for what it's worth

The Citadel is rather pliable. Even if we assume super-compressed and specialized fictional alloys, its internal joints and struts aren't going to handle the pressures put on the overall structure. That is to say, it's going to warp almost immediately once it hits atmo, and the heavy heating its internal air pockets experience along with general ablation means it's going to rip and blow itself apart into smaller pieces very, very quickly.

The cinematics in Mass Effect 3 after the destroy ending show the lower-right ward separating at what I estimate to be around 2.5 km/s. Considering the Citadel is in stationary orbit over London (51° N latitude) and is pointed generally towards Pluto/Charon (17° inclination in 2186), that gives an angle of around 34°, so we can assume a vertical space-to-surface motion of around 1.3 km/s. We have to note the stationary orbit, as it means initial v is zero.

Impact models don't really do non-round objects with fiberglass densities, so we're going to have to take some liberties here. Note that I'm an astrophysicist and not a materials scientist or structural engineer, so accept that there's going to be some leeway in what I expect.

The ward is going to just start falling while maintaining a north-to-south general motion of about 2.1 km/s. Where the ward starts impacting the atmosphere depends entirely on how high above it Earth it actually was. I'm going to make things easy for myself, and assume it was just starting to kiss the atmosphere.

It's going to start experiencing intense bending forces between 30-and-40 km up, at which point it will start to split apart very rapidly. Dynamic pressures are around 60 kPa at 30 km of altitude, rapidly increasing to 150 or so by 20-25 km. By that point, heated pockets should have ripped apart most of the structure and torn the ward into pieces. Your negative drag acceleration is going to be between 1-2.5 g that isn't evenly distributed across the surface, so even long intact structs are going to bend like crazy.

As the structure breaks up, pieces with greater average density will fall faster as their terminal v is greater than the structure's as a whole. Intense heating will ablate these pieces into molten blobs of metal or vaporize as a polymer (I was going to text Sash and ask her the chemistry part of this, but she's teaching a class right now). This leads to the ward pieces leaving a long chain of airbursts and impact craters along its length and N-S direction of motion, probably a few hundred km long. Those airbursts are the biggest issue, as they'll flatten anything in their vicinity when they finally dump most of their energy into the surrounding air. Think of a long chain of thousands of kiloton explosions with a scattering of megaton explosions thrown in. As pieces impact at different times, pressure bursts will accelerate other parts into a wider band that spreads the damage outward.

The heat of the deorbiting is surprisingly small. A basic E=(1/2)m*v^2 gives about 4.4 x 10^18 J, which is a little over a gigaton. Not all of that is going to be thermal, so expect 200-400 megatons in equivalent thermal energy. The atmosphere along this path is going to be hellish, but overall it's not a doomsday event. Most of the ward materials are going to be vaporized and distributed throughout the atmosphere, so climatological effects are going to be worse as those materials will likely reflect sunlight or block it completely, causing a global cooling event. People on Earth are going to suffer some intense breathing problems for a few months until the particulate matter settles.

Fortunately, with the Citadel over London, the direction of motion is south over the Atlantic ocean. There will be some major tsunamis, but most of the material damage falls into the ocean and doesn't leave too much damage over land. Earth is going to be changed drastically, but this disaster is nothing compared to the Reapers' destruction across the planet in the months prior.

The derelict Reaper orbiting the brown dwarf in ME2 is around 2 km long, or twice the length of a standard Council dreadnought. The game states that dreadnoughts are unable to land on planets due to their immense size, but capital-class Reapers are able to do so freely due to having extremely large drive cores at their center. In ME1, other characters comment on how Sovereign is capable of pulling off sharp maneuvers that would tear apart other ships, and their shields can withstand most Council capital-class main guns. Even the SR-2's well-oversized drive can't manage to make the frigate land on every planet. Reaper cores are very, very big compared to other ships.

Following the firing of the Crucible, those Reaper ships are just… orbiting Earth. Hundreds or thousands of them. They still have whatever velocity they had prior to activating the Crucible, but they are completely shut down unlike the derelict Reaper. They'll experience atmospheric drag, start deorbiting faster, and finally start burning up in the atmosphere. We know from the derelict Reaper that they are hollow inside (see my earlier reblog for why this is a Very Bad Thing), and their outer hulls don't seem to be all that much stronger compared to a normal ship judging by how the SR-1's standard gun made short work of Sovereign at the end of ME1. They're going to break up, exposing those giant drive cores to the outside.

Fun things happen to metal when it descends through the atmosphere at high speeds, but the most important one here is that friction and heat on a metal surface generate current. That current is going to dance over the core, causing random fluctuations in the core and break that concentrated eezo apart if it hasn't already been torn up by the initial deorbiting. You have to shut down the derelict Reaper's core by shooting at it in ME2, so we already know they don't handle normal stresses all that well. Human biotics were first discovered following starship accidents that dumped eezo dust over populations, and it's stated in ME1 that the SR-1's drive core could have been at the heart of 12,000 Alliance fighters. We can assume from this that very small amounts of purified eezo are all it takes to affect biology, so having those big drives cores burning up and blowing apart in the atmosphere is going to scatter eezo pretty far across the immediate hemisphere below, with air currents spreading fine dust readily around the planet. Adding to the issue are the thousands of smaller Council and Terminus ships that were destroyed during the battle, as they will be even less likely to handle the stresses of deorbiting.

These problems could be partially mitigated if there were many large enough ships around Earth to tow upper-atmosphere Reapers away so only the lower-atmosphere ones were a problem. But, the remnants of the allied fleet fled when the Crucible fired. Many of those ships likely suffered damage similar to the SR-2, and won't be able to return for some time. What's left is too few to make much difference, and it's unlikely the allied fleet brought utility ships that focus on things like towing - those vessels are far away, and that expanse is even more distant now that the relay network has been destroyed. The people on Earth and the ships that may or may not return are the only ones that would know there's a problem. So the Reapers are going to deorbit, and there's really no one that can stop it from happening.

Every week I ask a basic question about your Shepard, vote and elaborate in the tags!

Note on this week's poll: I assume every Shepard does the requisite work to stay in shape. I'm asking about what they like to do, what they would still do even if they had a completely different job.

BANG!

Heavily inspired by Cowboy Bebop "Bang" pose.

Reference pose photos and progress sketch under the cut if anyone is curious :)

Scenery in Mass Effect 12/??

Reaperized Benezia 2: Electric Boogalo