These are the most frequently used words in this book.
actually
advances
against
already
another
argument
asteroid
atoms
billion
cannot
catastrophe
century
chance
changes
come
computers
countries
course
different
during
earth
energy
enough
entire
even
event
ever
experiments
far
few
first
future
groups
happen
however
human
humanity
hundred
impact
indeed
instance
kind
know
large
least
less
level
life
long
machines
mars
may
might
million
natural
need
new
now
nuclear
number
people
perhaps
physicists
planets
population
power
present
rather
research
risk
science
scientific
scientists
seem
should
small
solar
space
species
stars
still
sun
system
take
techniques
technology
ten
themselves
theory
though
thousand
threat
time
two
universe
war
weapons
whether
world
years