Why Metals Spontaneously Fuse Together In Space

Shall I put this down? Yeah, of course. Ahhh, alright, we're about to do some welding.

Go on. Here on Earth, if you want to get two metals to fuse, you have to heat them up past their melting point. But in space, it's a different story, as we found out the hard way on the third of June, nineteen sixty-five. Astronaut Ed White conducted the first ever American spacewalk aboard Gemini Four. Tethered via a long gold-plated hose, he propelled himself outside the capsule by shooting pressurized oxygen from a handheld gun. He enjoyed it so much that flight control literally had to order him back into the spacecraft.

"The flight director said get back in."

"Back in? Back in."

"Roger, we've been trying to talk to you for a while here."

"Returning to the vehicle," he commented. "It was the saddest moment of his life." Little did he know what would happen next inside the spacecraft. The crew was faced with a problem: the hatch would not close. For about an hour, there was no word from Gemini IV.

"He said he was busy and he'd rather not talk to us."

Flying over Africa, they actually went out of range of communication with the ground.

"Gemini IV, Houston Capcom, give me your status."

But after a lot of wrenching and brute force, they got it closed and everything was fine. The original mission called for another depressurization where they would open up the hatch and throw away a bunch of bulky gear, things they needed only for the spacewalk. But the pilot, McDivitt, radioed to say that under no circumstances were they going to open the hatch again. Flight thought it was because the astronauts wanted to keep the EVA suit and gas guns as souvenirs.

The astronauts eventually returned to Earth safely with the extra gear literally stuffed into the footwells of their cramped craft.

So what caused the hatch to stick? Well, NASA engineers at the time identified that the failure was due to something called cold welding. You see, in space, if two metals come into contact, they can actually fuse together without the need for heat or melting of either piece. And the reason for this is due to the fundamental structure of metals, which is a little bit like this candy bar in that they contain a lattice of positively charged ions, like the peanuts, embedded in a sea of freely moving negative electrons, like the caramel.

Now here on Earth, the surface layers of a metal react with the oxygen in the atmosphere to create a protective oxide layer. This actually prevents two pieces of metal from joining together. But in space, this oxide layer can be worn away, like if two pieces of metal aren't sliding over each other, as in hinges. Then the bare pieces of metal, with a little bit of force or impact, can actually fuse together. So the electrons from one piece of metal can flow into the other.

In this business, as Richard Feynman put it, the atoms have no way of knowing that they're in different pieces. This obviously has huge implications for the construction and maintenance of spacecraft like the International Space Station.

So why don't we hear about more disasters like this happening in space all the time? I mean, why isn't the ISS already a big welded mess? Well, the truth is cold welding is not as big of a problem as scientists originally thought it was. I mean, experiments on the ground in vacuum chambers and in space have shown that perfectly clean metal surfaces, when pressed together in the absence of atmosphere, will weld together. But in practice, the metals that are used in spacecraft are never that clean; I mean, they have those oxide layers on them, not to mention other contaminants, dirt, and grease. It would take a long time for all of those things to be eliminated so that bare metal touches in space. So the Gemini IV hatch problem was not actually caused by cold welding; it was just a sticky door.

But that's not to say that cold welding never happens. For example, in 1991, the Galileo spacecraft was on its way to Jupiter. Now, when scientists tried to unfurl its high-gain antenna, which was meant to open like an umbrella, it got stuck. Three of the 18 ribs of the antenna refused to open, and this was due in part to the fact that they were cold welded; the pins were stuck in place and nothing the scientists could do would open it. In the end, they had to scramble to find a way to use the low-gain antenna to send back data from Jupiter, something that it was never meant to do.

A 2009 report by the European Space Agency recommended three main ways to reduce the risks of cold welding. Number one: where possible, use plastics or ceramics to avoid sliding metal on metal contacts. Second, if you have to use metal and metal, try to use two different metals, maybe two different metal alloys, because that reduces the risks of those metals melding together. And third, use durable coatings that resist wear to avoid bare metal on metal contact.

Cold welding is not always a bad thing. Scientists have actually found it incredibly useful in fabricating nanotechnology. You know, traditional welding techniques used on that sort of tiny scale can often go wrong because you're trying to pinpoint heat onto tiny nanowires, and you can end up pretty quickly with a melted mess on your hands. So in conditions very similar to the vacuum of outer space, scientists have shown that single crystal gold nanowires fused to each other in seconds, using no heat—just the cold welding technique—and those welds are perfect.

The crystalline structure is identical to the rest of the nanowire, as are the mechanical and electrical properties. So although cold welding is a bit of a potential problem in space, it's actually incredibly useful here on Earth to manufacture nanotechnology.

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