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The Backstory

The ship in question is a massive matter-antimatter annihilation powered starship, built by an imperialist and tyrannical government with basically unlimited budget.

The Ship

The interstellar long-haul ship, is a matter-antimatter beam-core powered ship, burning diamagnetic anti-hydrogen snowballs and normal liquid hydrogen.

The tractor section is made up of two long, rectangular radiators in parallel, with the reaction mass tanks (liquid hydrogen) between them. At the base of each radiator is the antimatter storage, accelerator and reactor chamber. They are both aimed ever so slightly away from each other as to not cook the truss.

The cargo section is towed four kilometers behind the main engines, on the end of a ceramic tile-cladded rigid truss. The forward bays are for main and backup computers, control systems and docking for autonomous maintenance drones.

Behind it are the cargo modules for repair parts needed for the journey, and at the very back is the cargo space for the measly 300 tonnes of cargo capacity, of which most is just dedicated to the planetary decent vehicle, and the transport of the 20 gram sample of the Bio-mechanical Terraforming Agent, the Typhon.

In the rear are the three shield-mirrors that are docked to one another, and reflect the petawatt-power hyper laser powered by a dyson sphere from earth, which drives the photon sail.

Flight Plan

As the ship departs its orbital docks, it will be pushed a safe distance from Mercury's orbit, where it will extend its photon sail, which is anchored to a mast between the radiators, which is a section of the truss that extends the entire length of the ship.

It will accelerate at a comfy 2 gravities until the particle-photon laser can no longer beam power to the sail, accelerating to a comfortable margin the speed of light in the process.

At this point the drones will fold the photon sail up and stow it in the cargo module for ship parts. After everything is secure, it will engage its antimatter engines and start accelerating, starting at less than 1 gravity and pick up speed as it burns though roughly 20% of its fuel and achieves its target 0.7C

Finally, the drones will detach the shield mirrors from the rear of the ship and position them hundreds of kilometers ahead of the engines, to vaporize and disperse any interstellar dust it hits.

It will now coast for the next 4 decades, traveling around 30 lightyears to the destination star system.

On arrival, the shields will be reattached and antimatter engines will fire once more, starting at 2.4G and ramping up to 5G at the end of the burn, sliding into orbit around the star.

After its payload is deployed, it will be repurposed as an interplanetary ferry, or a gas harvester, as needed.

Fuel

The ship runs on the most efficient engines reasonably possible to build, anti-hydrogen snowballs and molecular hydrogen reaction mass. I haven't taken the time to calculate the exact quantities of fuel needed, but around one kilogram of fuel and 5 thousand metric tonnes of reaction mass will be used in its engines.

As if this wasn't hard enough for a much more advanced humanity to build, they built 8 of these.

The Question

For the sake of argument, lets assume the fuel tanks are about 80% full, and we still have a kilogram of antimatter.

Two of these ships were sent to each star system, as at least one of the eight wasn't expected to survive the crossing, and forty odd years is a lot to gamble with.

What if an unfortunately angled speck of dust misses the hit-shield ahead of the ship, misses the engines and truss, and punches a hole in the control computers on the forward bays of the ship, ripping a big hole in it.

The computers fail and the delicate matter-antimatter reaction spirals out of control. What will happen?

If the engines were to explode, what would happen? How much of the ship would be vaporized? How big would the blast be? What would it look like for the curious oligarchs that payed for this, though a telescope back on earth? How bad would the gamma flash be to the rest of the ship? Is there an off-chance that little lander carrying the specimen could do an injection burn and pull around the star, slamming into the planet anyway?

I know I'm only supposed to ask one question, so the question is: What will happen?

Might as well ask as much as I can, so I didn't waste your time reading this. Thanks!

