Updated and bumped: I will be discussing this story on the the John Batchelor Show tonight, February 17, Friday, 12:50 am (Eastern), and then re-aired on Sunday, February 19, 12:50 am (Eastern).
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Someday, humans will be traveling far from Earth in large interplanetary spaceships not very different than the International Space Station (ISS). Isolated and dependent on these ships for survival, these travelers will have no choice but to know how to maintain and repair their vessels whenever something on them should break.

And things will break. Entropy rules, and with time all things deteriorate and fail.

Each failure, however, is also a precious opportunity to learn something about the environment of space. Why did an item break? What caused it to fail? Can we do something to prevent the failure in the future? Finding answers to these questions will make it possible to build better and more reliable interplanetary spaceships.

ISS is presently our only testbed for studying these kinds of engineering questions. And in 2007, a spectacular failure, combined with an epic spacewalk, gave engineers at the Johnson Space Center a marvelous opportunity to study these very issues.

In November 2007, astronauts were preparing the station to receive its last of four solar panel arrays. To install the last array, however, required moving an array from the one end of the station’s truss where it had been deployed for seven years to its permanent home on truss’s port side. To do this, the array was retracted, folding up accordion-like as designed. Astronauts then moved it to its new home, where it was then to be reopened.

As the panel was unfolding, however, the array began to tear in two places. For some reason a guide wire had snagged on a grommet, pulling the paneling apart and causing two tears one and three feet in diameter respectively. The deployment was halted, and an emergency spacewalk was then improvised in which astronaut Scott Parazynski — hanging on the end of the shuttle’s inspection boom that was attached to the station’s robot arm and thus out there farther than any astronaut had ever hung — cut the guide wire and then used cufflink-like attachments and straps to sew the tears back together.

After this spacewalk the panel was unfolded as planned, so that the station’s construction could go forward.

While everyone enthused about Parazynski’s surgical repair that saved the station, engineers had a more mundane but equally important question. What caused the guide wire to come free and snag the array?

When Parazynski cut the wire, he stored it so that it could be returned to Earth. Engineers at the Johnson Space Center in Houston then inspected the wire’s frayed ends using scanning electron microscopes. You can read their paper here [pdf].

The results were quite unexpected: The guide wire had broken because it had been hit by a tiny piece of space junk, melting and splitting the wire but damaging nothing else.

This conclusion was supported by two pieces of evidence. First, the frayed ends of three wires in the guide wire bundle showed clear evidence of melting. As the engineers noted in their report,

Micrometeoroid and orbital debris (MMOD) particles typically impact at high speed and release a large amount of energy, resulting in the displacement of target material with a mass 10 to 100 times the projectile mass due to melting and plastic flow local to the impact site. The presence of melt is a clear indication that the damage to these three wires was caused by MMOD impact. Other wires in the bundle appear to have been broken by mechanical action.

A likely scenario that explains the observed damage to the guide wire is that [an] impact damaged and broke a few of the wires, which allowed the guide wire to snag in a … grommet during deployment. Subsequently, as the process of deployment continued with a snagged guide wire, additional wires in the guide wire were sheared as they were pulled against the grommet.

The next question: What was it that hit the wire? Spectroscopy of the melt points showed no evidence of micrometeroid materials. Instead, the engineers found evidence of bismuth metal as well as alloys of gold-copper-sulfur, gold-silver-copper, lanthanum-cerium, antimony-sulfur, and tungsten-sulfur. None of these materials are found in asteroids, and none in the wire itself. Instead, these kinds of materials would be expected from a piece of engineered material once part of a satellite and now space junk.

The odds of such a collision seemed unlikely if impossible. The wire is tiny. Orbital debris is generally made of larger pieces that would have caused greater damage. Instead, what happened was a very small piece of space junk hit the wire — and only the wire — at high speed, breaking it.

What can be done to prevent this from happening again? Well, in Earth orbit the best solution is to reduce the amount of space junk. In this sense, the proposal by a Swiss company to launch an orbital “street sweeper” makes sense.

For interplanetary travel the problem is less worrisome, though small micrometeorites do exist. Though engineering work continues [pdf] to develop better shielding to protect the skin of spaceships, you can’t cover solar panels with shielding.

A more practical solution is to design the spacecraft so that the occupants can reasonably access all of its parts, and provide them the tools to make repairs, as Parazynski did during his space walk. In this sense, having several different robot arms, spacesuits, repair tools, spare parts, and training to do this sort of work seems essential for any spacefarers traveling beyond Earth orbit to another planet.

Like the sailors of old, space travelers will need to able to repair and even rebuild their spaceships, wherever they are. Any interplanetary spaceship design has got to factor this reality into its design.