How could a generation ship "steal" energy or resources from star systems which it passed without changing velocity?
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2$\begingroup$ Just from passing through the star system the speed of the ship will change; it will go faster as it approachs (falls into) it and it will go slower as it leave (flies away from) it. $\endgroup$– SJuan76Commented Nov 12, 2019 at 21:54
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2$\begingroup$ Pirate all their digital radio. $\endgroup$– RogerCommented Nov 13, 2019 at 2:51
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5 Answers
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It could deploy a swarm of von Neumann drones well ahead of the ship to collect resources and then blast them up to speed so that they are ready for collection when the generation ship cruises by.
For example, if the ship is going 20% c (the speed of light) and it sends out a probes ahead that are going 22% c, then for a system that's 10 light years away that gives the probes years to arrive, replicate, and then start sending out resources at the required velocity.
answered Nov 12, 2019 at 19:02
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$\begingroup$ This (and also the original question) sounds suspiciously like the premise behind Stargate: Universe... $\endgroup$– MatthewCommented Nov 12, 2019 at 19:24
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1$\begingroup$ @Matthew The Destiny was a toy compared to von Neumann probes...FTL drive aside. $\endgroup$– DKNguyenCommented Nov 12, 2019 at 19:29
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1$\begingroup$ Fair enough, but it's the same general idea... send parasite craft out ahead of you to gather resources to be sent back to the mother ship. (Destiny used stargates to collect the material, you're using rockets which need less hand-waving, other options are probably possible...) $\endgroup$– MatthewCommented Nov 12, 2019 at 19:42
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It could deploy wide solar panels to adsorb star light and gain energy.
It could use a Bussard collector to gather interplanetary matter, which should be many times denser than interstellar matter, and be careful to maintain the same speed and direction, making corrections and adjustments as necessary if it slows down while collecting the interplanetary matter.
answered Nov 12, 2019 at 18:48
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2$\begingroup$ Collecting interplanetary matter whilst travelling at interstellar speeds sounds risky. If you're ramscooping though, you'll need some other substantial source of power up offset the scoop drag, and unless you're dangerously far into the system solar ain't gonna cut it. $\endgroup$ Commented Nov 12, 2019 at 21:33
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1$\begingroup$ @StarfishPrime What's really crazy is that if you imagine a ship traveling at 10% the speed of light going through the Solar System: everything would appear frozen in place out the window. It would take 52 hours to go from one side of the solar system to the other. $\endgroup$ Commented Nov 13, 2019 at 1:09
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It could deploy faster smaller, faster, ships ahead of time to do the job. The harvester ships are launched from an already fast ship so they get a head start on their acceleration to travel faster than the generation ship. They need only be able to decelerate in time to arrive at the target.
But picking up the resources is trickier. If the harvester ships are able to accelerate under their own power faster than the generation ship they could catch up to the generation ship long after it has already passed the target.
If not, things get...complicated. It starts to become a matter of degree as to how slow can something be traveling and still be safely caught by the generation ship.
Obviously, this would be a generation spanning endeavor for each target.
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If you are stealing matter, in the ideal case you start with an inelastic collision. Then, you must accelerate the new combined mass (depending on what % of your ship's mass they are picking up, and how tight your "without changing velocity" constraint is). It isn't a super tricky calculation -- since this is a generation ship, you'll have a class of highschool students, hopefully the brighter ones will be able to solve it with little difficulty if you pose it for them properly.
If you are "stealing" energy, probably just use solar panels (I'd argue that stealing energy that was just going to be blasted into space isn't really stealing). Technically the photons will impart a tiny change in velocity, if you want to be really strict about the requirement to not change velocity, you'll have to accelerate to compensate. But note that these photons are going to hit you anyway.
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$\begingroup$ if you're going at relativistic speeds, anything in a solar system is effectively standing still and every collision could be treated as inelastic within reasonable error margins :) The Bremms radiation from the collisions would become a major problem of course. $\endgroup$– jwentingCommented Nov 13, 2019 at 4:46
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The video game Elite (1984) had the concept of Fuel scoops which would scoop up ambient hydrogen when flying through a star system. something like that.
Resources heavier than H2 would be more difficult, as you'd have to overcome inertia, but again, a scooping concept would probably work best, trace amount of resources through dust clouds, or a system that could crush small space debris into dust, and then stored would be practical
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$\begingroup$ That is what is referred to in M.A. Golding's answer - their "Bussard collector" is from Bussard ramjet concept, which I'm fairly certain is what inspired the fuel scoops for Elite. (I highly suggest your look into the Bussard ramjet concept - it is a well-established idea in classic sci-fi) $\endgroup$ Commented Nov 12, 2019 at 20:23
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$\begingroup$ @pluckedkiwi it's been a while since I've read any hard science. Decades in fact. $\endgroup$– user20762Commented Nov 12, 2019 at 20:33
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$\begingroup$ @pluckedkiwi the fuel scoops in the Elite series games aren't Bussard ramjets, the ships skim the outer atmosphere of a star, sucking hydrogen into their tanks (supposedly through filters I guess to get rid of the heavier elements). How they work isn't made clear, some people envision a physical scoop that gets deployed, others a semi-permeable skin for the ship that selectively allows hydrogen to pass. $\endgroup$– jwentingCommented Nov 13, 2019 at 4:43