Satellites That Bleed: The Future of Self-Healing Spacecraft
Hollow fibres just 40 microns in diameter tread the new material. When damage occurs, the fibres break releasing liquids that seep into the cracks and harden, repairing the damage.
Credit: ESA.

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."