Energy and Human Ambitions on a Finite Planet, 2021a

4 Space Colonization 56
Body Symbol Approx. Radius Distance (AU) Alt. Distance
Earth ⊕ R ⊕ ≈ 6, 400 km —
Moon ☾
1
4
R ⊕
1
400 60R ⊕ ≈ 240R☾
Sun ⊙ 100R ⊕ 1 240R ⊙
1
Mars
♂
2 R ⊕ 0.4–2.7
Jupiter ♃ 10R ⊕ ≈
10 1 ⊙ 4–6
Neptune 4R ⊕ ∼30
Proxima Centauri — 0.15R ⊙ 270,000 4.2 light years
Table 4.2: Symbols, relative sizes, <strong>and</strong> distances
in the solar system <strong>and</strong> to the nearest
star. An AU is an Astronomical Unit, which
is the average Earth–Sun distance of about
150 million kilometers. The fact that both
the sun <strong>and</strong> moon are 240 of their radii
away from Earth is why they appear to be a
similar size on the sky, leading to “just so”
eclipses.
7: For this, picture a grain of s<strong>and</strong> sitting
on the bridge of your nose representing the
earth, <strong>and</strong> a speck of dust in front of one
eye as the moon.
be anywhere from 4.5 meters (15 feet) to 30 meters (100 feet) away. Reflect
for a second that humans have never ventured farther from Earth than
the moon, at 3 cm (just over an inch) in this scale. 7 Mars is outl<strong>and</strong>ishly
farther. Neptune is about four-tenths of a kilometer away (on campus
Glance over to where Mars would
at this scale), <strong>and</strong> the next star is over 3,000 km (roughly San Diego to
be if the earth is a grain of s<strong>and</strong> on
Atlanta). So we’ve already busted our easy intuitive reckoning <strong>and</strong> we
your nose.
haven’t even gotten past the first star. Furthermore, this was starting
with the earth as a tiny grain of s<strong>and</strong>. We’ve only ever traveled twofinger-widths
away from Earth on this scale, 8 <strong>and</strong> the next star is like 8: The last time we went this far was 1972.
going on a giant trip across the country. For apples-to-apples, compare
how long it takes to walk a distance of two-finger-widths (3 cm) to the
time it would take to walk across the U.S. The former feat of traveling to
the moon was super-hard; the latter is comparatively impossible.
Box 4.1: When Will We Get There?
It took 12 years for Voyager 2 to get to Neptune, which is “in our back
yard.” The only spacecraft to date traveling fast enough to leave the
solar system are the two Voyagers, the two Pioneers, <strong>and</strong> the New
Horizons probe [23]. The farthest <strong>and</strong> fastest of this set is Voyager 1
at about 150 times the Earth–Sun distance after 43 years. The closest
star is about 2,000 times farther. At its present speed of 17 km/s, it
would reach the distance to the nearest star 9 in another 75,000 years.
The fastest spacecraft on record as yet is the Parker Solar Probe,
which got up to a screaming 68.6 km/s, but only because it was
plunging (falling) around the sun. Because it was so close to the sun,
even this amount of speed was not enough to allow it climb out of
the sun’s gravitational grip <strong>and</strong> escape, as the five aforementioned
probes managed to do. Even if Voyager 1 ended up with 70 km/s
left over after breaking free of the solar system, 10 it would still take
20,000 years to reach the distance to the nearest star. Note that human
lifetimes are about 200 times shorter.
Pushing a human-habitable spacecraft up to high speed is immensely
harder than accelerating these scrappy little probes, so the challenges
are varied <strong>and</strong> extreme. For reference, the Apollo missions to the
very nearby Moon carried almost 3,000 tons of fuel [24], or about
[23]: (2020), List of artificial objects leaving
the Solar System
9: It does not happen to be aimed toward
the nearest star, however.
10: It only had 17 km/s left.
[24]: (2020), Saturn V
© 2021 T. W. Murphy, Jr.; Creative Commons Attribution-NonCommercial 4.0 International Lic.;
Freely available at: https://escholarship.org/uc/energy_ambitions.