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  • $\begingroup$ What is meant by "around one kilogram of fuel and 5 thousand metric tonnes of reaction mass will be used"? Beam core rockets typically react equal parts matter-antimatter, but this sounds like a miniscule amount of antimatter is being used. $\endgroup$
    – BMF
    Dec 29, 2022 at 1:02
  • $\begingroup$ Hello Sam. I try not to vote to close new user questions, but I'm having trouble with this question. Humanity's experience with antimatter is breathtakingly limited. We know the collision releases a lot of energy, but what does that look like? That's my problem - you're asking for what, for all intent and purposes, is an aesthetic, which is opinion-based. Your ship might melt. It might sparkle like tinfoil in a microwave. It might explode like the Death Star. It's all opinion. In other words, this question is too story-based. (*Continued*) $\endgroup$
    – JBH
    Dec 29, 2022 at 1:11
  • $\begingroup$ ... The question might be on-topic if you told us what outcome you want and, as you've done, the basic nature of the engine, and we could help you with the rules of what would have to happen to achieve that outcome. But so long as you're asking us to tell you what the outcome is. I believe that's off-topic. Can you edit your question to make it less opinion-based and story-based? (I.E., less, "it can be anything you want because we don't know..."?) BTW, it might help if you read out Meta post about high concept questions. $\endgroup$
    – JBH
    Dec 29, 2022 at 1:12
  • $\begingroup$ @JBH Doesn't seem to me like OP is asking about the aesthetic, but rather the magnitude of the explosion. (Will it destroy the ship, will it be seen from Earth? Can something small survive the explosion?) $\endgroup$
    – BMF
    Dec 29, 2022 at 1:14
  • $\begingroup$ @BMF Possibly, but the OP would need to provide the mass of the ship, the quantity and composition of any flammables, the composition of the ship's structure, the nature of the antimatter engine (to a substantially greater degree than given here), the distribution and storage of energy in the ship (like batteries). It's a whole lot more than, "I have a pound of electrons and a pound of antielectrons, what happens next?" A better question might be, "given a ship of mass X, how much antimatter/matter would I need to produce an energy release so large that the details no longer matter?" $\endgroup$
    – JBH
    Dec 29, 2022 at 1:16

2 Answers 2

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I don't know the math, but I can do visuals

Due to the nature of mater-antimatter reactions, the explosion will make lots of really excited photons (or, in other terms: Bright, deadly light). Initially, the photons will be in the Gamma-ray end of the EM spectrum and shift toward visual light. This ball of pure energy and yet-to-be-energy matter and antimatter will be bright, brighter than anything anyone has ever witnessed, up to a certain distance at least. Any crew that was on the ship at this point either no longer exists, are dead, or feeling what standing in the middle of Chornobyl's reactor #4 is like

Viewing from a distance:

Depending on how far the explosion happens from the observer, the light from the explosion would take between a few seconds to a few centuries to arrive to the observer, if you don't have any handwavium superluminal communications device, Earth observers wouldn't know it happened until the light hit them.

By then, the light will be red-shifted and look like a faint star that soon dies out, depending on the distance.

PLEASE NOTE: This is all speculation, taking what I know about light and energy and applying it here

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  • $\begingroup$ HAH! Well take some free XP! Good answer, but for reference, yes they do have quantum entanglement communication, but the last they'd hear from the ship is sudden radio-silence. And it would only be about half a light-year away, so... how would red-shifting work then? It would be going at about 0.7C, too, if thats helpful. $\endgroup$ Feb 24, 2023 at 0:48
  • $\begingroup$ I'd like to point out that detonating 1 kg of antimatter has about the same yield as the Tsar Bomba. We would not be able to spot that at interstellar distances. $\endgroup$
    – BMF
    Feb 24, 2023 at 6:27
  • $\begingroup$ @SamKitsune I have no idea, I'm more of a biology nerd, optical physics is not really my thing, but I'm assuming the redshift wouldn't be that much with how close the detonation would be, and Im unsure how speed would affect it (my guess would be not that much) $\endgroup$ Feb 24, 2023 at 16:42
  • $\begingroup$ @BMF you forget that the Tsar Bomba had to deal with atmospheric scattering and a lot of the energy from the detonation was mostly transferred as heat (again: not a physic guy but I do know the basics) $\endgroup$ Feb 24, 2023 at 16:45
  • $\begingroup$ @redfrogcrab That doesn't factor into my consideration at all. On interstellar distances, Tsar Bomba's output is simply pitiful. 1 kg of antimatter annihilating releases 18e16 Joules of gammas. I calculate the lumosity of the blast 1 light-year away to be 2e-15 W/m^2. An order of magnitude less than the cosmic microwave background radiation. At multiple light-years it gets exponentially worse. From the nearest star you'd be lucky to receive even a few photons from the explosion. $\endgroup$
    – BMF
    Feb 24, 2023 at 17:06
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Explosion energy

Parsing the relevant details out of your question, what I think you're asking is whether 1 kg of antimatter annihilating with ship material is enough to totally destroy your ship, and whether the explosion would be visible from Earth.

$$E=Mc^{2}$$

Assuming all of the antimatter annihilates (a most likely scenario as its surrounded by containment machinery), I get around $9\cdot10^{16} \text{J}$ of energy released (90 quadrillion Joules). That comes out to around 22 megatons of TNT, or about half the yield of the largest nuclear device ever detonated, Tsar Bomba. You don't give a mass estimate for your ship, but I can give you some rough numbers to judge whether the explosion would annihilate the ship.

