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In 1991,  NASA’s Galileo probe was two  years into its journey to Jupiter when it hit a snag. It was supposed to unveil an umbrella-like  antenna to transmit data back to Earth, but it couldn’t do that… because some parts that were supposed  to unfold had literally welded together.

That’s right, the process that normally  happens only very intentionally in extreme heat here on Earth had happened in the  cold vacuum of space… automatically. Which was bad news for the probe. But the good news is, by understanding  how welding can happen in the cold, we can save other spacecraft from the same fate, and potentially also put the  process to good use here on Earth.

Now, when you think of welding, you  probably think of red-hot metal and sparks flying, because here on Earth,  that’s how welding is usually done. Basically, you put two pieces of  metal together and heat the ends until they melt and mix together. Then when they cool, they solidify as one piece.

But there’s another kind of welding  that doesn’t require heat at all. It’s called cold welding, and  it’s something that happens because of the unique  chemical composition of metal. Like any other material, metals are  made of lots of atoms bonded together.

But the bonds in metals are a little unusual. See, in many materials, like plastic  or salt, an atomic bond forms between just two atoms. For instance, you get salt when a  sodium atom gives its outer electron to a chlorine atom, creating  a bond between the two.

But metallic bonds are a  completely different story. In metals, atoms’ outermost  electrons wander in between atoms. So instead of belonging to any  particular atom, they belong to a communal sea of electrons.

As a result, you don’t just get two  atoms bonded together, you get an entire collection of atoms that are  connected by this sea of electrons. So, if you can manage to put two  blocks of metal close enough together, those free-floating electrons can  spread from one surface to the other, without breaking any atomic bonds. And at that point, if the atoms on the  two surfaces are sharing electrons, who’s to say where one block  begins and the other ends?

The two blocks of metal  essentially become one big block. But this doesn’t typically happen  under regular conditions on Earth. Like, you’ll probably never  open your silverware drawer and find all your spoons have welded together.

Thank goodness...or rather,  thank the Earth’s atmosphere. First of all, you can’t push two  metal surfaces close enough together for welding when there is air in the way. Second, oxygen reacts very easily with  many metals to form new compounds, which quickly cover pure metallic  surfaces and act as a boundary between two pieces of metal.

And finally, any contaminants, like grease, that get on metal surfaces can  stop them from combining too. So on Earth, if you want to weld some  metals together, you typically have to heat them up and weld them together  the old-fashioned way. But outside of Earth’s atmosphere, as  long as you have a clean and pure surface without air in the way, metals can  undergo cold welding pretty easily.

And that’s exactly what  happened with the Galileo probe. The doomed antenna had a pin in a socket, and both the pin and socket were made of metal. Originally, they were separated by a  lubricant, but the pin had been grinding against the socket while the parts traveled  around the country in various trucks, and it wore the lubricant away.

That left the two pieces of metal in  direct contact by the time of the launch. Then, after launch, the pieces continued  rubbing against each other, which eroded down the surfaces until it  was pure metal on metal. And now that they were in the vacuum of space, those two pieces of metal  just welded right together.

Once this main antenna got stuck, the Galileo team was forced to  use a much weaker backup antenna. This antenna received signals thousands  of times slower, but fortunately, it saved the mission, this time. Spacecraft designers have to take lots of  precautions to prevent problems like this from happening again.

They can reduce the risk of cold welding  by minimizing how often metal parts touch, or coating them with some  material that acts as a barrier. In some cases, it also helps to  use different types of metals whose atoms are arranged differently,  because their electrons don’t move as easily from one surface to another. While cold welding is an  enormous hassle for spacecraft, here on Earth, cold welding  could actually be pretty useful.

It’s not easy to get around the problems  the atmosphere creates for cold welding, but in 2010, scientists welded  nanometer-length gold wires together under vacuum conditions, just by touching  the ends together for a few seconds. And that was pretty exciting  because it avoided the need to subject the wires to heat, which could  easily damage them at that small scale. So it’s possible that cold  welding could help us make the next generation of  nano-sized electronic devices.

In the meantime, scientists are still  looking for new ways to use cold welding. And someday, it may not just be a  spacecraft designer’s nightmare, but a valuable engineering technique. Thanks for watching this episode of SciShow Space!

And thanks to our patrons for making  it possible for us to dive deep into science and make more content like this. And here at SciShow Space we  have our own independent Patreon so we can continue exploring the marvels  of the universe outside of our planet. And if you want to learn more about  becoming a part of that community, you can go to patreon.com/scishowspace. [♪ OUTRO].