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Let's say that I've developed a new engine that can accelerate at 1G continuously. Within about a year, I'm nearing the speed of light. Naturally, I recognize the danger of collisions with gas and dust. I have magnetic fields to sweep away ionized matter, but it doesn't work for non-ionized matter and it adds drag.

My chief engineer had an idea. He suggested we build a massive but thin solar sail and keep it in front of our ships, using our lasers to accelerate it at the same speed as our ship. This sail would suffer the brunt of any impacts from the interstellar medium.

So, is this a good idea, or are we just going to get hit by chunks of shattered solar sail?

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    $\begingroup$ This is an extreme example of en.wikipedia.org/wiki/Whipple_shield. It could work but needs tought (keep stable sail-shield attitude, etc...) $\endgroup$
    – b.Lorenz
    Jan 2, 2019 at 20:22
  • $\begingroup$ Nope, it will just make you get hit more. And I would advise to go 50-90% speed of light at max.. It is better to "waste" time and decrease energy of impacts and have ability to detect what is around you. $\endgroup$
    – Artemijs Danilovs
    Jan 2, 2019 at 20:35
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    $\begingroup$ Related: How to avoid objects when traveling at greater than .75 light speed. or How Not to Go SPLAT? $\endgroup$
    – Alexander
    Jan 2, 2019 at 21:23
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    $\begingroup$ Pedantic note: You can't approach the speed of light--You can't even move relative to light. You can only move at "Near the speed of light" in relation to most other objects in the region you are traveling through (which is obviously what you meant so I'm really just being annoying I guess, but since I actually started to understand that, anyone talking about moving "near the speed of light" as an absolute really bothers me) $\endgroup$
    – Bill K
    Jan 3, 2019 at 0:17
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    $\begingroup$ In Clarke's The Songs of Distant Earth interstellar ships used a thick ablative shield of cheap ice. instead of a thin shield of expensive engineered material. $\endgroup$
    – user535733
    Jan 3, 2019 at 1:12

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Your shield is moving at nearly the speed of light. The peanut-sized chunk of space debris you're approaching isn't. Best case, the peanut rips through the shield at nearly the speed of light and hits your ship anyway. Worst case, when the peanut hits the shield, the shield burns up in a glorious glow of fusion, which you appreciate just before being consumed by the firestorm. (That XKCD is probably the most commonly linked XKCD on this site.)

The basic problems of shields have been known or guessed-at by SciFi writers for decades. It's why they stick with the ubiquitous "deflectors," which are never actually explained and magically move all potential debris out of the way, much like cows vs. the cow sweeps on old steam trains.

The problems with your proposed shield are (at least) ...

  • It's not impenetrable (a massive + dense enough object will always go through it).
  • It's consumable (it's not self-repairing or self-replacing).
  • It will react to impacts (deliver enough energy over a large enough amount of area and it'll burn up like any other solid material).
  • The energy needed to keep it in front of you must be at least equal to the impact energy of anything that hits it (you're using lasers for this...) or it falls back onto you.

Does this mean you can't use it in your story? Not at all. Most scifi readers either don't know enough about the science to realize these problems, or they're more like me where I'm in it for the story and don't worry about the little things.

However, to give you an idea about how issues like this can be resolved. You can adopt Larry Niven's solution: the General Products hull, which is basically impervious to everything other than visible light, antimatter, and gravity. Designed as a single molecule with "strengthened inner bonds," it's simply the best built armor in the universe. Can such a material exist? Not that we know — but that doesn't stop anyone from enjoying his stories.

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    $\begingroup$ I've always wondered how you fit the interior of a GP hull. I mean, it's not like you can drill into it... Doh! As I write that, I realize: You just coat the inside with something that sticks to it (or something that's solid and the right shape to stay in place) and drill into that. Rubber-duck debugging... $\endgroup$
    – T.J. Crowder
    Jan 3, 2019 at 12:16
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    $\begingroup$ @T.J.Crowder I suspect that it's an attribute of manufacture: mounting brackets as a natural component of the hull. I suspect nobody ever asked Larry about that. :-) $\endgroup$
    – JBH
    Jan 3, 2019 at 16:21
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    $\begingroup$ Small enough impactors would be turned into plasma after impacting the sail, and said plasma would mostly disperse before hitting the ship if the sail is far enough, especially if you use a magnetic field to deflect the remainder. Assuming the sail can stretch its remaining self to close the puncture and the ship regularly launch new sails as the old ones are eaten up, this could work - not necessarily the most efficient scheme, though. And for bigger debris, well, I'm sure your insurance company is OK with a ships going missing from time to time. $\endgroup$
    – Eth
    Jan 4, 2019 at 16:33
  • $\begingroup$ at relativistic speeds things don't punch though all that well, energy transfer physics means the impactor simply explodes. The more powerful the impact the more circular the dispersal. The energy needed to keep it in front is minimal, since relativistic impactors are really bad at transferring energy. $\endgroup$
    – John
    Jan 4, 2019 at 17:18
  • $\begingroup$ @John, Notwithstanding the lack of empirical proof. Cheers. $\endgroup$
    – JBH
    Jan 5, 2019 at 4:03
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It is a good idea except for the sail.

You are accelerating a shield in front of you. You are using lasers. If you turn off the lasers you will accelerate through the shield, so the lasers have to stay on. It must be more than one laser to accelerate all parts of the shield uniformly.

