Take a large spacecraft. I'm thinking about 1,000,000 metric tons and 3,000,000 cubic metres, but I don't think a factor of 10 either way would change things.

The ship is heavily designed and capable of atmospheric flight. There are technobabble FTL and STL drives. The ship can maintain several G for several days. The drives do not operate during this scenario, I mention the acceleration and delta-V to give an idea how strongly the ship is constructed. Think of the Nostromo from the Aliens movie -- steel frames and hull plates, not lightweight composites. Under ordinary circumstances, the ship is capable of controlled reentry, atmospheric flight, and takeoff.

Now assume that this ship is left in low orbit, which decays over time because of atmospheric drag. (This will take some time, given the mass to surface ratio, but how long isn't the question.) At some point, the aerobraking accelerates and the ship crashes on the planet.

What kind of wreckage will result? Will most of the wreckage be in a confined area? Can there be something which intrepid adventurers can actually enter and explore?

Historically, there was the Columbia disaster. The Columbia disintegrated into relatively small debris. So did Skylab, yet the tank was recognizable. I would like a answer, but I realize that's too much to expect. So I made it instead.

Alexander asked in the comment if it was tough and relied on aerobraking, or if it relied on braking with engines. My assumption is that it is tough, yet usually uses engines to brake -- toughness so that it doesn't need an overhaul after each landing, engines for controlled flight.

Separatrix assumed that there would still be a crew. I was thinking of a derelict, no crew and the power for the computers and maneuvering thrusters is long gone.

  • $\begingroup$ The answer depends on what kind of "controlled reentry" it is designed for. If the ship is very tough and relies on passive aerobraking, it may land in one piece. If it needs to use its engines to drop its speed to manageable levels, it will likely disintegrate at high altitude. $\endgroup$ – Alexander Apr 22 at 8:46
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    $\begingroup$ Not exacly the same sort of question, but I asked about something similar a bit back.worldbuilding.stackexchange.com/questions/196007/… You might find the details usefull. $\endgroup$ – MrDracoSpirit Apr 22 at 9:55
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    $\begingroup$ Being "heavily designed" works against the ship; it remains in one piece all the way to the surface where it impacts at meteorite velocities of several kilometers per second (i.e. dozens of times the speed of sound). The result is, predictably, a very large crater with no pieces of debris larger than a fist being found. $\endgroup$ – GrumpyYoungMan Apr 22 at 15:46
  • $\begingroup$ Such a nicely written question, wow $\endgroup$ – MolbOrg Apr 25 at 9:08

Stronger is not better, unless you can reach really ridiculous strengths.
If a ship reenters in an uncontrolled way, it will tumble and fragment.
What can vary is what then happens with those fragments.

Anything that is built like an anvil will just experience some surface melting, leave a long trail in the air, and reach the ground with a rather loud thump. Very much as a metallic meteorite does, if it enters at a very shallow angle.

Anything that is build like a balloon (such as the tank from Skylab) will slow down quite rapidly in the upper atmosphere, losing only a bit of its outer layer, and the gently fall down to the ground. Satellite fueltanks are notorious for doing so, as they combine the requisite high strength and very low density needed for this to work.

Note that the manned capsules such as Apollo, Soyuz, Dragon also fall in this category. They are very low-density, reasonably aerodynamic shapes that lost their worst velocity very high up, where they can do so safely without experiencing too much heat or deceleration at one time. They are also guided, a bit, to make sure they stay in the correct regime during reentry.

Anything in-between will go through hell. Large panels, regardless of material, will very rapidly become small (sub-meter-size) pieces of panel. They are then subjected to blast furnace heats. Bits may survive, but not as much recognizable.

For your vessel, which sounds as if it is build much like a navy frigate (heavy build, strong, but not massively armored), and which is reentering at the absolute minimum speed and angle possible due to natural orbital decay, I expect to see a long, wide debris field of the shredded stuff, possibly as much as 600km long and 20km or so wide. Followed by the heavy bits impacting in an ellipse maybe 10km wide by 30km long at the end of the path. This heavy impact field will experience many hundreds of impacts, each capable of digging itself well into the ground but not making actual craters of impact. Velocity at impact high subsonic, maybe 800km/h or so and each chunk massing a few hundred kg? Possibly a few heavier but no faster impacts from the big structural beams, engines, or any other dense objects.

One possibility: Your vessel is designed to reenter. If the design is good enough, that might include passive stability sufficient to stabilize it! In this case the debris scenario is all out the window. Your ship will reenter, "glide" down at a shallow hypersonic path, and impact the ground while still at high supersonic speed. In this case, better not live in the city where it lands, neither the city nor the ship will be recognizable afterwards.

