| < Precious gravity |
| If
you want gravity on a space ship, you may do as Cpt. James T. Kirk and
press the respective button. But in the real world, this feat is much
harder to accomplish. In a famous treatise, the physicist Gerard K.
O'Neill suggested a rotating cylinder measuring (diameter x length) 4 x
20 and another with 5 x 20 miles. The latter would supply the
inhabitants on its inner surface with gravity similar to earth. All you
need is to turn this torus a little more than 28 times an hour around
its longitudinal axis. As icing on the cake, Prof. O'Neill added a
modest 'outer agricultural ring' with another 20 miles diameter. |
| The
resulting living space might accomodate several million
people. Probably the designer imagined to provide room to the
population of New York, for him (who was from there) an obvious
example. I wonder if such a dimension would serve our purpose well, if
it comes to saving mankind in a planetary emergency situation. It would
rather be a matter of survival as species after a catastophic asteroid
impact on
earth than to send colonists into space. After having lost technical
support from a devastated earth, the space survivors must have a chance
to maintain their living resources themselves. |
| Therefore,
the proposal should be scaled down to a more human size. Motivation and
organization in a population of a few thousand will be better and
easier to accomplish; and a few thousand would still allow for a
healthy blend of genes. It may be possible to survive a few years in a
crowd of microgravity balloon stations as presented in the preceding chapters.
But to persist for generations, we probably would need something
similar to gravity. For this purpose, a smaller cylinder with (diameter
x length) 1 000 x 400 m should do. If rotated once per
min, the crew will experience nice 56% of the earthern gravity. It
might be enough to prevent negative health consequences of 'microgravity'. |
| Scientific progress may allow in
the near future to erect in space light-weight cylinders (specific mass
< 2 g/cm³). Gravity, be it natural or 'artificial', always presses
against any obstacle, be it the earth below our feet or the hull of a
rotating cylinder, with a force proportional to the mass of the object
in case. While we usually build floors from steel and concrete, for the
open space carbon fiber or even graphene should be the material of
choice. The wall of a rotating cylinder has not only to support the
objects pressing against it; it also has to carry itself and to
provide radiation protection, heat isolation, and perfect tightness. |
| A
first step could be the construction of a hexagonal lattice made of
carbon fiber (or a better material yet to be developed). It has three
layers of compartments for the insertion of panels with 1 m diameter.
They are 10 cm thick and spaced 15 cm apart, creating two
equal-sized cavities between them. This saves weight and provides good
thermal insulation. The result is a huge number of chambers,
conceivable as a large-scale rigid foam. Total thickness 0.6 m,
average density 1 g/cm³ (ca. 45% void space). While the panels ensure a
tight seal, the lattice provides stability. This could be accomplished by a
tensile cable that clamps seven elements together, running over
bearings in the hollow rods (see link to pptx file after this text) . |
| Outside
the earth's magnetic
field, that diverts charged particles of the 'solar wind', we would be
exposed to harmful radiation. For that reason, habitats proposed e. g.
on mars (without such a field) require thick compact shielding of at
least one meter. In our case, solar irradiation exclusively enters by
the front. Instead of putting unnecessary mass into the whole
hull of the cylinder, we should focus radiation protection measures on
this front. Inhabitants will dwell in the most peripheral districts of
the rotating torus. To protect them from irradiation, a non-rotating
ring (50 m strong) might be installed in the front, in some distance (ca. 10 m) from
the rotating mass. If its mass density was high, a thickness of 1 m
could be sufficient. |
| My
rough estimate results in a total mass of ca. 3 million metric tons (O'Neill: ca. 3 000 mmt). The object
may be set up on the orbit of the earth around the sun at one of the 5 Lagrangian
points, the axis of rotation pointed to the sun. The colony
has a circular window (100 m across) at the front, transparent to as much energy as possible (not only to visible light).