It takes around 6 megajoules to flash boil one kilogram of steel. Dividing through, I get about 15 million metric tonnes. Wasn't really expecting that big of a number, but unless the heat of vaporization estimate is extremely low (which it doesn't seem to be), there's enough energy there to vaporize an extremely large vessel. Of course, that's all a rough approximation. The energy isn't spread uniformly. Steel near the blast doesn't stop heating once it hits 3000 K, stepping aside for the next steel atoms to take their fair share.

Such an explosion wouldn't be visible from Earth at any interstellar distance. Probably within cis-Lunar space, but not that far out.


Critique on ship design

As an aside, I doubted the plausibility of such a ship decelerating from 0.7 c on a handful of antimatter. I calculated the delta-v for an antimatter beam core rocket, which reacts equal parts matter-antimatter. Here are the stats:

  • Thrust power: 500 TW
  • Exhaust velocity: 100,000 km/s (0.33 c)
  • Engine dry mass: 100 tonnes
  • Thrust: 10,000,000 N
  • Remass: Hydrogen, antihydrogen

(These are extremely liberal numbers, not taking into account containment mass, radiator mass, etc., which would scale with mass ratio., and assuming the best possible beam core engine possible (also look into Winterberg's beam core & other designs).) With a mass ratio of $R=4$, delta-v comes out to almost 50% c. Remass comes out to 300 tonnes, 150 tonnes of which is antihydrogen. In your design, you state that you inject more hydrogen into the antimatter reaction. This means that exhaust velocity is decreased as the annihilation products exchange momentum with more remass. Delta-v is directly proportional to exhaust velocity by:

$$Δv=v_{e}\ln\left(R\right)$$

If exhaust velocity decreases, the mass ratio must increase, but that natural log on the mass ratio really puts a damper on things, so it's not as easy. Injecting hydrogen gets you higher mass flow & higher thrust:

$$F=ṁv_{e}$$

but it also gets you lower burn time:

$$t_{burn}=\frac{m_{p}v_{e}}{F}$$

and lower delta-v:

$$Δv=\frac{F\ln\left(R\right)}{ṁ}$$

For a delta-v of 70% c, I get a mass ratio of 8 (that's 350 tonnes of antimatter). In short, 22 megatons of energy is not getting a sizeable spacecraft down from 0.7 c. Dumping hydrogen on it doesn't give you more energy than the raw matter-antimatter reaction.
(Depending on engine design, it might allow you to use more of it (i.e. wasted gammas), but this is a non-trivial problem. See Winterberg's photon-core antimatter rocket.)

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    $\begingroup$ Holy- wow, thank you!! <3 $\endgroup$ Dec 29, 2022 at 3:21
  • $\begingroup$ Yes, you answered all of my questions, and now I have something more to work with when I fill out the real specifications listed, which I can work on at my own leisure. The plot of the book really doesn't pivot on any of the ships specifications per se, but it is important to me that everything is plausible enough to withstand scrutiny. $\endgroup$ Dec 29, 2022 at 3:23
  • $\begingroup$ But... In that case... how the hell does the Venture star pull off its 0.7C deceleration? And moreover, (canonically) it can't be a 1:1 ratio in that because for one, that's a ludicrous amount of antimatter to somehow contain (as we only see those big tankers in between the engines), and two, in the second movie, they use the ship as an atmospheric lander (I'm still confused as to how that would ever work). But the important part of that is that we know what the exhaust plume looks like (bright blue jet that scorches stuff). $\endgroup$ Dec 29, 2022 at 3:26
  • $\begingroup$ Antimatter produces some charged pions, I believe muons and a whole lot of gamma rays. Unless the blue jets are streams of gas from the atmosphere, pions and muons don't act like a gas, or glow for that matter. $\endgroup$ Dec 29, 2022 at 3:28
  • $\begingroup$ (This is unrelated to my novel, I'm just curious) $\endgroup$ Dec 29, 2022 at 3:29

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