Effects of a supermassive ship crash landing on a planet

Question

My planet is a 20% larger than the earth, assume a similar atmosphere, but it can be changed to fit the narrative. My ship is not so large as in the original question, a cylinder shape, about a million tons, but this also can be varied to fit the need. Its dimensions are: L952meters x H105m x W105m It is an interstellar vessel designed for travelling through wormholes and therefore was made of incredibly tough material, some sort of graphene / hardened carbon say, so it can withstand the phenomenal pressures and temperatures of wormhole transitions. It was constructed in space for space travel only, and not designed for planet landings. But the ship has to 'crash' land on an uninhabited planet, it can handle re entry and land intact, but I want the impact on the planet to hold some sort of reality check and I think I may have gone too far.

As it lands the ship gouges a scar 100 meters wide, 20m deep and 20 kilometres long, and pushes up banks of displaced soil etc as 'levees' 20 meters above the surrounding land, making the total depth of the gouge 40m. The idea was the gouge crossed a river which flowed into it, bringing water to the crashed ship. The ship also 'bounced/skimmed' a couple of times before making the final grounding.

Assuming soft enough rock and soil, is it possible with the right speed and angle of descent for the ship to have displaced this much material and made these sorts of features and not cause widespread damage to the region, let alone an extinction event on the planet, and have the ship and its passengers survive intact?

Any advise on the physics that I can try to fit to my story I would be very grateful.

Answer 1

Ditch it in the river.

The phrase "water landing" is also used as a euphemism for crash-landing into water an aircraft not designed for the purpose, an event formally termed ditching. https://en.wikipedia.org/wiki/Water_landing

I could not find an elliptical crater described in our solar system as long as what you describe. Something as massive as your ship could carve out a long crater but 20 km is really long.

Here is an alternative scenario. The ship is badly damaged and among other problems, an engine is stuck on. With the steering capabilities that remain, the pilot tries to slow the ship against the atmosphere of this planet by entering at a very steep angle, engine still blazing. On the ground, the engine continues to fire, pushing the ship along the ground like a giant bulldozer. The pilot continues to try to steer with whatever is available, heading for the water. Finally the ship gets to the river, falls in, and the water entering the ship shorts out the engine.

I am picturing the scene as pilot struggles with the ship to keep the nose up and the remaining crew watch the front view screen with huge gouts of earth and rock being thrown up and behind the ship.

Answer 2

I don't think this can be done

You're falling in a ship that isn't designed for atmospheric flight. The only way to achieve your goal is to hit the planet with a "glancing blow" (on an acute angle). The problem with this is gravity, which is pulling on the massive (absolutely massive) ship the whole way down. Without wings, ailerons, rudders, etc., this ship's only choice to hold to the right angle is by the application of raw force, which isn't designed to operate this close to a planetary surface and would only believably be overcome.

Consequence: the ship drops like a stone with a circular impact crater. Worse, if we assume the ship is dropped in free-fall and calculate a 100 km drop, then the handy-dandy splat calculator tells us it hits with 9.8x10¹⁴ Joules of energy, which is the equivalent of a 200 kt nuclear warhead. Every m/s of initial velocity makes that worse.

So, let's assume you could hit the planet at an acute angle, what then? You have to be going fast enough to overcome the planet's desire to pull you straight down. Vector math suggests you need, lets say, 3X the planetary gravity acceleration, perpendicular to the planet to achieve an reasonably acute angle. In that same 100 km drop, we're talking about 3X 1.4 km/s = 4.2 km/s. The ground is what's stopping you, so basically 4X the original problem or 0.8 mt worth of nuclear explosion. You're heating the ground as you go and you're not gently pushing anything aside. You're throwing dirt for kilometers around.

but that's actually very unrealistic. From what we know about atmospheric re-entry (especially uncontrolled re-entry), those last numbers are off by at least 300%. And given that your ship was traveling "normally" before its accident, they could be off by thousands of percent. At those speeds, your angle of descent is pretty much irrelevant. You'll sink far enough into the ground/water on first impact that it could be reasonably estimated as a direct-hit event.

And this all assumes the hull is indestructible. If you're one of Larry Niven's General Products hulls, that's a given, but closer to real life, the hull, the crew, and the cargo, are spread in pieces everywhere pretty much with the first impact — and that's assuming they didn't all die in the fireball of re-entry through the atmosphere.

Conclusion

It's hard for me to see any ship without atmospheric control hitting the ground at an acute angle to cause a long Hollywood-style gouge. It's impossible for me to see it happening to a million-ton ship. Sorry.

But don't let this stop you from having fun with your story. Hollywood has done this kind of impact a fair number of times. This is a moment in your story, something that sets up the events to follow. Just do it and don't worry about the handful of people who will realize it's improbable.