-Z- (Z@Gundam.Com)
Tue, 30 May 2000 20:15:23 -0700


At 09:47 5/30/2000, you wrote:
>>If the cylinder falls from west-to-east, it's following the orbit of the
>>Moon and thus the "path of least resistance" -- you don't have to bust a
>>gut overcoming both the cylinder's inertia and the orbital velocity that it
>>already has.
>
> So.. you are saying that the main reason why a colony crash will
>tend to be west-to-east is because the colonies were moving west to east to
>begin with, and hence will take less energy to get them going? Make sense.
>It's sometimes hard to keep the perspective that everything up there is
>moving...

The Moon and three of the five Lagrange points (L3, L4 and L5) are all
moving west-to-east at the same speed. The colonies would be parked in
elliptical "halo" orbits around the Lagrange points, with a major radius of
about 145,000 km (90,000 miles) and a period of 29.5 days. These halo
orbits will be in the plane of the Moon's orbit, which is about five
degrees off the ecliptic plane, and they could conceivably be "retrograde"
and transit east-to-west around the Lagrange point as that point transits
west-to-east around the Earth. Indeed, one of the graphics that I cadged
from SETTLEMENTS IN SPACE for my Gundam: High Frontier page shows just that
arrangement for L5:

http://www.DyarStraights.com/msgundam/frontier.html#Lagrange

But when you transfer from the halo orbit to an Earth orbit -- GEO or LEO
or re-entry state vector -- you'll undoubtedly use a Hohmann minimum-energy
doubly-tangent "lazy-S" transfer orbit. As the name implies, the
trajectory will start out west-to-east until it reaches a point midway
between the Moon and the Earth, then arc over into an east-to-west
trajectory that will carry it down until it matches speed with the desired
orbit.

Think of it as cosmic surfing -- you catch the wave and shoot the curl, in
this case sliding down the gravity well by following the path of least
resistance or, in ballistic terminology, the minimum-energy gradient.

> BTW, I've been wondering... In 0083, we see the hijacking of the
>cylinder. Cima's Marines/ engineers spun the cylinders together, then the
>two rebounded off each other. The one that went Earthwards should be
>spinning faster than usual and moving at a tangent -- I assume that the
>calculations are such that the tangent sends the cylinder into a suitable
>trajectory towards the moon.

This is anime physics. The concept works because the artists make it work,
but that's not what would happen in real life. The idea that shooting off
one of the three mirrors would "unbalance" the cylinder and cause it to
slew "sideways" might hold up if the cylinder was supported at both ends,
like a rotor in a motor, but it's a free-falling gyroscope, so the change
in mass distribution would have no effect on its trajectory. It would only
cause an oscillation along the long axis and that would probably not exceed
sixty degrees. In any case, the cylinders wouldn't move toward one
another, but rather continue along their way with their noses and tails
gyrating from side to side.

But to answer your question as to what happens in the anime, Island Blade
bounces off Island Ease and they go in opposite directions, with Island
Blade veering off into open space as Island Ease veers toward the Moon. Of
course, any collision of sufficient force to give Island Ease sufficient
orbital velocity to catch up with the Moon, which leads it in orbit by some
400,000 km (250,000 miles), would probably smash them both to smithereens.

> But the spin? IIRC, the later (brief) shots of the cylinder doesn't
>seem to show the cylinder spinning. Did Delaz/ Cima stop the cylinder's spin
>to prevent damage, or was it left out because it was too much to animate?

Half the time they show the colonies spinning at two RPM, four time their
actual half RPM (one revolution every two minutes) and the other half they
show them just hanging there, with no rotation whatsoever. Hell, many of
the colony shots are "freeze frame" stills, with the "camera" doing all of
the movement.

Again, consider the inertia these things must have. Stopping them from
rotating would probably take hours if not days and expend tons of
propellent -- and to what end?

> I seemed to have asked about the other cylinder sometime back too,
>but couldn't remember where it would have ended up... =)

They say off into space, I say that, at best, it ends up orbiting the Sun,
somewhere slightly above or below the Earth's orbit, and most likely falls
back into orbit around the Earth, somewhere above or below the Moon's orbit.

-Z-

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