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Comment by hamter | Hacker News Reader

For those like me who need some kind of reference for the distances mentioned (548-900 AU), Voyager 1 is 156.5 AU from earth today.

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Yeah, the solar lensing point is WAY the hell out there: since 1 AU is 8 light-minutes, so the 548 AU minimum works out to 73 light-hours away. When LIGHT takes half a week to make a one-way trip, you're in the deep space boondocks, folks.

I haven't read the paper yet, but this thing would have to have a fair amount of nuclear power, and comms would be a challenge as well. As the abstract mentions, though, while the project has a high degree of difficulty, there appear to be no complete technology showstoppers to actually doing this, so it's at least as doable (and considerably cheaper than) a von Braun-style centrifugal space station in Earth orbit.

It'll be interesting to see if the idea gets any traction...

For those looking for a live look at Voyager status: https://voyager.jpl.nasa.gov/mission/status/

The solar system explorer view they have on that page is fantastic!

I recommend clicking on the "solar system" toggle in the bottom middle of the view. It gives you a real sense of the planets, probes, asteroids etc that are flying around our solar system.

Also reminds me of looking at air traffic control maps and what that might look like once intra-solar system space travel becomes routine.

and 900 AU is 0.014 light years. so you can get 10km resolution on a ~7100:1 object to focal-point distance (what’s the right term for this?).

seems pretty good.

I wonder how much faster we could get there with solar sail and laser boost from Earth

One issue would be slowing down when you get there.

You'd need to carry a deployable/detachable mirror with you to reflect the laser back at the craft, but that mirror itself would also get accelerated further out, which means having to correct for that, etc., etc.

For this mission, you don't need to slow down or stop. Just keep taking image data starting at 548 AU and keep going until you're at 900 AU.

I thought you have to move laterally to get the pixels? "The data are acquired pixel-by-pixel while moving an imaging spacecraft within the image."

You can already do that while slowing down. Might require some image correction though to account for that in the pictures.

With a solar sail you can just start tacking, right?

I think a solar sail mostly works at a broad reach or a run, so it would be more of a jibe than a tack ;)

I don't know anything about sailing but doesn't tacking depend on some sort of keel?

With a solar sail you can "tack" by reflecting photons against your tangential velocity – but that only works if the tangential component is indeed what you want to shed. Getting rid of radial velocity is more difficult, and in a hyperbolic (escape) orbit radial is mostly what you have.

Probably not at 540 AU. Not many photons out there for the solar sail to grip.

Lol, nice

Can't you use the same lens to focus the propulsion laser?

Without specifying the size of the laser, anything from "even slower" to "whole trip in just under 16 days".

https://en.wikipedia.org/wiki/Breakthrough_Starshot

http://www.wolframalpha.com/input/?i=548%20AU%2F0.2c

That's 16 days if the average speed is 20% lightspeed. In reality you need to speed up, and then when you're halfway there, slow down again.

Which raises the other question: how would we slow this thing down so it just doesn't keep going past the 900AU mark?

Regardless, I guess we would be fine picking a slower speed that would get it there in a few years, which might be significantly easier to achieve.

Breakthrough Starshot aims to do all that acceleration in 10 minutes.

It’s an ambitious project.

Hah, the latter number requiring some significant percent of earths mass being converted to energy or something?

Not really, it's the velocity the "Breakthrough Starshot" probes would reach. They propose[1] that launching each probe would take 84 GWh, which is not super much (about 15 times more than a space shuttle launch), but of course the Starshot probes would be much lighter than this proposed telescope so it's not directly comparable.

[1] https://youtu.be/KIDuXQHt8pk?t=1562

Accelerating a meter class telescope to .2 c is well beyond our current capabilities, but it's nothing like that fast.

If we assume the meter class telescope + power supplies and whatever else masses ~ 1 metric ton, it would take 1,853,298,442,530,598,439 Joules - 1.8 quintillion - 1.853 * 10^18 - to accelerate it to .2c. Only ~ 442 Megatons of TNT. Keep in mind, that is assuming 100% efficiency, which would be impossible with a light sail or any other known technology.

With all the various inefficiencies in power collection/generation, laser generation, momentum/power transfer, etc. we'd be talking probably somewhere around 5% end-to-end power transfer - if we were lucky. Which is still way better than the rocket equation (probably).

So to get the required 1.853 * 10^18 J at 5% efficiency, we'd need say 20x more power at earth to accelerate it (if 5% efficiency). So 3.706 * 10^19 J. Which starts get more concerning, at 8,840 gigatons of TNT.

Let's take the most efficient means we can imagine to produce energy, direct matter annihilation. Annihilating 1KG of mass (using 500g of Anti-matter, 500g of Matter) produces 8.986 * 10^16 J of energy. If we could somehow feed the resulting energy directly into the laser for accelerating the craft, and assume near 100% efficiency in doing so, before laser losses - we'd only need roughly 1000 KG of matter/anti-matter to do so.

Not bad!

But wait, our more likely end-to-end efficiency is at best 1%. Hmm. Which would require 100 times the input energy to spacecraft acceleration. 44 Gigatons of TNT or 1.853 * 10^20 J.

5KG of matter/antimatter.

Which is definitely not a significant fraction of earths mass, but yikes. I wouldn't want to pay that energy bill!

51,480,512,292,500 kWh (the 'wall plug equivalent') at my current rates would be $26 trillion dollars!

The fastest way there would probably be with a nuclear propulsion engine, as researched in Project Orion.

https://en.wikipedia.org/wiki/Project_Orion_(nuclear_propuls...

Princeton FRC

There's a nice article on centauri dreams just published looking at solar and nuclear options for such a mission.

https://www.centauri-dreams.org/2022/07/26/getting-there-qui...

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