For millennia, humans have fantasized of being propelled at
breakneck speed through huge expanses of space by light breezes.
As bizarre as it may sound, pushing reflecting sails
steadily towards the speed of light with nothing more than the punch of photons
may be our sole realistic chance of reaching another star inside a single human
lifetime.
It's also a lot simpler to say than to do. Light particles are rapid, yet they
don't push very forcefully. If you design a sail light enough to feel the
inertia of radiation, the continual onslaught of photons may harm the sail's
material accidently.
Making a sail that can endure the dangers that face
spacecraft on such a lengthy voyage would necessitate some ingenious solutions,
such as those described in two recent research published in the journal Nano
Letters.
The technologies, developed as part of the Breakthrough Starshot program by
engineers from the University of Pennsylvania and the University of California
at Los Angeles in the United States, aim to find ways to achieve the balance
between endurance and mass necessary for an interstellar spaceship.
"We're only now working out how to make sure those
designs survive the voyage," says Igor Bargatin, a mechanical engineer at
the University of Pennsylvania.
Waves of radiation, like air particles slamming onto a sail made of cloth,
exchange momentum with anything they collide with. Photons, or light molecules,
unlike air molecules, have no resting mass, therefore whatever force they exert
will be minor.
The light bouncing off your body when you sunbathe, for example, is about
comparable to one thousandth of a gram in force.
There are a few techniques to increase this pressure in
order to move an object. One option is to construct a larger sail that can
capture more light. Another option is to increase the intensity of the light
striking it, such as by focusing a large number of lasers at it.
However, there are some issues here. Bigger sails mean more
mass. Trimming the mass would make it simpler to push, but at the risk of
making the sail less strong and putting it at risk of rupture.
More light, on the other hand, raises certain concerns. The wavelengths of
radiation striking the sail, for example, will appear to move gently towards
the red end of the rainbow as the sail speeds, limiting the types of material
that won't absorb too much infrared and overheat.
Previous research has focused on finding the correct
material to make the sails robust, lightweight, and capable of bearing the heat
generated by gigawatts of stretched out laser light. However, no one has truly
looked into the trade-off between keeping absorption low while maintaining high
momentum over a particular distance in order to speed the craft.
Engineers propose building a sail composed of two layers made of the materials
molybdenum disulfide and silicon nitride, which can both be formed into sheets
and have the optical qualities needed to balance low light absorption and
emission as it expands out.
A second research looked at the challenge from a structural
standpoint, to handle the strain that a laser array would exert on the photon
pressure.
Curving the sail, like in parachutes, might increase
stability, but as the study's authors point out, little research has been done
into the stresses that light pressure would exert on this type of structure.
The researchers established that a sufficient curvature is
the way to go by modeling a circular, spherically-curved sail on a size of
square meters that could pull a cargo of a few grams.
Similarly to the previous study, the researchers
experimented with different acceleration times in order to discover the best
balance of mechanical and thermal loads as well as trip duration.
Breakthrough Starshot hopes to build a spacecraft that is
light enough to travel at roughly 20% the speed of light, which would allow it
to travel the 4.2 light years to Proxima Centauri in just a few decades.
It's vital to keep in mind that this technology may never be able to carry
people. That will continue to be science fiction material for a long time.
But it's possible that during our lifetimes, we'll get a
close look at a planetary system that isn't our own.
