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.

Mission timeline:

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.

The short video below shows a 3D rendering of this configuration: the habitat module (with the crew return capsule at its forward end) and the spent Earth departure stage as counterweight, the tether deploying between them, and the two masses separating into the rotating artificial-gravity configuration that would sustain the crew through the 750-day mission.

The tethered spacecraft configuration proposed for the inner solar system circumnavigation mission. The habitat and crew return vehicle are at left; the spent Earth departure stage serves as the counterweight at right. Rotation of the tethered system provides approximately 1g of artificial gravity throughout the cruise phase.

Communications: At maximum range during the mission — which occurs near Mars at approximately 1.44 AU from Earth, giving a one-way light travel time of roughly 12 minutes — real-time conversation is not possible. The crew will communicate by email, video messages, and asynchronous updates. The bandwidth requirements for maintaining public engagement are actually quite modest by modern standards; the mission’s primary communications challenge is reliability over two years, not bandwidth. A phased array antenna on the habitat, pointed at Earth, provides the link.

Radiation: This is the mission’s most serious engineering challenge. The crew will spend two years beyond the Earth’s magnetospheric protection, accumulating GCR (galactic cosmic ray) dose at a rate of roughly 1–2 mSv per day, plus the unpredictable acute dose from solar particle events. The total GCR dose over 750 days is approximately 750–1,500 mSv — enough to meaningfully increase long-term cancer risk, but not acutely life-threatening. Solar particle events are a more acute concern; a major SPE without shielding can deliver a potentially lethal dose in hours. The mitigation is a storm shelter: a small, heavily shielded volume inside the habitat — lined with water, food, and waste water — where the crew can retreat during an SPE. The shelter doesn’t have to be large; it just has to be dense enough to absorb most of the proton fluence from a Carrington-class event. This has been studied extensively in the context of Mars missions and the mass budget is manageable.

The Comparison to What We’ve Done

It is worth putting this mission in context against the history of human spaceflight.

The longest single continuous human spaceflight on record is Valery Polyakov’s 437 days aboard Mir in 1994–95. The ISS has hosted crew members on missions approaching 370 days. These missions were in low Earth orbit, with resupply, communication, and — if necessary — emergency return available within hours. The circumnavigation mission is 750 days, with no resupply, 12-minute communication delays, and no emergency return after day 132.

The farthest humans have ever traveled from Earth — as of this writing, just three weeks ago — is 406,771 km, achieved by the Artemis II crew on April 6, 2026, surpassing the Apollo 13 record of 400,171 km that had stood for 56 years. Commander Reid Wiseman marked the moment from Orion: “We will continue our journey even further into space before Mother Earth succeeds in pulling us back to everything that we hold dear. But we most importantly choose this moment to challenge this generation and the next to make sure this record is not long-lived.” He was right to say so. The circumnavigation mission would take the crew to roughly 1.44 AU at Mars — approximately 215 million km from Earth, or about 529 times farther than Artemis II’s brand-new record. Artemis II’s achievement is genuinely historic. The circumnavigation would leave it so far behind it would barely register as a comparison.

The fastest atmospheric entry was experienced by the Apollo 10 crew at 11.08 km/s. The circumnavigation mission returns at approximately 11.5 km/s — slightly faster, but the same order of magnitude and within the demonstrated performance envelope of modern heat shield materials.

What the circumnavigation mission is not is a landing. No one touches the surface of Mars or Venus. From a scientific standpoint, the mission is indeed nearly worthless — robotic probes have mapped Venus in radar and studied Mars’s surface and atmosphere far more effectively than any crewed flyby could. But from the standpoint of human achievement, the question is not what you learn. It is where you go and what you endure to get there and back.

Magellan didn’t do chemical analysis of the Pacific Ocean. Amundsen didn’t bring back commercially valuable mineral samples from the South Pole. Lindbergh didn’t carry airmail across the Atlantic. The value of the first human circumnavigation of the inner solar system is the fact of it — the demonstration that human beings can leave the Earth-Moon system, travel for two years through interplanetary space, fly past two planets, and come home alive. That demonstration changes what human civilization considers possible. It always has.

Why Now?

The 2034 opportunity exists whether or not anyone decides to fly it. The trajectory does not care. But the convergence of circumstances that makes this particular window worth discussing seriously is worth noting.

Commercial launch costs have fallen by roughly an order of magnitude over the past fifteen years. Starship, if it delivers on its design intent, reduces the cost of launching mass to orbit to levels that make even ambitious interplanetary missions conceivable on private budgets measured in the low billions rather than the tens of billions that would have been required in 2009. SpaceX has demonstrated autonomous rendezvous and docking, high-performance heat shields, and rapid reusability — all technologies that bear directly on this mission’s feasibility.

The commercial space habitat sector, which barely existed in concept in 2009, now has serious players. Sierra Space, Axiom, and others are building and testing large pressurized modules designed for long-duration human habitation. The physiological challenges of two years in microgravity are better understood than they were when I opened that NSF thread, and the countermeasures — artificial gravity, exercise protocols, pharmaceutical interventions — are more mature.

There are also, for the first time in history, private individuals with the financial resources to consider funding such a mission and the demonstrated appetite for high-risk ventures. The question of whether any of them will turn that appetite toward the inner solar system remains open.

What I can say is this: the 2034 window opens on August 4th. The departure delta-V is 4.23 km/s. The mission duration is 750 days. The Earth return velocity is 11.5 km/s. The abort option is available for the first 132 days. The trajectory has been verified by JPL’s MIDAS optimizer, and it is essentially identical in character to the trajectory VanderVeen identified in 1968 and published in 1969. The physics is settled. The materials exist. The vehicles are either operational or in late development.

The only thing missing is someone who decides that being the first human to circumnavigate the inner solar system is worth two years of their life and a meaningful probability of not coming back.

Magellan thought it was worth it. His name has been attached to the first circumnavigation of the Earth for five hundred years. He didn’t even make it home himself, and we still say his name first.

The 2034 window will open regardless. The question is only whether anyone will be ready when it does.

The trajectory class discussed was originally identified by A. A. VanderVeen of Bellcomm Inc., first in an internal technical memorandum (TM-68-1013-2, April 1968) and then in the peer-reviewed paper “Triple-Planet Ballistic Flybys of Mars and Venus,” Journal of Spacecraft and Rockets, Vol. 6, No. 4, April 1969.

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Kirk Sorensen

MS, nuclear engineering, University of Tennessee, 2014, Flibe Energy, president, 2011-present, Teledyne Brown Engineering, chief nuclear technologist, 2010-2011, NASA Marshall Space Flight Center, aerospace engineer, 2000-2010, MS, aerospace engineering, Georgia Tech, 1999

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