If considering one gram of steel and one gram of foam, which one is the biggest threat and would cause most or least damage to the orbital structure it hits?

Radii of these debris is the only thing that will differ, yet depending on impact velocity, damage should differ too.

How does relation between relative impact velocity and damage caused by impact evolve for the gram of steel and the gram of foam, since there is a wide range of possible impact velocities?

(for instance at 15 km/s (prograde vs retrograde LEO frontal impact) the foam should punch a bigger hole in a solar panel or a hull.

At 0.1 km/s the steel may produce a small bump on a hull while the foam may do nothing).

  • $\begingroup$ Interesting question. I presumeit will depend on the material of the thing it hits too and its size, e.g. is that several meters across/several cm thick. That said my suspicion is that space agency led tests won't have done as detailed a characterisation as the question needs yet, i.e. it could be ahead of the research. Well worth asking here though, someone might have some ideas. $\endgroup$
    – Puffin
    Commented Sep 5, 2022 at 20:28
  • $\begingroup$ I expect this to vary widely with the type of material being hit. Compare a piece of glass developing cracks, shattering into pieces and some kind of metal that just gets punched through. $\endgroup$
    – asdfex
    Commented Sep 6, 2022 at 8:46
  • $\begingroup$ Kinetic energy is 0.5mv^2. The energy with which a object hits another object is dependent only on the mass & velocity of the object. If the foam & steel have the same velocity that hit with the same energy, the difference will be the area of contact & that the area of damage. $\endgroup$
    – Fred
    Commented Sep 6, 2022 at 13:58

1 Answer 1


Undoubtedly the steel because it is smaller. Compare how much damage you can do to a piece of wood by hitting it with a mallet compared to putting a nail against it and hitting the nail.

That said, foam is more dangerous than steel in a psychological (as opposed to a physical) sense. We all know that being hit with a 1kg bit of foam is harmless while being hit with a 1kg lump of iron is dangerous. It is easy to carry this intuition over into space - “no need to worry about foam, it doesn’t matter”. But the intuition we have comes not from the nature of the foam, but from air resistance. Foam and feathers float down, lumps of metal crash. In a vacuum there is no difference between the two - and not thinking about foam because “foam can’t matter” is deadly.

  • 2
    $\begingroup$ As noted in the question, I'd suspect a 1g foam ball to do more than a 1g ball bearing to something like a solar panel in terms of power loss, since both punch through but the foam takes more with it out the other side. $\endgroup$ Commented Sep 6, 2022 at 7:55
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    $\begingroup$ At hypersonic speeds the density of the material only influences over what area the impact force is distributed. It doesen't really matter anymore if it's hard, soft, brittle, elastic or whatever as soon as you move above the sonic speed inside the involved materials... $\endgroup$
    – TrySCE2AUX
    Commented Sep 6, 2022 at 9:41

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