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Comment by hermitcrab | Hacker News Reader

Summary, as I understand it:

-anti-matter is incredibly difficult to produce (the total produced so far in labs is about enough to boil 1 litre of water)

-anti-matter is incredibly expensive to produce (currently around 1 trillion USD per milligram)

-anti-matter is very difficult to store and manage

-a lot of the energy will be given off as high energy gamma rays, which will be difficult to convert into thrust

-the high energy gamma rays will be very destructive to the spacecraft and crew (biological or robotic) without lots of shielding

-over a ton of anti-matter would be required to get a ton of payload to a nearby star at ~40% the speed of light

So don't start saving up for a ticket on an anti-matter spaceship. Or maybe do start saving, so your ancestors can take advantage of compound interest to buy a ticket in a few millenia, if we ever manage to develop the technology.

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> -over a ton of anti-matter would be required to get a ton of payload to a nearby star at ~40% the speed of light

It all sounds awful until you get to this line. What are the equivalent numbers for fusion? All else being equal, 250 tons of fuel per 1 ton payload? And that payload has to include the reactor and storage and shielding and everything.

Yes, but hydrogen is cheap. Especially compared to anti-matter.

Shielding is also going to be a huge issue for anti-matter drives (even without a human crew). Anyone know how the energy of EM radiation emitted from a fusion reaction compares to that emitted from an matter:anti-matter reaction?

Probably only space based antimatter production makes sense, the risk of blowing up the most expensive and dangerous stuff on Earth is just too big. Fusion sounds like a clear winner as opposed to antimatter.

I've found direct fusion drive as a concept a company is working on: https://en.wikipedia.org/wiki/Direct_Fusion_Drive

I assume any future antimatter based propulsion would rely on some sort of metastable particle pair that are made up of matter-antimatter components like protons, except one that can be more easily/efficiently split and recombined. Otherwise the factories will just blow up the first time there's a serious accident or containment failure.

Just as an example: imagine particle A made up of up, bottom, and strange quark and particle B made up of down, charm, and top quarks (I'm ignoring charge) that can be stored without interacting. The engine splits them into quarks that then can annihilate each other (AFAIK that's not how quark plasmas work but you get my drift).

> Probably only space based antimatter production makes sense

And definitely not too close to Earth. Maybe a Lagrange point.

I think hydrogen can be simply found in space, along the way. Not sure about the concentration of deuterium and tritium. I suspect once you get to 1% of c, you can find enough.

If you need to lift all the fuel from Earth, sure, antimatter all the way. Although, they already suggest making it in space for some reasons.

> I think hydrogen can be simply found in space, along the way. Not sure about the concentration of deuterium and tritium. I suspect once you get to 1% of c, you can find enough.

That's called a Bussard ramjet. Long story short:

> A 2021 study found that, while feasible in principle, the practical construction of a useful Bussard ramjet would be beyond even a civilization of Kardashev type II. [1]

[1] https://en.wikipedia.org/wiki/Bussard_ramjet

The only way to go is to go is slowly and by not sending a ton, ie inject genetic material into a rock that's already on its way, hope for the one in a trillion chance that it takes root somewhere. That we're so alone in the galaxy is proof of this.

I don't think there is any real advantage of matching velocity with a rock and transferring material to it, compared to sending your own spacecraft at that velocity.

A big rock with its own momentum is more likely to come down on the surface (wherever it is) intact enough. But yeah there may be other ways to spread our seed. My point is that it will unpropelled for most of the journey, and a seed not an ape.

Fun fact, the technology needed to accelerate a spacecraft to 10% of the speed of light already existed in the 60s:

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

> Or maybe do start saving, so your ancestors can take advantage of compound interest to buy a ticket in a few millenia

My apologies if this is off-topic, but I have recently read and heard a couple cases of "ancestors" being used instead of "descendants", which has sparked my curiosity. Is this a common brain-lapse result?

I don't know. But I definitely meant descendants!

1/1 Fuel/Payload to hit 40% C is why it's going to be a trope forever.

In Count to a Trillion, a small group found a way to siphon antimatter off a natural source and effectively ruled the world forever due to the near limitless energy and fast interstellar travel.

I think the issue is more the cost of the anti-matter than the fuel to payload ratio. Chemical rocket people can only dream of 1:1 fuel to payload. Look how much Saturn V + fuel to took to get a tiny payload to the moon and back.

The issue is the cost. But the trope is "Well if we could manage the cost, 1/1 will actually get us places."

As opposed to more conventional solutions, which require 1e5 / 1, or more, or more magical ones, that make no sense.

Ok, I misunderstood what you were saying. I think we agree. ;0)

Perfectly understandable, now that I re-read it.

The 1T$/mg figure also is only based on electricity costs, since energy efficiency of antimatter is currently 0.4*10^-9 according to the paper.

I think we have all knowledge we need to produce and store it much more cheaply.

It’s just that governments and research institutions don’t have that incentive. Look how fast reusable rockets, and fairing reuse happened once money was on the line. (To use some space examples)

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