I understand the Interplanetary Transport Network allows for travel at low speeds between different planets in the solar system using very little energy. Is there flexibility in how long it would take to travel between planets (with the same orbital configuration) on ITN? Does the Δ𝑣 determine how long it would take to travel between different bodies?

Is there a minimum time it would take between Jupiter and/or Saturn to travel to Mars and/or Earth using the ITN?

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    $\begingroup$ posts as helpful resources 1, 2, 3, 4, 5 $\endgroup$ – uhoh Sep 29 '20 at 5:08
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    $\begingroup$ with special attention to @HopDavid's posts $\endgroup$ – uhoh Sep 29 '20 at 5:14
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    $\begingroup$ @uhoh So if I'm reading HopDavid's answer to the first link you posted correctly, it would take centuries to travel between those gas giants and Earth or Mars via the ITN. I'm I reading it correctly? $\endgroup$ – Bob516 Sep 29 '20 at 22:42
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    $\begingroup$ @uhoh Thanks for all the information. I'm looking for the way to travel from Jupiter to Earth using minimal delta-v. What I really interested in is the slowest transfer from Jupiter or Saturn to Mars and/or the Earth. Maybe I should be asking this in WorldBuilding SE. $\endgroup$ – Bob516 Sep 30 '20 at 0:04
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    $\begingroup$ I think step #1 can be simply updating this post and ask that more specific question, or to ask it as a new one here. While reading about ITN may have gotten you thinking about this, it might not be the basis of an answer you'll be glad to receive. Maybe ITN can be "background" or just one example. $\endgroup$ – uhoh Sep 30 '20 at 2:21

A dynamic system, with at least 3 massive bodies, will have chaos that can, in theory, be exploited to reach (almost) arbitrary positions within said system at close to zero $\Delta v$ over very long time spans. This is the "Interplanetary Transport Network".

This sounds very alluring, but it's easy to be misled into believing this has much relevance to space-flight. It may often even be intentionally presented in a misleading manner.

The following models of trajectories are used in space-flight involved multiple bodies, with quickly diminishing returns for the additional "tricks" their increasing complexity contribute.

  1. The patched conics approximation. A spacecraft is always assumed to be orbiting a single body, and when it reaches another one, the frame of reference is changed. For the solar system, this is usually very accurate, as the gravitational influence of the closest body is in almost any location dwarfing all the other ones.

  2. The CR3BP, which takes into account the gravitational influence of two bodies at once. This is only really relevant close to the border regions of the patched conics approximation, but it give rice to some interesting artefacts such as Lagrangian points

  3. True n-body physics.

The ITN deals with the effects of the third one. Unfortunately, the gravitational influence of the "third strongest" body or lower is extremely small in almost any part of the solar system.

We actually happen to live close to one of the regions where true n-body physics is measurable, that is, the region where the Earth, the Moon and the Sun all contribute some meaningful amount of gravity.

In particular, The Sun-Earth L-points SEL1 and SEL2 and the Earth-Moon L-points EML1 and EML2 can be shown to be connected with the low energy pathways of the ITN.

Beyond that region, the effects of the ITN become almost unmeasurable small. There's no point in space where the gravitational influence of the Earth, the Sun and Jupiter all have comparable influence. One of those three will always be much much weaker than the strongest one, leading to ITN pathways in the order of millions of years.

The ITN is not relevant to interplanetary spaceflight

This has to be clearly stated, as many others fail to say so.

Often confused with the ITN is gravity assists. The difference is that they are actually relevant to space-flight, and can be adequately modelled by patched conics.

Those can be effectively used to trade transfer time for $\Delta v$ savings.

  • $\begingroup$ Excellent answer! That mission designers "exploit chaos" to get where they want to go is an interesting choice of words. I want to say it's wrong but I can't. :-) However I disagree with "...models of trajectories... with quickly diminishing returns for their increasing complexity." How do n-body models have diminished returns for those who model spacecraft trajectories? Perhaps you mean that n-body "tricks" to getting there with lower energy has diminishing returns; I would hope all trajectory modelers always use n-body models for gravity. $\endgroup$ – uhoh Oct 1 '20 at 23:29
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    $\begingroup$ @uhoh I quite like the expression "exploit chaos" :) By picking a flyby altitude in advance of a Titan flyby, the Cassini spacecraft could chose its next Titan flyby. It's a butterfly effect. Yeah, there should probably be a "tricks" in there. $\endgroup$ – SE - stop firing the good guys Oct 1 '20 at 23:36
  • $\begingroup$ Our of curiousity how do Saturn/Titan/Sun or Sun/Jupiter/Ganymede stack up on the "all three bodies have significant effects" scale? $\endgroup$ – Steve Linton Oct 2 '20 at 7:40
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    $\begingroup$ @SteveLinton The ratios are 505 for Sun/Jupiter/Ganymede, and 393 for Saturn/Titan/Sun in the best scenario. Compare to 2.20 for Sun/Earth/Moon. Outer moons may score a bit better, but at those distances, the actual magnitude of the forces is very small. $\endgroup$ – SE - stop firing the good guys Oct 2 '20 at 8:20

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