# How does an extraterrestrial spacecraft survive a nuclear explosion directly impacting it in LEO? [closed]

I am trying to think of some materials that an extraterrestrial spacecraft owned by an extraterrestrial species, known to humans as the Visitors, can use to survive a nuclear explosion in Low Earth Orbit.

The spacecraft is capable of hosting at least ten railguns, six nuclear torpedo launchers, each carrying a total of 20 nuclear torpedoes each, and three particle beams as spacecraft to spacecraft weapons, while also being able to store 40,000 large 20 metre long and 2 metre in diameter tungsten rods, and additional 200,000 0.30 metre in diameter and 6.1 metre long tungsten rods ready to bombard a planet (in this case, Earth). This thing is also capable of sustaining a total of up to 4 million scientists, engineers, security personnel, crewmembers, etc. and 10 million robotic troops and drones combined. It also has 200 km/s of delta-v for interplanetary travel using its ICF engines, as well onboard research labs, manufacturing facilities, habitat modules and storage facilities full of enough raw materials and 3D printers to get small operations elsewhere in the Solar System started. It is 10 km long and 6 km in diameter. Each antimatter beam core stage has a total length of 724 km and is basically ~72,000 Mt in dry mass, and capable of pulling 0.35 Earth gees of acceleration, and reaching a cruising velocity of 0.75 c.

This spacecraft has a magnetic shield to protect against radiation, not to mention the spacecraft has four layers' worth of armour, mainly 100 metres' worth of Inconel, 50 metres' worth of carbon nanotubes, 130 metres' worth of Grade 38 titanium alloy, and a 30 metre thick layer of reinforced carbon-carbon.

Do take note: this is a K2 civilisation. If you happen to not be aware of the Kardashev Scale, here is some links to the scale itself.

Wikipedia - Kardashev Scale

Issac Arthur - Kardashev Scale

Kardashev Scale Brief Explanation - Futurism.com

Briefly explaining, however, it is basically a civilisation that can harness the energy of an entire star, spam multiple star systems at once, build large-scale megastructures like Dyson Swarms and the like to harness the energy of the star itself for whatever purpose it can come up with, as well as manufacture billions of spacecraft at any given time, not to mention use relativistic missiles (aka: RKKVs/RKVs/relativistic bombs), advanced AI, Von Neumann probes and the like.

So my question is how does the Visitor spacecraft (nicknamed the Leviathan by the pesky humans) survive a 475 kt nuclear explosion at an altitude of 1,500 km above the Earth's surface?

## closed as off-topic by Mołot, Azuaron, Radovan Garabík, SPavel, AndonJan 20 '18 at 0:36

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• Comments are not for extended discussion; this conversation has been moved to chat. – James Jan 25 '18 at 18:01
• extraterrestrial spacecraft owned by an extraterrestrial species, what if they are only leasing? Sorry I had to ask. Hope they bought the extra insurance. – ArtisticPhoenix Feb 2 '18 at 10:23

The alien starship has survived near-lightspeed interstellar travel. That means it must be well-shielded against radiation.

When nuclear anti-satellite weapons are proposed, they use radiation rather than direct hits (see effect of high-altitude nuclear explosions, the Starfish Prime test, and Program 437).

So when the humans hurriedly turned their ICBM into ASAT systems, which involved a major rewrite of their fire control, somebody cut corners and used the old fire control software without really thinking about what it does and why. The old programs triggered the nuke as soon as it was close enough according to the old targeting criteria.

Without an atmosphere, there was not much of a shockwave.

• Not even in a direct impact? – Future Historian Jan 19 '18 at 15:56
• @FutureHistorian: What's a "direct impact" and how is it different from detonating really close? A nuclear bomb in space generates basically only electromagnetic radiation (in the infrared, visible, ultraviolet, X ray and gamma ray range). It may melt a little bit of the ship's exterior armor, but no a large part. It may do some damage to some ship compartments -- presumably the ship is divided into compartments like any warship. It may kill a hundred thousand invaders. But the question says that the ship is 10 km long and 6 km wide and there are 4 million invaders; that's a lot of ship. – AlexP Jan 19 '18 at 16:27
• @FutureHistorian, the OP wants the alien ship to survive. So I gave a reason why the missile exploded at the optimal distance for an ASAT strike against Earth-made sats, which is not the optimal distance for a countership strike. – o.m. Jan 19 '18 at 16:47
• @o.m.. Oh. Now I get it. – Future Historian Jan 19 '18 at 17:34

Here is a very good article about what would happen to Los Angeles if a nuclear ground burst the size of Hiroshima went off at Walt Disney Studios (presumably because some leader of a Nuclear Power really hated Hannah Montanna).

