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Inspired by a thread on plausible alien invasions.
As I think I've blogged before, in a sense we may be quite close to being technically capable of sending a ship to a nearby star. At the raw physics level, we already have the energy sources. The Newtonian kinetic energy of mass at 0.03 c is comparable to the energy density of fission fuels, and means getting to Alpha Centauri in 140 years. The tricky part is delivering the energy into exhaust of such speeds; thermal engines melt, mass drivers quench, ion drives I'm not sure about, plasma drives ditto, photon drives have too much exhaust 'velocity' to be efficient, fission fragment rockets would be just right but the atoms that want to fission aren't the ones in a good surface position to send fragments out the back.
Still, it's possible that an ion drive, or plasma drive, would in fact work. The extreme case is Project Longshot, where a fission reactor is used to force D-He3 fusion pulses, getting you the energetic plasma needed (and more, it's like a fusion afterburner) while ducking the problem of fusion power reactors being among the hardest things the human race has ever tried to do. (Here, plasma squirting out is a feature, not a bug.) And of course there's always Project Orion, another fission-fusion combination, and maybe one that could use the much cheaper D-D reaction. Or fragments.
Of course, then there's the matter of having something that lasts 95 (Longshot) to 140 (pure fission) years, in hard radiation to boot; this might well be harder than simply making something go fast. Even more so if you want to send live beings.
But... there's a common assumption that if you can send a ship like that, you don't need to invade, you can build space colonies and such. But it's not true. Leaving aside whether people want to live in space colonies, the problems are different. The ship 'just' needs to last over a century; air leaks can be replenished from ice supplies, breakdowns can be compensated for by redundancy, spare parts, and a portable machine shop; people need some combination of a few generations, stasis, or longevity (possibly including partial longevity through partial stasis, or slowdown.) While a colony needs to be more permanently robust, and to contain or have access to a complete industrial ecology.
So invading your neighbors with the desperate hope and need of taking them over, and using their labor and industry, may in fact be easier than a self-contained space colony, and at any rate is a different problem.
Another key note: the sort of "we could expensively build it soon" fission-fusion interstellar ship above does not include ground to orbit capability for Earthlike planets. Moon landers sure, Mars maybe, but for anything we could send, taking capsules down to the surface of an Earth would be a one-way trip. We don't know how to get off again without an army of thousands building the return vehicle.
And of course for any rocket a one-way trip is a lot cheaper than a planned round-trip without guaranteed refueling. And if you need lots of fissionables, refueling may be hard and chancy.
So while the probability of having near neighbors to invade seems very low, and it'd be expensive, there's actually a certain plausibility to would-be conquistadors not much more advanced than us coming and trying to bluff/conquer/trade their way in, without any option to go back home, or even get back off the surface once landed without help. Not very plausible -- but the alternatives, that anyone crossing interstellar distances must be magically more advanced, are not clearly true. You just need fission, you don't need indefinite life support (if you're counting on another ecosystem you've observed with telescopes), you can't necessarily get off the planet, or zip around a solar system arbitrarily.
As I think I've blogged before, in a sense we may be quite close to being technically capable of sending a ship to a nearby star. At the raw physics level, we already have the energy sources. The Newtonian kinetic energy of mass at 0.03 c is comparable to the energy density of fission fuels, and means getting to Alpha Centauri in 140 years. The tricky part is delivering the energy into exhaust of such speeds; thermal engines melt, mass drivers quench, ion drives I'm not sure about, plasma drives ditto, photon drives have too much exhaust 'velocity' to be efficient, fission fragment rockets would be just right but the atoms that want to fission aren't the ones in a good surface position to send fragments out the back.
Still, it's possible that an ion drive, or plasma drive, would in fact work. The extreme case is Project Longshot, where a fission reactor is used to force D-He3 fusion pulses, getting you the energetic plasma needed (and more, it's like a fusion afterburner) while ducking the problem of fusion power reactors being among the hardest things the human race has ever tried to do. (Here, plasma squirting out is a feature, not a bug.) And of course there's always Project Orion, another fission-fusion combination, and maybe one that could use the much cheaper D-D reaction. Or fragments.
Of course, then there's the matter of having something that lasts 95 (Longshot) to 140 (pure fission) years, in hard radiation to boot; this might well be harder than simply making something go fast. Even more so if you want to send live beings.
But... there's a common assumption that if you can send a ship like that, you don't need to invade, you can build space colonies and such. But it's not true. Leaving aside whether people want to live in space colonies, the problems are different. The ship 'just' needs to last over a century; air leaks can be replenished from ice supplies, breakdowns can be compensated for by redundancy, spare parts, and a portable machine shop; people need some combination of a few generations, stasis, or longevity (possibly including partial longevity through partial stasis, or slowdown.) While a colony needs to be more permanently robust, and to contain or have access to a complete industrial ecology.
So invading your neighbors with the desperate hope and need of taking them over, and using their labor and industry, may in fact be easier than a self-contained space colony, and at any rate is a different problem.
Another key note: the sort of "we could expensively build it soon" fission-fusion interstellar ship above does not include ground to orbit capability for Earthlike planets. Moon landers sure, Mars maybe, but for anything we could send, taking capsules down to the surface of an Earth would be a one-way trip. We don't know how to get off again without an army of thousands building the return vehicle.
And of course for any rocket a one-way trip is a lot cheaper than a planned round-trip without guaranteed refueling. And if you need lots of fissionables, refueling may be hard and chancy.
So while the probability of having near neighbors to invade seems very low, and it'd be expensive, there's actually a certain plausibility to would-be conquistadors not much more advanced than us coming and trying to bluff/conquer/trade their way in, without any option to go back home, or even get back off the surface once landed without help. Not very plausible -- but the alternatives, that anyone crossing interstellar distances must be magically more advanced, are not clearly true. You just need fission, you don't need indefinite life support (if you're counting on another ecosystem you've observed with telescopes), you can't necessarily get off the planet, or zip around a solar system arbitrarily.