Supportive aggregates of photosynthetic ballons orbit around in a chain
of 365 (17 400 m above the center of the colony), with
circulation period 365.26 d. Each balloon aggregate measures about 100
m
across and dims the entry of sunlight during 'night'. The mass is
estimated to three thousand metric tons (ISS: 450). The gravitational
force between each balloon aggregate and the cylinder amounts to 2.1
milli-Newton (for comparison: a mass of 1 kg on earth is pressed
downwards with a force of 9.81 Newton). |
| The
balloon aggregates follow each other on their circular orbit around the
colony. The free distance between them is about twice to their
extension.
This creates a circadian rhythm with 16-h-days of stable brightness and
slowly coming and going 8-h-nights. Sunlight conducting fibres will
provide additional illumination where needed. Balloon aggregates
may be visited from
time to time, but in general will be controlled by robots. For
transport, small spaceships are docked at the dark end of the habitat.
High-energy radiation will help micro-organisms in the most peripheral
balloons to cleave water into oxygen and hydrogen as fuel for jet
engines. For an ample stockpile of water: see overnext paragraph. |
| The space refugees will need two
sorts of spaceship. The small ones transfer humans and material between
stations in orbit. I imagine at least one thousand balloon aggregates
alltogether. A few hundred of 'small spaceships' may be sufficient to
serve their transportation needs. They may provide room for up to 20
passengers, but usually will transport only 2 persons plus cargo. The
second type of ship is foreseen for the prospective return to earth.
This is going to be a one-way-trip for a few thousand. Maybe 10 such
ships, each for a few hundred passengers, will be parked at L2 (see
overnext paragraph). |
| Blocks
of water-ice in gas-tight
envelopes may be stored in the extreme cold exterior. In the absence of
regular supply from earth, we must have local stores. Luckily,
each balloon aggregate orbiting the sun provides a shady corner in its
neighborhood. A balloon station with 3 000 metric tons circling the sun
in 365.26 d will be orbited itself by any object in the same time, if
the object was positioned at a particular distance (independent of its
mass; my estimate goes from 1 190 to 1 716 m; just try). Given both
orbiting in the same plane, this opens the possibility to place a
bundle of ice blocks where the sun never shines:
in the shade of the station. They hopefully will stay there: Since only
weak forces are involved, the
position will have to be readjusted at times. Unlike the mass of the
ice, the mass of the station will be of influence and will fluctuate
depending on visitors, plant growth and harvest. Alternatively, blocks
of ice may also be stored at the dark center (instead of a balloon). |
| A large stock of ice blocks can also be placed behind the rear (dark) end
of the cylinder (not taking part in the rotation, but contributing to
its mass). Much more water-ice could be provisioned at Lagrange point 2 (L2). Since 2022, the James Webb
Space Telescope (6.2 metric tons) is operative in this district. L2 lies on a straight
line through sun and earth, one and a half million km further than earth (1% of
the sun-earth distance). Due to the enormous size of the sun (109 x the
earth diameter), the core of the earth shadow just doesn't reach L2
(annular eclipse). Nevertheless, this might be a good place for
hoarding enough water-ice to supply several generations of a few
thousand people using water sparingly. Also spare parts for the
photosynthetic stations and the cylinder may be deposited there. |
| For
security reasons, the gigantic inner hall of the cylinder should be
tightly separated into 2 compartments, one towards the sun,
the other in the rear part. The reason is obvious: in case of damage to the outer containment, the crew may find shelter in the
intact half. The
partition will have a window of same
size as the front, equipped with tiles transducing the energy of incoming radiation into visible light. This will
diffusely be scattered at both sides in all
directions (maybe such a material is still to be developed). A collection of spare parts should be in stock, including
also such for the containment. The outer hull will be composed from
standard elements of a size compatible with transport in conventional
space craft and assembled at the final location. Exchanging damaged
elements must be possible with local means. Also the windows will not
be put in place as one single piece, but rather composed from hexagonal
tiles. |
| Although
O'Neill-type cylinders are sometimes shown with streets and little
houses arranged in villages, at our reduced scale
discreet housing should not be necessary. The environmental conditions
in the halls
should allow some camping-style of living up to the taste of the crew.
