If the
nearest star system, Alpha Centauri, does harbor rocky planets similar to Earth
as new findings suggest, there exist a host of ways to get us there, in theory.
Sending a
person to Alpha Centauri within a human lifetime wouldn't be easy. Alpha
Centauri is 4.37 light-years away — more than 25.6 trillion miles, or more than
276,000 times the distance from the Earth to the sun.
"Interstellar
travel is extremely hard," said science fiction author and NASA physicist Geoffrey
Landis.
But the
lure has never been stronger. Scientists last week said the Alpha Centauri
system has the ingredients for an Earth-like
planet, and they think they can spot it.
Conventional
rockets are nowhere near efficient enough. At a maximum speed of about 17,600 mph (about 28,300 kph), it would take the space
shuttle, for example, about 165,000 years to
reach Alpha Centauri. In any case, "the problem with conventional rockets
is that if you're carrying fuel, you need fuel just to carry all the fuel you
bring with you, and it just gets exponentially worse," Landis said.
But antimatter
engines might work. These drives rely on the extraordinary amount of energy
released when antimatter
and matter annihilate each other. The problem, however, is creating enough and
storing any antimatter for the trip.
"All
of the current methods of manufacturing antimatter require enormous particle
accelerators and produce antimatter in very small quantities," Landis
said. "And to store antimatter, if you need a ton of magnets for one gram
of antimatter, the entire idea of a lightweight way to store immense amounts of
energy is no longer lightweight."
Although
one could in principle freeze anti-hydrogen and thus bypass the need for
magnets, "if even the tiniest amount ever leaked out and touched the walls
surrounding it, you'd produce a lot of heat, which in turn would heat up the
frozen anti-hydrogen, and the whole thing catastrophically goes away,"
Landis said.
Antimatter
could nevertheless perhaps find use in interstellar spaceships as a way to help
trigger nuclear reactions. "That's something that hasn't been ruled out
yet, and a little antimatter could help go a long way," Landis said.
Suck it
up
Instead of
rockets that carry all their fuel with them, spaceships might scoop it up along
the way. One design proposed by physicist Robert Bussard (who died last year)
would employ giant electromagnetic fields to suck in hydrogen to fuel a nuclear
rocket.
Unfortunately,
this "ramscoop" or Bussard ramjet, probably could not work. "The
interstellar medium is not as dense as Bob Bussard thought it would be,"
Landis said. "And so far all attempts to design some kind of scoop had the
unfortunate effect of producing more drag than you get back thrust, working
kind of like parachutes."
Moreover,
"we don't really have any notion of how to use the pure hydrogen we find
in interstellar space as fusion fuel," Landis added. All of the proposals
for fusion in the lab use deuterium-tritium (two isotopes of hydrogen) or
deuterium with helium-3 (an isotope of helium) — "we don't have any
suggestions for pure hydrogen in a fusion reaction," he said. "It was
a clever idea, but the devil's in the details."
Sail
away
Light sails
might be another way to go — giant,
thin, lightweight reflective sails that rely on the slight push provided by
light beams. "The point is to not carry the energy you need for propulsion
with you, but to get it transmitted to you," explained Jordin Kare, a
Seattle-based technical consultant on advanced space systems.
Instead of
relying just on the enormous amount of light given off by the sun, light sails
to Alpha Centauri could also ride laser beams
that earthlings would fire carefully at those ships to give an extra boost,
especially when sails were too far away to catch much light from our sun.
The idea
with a laser sail is that the sky is the limit in regards to speed. You just
keep accelerating, albeit gradually.
The problem
with interstellar travel with laser sails is that a lot of light needs to be
used for a long time to get fast enough to get to Alpha Centauri within a human
lifetime. This means very powerful and extraordinarily large lasers are needed in
order to focus on sails that get farther and farther away, Kare explained.
An idea
similar to light sails that Landis helped come up with involved firing a
particle beam at a spaceship that would ride that energy. "The problem
with laser beams is that they disperse over distance, so we thought about
particle beams," Landis explained. The beam would have to have a neutral
electrical charge so as not to disperse itself over time. "It would be a
feasible idea," he said.
Bombs
away!
Another
idea for space travel would involve riding explosions through space. Such
"pulsed propulsion" would hurl bombs behind a ship, which is shielded
with a giant plate. The explosions would push against the plate, propelling the
ship. Project
Orion suggested using nuclear bombs, while other proposals have since
proposed smaller explosives.
"Nuclear
pulsed propulsion works best for really big systems. If you want to send a
colony of 1,000 people to space, an Orion-type ship is definitely the way to do
it," Kare said. "If you want to send a one-ton probe, I would say a
laser system is the way to go."
A variant
on both the laser sail and pulsed propulsion idea that Kare came up with was
the "sail
beam." Essentially, a laser would propel lots of miniature sails like
bullets at a distant ship. The impact of these sails would propel the
spacecraft.
"The
idea is to get a craft up to about a tenth of the speed of light that
way," Kare said. "It could get you to Alpha Centauri in 60 to 70
years."
So far no
one has created technology that is widely agreed upon as capable of caring for
or preserving humans across the lifetimes it might take to get to Alpha
Centauri. It might easily take more than one lifetime to reach the star system
— one antimatter engine design would take 200 years to
send humans there. If that proves so, mission designers might have to take sex
and family into account so offspring of the original crew would be around
at the end of the trip, unless someone successfully invents a technique for
placing people in suspended
animation.
Then again,
warp
drives and similar far-out ideas might one day zip us faster than light to
Alpha Centauri and beyond. "We don't know all the physics there is to know
yet, and something we don't know yet might give us tremendous
capabilities," Landis said.
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