Space can
be an unforgiving environment for elderly satellites, with temperature
extremes, tiny rocks and other hazards threatening to breach spacecraft hulls.
But future
spacecraft may be able to extend their mission lifetimes by borrowing a human
trait to heal minor nicks and scratches.
"The
analogy is the human body," said Ian Bond, of England's University of Bristol, in an interview with SPACE.com. "Think about cutting yourself. There, capillary action will draw
blood out to block the cut."
Bond and
his colleague developed a similar system, but replacing blood with resin and
veins with tiny glass tubes, to fill in cracks or small holes in satellite
"skin" as part of a European Space Agency (ESA) program to study technology for
self-healing spacecraft. By closing small cracks or micrometeorite punctures
themselves, satellites could stave off more serious structural problems the
initial damage may lead to, researchers said.
"We were
surprised at how well it worked," said Bond, a senior lecturer in of the
year-long study. "We're trying to develop an autonomous system...so you don't
have to initiate the healing process for a spacecraft."
Cracking
down on cracks
In their
experiment, Bond and his colleagues packed a gooey, resinous adhesive and a
hardening agent inside tiny, hollow tubes embedded in a composite material
commonly used in spacecraft hardware.
"We have
hollow fibers in structure which are then breached, and the resin bleeds out
into the damage," Bond said. "It's quite viscous."
The adhesive
resin flows through a 40-micron wide space inside the glass fibers, Bond said,
adding that other fibers filled with hardening agent are intermixed among their
resin-full counterparts to cure and close a crack or hole. One micron is
one-millionth of a meter. For comparison, a human hair is about 100 microns
thick.
The method
successfully sealed breaches in material across a wide range of temperatures,
from -148 degrees to 212 degrees Fahrenheit (-100 degrees to 100 degrees
Celsius), in a vacuum chamber, Bond said. It also sealed cracks within about 90
minutes, about the time it takes a spacecraft to complete one full orbit around
the Earth, he added.
"It's
really useful for cracks of small holes, that sort of thing," Bond said. "If
there's like a big hole there, we wouldn't be able to repair it."
Fibers
or microcapsules
Lining a
spacecraft's skin with a wound-sealing resin is not the only way to ward off
stress maintain hull integrity.
Researchers
at the University of Illinois Urbana-Champaign are seeding composite materials
with tiny capsules of healing agent and hardening catalyst to test their
self-repairing properties.
"There are
certainly applications in microelectronics, as well as with materials that
suffer from thermal or mechanical fatigue," said Scott White, who heads the
Autonomic Healing Research project at the university. "One of the problems with
composites is that, if there's internal damage, it's hard for us to see it."
Adhesive-laden
microcapsules or fibers could fill in microcracks or gaps between layers of a
composite material that separate over time due to age or delamination, he
added.
White's lab
can fabricate microcapsules ranging in diameters from 100 microns down to the
sub-micron level.
Vascular
mimicry
Both
microcapsules and hollow fibers suffer from a limited supply of healing agent.
Once the microcapsules or resin fibers near a damage site are exhausted, the
healing process stops regardless of whether it's complete, the researchers
said.
"What we
have now, it's a one-shot system," Bond said.
Both teams
are working to develop transport systems that would shift healing agents
through a material. The next step for the fiber-based method is the development
of a pumping setup akin to the human body's vascular system that could
circulate healing agent throughout a spacecraft to ensure a constant supply,
Bond said.
Meanwhile,
White and his colleagues - a group that includes Bond - are studying the
potential of building channels directly into a material, where microcapsules
could flow from a central reservoir.
"We're on
the first rung of the ladder," Bond said, adding that the system could one day
evolve beyond unmanned satellites to help astronauts safeguard their vessels
against small hull breaches. "For manned spacecraft...it could mitigate things
like [extra] spacewalks."
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