passes over Venus’s poles rather than its equator.

This is also the last moment at which a return to Earth on the abort trajectory remains practical. After Venus #1, the spacecraft is committed to the full mission.

Mars flyby (Day 286): This is where the trajectory’s elegant character becomes most apparent. The spacecraft passes Mars at a distance of 71,855 km from Mars’s center — roughly 68,600 km above the surface, well outside even Deimos’s orbit at 23,459 km. The flyby velocity relative to Mars is 9.48 km/s, and the gravity assist bends the trajectory by only 0.76°. Mars barely deflects the spacecraft at all.

This seems counterintuitive — why fly past Mars at such a large distance, with such a tiny bend angle? The answer is that in the #5/#5 trajectory class, Mars is not providing most of the gravity assist energy. Its role is primarily geometric: the spacecraft’s encounter with Mars sets the timing and phase for the return trajectory back toward Venus. The nearly tangential Mars encounter is what keeps the flyby velocity low (and thus the mission energy moderate), and it’s also what makes the trajectory profile nearly symmetric — the outbound and inbound legs have similar shapes.

There is an important practical implication of the large flyby distance: the crew will see Mars at roughly half the angular diameter of the Moon as seen from Earth, rather than the overwhelming close-up view that a low-altitude flyby would provide. That said, they will be the first humans ever to see Mars with their own eyes from interplanetary space, and for 286 days of transit they will have watched it grow from a dot to a resolved disk. That still counts.

Venus flyby #2 (Day 589): The return Venus encounter passes at 8,318 km from Venus’s center — about 2,266 km above the surface, slightly higher than the first flyby. The flyby velocity is 6.01 km/s and the bend angle is 62.6° — again a substantial gravity assist turn. This second Venus encounter redirects the spacecraft from its outbound trajectory back toward Earth for the final 161-day coast to arrival.

Earth arrival: The spacecraft returns to Earth on August 23, 2036, with a hyperbolic excess velocity of 3.950 km/s. This corresponds to an atmospheric entry velocity of approximately 11.5 km/s. For comparison, the Apollo capsules entered at about 11 km/s returning from the Moon. This mission’s entry velocity is slightly higher, but the heat shield technology has advanced enormously since 1969, and SpaceX has demonstrated Dragon capable of hyperbolic Earth entry in testing. The entry speed is challenging but not exotic.

What Makes This Trajectory Special The numbers above describe a specific trajectory, but they don’t fully convey what makes this class of mission so architecturally attractive. Let me try to do that more directly.

No deterministic burns after departure. Once the spacecraft leaves Earth orbit, the only propulsive maneuvers required are small course corrections — trajectory control maneuvers of perhaps 10–50 m/s per leg, well within the capability of any spacecraft with a modest propulsion system. The gravity of Venus and Mars do all the real work. This is not a consequence of clever mission design; it’s a fundamental property of the trajectory class that VanderVeen identified. The planets are in the right configuration to make the whole thing work ballistically. This dramatically simplifies the spacecraft design, reduces the propellant mass required, and eliminates the mission-critical single-point-failure events that would otherwise punctuate a two-year interplanetary flight.

The abort option. For the first 132 days — more than four months — the mission can be aborted by simply not performing the trajectory correction that converts the Venus flyby profile to the triple-planet profile. The abort costs less than 150 m/s, comparable to a routine midcourse correction. The crew continues past Venus, swings back toward Earth, and returns in about 13 months instead of 25. This is not a perfect rescue — 13 months is still a long time to wait for a crew in trouble — but it is a genuine abort option of the kind that most interplanetary mission concepts lack entirely. After Venus #1, there is no abort. The trajectory is committed. But for nearly one-third of the outbound journey, the crew retains a meaningful option to come home early at very modest cost.

The record book. I laid this out in the NSF thread in 2009 and it still applies. The crew of this mission would be:

The first humans to leave Earth’s gravitational sphere of influence (the Moon is technically still within it) The first humans to fly past Venus The first humans to fly past Mars The holders of the longest crewed spaceflight in history (~750 days, more than double the current record) The humans who have traveled farthest from Earth The humans who have traveled closest to the Sun The humans who have experienced the highest atmospheric entry velocity That list is not diminished by the fact that the mission is a flyby rather than a landing. Magellan’s circumnavigation of the Earth was a surface-skimming voyage, not a systematic exploration of every coast. The historical significance of being the first humans to leave the Earth-Moon system and traverse the inner solar system is independent of whether any surface was touched.

The Mission Architecture I’ve been thinking about mission architectures for this kind of flight for fifteen years. The core argument I’ve made — and still make — is that this is fundamentally a private mission, not a NASA mission. NASA will not voluntarily accept a mission where the probability of crew survival is meaningfully below 99%, where there is no possibility of rescue, and where the scientific return is essentially zero. These are all true of a circumnavigation mission, and they are precisely the properties that make it historically significant. The risk, the isolation, and the absence of any purpose other than the journey itself are the whole point.

The architecture that makes the most sense to me is still the one I sketched in the NSF thread, updated for the hardware that now exists or is in late development:

Habitat: A large inflatable habitat module — Sierra Space’s LIFE module or a Bigelow-derived design — providing 300–500 cubic meters of pressurized volume for one or two crew members. Two years in a telephone booth is a sentence, not a mission. The habitat needs to be genuinely habitable, with room to exercise, sleep comfortably, maintain a greenhouse, and maintain sanity. The mass penalty for adequate living space is trivial compared to the mass of propellant, and the mission duration means that life support reliability is far more important than any single hardware mass optimization.