Pack up the sail for some later use. Leave the lasers on; you have already budgeted for them. They will spray impressively in front of you off into space. They will hit obstacles in your way. Those obstacles will be illuminated. From your perspective they will be coming at you very fast, so fortunately your lasers are governed by fast-thinking computers.

Your many lasers are different wavelengths. The reflected wavelengths allow the computer to calculate the trajectory of the incoming mass according to which beams are interrupted. Your computers fire a BB at the approaching mass. It will become an XKCDesque shower of radiation and charged particles, which your magnetic shield and bulk shield can deal with. There is an option to fire another BB at recalcitrant masses.

For very tiny particles, the lasers themselves might be enough. They were going to push a sail, after all. They can push particles out of the way too.

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    $\begingroup$ Icebreaker spaceships, +1. "Peanut Cruncher 7, you're clear for launch. All other vessels: window of opportunity to join convoy is 12.4 seconds, on my mark..." $\endgroup$
    – Mazura
    Jan 3, 2019 at 4:19
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    $\begingroup$ Sounds like what-if.xkcd.com/119 exposed to space. Also, at speed close to c your "fast" computers will be far too slow. $\endgroup$
    – val - disappointed in SE
    Jan 3, 2019 at 12:55
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    $\begingroup$ @val With no atmosphere to transfer the heat and with intentionally trying to slightly deflect the debris instead of increase its temperature or actually vaporize it, it shouldn't be quite as bad as that XKCD What-If. $\endgroup$
    – reirab
    Jan 3, 2019 at 17:16
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What you have described is a "flimsy shield", which, by itself, can not be effective at relativistic speeds.

Incoming hydrogen and helium atoms, upon colliding will turn into potent radiation, for which this shield will provide no protection. Putting the shield at sufficient distance in front of the ship will help to disperse the radiation, but then there is a question of generating protective magnetic field in front of the shield (way ahead of where the field generators can be located).

The solution (within the realm of existing science) is having a massive shield. Unfortunately, calculations show that this shield has to be so massive that accelerating our spaceship to relativistic speed is becoming practically impossible.

But of course if you are willing to introduce new science (as of now, your question is neither "hard-science" nor "science-based") you can make this sail shield design work just as you want it to.

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The classic shield for a relativistic spaceship isn't a solar sail, but a massive object built from engineered materials. The cheapest one would be mostly ice, but the ice is carefully cast and frozen so there are no bubbles, voids or cracks, and it is likely reinforced with high tech rebar like reinforced concrete. Other choices might be the sintered slag from asteroid mining. At any rate, this is an iceberg or mountain sized object moving ahead of the ship.

The benefit for the ship is you have created a wakefield shield, and the ship is running in a very clean "void" which has been created by the passage of the shield. Even the vacuum is harder than the space around it. The downside is the amount of energy needed to move a passive shield of that size is going to be massive. One possible solution for that is to build a huge mass driver in the solar system and fire the shield on the ships trajectory ahead of the launching of the ship. This will clear a path for the ship, so long as the launch isn't delayed too long after the mass driver fires the shield.

However, modifying the initial solution by eliminating the solar sail works to your advantage as well. The high energy laser shining ahead of the ship will illuminate everything in front of the ship. Gas molecules will be ionized and thus can be thrown to the side by a magnetic or electrostatic field surrounding the ship, and small physical objects will either be vapourized (the laser will act like a "laser broom") and turned to an ionized gas to be swept away by the ships field, or if large enough, the heating will blast enough matter from the object to act like a rocket and propellor it on a path away from the ship. If the object is too large, the heating by the laser will make it glow in infrared frequencies, and provide warning for the ship to change course and miss the object instead.

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Is this a good idea, or are we just going to get hit by chunks of shattered solar sail?

You are describing a very special kind of ablator.

The problem you have is that a collision at near light speeds will end up in thinga like nuclear fission and fusion. Then, instead of impacting against a rock at nearly light speeds, you will collide with your solar sail plasma debris + rock's plasma debris at near light speed. It might be less dense, but it might also be larger than the original rock, so you will be in trouble anyway.

But there is an alternative. Like in so many other aspects of life, attacking is the best form of defense in space travel as well. Use an Alcubierre Drive. It involves travelling inside a bubble of spacetime that destroys everything ahead of it, including the ship's destination, so it should also disintegrate any pesky space stones that happen to be in your way.

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    $\begingroup$ It may also, for some interpretations of the equations, superheat everything inside the bubble to stellar temperatures en-route or deliver a life-scouring gamma ray burst at your destination, but hey! $\endgroup$
    – Joe Bloggs
    Jan 2, 2019 at 21:07
  • $\begingroup$ Interesting. I though that sublight Alcbierre drives weren't subject to these problems, that they only occurred at FTL speeds? $\endgroup$
    – AskerOfQuestions
    Jan 2, 2019 at 21:35
  • $\begingroup$ @AskerOfQuestions yes. $\endgroup$
    – The Square-Cube Law
    Jan 2, 2019 at 21:42
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One solution to the problem has been proposed, which is to use a high power laser to ionize all material ahead of the ship so that it interacts with the magnetic field. Very substantial power is required, but given you already have enough to accelerate to near light speed its probably not a show stopper

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