Energy and Human Ambitions on a Finite Planet, 2021a

4 Space Colonization 57
80,000 times the typical car’s gasoline tank capacity. It would take
a typical car 2,000 years to spend this much fuel. Do you think the
astronauts argued about who should pay for the gas?
Let’s relax the scale slightly, making the sun a chickpea (garbanzo bean).
Earth is now the diameter of a human hair (easy to lose), <strong>and</strong> one meter
from the sun. The moon is essentially invisible <strong>and</strong> a freckle’s-width
away from the earth. The next star is now 300 km away (a 3-hour drive
at freeway speed), while the Milky Way center is 1.5 million kilometers
away. Oops. This is more than four times the actual Earth–Moon distance.
We busted our scale again without even getting out of the galaxy.
So we reset <strong>and</strong> make the sun a grain of s<strong>and</strong>. Now the earth is 10 cm
away <strong>and</strong> the next star is 30 km. 11 Think about space this way: the
swarm of stars within a galaxy are like grains of s<strong>and</strong> tens of kilometers
apart. On this scale, solar systems are bedroom-sized, composed of a
brightly growing grain of s<strong>and</strong> in the middle <strong>and</strong> a few specks of dust
(planets) sprinkled about the room. 12 It gets even emptier in the vast
tracts between the stars. The Milky Way extent on this scale is still much
larger than the actual Earth, comparable to the size of the lunar orbit.
In fairness, fuel requirements don’t simply
scale with distance for space travel, unlike
travel in a car. Still, just getting away from
Earth requires a hefty fuel load.
11: ...along day’s walk
12: Even a solar system, which is a sort
of local oasis within the galaxy, is mostly
empty space.
Box 4.2: Cosmic Scales
It is not necessary to harp further on the vastness of space, but having
come this far some students may be interested in completing the
visualization journey.
As mind-bogglingly large as the solar system is, not to mention
that it itself is dwarfed by interstellar distances, which in turn are
minuscule compared to the scale of the galaxy, how can we possibly
appreciate the largest scales in the universe? Let’s start by making
galaxies manageable. If galaxies are like coins (say a U.S. dime at
approximately 1 cm diameter), they are typically separated by meterlike
scales. The edge of the visible universe (see Sec. D.1; p. 392) would
be only 1.5 km away. Finally, the picture is easy to visualize: coins
as galaxies separated by something like arm’s length <strong>and</strong> extending
over an area like the center of a moderately-sized town. We can
even imagine the frothy, filamentary arrangement of these galaxies,
containing house-sized (5–50 m) voids empty of coins (galaxies). See
Figure 4.3 for a visual explanation.
But penetrating the nature of the individual galaxies (coins, in the
previous example scale) is extremely daunting: they are mostly empty
space, <strong>and</strong> by the time we reduce the galaxy to a manageable scale
(say 10 km, so that we can picture the whole thing as city-sized),
individual stars are a few tenths of a meter apart <strong>and</strong> only about 50
atoms across (roughly 10 nm). Cells <strong>and</strong> bacteria are about 100–1,000
times larger than this. So it’s nearly impossible to conceive of the
Figure 4.3: Galaxies are actually distributed
in a frothy foam-like pattern crudely lining
the edges of vast bubbles (voids; appearing
as dark regions in the image). This structure
forms as a natural consequence of gravity
as galaxies pull on each other <strong>and</strong> coalesce
into groups, leaving emptiness between.
This graphic shows the bubble edges <strong>and</strong>
filaments where galaxies collect. The larger
galaxies are bright dots in this view—almost
like cities along a 3-dimensional web of
highways through the vast emptiness. From
the Millennium Simulation [25].
© 2021 T. W. Murphy, Jr.; Creative Commons Attribution-NonCommercial 4.0 International Lic.;
Freely available at: https://escholarship.org/uc/energy_ambitions.