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I read Songs of a Distant Earth and in the book ice-water is used as the shield on a ship that travels close to the speed of light.

Realistically, what material might actually work to protect a ship traveling at relativistic speeds from interstellar debris?

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    $\begingroup$ Is there a worldbuilding problem here? (a) Questions about 3rd party or commercial worlds are off-topic. (b) We allow real-world questions, but only in the context of solving a worldbuilding problem. (c) What's wrong with ice/water? Any substance will work so long as there's enough of it (your question is missing the mandatory requirements/conditions, see the help center). Remember that economics are as important as engineering. Tungsten might make a good shield - but it might be cost prohibitive or weight prohibitive. (*continued*) $\endgroup$
    – JBH
    Jun 9, 2022 at 4:38
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    $\begingroup$ Finally, please remember from the help center, "To prevent your question from being flagged and possibly removed, avoid asking subjective questions where … every answer is equally valid." Considering (as I said) every material can work (which makes (c) a reason to close your question), can you edit your question to improve it? What are our limitations? $\endgroup$
    – JBH
    Jun 9, 2022 at 4:40
  • $\begingroup$ What sort of limitations are you dealing with regarding technology of your society? It would be helpful to get some context there too. $\endgroup$ Jun 9, 2022 at 18:55

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Preemptive strike

The biggest threat is from objects in its path. These might be coming so fast that there is no time to see the thing and act. The ship periodically fires an explosive shell ahead of it. The shell has a proximity sensor. If it gets close to something it sends a signal back, and explodes. The light from the explosion could be the signal.

The ship has other guns. If a shell ahead of the ship explodes, more shells are fired along the path of the ship. The object of these shells is to produce an explosion that moves the obstacle out of the way.

Objects coming in from the side are moving less fast. The ship watches for these with active sensors: big lights. Visible light works great in space.. If something is seen on a collison course, the ship opens fire on it to divert it.

As regards something in front of the ship that could withstand impacts: something cheap and dense. A nickel/iron asteroid you find will suffice. Maybe make it pointed, so particles will be deflected and not dump the entirety of their kinetic energy into your shield. The equivalent of aerodynamic, but as regards space particles not aer. If the shield starts getting thin, get another one.


I was thinking about the metal asteroid sharpened to a point. This would be a good look. The long point would be sharp and shiny with etched grooves along its length carved by deflected particles.

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    $\begingroup$ And if the ship is using an Orion Drive, the EMP from the drive will magnetize the iron of the asteroid, improving its ability to deflect some kinds of materials! $\endgroup$
    – JBH
    Jun 9, 2022 at 4:43
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Ice / water can't really be topped for this purpose, radiation, impact shielding, augmented fuel source, many ancillary benefits.

You could improve the ablative characteristics of the ice. Something like pykrete comes to mind. Finding sawdust in space may be an issue but a synthetic substitute that performs even better that sawdust is not out of the question.

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Here's a link to a paper "Radiation Hazards of Relativistic Interstellar Flight" which suggests a number of options. One of them which seems particularly efficient (as least as far as it's effect on your mass budget, if that's a concern) is to use a metal foil which causes incoming interstellar medium to ionize, then deflect the ionized gas with electromagnets. But you might need additional shielding to deal with dust grains or micrometeorites.

https://arxiv.org/abs/physics/0610030

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