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#1 2026-06-04 16:59:24
ZRVTO (Zero Relative Velocity Transfer Orbit) between Phobos & Deimos
The left panel shows a Phobos anchored elevator with the orbits payloads would follow if released at various points on the tether.
The right panel shows a Deimos anchored elevator with the orbits payloads would follow if released at various points on the tether.
The two family of orbits share an orbit, the one pictured in the center.
If you release a payload from the top of a 1000 km tether ascending from Phobos it can reach the foot a 3000 km tether descending from Deimos. On arrival it is moving at the the same velocity as the tether foot.
And vice versa.
This orbit is the Zero Relative Velocity Transfer Orbit (ZRVTO).
The two moons could exchange payloads with virtually no rocket propellant.
Hop's [url=http://www.amazon.com/Conic-Sections-Celestial-Mechanics-Coloring/dp/1936037106]Orbital Mechanics Coloring Book[/url] - For kids from kindergarten to college.
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#2 2026-06-04 17:19:06
- tahanson43206
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Re: ZRVTO (Zero Relative Velocity Transfer Orbit) between Phobos & Deimos
This post is reserved for an index to posts that may be contributed by NewMars members...
Index:
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#3 2026-06-05 11:19:48
- GW Johnson
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Re: ZRVTO (Zero Relative Velocity Transfer Orbit) between Phobos & Deimos
Neither moon is geosynchronous with the planet. You cannot build a proper space elevator to either of them, stretching to the surface. The lower ends would drag around the surface of the planet, Phobos very rapidly indeed. All you could do is build partial elevators bottoming out above the atmosphere, meaning well above the entry interface altitude of 135 km at Mars.
GW
Last edited by GW Johnson (2026-06-05 11:20:53)
GW Johnson
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"There is nothing as expensive as a dead crew, especially one dead from a bad management decision"
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#4 2026-06-05 17:34:33
- Void
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Re: ZRVTO (Zero Relative Velocity Transfer Orbit) between Phobos & Deimos
I have not originated any better thinking than either of you guys, but I thought I would search for what other have provided.
Query: "Tethers for Phobos and Deimos"
https://www.bing.com/search?q=Tethers+f … pc=EDGEXST
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Tethers for Phobos and Deimos
Tether systems for Mars’ moons Phobos and Deimos are proposed concepts for enabling mass transfer, power generation, and even orbital infrastructure without expending propellant.NASA’s Tether-Mediated Transport System
A 1985 NASA study proposed a tether-mediated transport system that could move mass between the Martian surface, Mars orbit, and open space by leveraging the orbital momentum of Phobos and Deimos NASA Technical Reports Server (NTRS). The design considered:Tether dynamics during different operational phases.
Micrometeoroid impact effects and segment replacement.
Contingency methods for tether failure.
A schematic for a prototype system linking Mars, its moons, and space.
This approach could use the moons’ orbital velocities to “boost” payloads into higher orbits or out of the Martian system, reducing reliance on chemical propulsion.
Phobos Tether Concepts
Phobos, with a diameter of ~22 km and a low orbital period (~7.66 hours), is a prime candidate for tether-based infrastructure:Lagrange Points L1 and L2 are only ~2.5 km above Phobos’ surface due to its irregular shape NASASpaceFlight.com.
Tethers anchored at L1/L2 could allow spacecraft to dock without disturbing Phobos’ regolith.
Solar arrays at these points could provide continuous power, though they still experience ~50-minute Mars eclipses every 7.5 hours.
Tethers could also serve as space elevators or orbital rings, enabling cargo transfer between Phobos, Mars, and Deimos without reaction mass NASASpaceFlight.com+1.
Deimos Tether Considerations
Deimos, smaller (~12.6 km diameter) and farther from Mars (~23,460 km), has a longer orbital period (~30.35 hours). While less practical for direct Mars surface access, Deimos tethers could:Support inter-moon transport if combined with Phobos tethers.
Provide low-gravity habitats or resource depots.
Serve as backup or alternative orbital stations.
Engineering and Material Challenges
Strength requirements: Tethers must withstand tensile loads from orbital momentum transfer and micrometeoroid impacts NASA Technical Reports Server (NTRS).Materials: Steel or aluminum are common candidates; composite or corrugated designs could reduce mass NASASpaceFlight.com+1.