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    $\begingroup$ Yes, it is designed for controlled reentrry and also to survive survivable accident scenarios. So passive stability sounds good. If it comes to the shallow hypersonic ditching, will the ship really be unrecognizable or just visibly dented? $\endgroup$ – o.m. Apr 22 at 10:30
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    $\begingroup$ @o.m. in the case of it entering the atmosphere within normal operating parameters, you're looking for impact resistance. The chances are it'll still smear itself across a considerable swathe of planet, even if it makes a water landing. If you're using a General Products Hull, nothing inside will survive, though the hull itself may be intact. Add Inertial Dampers and you can probably have the crew walk away. $\endgroup$ – Separatrix Apr 22 at 11:40
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    $\begingroup$ @o.m. When running into the ground at several thousand kilometers per hour, *Splash* happens. It does not matter what you are made of, you splash. Even a solid anvil hitting the ground at that speed will splash. You would need some technomagic doubletalk (like abovementioned General Products Hull) to excuse anything other than Splash happening. $\endgroup$ – PcMan Apr 22 at 13:29
  • $\begingroup$ @Separatrix: A GP hull equipped with a stasis field can collide with a star and the occupants will be just fine. I expect that high-speed lithobraking would be comparatively little trouble. $\endgroup$ – Codes with Hammer Apr 22 at 19:16

It's not the fall you should be worried about, it's the stop at the end

Your ship is designed for atmospheric flight, which suggests that it's some kind of aerodynamic space plane. If it's designed carefully, it could theoretically be built to survive an unguided trip through the atmosphere. The aerodynamics of the ship could be designed so the gradually increasing atmospheric force would guide it into a certain position that maximizes survivability. Also, if you have the technology to build such a massive ship, you probably have the technology to create a massive heat shield. Even if you used the technology available in the 1980s for the Space Shuttle (and just a little hand waving), you could come up with a scenario where reentering in one piece is possible.

The problem is going to be the landing. At the risk of stating the obvious, one million tonnes is heavy. Even if the vehicle slows itself through clever design and survives reentry, you're going to have a huge amount of mass hitting the earth. I suggest playing with this calculator to estimate the impact force. Plugging in some dummy numbers suggests your steel ship splats and buries itself in a big hole. I don't think you're going to have much left to explore.

If you want debris that's explorable, you might actually want the ship to break up in reentry. Maybe the designers built certain compartments with emergencies in mind. For example, the bridge may have been surrounded by armor, like how the A-10's cockpit is surrounded by a "titanium bathtub" to protect the pilot. That way the commanders would survive a meteorite impact or enemy attack that would rupture the hull and depressurize parts of the ship. So as the ship passed through the atmosphere, it would break into many pieces, with the armored bridge surviving in one piece. Maybe the builders planned for this scenario and added parachutes that automatically deploy in the event of reentry. That would give you a large compartment to explore after the ship crashed to earth.

EDIT: Changed "crater" to "hole" and adjusted other wording per comments from Keith Morrison.

  • $\begingroup$ A ship in a decaying orbit is not going to leave a crater much larger than what you'd get from dropping it from high above the ground.. $\endgroup$ – Keith Morrison Apr 22 at 15:16
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    $\begingroup$ The details of the fall will matter because it might (or might not) shed a debris track pointing to the crash site. $\endgroup$ – o.m. Apr 22 at 15:31
  • $\begingroup$ @KeithMorrison Dropping a million tonnes on soft ground at terminal velocity will leave a mark. $\endgroup$ – Andrew Brēza Apr 22 at 19:24
  • $\begingroup$ @o.m. One option would be for the vehicle to break up into lots and lots of pieces and have the armored bridge survive for the reasons described above. That would give you a debris trail leading to a compartment that's ready to explore. $\endgroup$ – Andrew Brēza Apr 22 at 19:25
  • $\begingroup$ +1 for making me smile with "one million tonnes is heavy" :-) $\endgroup$ – Tom Apr 22 at 21:01

IF the ship is aerodynamically stable, both during the hypersonic and supersonic parts of reentry, and after atmospheric drag slows it to subsonic speeds (think Apollo or Crew Dragon), then it'll be mostly intact when it hits the ground -- perhaps completely intact, unless it depends on retracting fragile parts like antennae and solar panels to protect them during atmospheric flight.

Do note that, per the question, this ship is only about 1/3 the density of water; this is more comparable to an oceangoing freighter (running empty) than a bullet or meteoroid.