Anyway, it's a good read for what goes on in a nuclear blast. You will want a ground burst here because you lose some effect if you don't connect it to the hull of the ship. In the initial explosion there are two destructive elements and it's basically similar to a Lightning Strike... but much worse. First is the Flash which will at the point of impact hotter than the surface of the sun (for a few seconds). This heat will cook anything in line of sight of the explosion.

Next and nearly instantly, comes the boom or over-pressure. This is basically displaced air being pushed away from the explosion. Since you're in space, this will likely do very little damage if you don't impact the hull. If your ship can tank these two elements, then you have one more thing to do survive.

During the flash, there is one more destructive element that will occur, though it's not really a loss of life destruction... the EMP. The flash is basically all sorts of electromagnetic radiation and will essentially stop any unshielded electronic device from ever working again (fun fact... if you get a missile warning and want to be able to check yourself as safe on Facebook after things stop exploding, throw your phone, tablet, and laptop into a microwave, close the door, and for the love of Jobs, do not turn on the Microwave. Microwaves interiors are shielded and prevent radiation from going out and in this case, going in). Given that this is military hardware and equipped with nuclear armaments, it probably is because if you are going to go breaking Light Speed, you probably know a thing or two about Radiation.

So my question is how does the Visitor spacecraft (nicknamed the Leviathan by the pesky humans) survive a 475 kt nuclear explosion

The spacecraft does not even shrug. The specifications you quote are wildly in excess of those of a Orion propulsor "pusher plate", which was designed to withstand several hundreds of megaton-yield explosions. A single 475 kt explosion (more or less the warhead of a P700 Granit cruise missile) isn't going to do much more than scratch the paint, and contaminate a couple of square kilometers of the craft's surface.

Indeed, should I decide to attack such a spacecraft, I'd go exactly with that - an Orion-propelled missile with ultradense penetrating warhead, and the largest inertialized fusion device I could fit. Once decohesion velocity has been attained, the penetration depth is $L=\frac{density_{penetrator}}{density_{armor}}$, and a density of about 2-3 times the starship's armor can be achieved.

This means our shipkiller missile is a tungsten-osmium layered rod about 120 meters in length, with an armored chamber about 2/3 of its length hosting a fusion warhead, propelled by a series of nuclear devices and shielded by a foliated aluminum sabot filled with ice. It impacts the starship at about 5000 m/s; while the chamber decelerates at about 300 G, the tungsten rod burns its way through the armor. The force of impact itself can be used to trigger a long-gun ("Little Boy") type fission initiator. Then, the armor itself amplifies the blast, directing it towards the inside of the starship.

A suitable defense might involve reactive armor or sacrificial plated armor such as NERA, designed not so much to defeat the penetrator but to redirect its path.

# The Visitor spacecraft is armored with 1 km of ice

Ice has a high specific heat capacity, high enthalpy of fusion, and high enthalpy of vaporization. Ice at 50 K takes 3.7 MJ to vaporize, per kilogram.

A 475 kt nuclear with a surface detonation could be expected to deposit less than half its energy in what it hit, due to some reflection. Lets say that it deposits 200 kt of energy into the target. That is $2.4\times10^{14}$ J of energy; enough to vaporize 65000 m$^3$ of ice, or a hole 50 meters in radius and 4 meters deep.

If the ship has a 1 km ice barrier, it will need to be hit with at least 250 475 kt nuclear weapons sequentially in exactly the same spot to penetrate the ice armor, under optimal conditions and very favorable assumptions.

• So, how much would a B83 cause in comparison (which can release 1.2 Mt of TNT)? – Future Historian Jan 19 '18 at 15:37
• @FutureHistorian What is the ratio between 1.2 Mt and 475 kt? Scale appropriately. – kingledion Jan 19 '18 at 15:42
• So, basically, same results times 2.53. – Future Historian Jan 19 '18 at 15:44