Comment by samplatt | Hacker News Reader

Comment by samplatt | Hacker News Reader

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>A century ago, almost everyone thought that stepping on a divine body like the moon is absolutely impossible for humanity.

A century ago we DEFINITELY conceived of visiting a divine body. 121 years ago a science fiction movie about landing on the moon was released, based on a scifi book released _158_ years ago.

>if a ship constantly accelerates in space, it can reach another star system in a few decades

Try a few dozen thousand years, my dude... https://www.universetoday.com/141407/how-big-would-a-generat...

Even Project Orion, which is literally strapping many successive nuclear bombs onto the bottom of your craft, would take 130 years to get to our closest star.... and that's not taking into account stopping time, so it'd take 130 years to do a flyby of Alpha Centauri at 3% of the speed of light.

Not to mention the fact that ALL spaceships bleed air even under optimum conditions. A generational ship would need to be HUGE and have even bigger reserve supplies of air and water than you'd ever think possible. 50,000 years worth of air and water for 100-500 people? That's a LOT of mass.

Beyond ion propulsion and Project Orion we start to get into wormholes, black-hole-gravity-assists, remote-quantum-entanglement, time travel. Stuff that's theoretically possible kinda sorta if you completely ignore some fairly fundamental laws... the tricky thing is EVEN IF you choose to ignore those fundamental laws, the amount of sheer energy you're talking about is like the many multiples of the energy output of our sun over its _entire lifetime_.

> A century ago we DEFINITELY conceived of visiting a divine body. 121 years ago a science fiction movie about landing on the moon was released, based on a scifi book released _158_ years ago.

I said "almost everyone". If you go to 1920s and ask a million people if humanity will be able to walk on the moon within their lifetimes, not a single one would say yes. And yet, we were able to do it. Similarly, we have been conceiving of interstellar travel for decades.

> Try a few dozen thousand years, my dude

With constant acceleration, ship time is very different. With constant acceleration using controlled nuclear fusion, we can do it in less than a century. Project Orion is not constant acceleration. Now before you point out issues in constant acceleration, remember that if you were in 1920s, you (and I) would also point out issues in moon landing. How can humanity ever leave Earth's gravity, go accurately to the moon, decelerate, LAND without crashing, walk, LAUNCH from moon again, go accurately to the earth, handle extreme atmospheric temps, and LAND without crashing.

Well, we did all of that. So surely, given a century, we can figure out interstellar travel.

I suspect you're getting ion propulsion confused with nuclear fusion. In order for deuterium and tritium to be useful for fusion, you're going to need kilograms of it, not individual atoms. And tritium is required to be radioactive for it to be useful for fusion, but it has a halflife of about 12 years, meaning any fuel we take with us will be useless pretty quickly. And that's even before you take into account the idea of "constant acceleration"... some sort of magical sustained explosive reaction that would go on for 100 years? And that's after sourcing the unobtanium required to shield/support the "constant" detonation.

I suspect you're thinking of an ion drive, which uses fancy magnets to accelerate mass (pea-sized pellets all the way down to individual molecules), which is a slow start but is fine for constant acceleration over tens or thousands of years.

The problem there though is that the acceleration is SLOW, much less than 1G, hence the 20-80-thousand-year estimate to get to our closest star.

>If you go to 1920s and ask a million people if humanity will be able to walk on the moon within their lifetimes, not a single one would say yes.

I'm not sure I agree with that. Spaceflight and lunar (and even Martian!) colonisation was a pretty common scifi idea by 1920. Cars and planes were already 20 years old by that point. If you asked a million people in 1920 I'm pretty confident you'd get a decent chunk of people saying "yes".

Try this : https://en.wikipedia.org/wiki/Project_Daedalus

Also, I agree that the idea for interstellar travel has some holes. Let me rephrase my argument as follows : for every accomplishment of mankind (airplanes, spaceflight, moon landing, nuclear, computers, internet), there have been just as many critics claiming that it isn't possible (they had their own set of reasons). And all of these technologies were much more infeasible compared to interstellar travel (and yet we did that).

5 years ago, would you have imagined ChatGPT or Stable Diffusion? Tech moves fast. There is no doubt in my mind that in a century, we will see interstellar travel like we do airplanes today. But this may well be wishful thinking because I really want this to be true.

>Project Daedalus

That's not a manned craft, which is what we've been talking about so far. It also doesn't have the deceleration stage - Daedalus would flyby Barnard's Star at 12% the speed of light. If you hit a rock along the way at that speed, you're not gonna get any pretty pictures :-)

>5 years ago, would you have imagined ChatGPT or Stable Diffusion?

Absolutely! I think most people in my industry would have imagined ChatGPT or Stable Diffusion as they appear now 20-30 years ago, let alone 5.

>And all of these technologies were much more infeasible compared to interstellar travel (and yet we did that).

See... again, most of the problems faced by the "smaller" feats like airflight are engineering problems, not "this breaks Einsteinian physics" problems. When you're talking about interstellar flight you're running up hard against many fundamental constants of the universe, not just "we can't do that yet"

Any serious fusion proposal will breed tritium. Power to weight is going to be abysmal though.

Also slow acceleration is not the millenia long problem you're making it out to be, 0.01g will get you to 0.1c in under a century. Ion drives are largely limited by the weight of the power source even with current tech.

Any fusion reactor capable of working without prohibitively large radiatiors will have enough control over the plasma to just eject it for thrust directly. ISP will be your limit, not thrust.

Q-drive is a fairly compelling proposal compare to other options. Use some kind of magnetic solar sail to soar along the heliopause until you can make the trip in decades.

>0.01g will get you to 0.1c in under a century

Okay, cool, but that's TWO centuries including the flip-&-burn halfway through.

Also, an NSTAR thruster gives 0.000092 m/s2 for a single-ton payload. That'll take roughly 10,000 years to get to 0.1c, not 100 or 200 years.

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