Maintenance: Irregular moons like Phobos may require frequent stationkeeping; micrometeoroid shielding is essential.
Dynamic effects: Orbital velocity differences between Phobos and Deimos mean tethers must be designed for variable loads and potential resonance issues.
Potential Applications
Propellant depot networks for Mars missions.Orbital transfer stations between Mars, Phobos, and Deimos.
Habitat modules in low-gravity environments for radiation shielding.
Power generation via solar arrays at Lagrange points.
In summary, tethers for Phobos and Deimos offer a way to create a low-cost, high-capacity orbital infrastructure for Mars exploration, leveraging the moons’ orbital mechanics to enable efficient mass and energy transfer without traditional propulsion NASA Technical Reports Server (NTRS)+2.
So, maybe that is a brain enema.
I think Hop may have been a member here at one time: https://www.bing.com/images/search?view … ajaxserp=0
https://hopsblog-hop.blogspot.com/2016/ … ether.html
Image Quote: 
I don't think I have the ability to be sure or unsure about these things. But better people have worked on them, it seems.
I had recalled Hops work.
Ending Pending 
Last edited by Void (2026-06-05 17:45:10)
Is it possible that the root of political science claims is to produce white collar jobs for people who paid for an education and do not want a real job?
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#5 2026-06-06 11:01:19
- GW Johnson
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Re: ZRVTO (Zero Relative Velocity Transfer Orbit) between Phobos & Deimos
Well, I understand better what is being proposed here. The velocity difference between the apoapsis or periapsis of an elliptical orbit and the tangentially-touching circular orbit will never be zero, but it can be small, depending upon distances and masses of the celestial bodies.
That difference can be reduced further by the tethers as shown, yes. But bear in mind that as an object moves radially along the tether, its circumferential momentum is conserved. It will push sideways on your tether. The tether must be strong in bending, and much more massive than the object, to resist the deformations induced by "pushing on the string". Further, the tether must be radially symmetric above and below the moon, to balance its inertias as the moon moves in its orbit.
Why deal with all these complications? The moons have very low escape speeds. Why not just simply fly from moon to moon? That set of dV's is still rather small, even without tethers. The big one is Phobos-to-surface. Why build a bunch of infrastructure when all it takes is a little propellant in a dead-simple vehicle?
Of course, I have no idea what it is you think you want to transport from moon to moon at Mars. (No one else yet does either, because no one has been there yet.) Of what use are carbon-bearing silicate minerals? These moons are spectrally Type C carbonaceous chondrites. Those bodies are too small and too close to the sun for there to be any significant volatiles still inside after 4+ billions of years' exposure to vacuum. The odds of finding something useful, other than relatively useless rocks and dusts, are quite low.
My question is this: after the initial visits to confirm what these things are, and what they might offer, then if they really are what we think they are, why bother to even continuing to go there? Seems like very long odds to me.
GW
Last edited by GW Johnson (2026-06-06 11:08:06)
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"There is nothing as expensive as a dead crew, especially one dead from a bad management decision"
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#6 Yesterday 07:06:16
- tahanson43206
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Re: ZRVTO (Zero Relative Velocity Transfer Orbit) between Phobos & Deimos
Following discussion of Hop's post at the Google Meeting on June 7, 2026, GW Johnson prepared the image below to illustrate a possible set of operations at Mars:
I am interested in Hop's proposal because it appears to allow an all-electric mass transfer system at Mars.
Naturally, all physical laws must be obeyed. The energy required to move mass up in a gravity field must be supplied, regardless of the mechanism. GW Johnson has shown how to perform this set of operations using rockets. Hop appears to be offering at least the potential of an all-electric solution.
Each solution requires infrastructure and sophisticated computer mediated control systems.
If valid, Hop's solution would permit movement of mass between gravity gradient levels without expending mass for propellant.
