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It occurred to me the other day that when a spaceship is travelling at near relativistic speeds, encountering a tiny rock could rip it apart.

What could be done to protect from such incidents considering small rocks are very hard to detect, especially when travelling so fast? If you're going to mention forcefields, do you think that they can be run efficiently for long periods of time? Assume any tech level.

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    $\begingroup$ Non-relativistic travel speeds pose a hazard to spacecraft! It wasn't that long ago was it that a paint chip cracked the window of the Challenger space shuttle. Detection and tracking of 1cm and larger objects just in Earth orbit verge on 500,000. So yeah, you're going to need some serious hardware for relativistic speeds, if its a force field, you may as well find a way to power it indefinitely: turning it off would reduce your craft to swiss cheese! $\endgroup$ – Draco18s no longer trusts SE Dec 21 '15 at 21:08
  • $\begingroup$ The Star Trek answer is the "deflector array", pretty much a force field that shoves small particles out of the way while the ship's underway at impulse speeds (at warp speeds, the ship's travelling in a "bubble" of spacetime and most objects in the spacetime through which that bubble travels is moved around the bubble). This main purpose, however, is rarely mentioned in-universe, with the array being one of the principal Swiss army knives the Enterprise crew has at their disposal. $\endgroup$ – KeithS Dec 21 '15 at 23:21
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    $\begingroup$ Very similar question: How to avoid objects when traveling at greater than .75 light speed. or How Not to Go SPLAT? $\endgroup$ – Wingman4l7 Dec 21 '15 at 23:32
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A giant ablative shield, combined with EM-based deflection.

Current thinking for theoretical starships has been quite partial to using huge chunks of ice for debris protection, because it is already available outside our gravity well (just go snag a comet!), potentially re-shapeable (just melt it!), and it's a valuable source of hydrogen (for fuel) and oxygen (for breathing). Neal Stephenson's recent hard sci-fi novel Seveneves explores this concept, albeit at planetary-orbit, sub-light speeds.

Impact at relativistic velocities, on the other hand:

For a starship moving at 0.3 c, a typical grain of carbonaceous dust about a tenth of a micron in diameter should have a relative kinetic energy of 37,500,000 GeV. [Our ship] will run into about thirteen of these dust particles every second over every square meter of frontal area. Travel fast enough and even small grains of dust behave like energetic cosmic rays. (source)

For passive shields, several possibilities have been examined -- aluminum, titanium, graphite, graphene, even beryllium. This might be placed many kilometers ahead of the spacecraft. Any mass that makes it through the "bumper" would end up being high-energy spray of charged particles that could be deflected by some sort of EM beam or large magnetic field. One novel concept for an additional layer of defense was deploying a dust cloud in advance of your ship, which would theoretically vaporize larger particles and render them susceptible to the active (EM) shield, before they could damage the ship.

An equal or bigger concern than physical debris is the variety of radiation you'll encounter while moving at relativistic velocities. Metal hull shielding is workable up to a third the speed of light, but much faster and it has to be tens of meters thick. The old trick of encasing yourself in a water tank shell would also provide shielding.

The blog Centauri Dreams -- which I've linked to throughout this answer -- has at least three posts examining this exact problem and referencing whitepapers on the subject; I highly recommend checking them out for more details.

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  • $\begingroup$ It's an interesting concept, but at that energy level wouldn't you need a totally impractical amount of ice to prevent hull damage? Even further -- wouldn't the force of impact alone cause some serious challenges? The dust cloud or, more broadly, the idea of trying to vaporize debris before you hit it, feels like a much safer bet. $\endgroup$ – Avernium Dec 21 '15 at 21:43
  • $\begingroup$ @Avernium: Possibly... it was my gut answer but it may actually be unfeasible, a super thick chunk of titanium may actually be more practical. Still doing a bit of research. $\endgroup$ – Wingman4l7 Dec 21 '15 at 21:45
  • $\begingroup$ In any case, I suspect you're on the right track for a difficult problem. I'll be interested to see what else you find. $\endgroup$ – Avernium Dec 21 '15 at 21:50
  • $\begingroup$ @Avernium: Yeah, turns out that most theoretical solutions employ an active defense solution in combination with passive shielding. $\endgroup$ – Wingman4l7 Dec 22 '15 at 0:04
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    $\begingroup$ @Wingman4l7 The benefit of using ice over titanium would be, as you say, that ice or water should be fairly easy to find, and much easier to work with. $\endgroup$ – Smoj Dec 22 '15 at 13:06

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