# How useful is the Interplanetary Transport Network?

Wikipedia summarizes the Interplanetary Transport Network/Interplanetary Superhighway fairly succinctly, although it's pretty vague to someone who doesn't know what's going on:

The Interplanetary Transport Network (ITN) is a collection of gravitationally determined pathways through the Solar System that require very little energy for an object to follow.

ernestopheles gives a better summary in an answer to a related question:

The "Interplanetary Transport Network" may be a misleading term. When probes are sent into deep space, most of them make use of flybys or gravity assist manoeuvres. Virtually every celestial body can therefore be used for increasing the speed of a probe or decreasing it. The "network" refers to series of such manoeuvres.

I have two parts to my question:

• What's the minimum amount of $\Delta v$ needed to transfer between say, Earth and Jupiter, using the ITN? Is it possible to transfer with near-zero $\Delta v$ from say, a Sun-Earth Lagrange point?
• If savings are large, why don't more missions use the ITN? Wikipedia's page on Low-energy transfers suggests to me that not a lot of missions even do that, let alone a more complicated maneuver through the ITN. Is it just the length of time needed for the transfer? Or maybe there are very few windows to execute such a thing?
• – Erik Jun 5 '15 at 15:28
• A porkchop plot with a fine enough resolution will find all of these paths. – Erik Jun 5 '15 at 15:55
• @Erik So far as I know, most porkchop plots rely on Lambert Space Triangles. N-body mechanics are more complicated. – HopDavid Mar 7 '16 at 4:09

ITN usually refers to ballistic captures/throws via Weak Stability Boundaries (WSBs) to/from L1 and L2 necks. I don't like Ernestopheles' answer. Conflating this with swing by gravity assists muddies the waters.

Shane Ross and friends are credited with inventing the term Interplanetary Superhighway. From Shane Ross' page:

Impractical for Interplanetary Transfers Due to Long Transfer Time: Due to the long time needed to achieve the low energy transfers between planets, the Interplanetary Superhighway is impractical for transfers such as from Earth to Mars at present.

My terms:

SEL1 Sun Earth Lagrange 1

SEL2 Sun Earth Lagrange 2

SML1 Sun Mars Lagrange 1

SML2 Sun Mars Lagrange 2

My convention is to put the initial for central body first and orbiting body second.

The Sun Earth L1 and L2 points are about 1.5 million kilometers from earth. This is only 1% of an A.U.

The ellipse from SEL2 has a 1.07 A.U. aphelion. This doesn't come close to the ~1.52 A.U. aphelion needed to reach Mars.

An ITN defender might reply "But repeated gravity assists from earth might boost aphelion."

The 1.01 x 1.07 A.U. ellipse has a period of of 1.063 years. It's synodic period with regard to earth is 16.87 years. (think of two runners running almost the same speed -- it takes the slightly faster runner a long time to lap the other)

Does this mean a nice gravity assist every 17 years or so? Not quite. For a good gravity assist, it would need to pass by earth during perihelion. A flyby during perihelion would only occur once every 8th fly by. So gravity assists every 136 years or so.

And as the earth comes up from behind the payload, it pulls it backward. Then after it passes, it is pulling the payload forward. So usually the net effect of a fly by is nearly zip. It would take millennia to get an aphelion up to 1.52 A.U.

A key quantity in 3 body mechanics is (mass orbiting body)/(mass orbiting body + mass central body) often denoted μ

Some μ numbers for various orbiting bodies:

Earth Moon .012

Sun Jupiter .00095

Sun Saturn .00029

Sun Neptune .000052

Jupiter Ganymede .000078

Jupiter Europa 000025

Sun Earth .00000304

Can comets ride WSBs from one gas giant to another? Sure. Could a Jupiter orbiter ride WSBs between Galilean moons? Sure.

But a WSB from Earth to Mars? That is a bridge too far.

I go into more detail at my blog post Potholes on the Interplanetary Superhighway.

Notice the earth moon has a hefty μ. Using WSBs for travel about the earth moon neighborhood is much more interesting. I write about this at my blog post EML2

So in response to "How useful is the Interplanetary Transport Network" I'd reply "Not very".

• How is aphelium of 1.07 or anything bigger than 1.01 AU achieved from SEL2? Is it a hard reliable figure? And is it hard to find the best direction and timing for the "nudge"? – LocalFluff Aug 5 '15 at 8:15
• A 1.01 A.U. circular heliocentric orbit would move about 29.6 km/s, that is if it's an ordinary 2-body orbit. But SEL2 moves 30.1 km/s. When nudged loose from the earth's influence, that 30.1 km/s would send it to an 1.07 A.U. aphelion. – HopDavid Aug 5 '15 at 14:30

Regarding the second part of your question there are few windows where celestial bodies are lined up in such a way so that the gains outweigh the losses. The savings can be large only if there's a gravitational field in the right place, right time, and heading in the right direction.

I'd recommend also looking at Vallado's book Ch12 three-body mission design, as well as Jeff Parker's work. Low Energy Transfer, also topics on manifold. Problem with low energy transfer is that it will take a long time to fly which make it less useful/practical. Although, if possible one would pre-plan a mission by using these low energy trajectory, for example, if you have a optimal trajectory to Mars (for human) say in the next 20 years, you can potentially calculate for one of these low energy trajectory, and send any essential stuff for the Mars stay, and the Mars return part of the mission.

Another use of these low energy trajectory is a cislunar mission redirect when spacecraft is low on fuel or damaged. As you have probably already read from the wiki on Japan's Hiten.

• The WSBs from Sun Earth Lagrange 2 and Sun Mars Lagrange 1 do not overlap. There are no low energy transfers from earth to Mars. See hopsblog-hop.blogspot.com/2015/04/… – HopDavid Apr 12 '16 at 20:41