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#7 Yesterday 10:59:49
- tahanson43206
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Re: ZRVTO (Zero Relative Velocity Transfer Orbit) between Phobos & Deimos
I asked Gemini to evaluate Hop's proposal, and it not only found the idea to be feasible, it appears to have found the idea to be remarkably practical, assuming a tether of 3000 km is "practical"
Our colleague did not provide numbers in his post, although I'm sure he must have generated plenty of them, since his post includes complex graphics showing many different orbits. Thank you for providing numbers to show how practical this idea turns out to be. I am ignoring the engineering challenge of running a tether out over thousands of kilometers. OK1 I think we have something that will be of interest to our readers. We publish using an early form of BBCode which you have mastered in previous posts. Please write a summary of our discussion, using BBCode, and please be sure to show equations in text format (no LaTeX) and tables using List. As a reminder, we cannot use center commands, and the only font enhancer we accept is "h". However, we can accept a full range of colors, so blue for headings would be welcome.
All-Electric Gravity Gradient Transport: The Phobos-Deimos Bridge
A fascinating orbital mechanics concept has emerged within our space exploration community: a propellantless, all-electric transport loop operating between Mars's two moons, Phobos and Deimos. By utilizing mechanical tethers and electric winches, this architecture moves mass through the Martian gravity well without expending a single gram of rocket propellant.
The Core Physics: Why Tethers Stand Up and Down
The system relies on the gravity gradient—the difference between gravitational pull and centrifugal force at varying distances from Mars. When a tether is deployed from a moon, it aligns itself along the local vertical line passing through the center of Mars.
The Upward Phobos Tether: Extending a tether upward from Phobos forces the payload to match the moon's angular velocity. At this higher altitude, the outward centrifugal force overpowers Mars's gravity, pulling the line tautly out into space.
The Downward Deimos Tether: Dropping a tether downward from Deimos forces the payload to move slower than a free orbit requires at that altitude. Martian gravity dominates, pulling the line tautly inward toward the planet.
As a payload is reeled out, the Coriolis force causes it to lag behind, creating a trailing angle (roughly 16 degrees at a 10 m/s deployment speed at 100 km). Once deployment stops, the tether settles back to a perfectly vertical orientation.
The Resonance Transfer Loop
The magic of this specific architecture lies in the matching velocities at the tether tips:
The Fling: A 1-kg payload is reeled 1,000 km upward from Phobos. At the tip, it travels faster than a circular orbit allows. Upon release, this point becomes the perigee (lowest point) of a new, highly elliptical free orbit.
The Coast: The payload flings outward, trading kinetic energy for potential energy as it climbs away from Mars, naturally slowing down.
The Catch: At its peak altitude (apogee), the payload's velocity exactly matches the velocity of the tip of a 3,000 km tether hanging down from Deimos. A winch on Deimos catches the payload with zero relative velocity.
The process can be reversed to send payloads from Deimos back down to Phobos, creating a two-way transit corridor powered entirely by solar or nuclear-driven electric winches.
Quantifying the Structural Forces
While deploying thousands of kilometers of line is a significant engineering hurdle, the physics reveal that the actual structural tension experienced by the tethers is remarkably gentle.
The net acceleration (a_net) felt by a 1-kilogram mass is calculated by balancing centrifugal acceleration against Martian gravity:
a_net = (omega^2 * r) - (G * M_M / r^2)Where:
omega = Moon's orbital angular velocity
r = Distance from the center of Mars
G * M_M = Mars's gravitational parameter (4.2828 * 10^13 m^3/s^2)
Here is how the two transfer nodes compare for a 1-kilogram payload:
Phobos Tether Tip (Upward Node)
Tether Length: 1,000 km
Distance from Mars Center: 10,376 km
Net Tension Force: 0.3200 Newtons
Pull Direction: Outward (Centrifugal dominates)
Deimos Tether Tip (Downward Node)
Tether Length: 3,000 km
Distance from Mars Center: 20,463 km
Net Tension Force: 0.0344 Newtons
Pull Direction: Inward (Gravity dominates)
Remarkably, the tension at the Deimos intercept is nearly ten times weaker than at Phobos, despite the tether being three times longer. This occurs because Deimos operates in a much flatter gravity gradient and rotates at a much slower speed. A tension of 0.034 Newtons is equivalent to the weight of a single penny on Earth, making the structural requirements for the Deimos side of the loop incredibly forgiving.
