
Interplanetary Transport Network - ColinWright
http://en.wikipedia.org/wiki/Interplanetary_Transport_Network
======
evolian
If you find this stuff as fascinating as I do, I know of no better way to geek
out about orbital mechanics than to play Kerbal Space program. Ex-NASA people
quip that they learn more playing that than actually working at NASA.

It doesn't have multibody physics needed for ITN, but you'll learn tons of
crazy things about the technicalities of space travel without realizing you've
learned them.

That includes the realization that there are indeed ways to cross the universe
using zero fuel. Also, you can burn towards a body to delay your arrival
there. It's crazy, but the math checks out.

~~~
32faction
it's all about your delta V. Delta V is basically the currency of space
travel.

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kartikkumar
I worked on this for my Masters thesis [1], studying the nature of low-energy
transfers [2] to the Moon, using the Weak Stability Boundary (WSB) [3]. Turns
out that the WSB and invariant manifolds described by Dynamical Systems Theory
(DST) [4] in the N-Body problem [5] are intricately linked. There was a bit of
a "war" going on when I was doing my thesis between the group at Caltech that
pioneered the (DST) methods, including people like Wang-Sang Koon, Shane Ross,
and Martin Lo, and Ed Belbruno, who formalized the concept of the WSB, because
there was still an evolving view of chaos in the gravitational 3-body problem.

It's a fascinating area of research that has implications for planetary
formation and has been exploited for space mission design. I'm currently
finalizing a paper that draws attention to the fact that low-energy orbits are
likely an important component of a peculiar ring system around Uranus: the mu-
ring/Mab system. Mab [6] shows strange orbital behavior that's likely
intimately tied to interactions with a belt of moonlets that display
horseshoe-like motion.

There's been some interesting research using the concept of orbital cyclers
and the coincidental proximity of Lagrange points in the Earth-Sun and Mars-
Sun systems. The only issue is that these orbits are necessarily very slow, so
they're not suitable for time-critical applications.

[1]
[http://repository.tudelft.nl/view/ir/uuid%3Ae8c99c80-a25d-4c...](http://repository.tudelft.nl/view/ir/uuid%3Ae8c99c80-a25d-4c7b-b8c2-f0f27c69fc32)

[2] [http://en.wikipedia.org/wiki/Low-
energy_transfer](http://en.wikipedia.org/wiki/Low-energy_transfer)

[3] [https://video.ias.edu/gds/belbruno](https://video.ias.edu/gds/belbruno)

[4]
[http://en.wikipedia.org/wiki/Dynamical_systems_theory](http://en.wikipedia.org/wiki/Dynamical_systems_theory)

[5]
[http://www.cds.caltech.edu/~koon/book/KoLoMaRo_DMissionBk.pd...](http://www.cds.caltech.edu/~koon/book/KoLoMaRo_DMissionBk.pdf)

[6]
[http://en.wikipedia.org/wiki/Mab_(moon)](http://en.wikipedia.org/wiki/Mab_\(moon\))

edit: just edited the links so they display on separate links.

~~~
mturmon
This work occupies an odd niche that piques my curiosity. I knew Ed Belbruno
slightly, and I'm cordial with Martin Lo. I'm trying to figure out how to say
this neutrally: the people like the ones I mentioned do not seem to have much
influence on the actual world of mission trajectory design. They're off on
their own.

Yet when I read the summaries of their methods, they don't discuss any
drawbacks besides the large transit times required. I'm left thinking that
either there is something political/historical going on that I don't know
about, or that the large times in fact totally kill the idea, rendering the
theory moot. Come to think of it, another option is that reducing delta-v to
zero is not worth going to so much trouble over (from a mission design
viewpoint).