Unfortunately, when a ship built the way you describe does hit the ground, it'll be ugly. No pilot to slow down at the last moment, no one to extend landing gear or legs -- the result will be a crater.

Not the kind of crater you'd get from a meteoric impact, of course -- you're talking a few hundred meters per second (even a ship built as you describe is mostly empty space, after all) rather than several kilometers per second. What you'll get as far as ground effects is much like what you'd see when a disarmed bomb is dropped from an aircraft -- only multiplied by thousands, and quite likely modified by running out of soil depth so bedrock is involved. Think small strip mine -- and inside the crater, plus scattered downrange over a debris field of at least several hundred meters radius, will be ship parts and contents.

In the end, it'll look much like an oceangoing freighter or liner had crashed at airliner speeds. There will be a lot of recognizable (though probably not reusable) ship parts, a lot of twisted scrap, and a few small or very durable items (purser's safe, for instance, or a passenger's shaving kit) will be nearly intact -- the uppermost superstructure (i.e. the part traveling last when it strikes) will be the most intact, as the lower structure will have absorbed impact energy like the crumple zones in a modern car.

Of course, this depends on the shape of the ship. If it's shaped like Nostromo, other answers ("it'll tumble and break up at high altitude") will apply. If it's shaped to reenter with minimal propulsion, however, and more like a "capsule" than a spaceplane, this is how its likely to occur.

  • $\begingroup$ It is not likely to ever slow down to subsonic speeds. This is a Million -tonne aerodynamic hunk of metal hurtling to the ground. Terminal velocity is likely to still be high supersonic. $\endgroup$ – PcMan Apr 22 at 15:00
  • $\begingroup$ @PcMan if it's designed for reentry, it could presumably be assumed to be designed to slow down to subsonic during reentry. $\endgroup$ – Keith Morrison Apr 22 at 15:18
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    $\begingroup$ @KeithMorrison of course it is. by deploying control surfaces, changing its angle of descent, firing retrothrusters, etc.. these controls are NOT AVAILABLE... a boeing 747 is also designed to be stable, almost fly itself, Try flying at cruising with one, then switching off and ripping out every control system, every electrical circuit, every hydraulic line. I suspect the landing will not be very smooth!! The OP's spaceship is an unattended derelict, undergoig unintended reenttry $\endgroup$ – PcMan Apr 22 at 15:23
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    $\begingroup$ @PcMan That depends very much on the shape. The density given (about 1/3 that of water) and the fact it's a decaying orbit (giving a near-level entry, as opposed to coming in at a high angle) mean a suitable shape might easily slow well below sonic speeds by the time it's well down into the troposphere. The sectional density of an Apollo spacecraft or Crew Dragon are such that they have a terminal velocity around 200 m/s. Much faster than a skydiver, but a lot slower than, say, a gravity bomb. I did say "more like a capsule than a spaceplane" after all. $\endgroup$ – Zeiss Ikon Apr 22 at 15:52
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    $\begingroup$ @ZeissIkon if you were to scale up the Apollo capsule by 56 times (linear dimension), to match the mass of the OP's craft, it would fall a teensy bit faster. About mach 2.6, by my calc. $\endgroup$ – PcMan Apr 22 at 15:58

It might land almost intact, definitely recognizable as a ship

Have you seen how slowly the Spacex Starship glides down? If it wasn't fuelled and ready to explode when it's fuel tanks rupture I think it would crash mostly intact, albeit folded up. Look at how recognizable some of the wreckage is (although this is after lighting its engines and at least halving its freefall speed): enter image description here

At 120tons dry mass and around 30tons of fuel the density of the nearly empty Spacex Starship falling down is 160kg/m^3. The velocity of free falling objects is actually determined by their surface area, but as long as your ship isn't built like a spear, then density will at least give a good ballpark figure. Plugging the numbers for your ship, it's 300kg/m^3. You say you don't mind changing that by a factor of 10, so halve your mass or double your volume, or make it a huge flat and stable pancake. The classic space saucer shape should work amazing! Especially since it's also conductive to being stable and presenting a huge rounded surface to survive re-entry; like a scaled up version of today's capsules.