Crew vehicle: A crew return capsule capable of surviving Earth entry at 11.5 km/s. SpaceX’s Dragon has been demonstrated capable of hyperbolic entry. Orion was designed with lunar return velocities in mind and has significant margin. Either would serve, with appropriate heat shield upgrades. The crew vehicle remains docked to the habitat for the entire mission and is used only for the final Earth return phase.

Propulsion: The departure burn of ~4.23 km/s from a 200 km parking orbit requires an Earth departure stage. For a 30,000 kg mission stack, this implies roughly 50,000–70,000 kg of propellant for a cryogenic upper stage — achievable with a single Falcon Heavy or New Glenn launch, or with in-orbit propellant transfer using Starship. The departure stage is expended after the burn and can be retained on a tether as a counterweight for artificial gravity during the coast phases.

Artificial gravity: Two years of microgravity will destroy a human body, and the mission requires the crew to survive atmospheric entry at elevated g-loads at the end. Tethered rotation — the departure stage connected to the habitat by a cable, spinning around the common center of mass — can provide 1g at reasonable tether lengths and rotation rates. This is the Mars Direct approach that Zubrin advocated, and it is mechanically feasible with existing technology. The Canfield joint solves the course-correction-while-spinning problem by allowing the thrusters to fire in the appropriate direction regardless of the spacecraft’s rotational phase.

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A proposed 750 day flyby mission to Mars an Venus. I suggest Elon get planning for this in detail. Less than eight years to get ready.

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Never happen. 47 hates NASA - so no money there; Elon has yet to make orbit, let alone get anything man-rated - Elon will be lucky to have his Lunar Lander certified, man-rated, demo space refueling, and landing successfully at the Lunar South Pole by 2034.

Not to mention there is no capsule/ship designed or under design for 750 days of human occupation.

The only ship capable of that sort of venture was killed off in 1964 in favor of Apollo. Had it not been killed, due to failure of political will, ships like it would have already explored the entire solar system and larger models would be voyaging to the stars and back. Ship motto was: Mars by 1965, Saturn by 1970.

“...the first time in modern history that a major expansion of human technology has been suppressed for political reasons.” - Freeman Dyson

Buzz Aldrin a.k.a. Doctor Rendezvous has proposed a Mars Cycler, using these same principles. Using gravitational slingshots to toss planetary probes around has been used for at least 40 years, hmm, pushing 50 years. Oh, sez here 1959:

https://search.brave.com/search?q=gravitational+slingshot&summary=1

A gravitational slingshot, also known as a gravity assist or swing-by, is an orbital mechanics maneuver where a spacecraft uses the relative motion and gravity of a planet to alter its path and speed, typically to save propellant and reduce mission costs. This technique works by exchanging momentum between the spacecraft and the planet; the spacecraft gains or loses kinetic energy relative to the Sun, while the planet experiences a negligible change in its own orbit due to its vastly larger mass.

https://search.brave.com/search?q=aldrin+cycler&summary=1

The Aldrin cycler is a spacecraft trajectory concept proposed by Buzz Aldrin in 1985, designed to provide regular, low-propellant transport between Earth and Mars using gravity-assist flybys. This specific cycler completes a full orbit around the Sun every 2.135 Earth years (one synodic period), allowing it to encounter both planets repeatedly without requiring continuous propulsion.

The system typically involves two complementary cyclers, often referred to as “up” and “down escalators”, to minimize wait times for travelers:

Outbound Trip: A spacecraft travels from Earth to Mars in approximately 146 days (5 months).

Return Trip: The cycler spends the remaining 16 months beyond Mars’s orbit before returning to Earth in another 146 days.

Key characteristics and operational details include:

Taxis and Castles: Astronauts use smaller, specialized “taxi” vehicles to rendezvous with the massive cycler “castle” station at Earth or Mars orbit, attaching to hitch the ride to the other planet.

Radiation Shielding: Because the journey takes several months, the cycler can carry heavy shielding to protect crews from cosmic rays, which is difficult in smaller, conventional rockets.

High Approach Speeds: A major drawback is that ballistic cyclers approach planets at high speeds (e.g., 11 km/s at Mars for the outbound trip), requiring significant fuel for braking unless powered variants with ion engines or solar sails are used.

Efficiency: Once established, the trajectory requires minimal propellant for course corrections, making it potentially economical for establishing permanent colonization routes despite the high initial investment to launch the massive station into orbit.

AI-generated answer. Please verify critical facts.

Don’t be silly; we are not talking about Falcon 9 or Heavy or Dragon. This is about Starship. And no Falcon Heavy could not do the job for the simple reason there is no such thing as a ship that could carry one or more humans on a 2 plus year voyage anywhere.

Dragon could not, even of it were rated above LEO; Orion is only rated for a 10 days, not 2 years. And not beyond the Moon, were everything changes. Then there is crew safely - what happens if there is an emergency with a crew member?

Such a ship exists only in fantasy, not reality. 2034 is not enough time to make on either. 2134 is more realistic with today’s tech to go on such a pointless and extremely dangerous journey.

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