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Starships are big. Big things moving at atmospheric entry speeds do not bode well with planets. If a bus-sized asteroid can cause massive damage to an entire city, how much can a cruiser about 30 times the size of a bus with a powerful nuclear reactor do? It definitely isn't pretty, and that's a cruiser, forget a dreadnaught about 5 times the cruiser's size.

So, redundance for clarity's sake, how much damage could a starship really do if it crashed on an inhabited planet? A certain battle in my sci-fi story takes place partially in a massive dust storm kicked up by a starship crash relatively nearby as a result of the battle in space.

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    $\begingroup$ This heavily depends on the specs of the starship, as well as what the starship is carrying. $\endgroup$ – Aify Jul 5 '16 at 7:01
  • $\begingroup$ Well, coming up with precise specs for military hardware isn't really my forte, I'd say about 30 times the size of a bus, a crew of about 30-40, large nuclear fusion reactor, some heavy anti-ship conventional ordinance, (but nothing near the power of a reactor meltdown) and obviously reinforced alloy plating. (stronger and lighter than steel) $\endgroup$ – LUnacy45 Jul 5 '16 at 7:10
  • $\begingroup$ "Heavy anti-ship conventional ordinance" - is this all solid slugs, are these 'conventional' ordinance missiles with nuclear warheads, etc etc, those can change the answer heavily. However, if you just care about the size/weight stuff here's a handy site you can use: impact.ese.ic.ac.uk/ImpactEffects $\endgroup$ – Aify Jul 5 '16 at 7:15
  • $\begingroup$ Ahh... sorry. Perhaps a mix of high-yield conventional missiles and shells, with some relatively low-yield nuclear fission warheads. However, storage protocols would dictate that weapons with explosive ordinance be securely stored, but depending on impact velocity, it might not matter. $\endgroup$ – LUnacy45 Jul 5 '16 at 7:22
  • $\begingroup$ And the big question, how fast was it moving? The easy way to answer that is to break it down into what was it doing before it crashed? And why did it crash? $\endgroup$ – John Dallman Jul 5 '16 at 7:26
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If you tied 30 American school buses together and set them moving at a typical earth orbit speed, you would have 300 tonnes of matter moving at 10 km/s. A crash would equivalent to 3.5 kilotons of TNT. About 1/4 Hiroshima.

If instead of 30 school buses, you used one Ticonderoga class missile cruiser, the kinetic energy would be equivalent to 100 kilotons of TNT - about seven Hiroshima bombs.

References:

A School bus has a mass from 4,536kg to 16,329kg. For easy maths, I took a middle number of 10,000kg. Thirty of them thus mass 300,000kg.

The Vis-Viva Equation allows one to to calculate the velocity at one point in an orbit, given the highest point of the orbit.

The radius of the Earth is the lowest point of the orbit, where the ship touches the surface. That is 6,371,000 meters from the center of the Earth.

Geostationary Orbit is 35,000,000 meters above the surface. To pick a number for "in orbit", I chose 10,000,000 meters above the surface, which means a highest point of 16,371,000 meters from the center of the Earth.

The calculator I linked to gives the "standard gravitational parameter" a value of 3.986004418e+14 m^3/s^s for Earth. Wikipedia provides a list for other planets.

The calculator then gives a velocity when the object touches the ground of 10,039 m/s, which I'll call 10 km/s.

We can then plug the number into a Kinetic Energy equation and get the number of Joules of energy that will be converted when the object comes to a crashing halt. Since we kept the numbers easy, the result is 15,000,000,000,000 Joules.

Wikipedia tells me that one kiloton of TNT is 4,184,000,000,000 Joules, so simple division tells me that the energy release on impact is 3.585 kilotons.

For the Ticonderoga cruiser, the only change is the mass, and Wikipedia says that the Ticonderoga masses 9,600 long ton. A long ton is 1.016 metric tonnes. That means a Ticonderoga masses 960 times more than one school bus, or 32 times more than 30 school buses, so the impact energy goes up by a factor of 32. That actually gives you 114 kilotons, which I rounded down to 100 kilotons.

And finally, the "Little Boy" Hiroshima bomb had an estimated yield of 15 kilotons.

