Energy and Human Ambitions on a Finite Planet, 2021a

4 Space Colonization 65
4. Find an Earth globe <strong>and</strong> an object about one-fourth its size to i By doing this, you can get maybe 2% of
represent the moon, then place at the appropriate distance apart. the enjoyment of a trip to the moon for less
Report on how far this is. Take a personalized/unique picture to than one-billionth the cost: a real bargain!
Can you make out Florida? Japan?
document, <strong>and</strong> take some time appreciating how big Earth would
look from the moon.
5. Highway 6563 in New Mexico has signs along a roughly 30 km
stretch of road corresponding to the solar system scale from the
Sun to Neptune. On this scale, how large would Earth, Sun, <strong>and</strong>
Jupiter be, in diameter? Express in convenient units appropriate
to the scale.
6. Using the setup in Problem 5, how fast would you have to travel on
the road to match the speed of light, for which it takes 500 seconds
to go from Earth to the sun? Express in familiar/convenient units.
7. Note that the size of the moon in Figure 4.1 is about the same size
as the sun in Figure 4.2. Explain how this is related to the fact that
they appear to be about the same size in our sky. Hint: imagine
putting your eye at the earth location in each figure <strong>and</strong> looking
at the other body.
i This is why eclipses are special on Earth.
8. Use Table 4.1 to accumulate (multiplicatively combine) scale factors
<strong>and</strong> ask: which is a bigger ratio: the distance to the nearest star
compared to the diameter of Earth, or the distance to the edge of
the universe compared to that to the nearest star? Compared to
the large numbers we are dealing with, is one much smaller or
much bigger than the other, or are they roughly the same?
9. It may be tempting to compare Earth to a life-sustaining oasis in
the desert—maybe spanning 100 m. But this is a pretty misleading
view. One way to demonstrate this is to consider that in a real
desert, the next oasis might be a perilous 100 km journey away.
Using the ratio of distance to size (diameter of planet or oasis),
how close would another Earth have to be (<strong>and</strong> compare your
answer to other solar system scales) to hold the analogy? 29 How
long would it take to drive this distance? Do we have another oasis
or potential oasis within this distance?
10. Another way to cast Problem 9 is to imagine that the actual distance
between Earth <strong>and</strong> a comparable oasis is more like the distance
between stars. 30 In this case, how far would the next oasis be in
the desert if we again compare the 100 m scale of the oasis to the
diameter of the Earth? 31 How long would it take to drive between
oases at freeway speeds (cast in the most informative/intuitive
units)?
29: In other words, oasis size is to distance
between oases as Earth’s diameter is to how
far?
30: . . . since Earth is the only livable “oasis”
in our own solar system
31: In other words, Earth diameter is to
interstellar distances as a 100 m oasis is to
how far?
i The insight you develop will not depend
on exact choices for distance <strong>and</strong> speed, as
long as they are reasonable.
11. On the eighth bullet of Box 4.3 (the one that asks you to pause),
imagine someone from the year 2020 traveling back 50 years <strong>and</strong>
explaining that we have not been to the moon since 1972, <strong>and</strong> that
© 2021 T. W. Murphy, Jr.; Creative Commons Attribution-NonCommercial 4.0 International Lic.;
Freely available at: https://escholarship.org/uc/energy_ambitions.