Convection can be expected to be driven by warm air rising to the
middle at the front. This should lead to mild winds on ground level
from back to front. Buildings to separate different types of activity
from each other (agriculture, sanitary units, teaching, manufacturing,
scientific research, recreation, telecommunication, and more) will be
arranged close to the walls (for static reasons). Ideally, production
facilities for all modules constituting the balloon aggregates and the
cylinder should be at hand. At the
rear end, no large window is foreseen, but it would be nice to have a
look into the starred sky (even though its turning fast...). For that
purpose, some of the penultimate elements (for the
inhabitants subjectively at eye level) should be transparent. |
| For
the development of complex
technical solutions, panic is a bad advisor. Such a project must start
without any imminent threat at the horizon, driven solely by scientific
curiosity. A downsized cylinder as suggested here would be in the range
of budgetary feasibility for research organizations as NASA or ESA.
Even if never any deadly impactor will show up (let's hope so), it
would be fine to have such a comfortable place in space. We would start
with several balloon aggregates to explore conditions for
self-sustained life in space. The first cylinder will be erected for
plants only (no trees - too heavy). We need time to explore the optimum
setup. E. g., it might be explored whether a gigantic central rod
magnet rotating with the cylinder would divert the solar wind (maybe radiation alone would induce enough charge difference between front and back - Holmberg et al 2023, Michelagnoli et al 2024). Or
whether erection could be started in the shade of earth at L2 and the
construction site be transferred to a more sunny site after finishing
the radiation shield. Or how much IR transmission should be allowed by
the front and middle walls to arrive at an agreeable ambient
temperature. |
| When Prof. O'Neill proposed his cylinder
half a century ago, he considered two counterrotating cylinders to
prevent gyroscopic effects. In our times with fast and powerful
computers, this precaution may no longer be necessary. It is true
that a spinning top often exhibits pronounced precession around the
axis of rotation. On the other hand, it strives for its erect stance because it is spinning. Jet engines positioned at strategic spots and
controlled by fast computer processing should prevent any deviation
from
a steady and completely erect orientation. Similar programs will control
small jet engines in the balloon aggregates to keep them on their
regular path around the colony. |
| Each
of the 365 balloon aggregates causes one of 365 nights of a year on the
cylinder. It
may be essential for the mental well-being of the crew to see their
earthern clockwork at work. In an emergency situation, this will
nourish the hope of the survivors for a swift
return to their original home, if not for themselves, then at least for
their children or grandchildren. |
| Construction of the cylinder (powerpoint file) |
| Holmberg et al
(2023) Surface charging of the Jupiter icy moons explorer (JUICE)
spacecraft in the solar wind at 1 AU. J Geophys Res Space Physics 129:
e2023JA032137 Spacecraft Plasma Interaction Software was used to simulate the surface charging... for a typical solar wind environment the spacecraft will charge to
around 6 V, with the different dielectric parts of the spacecraft
charging to potentials from around −36 to 8 V.
|
| Michelagnoli et
al (2024) Surface charging analysis of Ariel spacecraft in L2-relevant
space plasma environment and GEO early transfer orbit. Aerospace 11: 988 Ariel (Atmospheric Remote-sensing Infrared Exoplanet Large-survey) is
the ESA Cosmic Vision M4 mission ... launch is scheduled by 2029 ... Surface
charging analyses conducted using the SPIS tool of the European SPINE
community. ... Under average solar wind conditions,the Ariel solar arrays charge positively up to about 14 V, due to the photoelectric effect for surfaces directly exposed to sunlight.
|
| Photoelectric effect. ... Hallwachs (1888) connected a zinc plate to an electroscope. He allowed ultraviolet light to fall on a freshly cleaned zinc plate and observed that the zinc plate became uncharged if initially negatively charged, positively charged if initially uncharged, and more positively charged if initially positively charged. From these observations he concluded that some negatively charged particles were emitted by the zinc plate when exposed to ultraviolet light. |
| MB (4/25) |
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