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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!
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