Does it take two forces to deform an object?

[tomR]

I have a question - if 2 forces are required to deform an object then why does a piece of space junk impacting an object in space cause deformation? What is the second force?

 

[PeroK]

Who says it takes two (external) forces to deform an object? Just look at a slow-motion video of bat hitting a ball.

 

[Nik_2213]

Action and Reaction ??

 

[Frabjous]

Ignoring the subtleties, inertia becomes dominant for high rate processes.

 

[jrmichler]

What is the second force?

Because $F = ma$.
The first force is $F$, the force applied by the impacting object.
The second force is $ma$, or the mass of the object being hit multiplied by the acceleration of that object.

inertia becomes dominant for high rate processes.

Typical space junk is moving roughly ten times the velocity of a high velocity rifle bullet. That's a high rate process. The whole impact happens so fast that only the material in the immediate vicinity of the impact is affected. Space junk impact is finished punching a hole (or crater) before the material away from the impact is affected by the impact force.