On the other hand, the scientific angle (that these trajectories can allow
long-range mass circulation in the solar system) is very interesting and
something I was not aware of. Thanks for your interesting reply.

~~~
kartikkumar
It has occupied an odd niche in my curiosity ever since my first grad-level
class in astrodynamics/celestial mechanics :) . It's a fascinating problem
because it's inherently tied to many physical phenomena in our Solar System
and in other planetary systems, yet the statement of the 3-body problem is
pretty much as simple as it gets.

And you're right, they are kinda off on their own, but that's mostly because
this niche is an interesting meeting of the minds for mathematicians,
astrophysicists and engineers. That means you get some "interesting"
discussions to say the least.

As for the "political/historical" aspects, have a chat with Ed sometime if you
get the chance. He's got an interesting backstory that's catalogued in his
books too. Basically, to some extent, there was a lot of resistance at JPL to
his ideas, and so he had to bypass them, which is what brought about the Hiten
mission.

As an objective researcher, I can say that low-energy transfers for mission
design are interesting, but really for a specific subset of scenarios. Time-
critical missions, e.g. manned spaceflight, falls outside the scope of low-
energy transfers. Additionally, it turns out that the maths is a bit finicky
and that low-energy transfers only lead to a significant Delta-V reduction if
you launch in the right "geometry". In the case of Earth-Moon transfers for
instance, it turns out that if you neglect the Sun, you can't actually reach
the Moon "for free", as is advertised by WSB transfers, because of KAM tori
around the Moon. The Sun is crucial, as it's perturbing effect ensures that
phase-space opens up and you can actually reach the Moon. This comes with a
BIG caveat though that the geometry of the Earth-Moon-Sun has to meet certain
requirements. Hence, launch windows are limited.

As you point out, I do think the greater interest in studying low-energy
orbits, WSB and invariant manifolds in high-order gravitational problems is in
using the theory to explain natural processes. This fits within the larger
context of dealing with resonances in the 3-body problem. Murray and Dermott
have an excellent textbook on the fundamental theory behind all of this [1].
It's a must-buy if you're interested in delving into this further.

All of this has tremendous potential to elucidate exoplanet systems. There are
plenty of systems discovered by Kepler, CoRoT etc. that require a deep,
fundamental understanding of the processes that shape(d) them.

[1] [http://www.amazon.com/Solar-System-Dynamics-Carl-
Murray/dp/0...](http://www.amazon.com/Solar-System-Dynamics-Carl-
Murray/dp/0521575974)

~~~
mturmon
Thanks for the detailed and thoughtful reply.

------
electromagnetic
One great visualization of the principles here is
this:[http://sajri.astronomy.cz/asteroidgroups/hildatroj.gif](http://sajri.astronomy.cz/asteroidgroups/hildatroj.gif)

The pink/purple ones bounce between the L3, L4 and L5 points. The green ones
are in orbits of the L4 or L5 points and have an orbital period that should be
the same as jupiter itself.

------
maxander
The sci-fi technology that will truly enable mankind to efficiently explore
space isn't fusion rockets, or space elevators, or any other grandiose
machinery- its cryogenic sleep, or hibernation, or otherwise the ability to
just conk out for long periods of time and wake up at your destination.
Getting places in space is relatively easy, if you don't mind taking next-to-
forever to get there. (In addition to the ITN, consider the bi-elliptic
transfer: [http://en.wikipedia.org/wiki/Bi-
elliptic_transfer](http://en.wikipedia.org/wiki/Bi-elliptic_transfer) .)

~~~
VLM
"if you don't mind taking next-to-forever to get there."

If you want to be a (permanent-ish) space colonist that isn't wasted time.

As a counter example its continually proposed as a training / colonization
system on short hops. If you want to test your 12-year rated life support
system for a run to Jupiter, rather than sitting in low earth orbit the whole
12 years (boring!), why not try a leisurely ITN flight to the moon, in fact
why not leave the ship/station in moon orbit when you're done, as a moon
orbiting station?

If something goes horribly wrong, keep some large delta-v chemically fueled
beast of a lifeboat around, worst case scenario on your "12 year mission to
the moon" you'll never be more than many hours (well, OK, days worst case)
from a splashdown on the earth via a very non-ITN path. Also your giant
station/ship could be resupplied from the earth, at a considerably higher than
ITN cost. Given your station/ship can move orbits very cheaply, your
"lifeboat" could be extremely high performance.

The ITN would be a great way to help a manned (anywhere) expedition by slowly
sending enormous quantities of cargo and base equipment long ahead of time.

A group that plans for the future would probably already be planning care
packages and emergency packages to likely destinations. If it takes people 20
years to get to eventually really quickly fly to mars, then its no big deal if
emergency O2 and first aid kits or whatever launched today take perhaps 15
years in transit.

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saalweachter
I'm still a fan of the Aldrin Cycler. Put an utterly massive, nearly self-
sufficient space station into an orbit that touches both Mars and Earth (or
presumably other pairs). Give it all the radiation shielding and spinning
gravity you'd need to survive indefinitely in space. Now your per trip cost is
just getting the astronauts to and from the surfaces of each planet.

------
ColinWright
A complete article giving an exposition of the ideas and techniques submitted
here:

[https://news.ycombinator.com/item?id=8579626](https://news.ycombinator.com/item?id=8579626)

------
gfrench27
space travel is not worth dying for

~~~
FromTheMountain
The people who risk their lives for it clearly disagree. They can dedicate
their lives to whatever cause they want.

ad astra per aspera

