Cold Welding: Can Asteroids Fuse?

[lilythmagebor]

"Cold welding" question

My husband posed an interesting question to me that I can't rightly answer, not being any kind of expert, when I told him about the fact that metals fuse if they come into contact with one another in space.

If an asteroid collides with another asteroid, will the two fuse? If that asteroid happens to pass through the tail of a comet, will it then be incapable of fusing? (I read that if you take a metal object from Earth to space that it will not fuse with another metal because the oxidation layer remains in tact.)

I know these are all very rare circumstances. I just thought the question was interesting, and was surprised to find no information (with a somewhat cursory Google search) about any asteroids having ever fused with another.

 

[ideasrule]

My husband posed an interesting question to me that I can't rightly answer, not being any kind of expert, when I told him about the fact that metals fuse if they come into contact with one another in space.

They actually don't, at least not under normal conditions. A cold weld--which is what this phenomenon is called--occurs when two components that fit perfectly come into contact. If you take two ridiculously flat pieces of metal, clean both so that they're absolutely free of any oxidation, and bring them together in vacuum, a cold weld will occur. Two arbitrary metal objects won't weld because, even if they appear smooth to the naked eye, they'll likely be very different on the molecular scale.

 

[Chalnoth]

Basically, for a fusion to occur, their gravity has to hold them together. But gravity is extraordinarily weak, so the pieces have to approach extremely extremely slowly, or one of the objects has to be big enough to keep most of the ejected material from escaping (which is pretty darned big). Even then, however, they won't have fused: they'll just be a collection of rocks moving together through space. It is only if the combined gravity is large enough that the pressure and temperature allow them to fuse that they actually do. And that requires quite a lot of mass.

 

[Chronos]

Even trace amounts of surface contaminants inhibit cold fusion.

 

[Chalnoth]

Even trace amounts of surface contaminants inhibit cold fusion.

Sorry, I wasn't trying to say anything opposed to ideasrule's post (if you were responding to me). I was just trying to describe what actually happens. I suppose I misused the word "fusion" in the first sentence. But I believe the rest is accurate.

 

[GeeDoubleYa]

Cold fusion can be a problem when working with gage blocks (also known as Johansson blocks or "Jo-blocks" for short). These are precise blocks of varying thickness made from tool or stainless steel (usually) that can be stacked together to create an exact reference for dimensional measurement. Their surfaces are polished smooth and flat. Simply pressing and twisting two or more blocks together by hand (a process called 'wringing') is sufficient to make a stable stack that resists gravitational forces (a properly wrung stack can be held sideways at one end!). No vacuum or magnetism is required.

Left wrung together for a period of time (weeks?), the blocks will permanently fuse together, rendering them useless for their intended purpose. The mechanism by which this happens is poorly understood.

Fusing two asteroids together would likely require an inelastic collision and a lot of heat: a very, very low probability event.

 

[lilythmagebor]

Thanks for answering my questions, all. Seems like this is just another one of those science facts that, while a fact, gets exaggerated. The article made it seem like any metal would fuse in a vacuum if it didn't have that layer of oxidation.

(I first read the fact here: http://goo.gl/UV1TA , and decided to look into it further here: http://goo.gl/JcH3u .)