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Since the hull must be a protective wall that
keeps the ship's crew compartment at room temperature against the
cold vacuum outside, hundreds of degrees below zero, one's first
feeling is that it must be made from an extremely strong material.
The obvious candidate might seem to be aluminum,
of which aircraft hulls are made, is about a third of the weight
of steel and is also used in the International Space Station. It
is also non‑flammable.
But aluminium hulls would have one fatal weakness.
Beyond the Earth's protective radiation they would offer no
defence against cosmic rays, those deadly sub‑microscopic
projectiles which arrive from all directions at close to the speed
of light and can do severe damage to the human body.
The danger mainly exists when an astronaut is in
flight from one planet to another. On the surface of the Moon or
Mars one can find shelter from the deadly rays. But when in space
they come from all directions. "Going to Mars with an
aluminium spaceship is undoable," says Nasser Barghoury
Project Scientist for NASA's Space Radiation Shielding Project.
Many solutions to this problem have been proposed.
Encasing the ship's hull in a lead shield would be effective, but
it would make the ship so intolerably massive that accelerating it
would require an unacceptable amount of energy.
Coating the hull with water would also work, since
water stops most cosmic rays, but the water would still make the
ship far too massive.
The solution now being considered by NASA
scientists is an astonishing one: polyethylene, of which plastic
rubbish bags are made. We may travel to Mars in plastic
spaceships.
Now Barghoury and his team have invented a
material based on polyethylene called RXF1 which they claim has
three times the tensile strength of aluminium, and yet is 2.6
times lighter. It also affords more than 50 percent more
protection from solar storms. (The details of how RXFl is made are
being kept secret because a patent on the material is pending. But
the private space industry should be reassured: my understanding
of US patent law and procedures is that such secrecy seldom lasts
more than a year.)
RXF1 is apparently strong enough to deflect
micrometeorites, which although travelling more slowly than cosmic
rays, are much larger and do just as much damage. "ince it's
a fabric," says NASA scientist Raj Kaul, "It can be
draped around moulds and shaped into specific spacecraft
standards."
It is hard to imagine that humble rubbish bags
might have such a destiny, but daft as this sounds, they may still
be the way to the stars.
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