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  • $\begingroup$ Could you elaborate on the formula you're using? $\endgroup$ – AmiralPatate Jul 5 '16 at 9:19
  • $\begingroup$ Kinetic energy E=0.5*m*v^2. This all has to go into the atmosphere or the ground to bring the ship to a stop. I then convert Joules to Kilotons of TNT. I ignore ablation in the atmosphere because we are talking about a military vessel which is presumably armored enough to resist damage from re-entry. Masses, speeds and the number of Joules in 1 kiloton of TNT all sourced from Wikipedia. $\endgroup$ – Simon G. Jul 5 '16 at 10:21
  • $\begingroup$ @SimonG. you got my vote, but it will still greatly improve your answer if you can link up to the wikis you are using in your answer ;) $\endgroup$ – Renan Jul 5 '16 at 11:14
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Ok, lets do the math for various different speeds.

First, lets see what happens when we crash a naval ship into New York at different speeds.

30 double decker busses have a volume of about 3375 cubic meters.

So I'm going to work with a ship with a similar displacement, Type 051 destroyer, Displacement: 3,670 tons

Destroyer

Lets look at the 2 extremes.

LEO orbit:28,080 km/h

A ship this size in LEO re-entering the atmosphere is carrying a mere 1.013×10^14 joules of kinetic energy or about the same as a 24 kiloton nuke. Most of that would be dissipated in the atmosphere as heat during reentry. I wouldn't want to be too close to the point of impact but it wouldn't be too bad.

Interstellar battle at 0.1c, the dead hulk then plows into a planet.

When the ship hits the atmosphere it's carrying 1.507×10^21 joules of kinetic energy. The equivalent of 3.602×10^11 tons of TNT, for comparison the Tzar bomba was 5.8000000x10^7 tons of TNT

If it hit New York the calculators for nuclear weapons break down, seeing it's glow as it hit atmosphere would likely blind you and it might just bust through the crust of the planet. That much energy released as a nuke would look like this:

enter image description here

but at those levels it's hard to say much since it wouldn't detonate like a nuke though it's safe to say that most people in the orange zone would be very dead.

Pick something between the 2 depending on how big a bang you want at the impact site.

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how much damage could a starship really do if it crashed on an inhabited planet? A certain battle in my sci-fi story takes place partially in a massive dust storm kicked up by a starship crash relatively nearby

Normally you wouldn't get a dust storm; a crashing starship at orbital speed (which for LEO is around 7.6 km/s) would have effects similar to a small nuke, depending on its mass. Thus, a fireball and overpressure front.

Depending on propulsion, it could have effects similar to a not-so-small nuke; the engines and reactors would probably have an almost fault-proof and disaster-proof scram mode to avoid an unlucky hit turning into a catastrophic kill, and there's plenty of time for scramming on the long way down (even at 11 km/s of terminal velocity, you still need half a minute from low Earth orbit to the ground). Even so, what's not catastrophic in space could be in an atmosphere capable of conducting a concussion/blast wave as well as oxidizing fuels. Again, larger fireball, possibly radioactive fallout from the equivalent of a dirty bomb, and other effects of onboard unsecured ordnance.

I suppose that a starship might be equipped with deflector shields engineered to deflect incoming stardust. They would probably do so by charging the stardust electrostatically and confining it electromagnetically, somewhat akin to a Bussard ramjet. If the electrostatic part did not scram on impact and continued operating on the surface, it might kick off a sort of dust storm from Hell - similar to Clarke's lunar haze but on a far greater scale.

The same effect would probably play merry hell with electronics, creating an area where both the Eyeball Mark One and most augmentations would be unable to "see" clearly.

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  • $\begingroup$ Yeah, the dust storm is due to the planet it crashed into, a (mostly) arid planet with big sand dunes and plateaus as its dominant biome. $\endgroup$ – LUnacy45 Jul 5 '16 at 16:34
  • $\begingroup$ Is this planet Arrakis? $\endgroup$ – Caleb707 Jan 6 '17 at 3:22
  • $\begingroup$ The space battle could be "near" the planet but the spaceships don't have to be in orbit. Possibly some of them are travelling at interstellar or interplanetary travel speeds. Possibly the crashing space ship was decelerating from such speeds when disabled and thus crashes into the planet at many times the planet's escape velocity. If the space ship might have an arbitrarily large velocity the effects could range from goodbye to a few square kilometers to goodbye planet. $\endgroup$ – M. A. Golding Jun 3 '17 at 16:00

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