

Special spots in empty space might hold the key to interplanetary exploration - dnetesn
http://nautil.us/issue/3/in-transit/point-and-shoot

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ColinWright
This is strongly related to the Interplanetary Transport Network, so I've
submitted a link to that in case people want to discuss the two topics
separately:

[https://news.ycombinator.com/item?id=8577596](https://news.ycombinator.com/item?id=8577596)

The basic idea is that multiple moving gravity wells combine in strange ways
creating lumps and bumps, and we can move around with much less effort and
much less fuel than expected. The "Gravity Assist"[0] idea is just the
beginning.

An interesting thing is that we can perform gravity assists around Lagrange
points, effectively bouncing off (or around) things that aren't there.

[0]
[http://en.wikipedia.org/wiki/Gravity_assist](http://en.wikipedia.org/wiki/Gravity_assist)

~~~
Tloewald
Given that the goal of a gravity assist is not to hit the source of the
gravity well, how is using a lagrange point going to be more useful than using
a planet?

The usefulness of the technique for moving a satellite around earth seems
clear (for unmanned probes). The idea of using this for manned flight seems
ridiculous (it's not like Arthur C. Clarke didn't know about Lagrange points).
The idea of sticking a space station at Earth/Moon L1 or whatever is
ridiculous. You need to decellerate into the point since it's (at best)
metastable, so there's a huge practical downside. Firing your lump of ice or
aluminum at a large gravity well with decent precision seems much more
practical.

~~~
tzs
The main point of ITN is that it is very low energy, at a cost of being slow.
You wouldn't use it for manned missions. You'd use it for unmanned missions,
including cargo transport to support manned missions.

For instance, suppose you have established a mining and manufacturing
operation on the Moon, and want to start a long term project to establish a
permanent Mars base. You can start out by putting together a bunch of supplies
that Mars base will need, launching that supply package into the Earth's
orbit, moving it to an appropriate Earth/Sun Lagrange point, and then nudging
it into an unstable orbit that intercepts an unstable orbit around a Mars/Sun
Lagrange point. When the package reaches that point, another nudge transfers
it to that second unstable orbit, where it eventually reaches a Mars/Sun
Lagrange point, from which it can move to Mars.

The only step in that trip that uses a lot of energy is the launch to get away
from the Moon. All the rest of the maneuvers (moving to/from Lagrange points,
entering and exiting the unstable orbits, and switching between the unstable
orbits) take very little energy.

It would take a while to get there...I don't recall for sure, but I think it
is something around 3 years.

You make the first package large, with enough supplies to support a Mars base
for 6 months or more. Then every 3 months you launch another package.

When the first package arrives successfully at Mars, and the other packages in
the pipeline are in good shape, you launch your manned mission to actually
establish the base.

Note that the manned mission only needs to carry enough food, air, and water
for the trip itself. Same goes for fuel--it only needs enough fuel to get to
Mars. Fuel and supplies for a return trip can be included in the supply
packages coming in on the ITN.

(You can do this starting from the Earth instead of the Moon, of course, but
it takes a lot less energy to get from the Moon to an Earth/Sun Lagrange point
than from the Earth, so it is probably worth getting mining and manufacturing
set up on the Moon first and then making that the starting point for
establishing bases elsewhere).

~~~
Tloewald
Don't you need to decelerate into the Lagrange point? If so, why not just use
conventional gravity assist to get it to Mars faster?

------
HarryHirsch
Voyages on low-energy transfer orbits last decades, but you can send really
heavy probes that would be impossible to fly on traditional trajectories.

This makes it into a political problem. Under the US system the budget for
space exploration is allocated every year. All other governments allocate
funds for the whole mission at the point when it is approved.

No government will commit funds for a flagship mission when they know that the
US may pull out halfway through the project. It has happened before with
Galileo, and no one will run the risk of it happening again, especially in the
present political climate.

~~~
jessriedel
> No government will commit funds for a flagship mission when they know that
> the US may pull out halfway through the project. It has happened before with
> Galileo, and no one will run the risk of it happening again, especially in
> the present political climate

Could you explain what you mean here? Are you talking about international
collaborative missions? Galileo was a successful NASA spacecraft to Jupiter
that, as far as I can tell, was a fully American financed mission.

Or maybe you're thinking of the Europa Jupiter System Mission, which was a
NASA-ESA mission to Jupiter recently crippled by NASA budget cuts? That seems
like a poor example, since only about $100 million or less had been spent on a
mission that would have cost $6 billion (of which nearly $5 billion would have
been American dollars).

~~~
HarryHirsch
Part of the scientific payload was European. The European collaborators were
not impressed with the delays on NASA's part. The combined Galileo/Ulysses
disaster was what cured ESA of signing Memoranda of Understanding.

