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MATTER
TECHNOLOGY
Roadmap to Alpha Centauri
Pick your favorite travel mode—big, small, light, dark, or twisted
VER SINCE THE DAWN of the space age, a
quixotic subculture of physicists, engineers,
E
for starships, propelled by the imperative for humans
to crawl out of our Earthly cradle. For most of that
time, they focused on the physics. Can we really fly to
the stars? Many initially didn’t think so, but now we
know it’s possible. Today, the question is: Will we?
Truth is, we already are flying to the stars, with-
out really meaning to. The twin Voyager space probes
launched in 1977 have endured long past their original
goal of touring the outer planets and have reached
the boundaries of the sun’s realm. Voyager 1 1s 124
astronomical units (AU) away from the sun—that
is, 124 times farther out than Earth—and clocking
3.6 AU per year. Whether it has already exited the
solar system depends on your definition of “solar sys-
and science-fiction writers have devoted their
lunch hours and weekends to drawing up plans
tem,” but it is certainly way beyond the planets. Its
instruments have witnessed the energetic particles
and magnetic fields of the sun give way to those of
interstellar space—finding, among other things, what
Ralph McNutt, a Voyager team member and planetary
scientist, describes as “weird plasma structures” beg-
ging to be explored. The mysteries encountered by
the Voyagers compel scientists to embark on follow-
up missions that venture even deeper into the cosmic
woods—out to 200 AU and beyond. But what kind of
spacecraft can get us there?
Going Small: Ion Drives
NASA’s Dawn probe to the asteroid belt has demon-
strated one leading propulsion system: the ion drive.
An ion drive is like a gun that fires atoms rather than
bullets; the ship moves forward on the recoil. The sys-
tem includes a tank of propellant, typically xenon, and
a power source, such as solar panels or plutonium bat-
teries. The engine first strips propellant atoms of their
outermost electrons, giving them a positive electric
charge. Then, on the principle that opposites attract,
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