  • $\begingroup$ Nope. Falling speed is not governed by volume density, but mass per frontal area. If OP's ship has the same shape as SpaceX StarShip, it will have 18 times the mass per surface presented to the wind. Thus terminal velocity of about 2.7 times as much. Starship stabilized at a falling speed of some 85m/s, so this thing will stabilize at 229m/s. High subsonic speed. if it is belly-flopping, but an atmospheric flight vehicle will be flying nose-first, not bellyflopping. So much less surface area presented, thus higher speed. $\endgroup$ – PcMan Apr 22 at 15:06
  • $\begingroup$ The problem isn't the landing, the problem is the reentry. If it's not built for an aerobraking reentry, it'll be shredded by a hypersonic blowtorch. $\endgroup$ – Mark Apr 23 at 3:08
  • $\begingroup$ @Mark re-entry might not be too big of a problem. It seems like heating per surface area decreases with the square root of the "nose" radius. Current capsules have a few meters in nose radius, while this could have a few hundred to 1km radius (I'm imagining it's the size of a large cargo container ship). The heating per surface area will be 10 times less so it might survive even with ordinary materials instead of heat shielding tiles. $\endgroup$ – csiz Apr 23 at 4:35
  • $\begingroup$ @PcMan you're right on the freefall speed scaling. It would have to be as thin as starship's diameter to have a similar freefall speed. But then if it's a 9m thick ship and 1km wide, it'll be much thinner than a pancake and probably even less structural. $\endgroup$ – csiz Apr 23 at 4:38
  • $\begingroup$ @csiz, reentry heating is complex, but in general, it's a case of "the faster you stop, the hotter it gets, but the less time you spend exposed to that heat". Something big and hollow (eg. a cargo ship) will likely stop fast enough to not melt, but those same braking forces (50-100 times the force of gravity) will simply shatter it. $\endgroup$ – Mark Apr 24 at 3:06

As has already been mentioned, you're going to splash. What you have to do is control the splash.

Other answers have discussed passive stability on a reentry glide path. Now we need to consider how the designers could have intended a crew to survive the sudden stop at the bottom. After all, any ship designed to go into a planetary orbit has a risk of a rapid unscheduled landing on the planet.

Once upon a time, cars were steel boxes such as the ship you describe, and people died in what are now considered entirely survivable collisions. Nowadays we design the car to sacrifice itself to protect the occupants.

Your passive stability puts the crew quarters at the back. The engines have been jettisoned to prevent an explosion killing the crew. Everything else about the ship is crumple zone.

  • The crew have crash pods
  • The crew quarters are crash pods
  • The crew have escape pods
  • The crew quarters are an escape pod

As a designer I'd choose the last, for a story perhaps the second is best as you want to see sufficient destroyed ship and sufficient surviving ship. This ship isn't an unsinkable Titanic, it's a modern family car designed to protect the crew at all costs.

  • $\begingroup$ I'll clarify: There are no crew. The computers are dead. The ship was designed to survive/mitigate ordinary disasters, but not this exceptional case. $\endgroup$ – o.m. Apr 22 at 15:32
  • $\begingroup$ @o.m. Even if there's no crew inside any more, the ship would still most likely be designed such that, if all controls fail out with the crew still inside (which I'd definitely consider an "ordinary desaster"), the crew still has a good chance of surviving. This type of design has a significant influence on crashng without a crew. $\endgroup$ – Egor Hans Apr 23 at 7:38

The ship could land generally intact, with proper foresight in its "heavily designed" plans. (In other words, I read Separatrix's answer and figured out how to control the splash.)

Other answers have already addressed a spaceplane / lifting body shape for passive stability. All that's left is the last few hundred kph before impact.

Given the assumption that there is no crew, no computer control, just passive systems, the solution to a slow landing is mechanically actuated parachutes.

A purely mechanical accelerometer detects when the ship goes through a passive aerobraking sequence, and at the proper point releases a drogue chute. A simple pressure-based system (I'm thinking something like a plug door) ensures that this release takes place only in an atmosphere. Then the drogue chute pulls out the main chute, and a less-than-crash landing ensues.

Much like the Apollo craft had 3 chutes and only needed 2 for a safe splashdown, this ship would be built with N independent emergency parachutes, where some number less than N will ensure a less-than-crash landing on Earth.

This system requires neither electronic computers nor active crew. It was designed as a final layer of fail-safe for an incapacitated crew to make it to the ground.

  • $\begingroup$ It is the unplanned crash of a derelict, left in orbit far too long. There are engines for braking, as written in the question. They are no longer able to operate. So no parachutes because it was designed with engines. $\endgroup$ – o.m. Apr 23 at 4:54
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    $\begingroup$ @o.m. As mentioned in the last sentence, parachutes would still have been reasonable to include, in case the crew is still in board, but unable to operate the engines for whatever reason. $\endgroup$ – Egor Hans Apr 23 at 7:43

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