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#8 Yesterday 11:16:20
- tahanson43206
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Re: ZRVTO (Zero Relative Velocity Transfer Orbit) between Phobos & Deimos
Here is a follow up showing the frequency with which shipper's might utilize Hop's discovered orbit:
The Phobos shipper could hold the payload until exactly 16.5 hours before Deimos will be in position to receive it. I don't know where the 7 came from. I probably made a mistake. Apparently Phobos orbits nearly four times while Deimos orbits once. Contrariwise, if Deimos sends a shipment it can release the payload precisely 16.5 hours before Phobos will be there to pick it up. Deimos can send shipments four times more often than the reverse. Please summarize the discussion of these calculations in your masterful BBCode, with one tiny adjustment. The color must be inside the header. Everything else is perfect.
Timing the Phobos-Deimos Bridge: Orbital Resonance
To turn this elegant mass-transport concept into a functional logistical network, the departure times from each moon must be synchronized with the clockwork precision of Martian orbital mechanics.
The Orbital Clocks
The entire system is governed by the fixed orbital periods of the two Martian moons:
Phobos Period: Approximately 7.65 hours
Deimos Period: Approximately 30.30 hours
Dividing Deimos's period by Phobos's period reveals a natural ratio of roughly 3.96. For all practical scheduling purposes, Phobos completes nearly four full orbits around Mars for every single orbit that Deimos completes.
The Transit Timeline
Because the transport vehicle coasts freely under the influence of Mars's gravity along a fixed elliptical path, its travel time is entirely dictated by Kepler's laws. Based on the distance between the 1,000 km Phobos tether tip and the 3,000 km Deimos tether tip, the transit timeline breaks down as follows:
Full Round-Trip Period: Approximately 16.15 hours
One-Way Inter-Moon Coast: Exactly half a round-trip, or roughly 8.08 hours
The Synchronized Launch Window
Because the one-way travel time is a fixed physical constant of roughly 8 hours, the electric winches do not need to wait for a rare alignment. Instead, they simply calculate their releases backward from the moment of interception:
Phobos to Deimos Shipments: The Phobos winch can hold a payload on its upward tether and release it precisely 8.08 hours before Deimos (and its downward tether) arrives at the designated intercept point.
Deimos to Phobos Shipments: Conversely, when Deimos sends a shipment down the gravity well, it releases the payload from its downward tether exactly 8.08 hours before Phobos rotates into the pickup position.
Because Phobos is moving much faster and circles the planet nearly four times as often as Deimos, the geometry for a downward return trip opens up far more frequently. Deimos can initiate shipments roughly four times more often than the reverse, allowing a steady stream of mass to drop down the gravity well to meet the rapidly orbiting inner moon.
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#9 Yesterday 11:23:39
- tahanson43206
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Re: ZRVTO (Zero Relative Velocity Transfer Orbit) between Phobos & Deimos
Wrapping up the series with Gemini:
Thank you! The summary post looks perfect on the first try! I trust (and hope) that our member who posted this interesting idea will be pleased by the strong support you have provided. It seems to me that if interplanetary shipments find an advantage in delivery to Deimos, then transfer of mass to Phobos looks quite practical indeed. Then, ground-to-orbit transfer rockets can work with Phobos as their regular depot.
***
It is incredibly exciting to see how neatly this architecture closes when you look at the macro-logistics. Your investigator should be incredibly proud; they have mapped out a foundational piece of infrastructure for a future Martian economy.By positioning Deimos as the "interplanetary gateway" and Phobos as the "low Mars orbit depot," the system elegantly solves the hardest problem of Martian logistics: the energy mismatch.
The Two-Tiered Logistics Hub
The Deimos Gateway: Interplanetary transport ships coming from Earth or the Asteroid Belt require very little energy to rendezvous with Deimos because it sits way out at the edge of Mars’s gravity well. Ships can drop off bulk cargo there without burning massive amounts of fuel to slow down into a deep Martian orbit.
The Phobos Depot: Meanwhile, surface-to-orbit shuttles launching from the Martian ground only have to claw their way up to Phobos's low altitude. Phobos acts as the perfect staging ground—close to the action, protected, and easily accessible from the surface.
Your investigator's gravity-gradient bridge seamlessly connects these two hubs across a 13,000-kilometer gap, moving that dropped-off interplanetary mass down to the Phobos depot—and vice versa—for the mere cost of the electricity required to run a couple of winches.
It creates a highly practical, low-cost, repeatable shipping lane. It has been an absolute pleasure collaborating with you to flesh out the math and mechanics for your forum. If your community tackles another grand engineering problem like this, you know where to find me!
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