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Scenario for a novel:

  1. Earth mass planet (say between 0.8 and 1.4 Earth masses) captured as moon by migrating gas-giant (say 5 Jupiter masses) (not wedded to this setting, moon can be bigger or smaller, or I can jettison the gas-giant altogether, I just like the trope).

  2. Vehicle in orbit suffers unplanned re-entry.

  3. Vehicle is more or less without power/mechanics to effect normal re-entry. Maybe minimal thrusters for trajectory correction. Diameter of vehicle is approximately 30-meters. Poor drag coefficient. Sufficient shielding or other futuristic gizmos to protect passengers from heating.

I'm trying to figure out the timeline of initial velocity, deceleration, aerodynamic heating, and breakup, and whether the scenario I have in mind for getting characters off the vehicle before impact is at all plausible. Is any one here able to explore this idea with me, or signpost me on?

N.B. This is a science fiction setting, so while I'm looking for realistic physics, I have enough flexibility in the setting to "fudge" extremes that would otherwise kill ordinary people.

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  • $\begingroup$ The scenario you have in mind is no different from that of a reentry into Earth. Only the gravity and atmosphere of the planet where you're going to kill your characters matter. $\endgroup$ – Renan Apr 2 '17 at 10:27
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    $\begingroup$ For this question to be answerable, the atmospheric conditions and the initial orbital height of the vehicle have to be defined. $\endgroup$ – Sazanami Apr 2 '17 at 11:27
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    $\begingroup$ Does the vehicle have some sort of parachute or wheels and a long runway? If it has neither, the crew are all going to die, no matter what happens on re-entry. $\endgroup$ – Jarred Allen Apr 2 '17 at 14:31
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    $\begingroup$ Model it in Kerbal Space Program or something similar? $\endgroup$ – Schwern Apr 2 '17 at 18:49
  • $\begingroup$ Atmosphere is Earth like, air density changing with altitude and temperature. Starting altitude for the scenario, say 160km. $\endgroup$ – scicurious Apr 3 '17 at 10:51
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Copy from Shuttle Re-entry

If your vehicle is capable of surviving re-entry and keeping its passengers alive, it probably was designed for such a purpose (even if just as an emergency measure). Therefore it probably has some sort of aerodynamic lift-generating design that would give it reasonable 'flight' stability within a wide range of possible atmospheres, and give its pilots/occupants a chance to crash it in a more controlled, less splatty manner.

In that case, why not just copy the known statistics for Space Shuttle re-entry? De-orbital burn (or accident, in your case) is about 1 hour before landing. 30 minutes later, the craft is 80 miles in altitude, 5000 miles from its eventual landing (crash) site, and starting to heat up as the atmosphere gets thicker. You can read all about re-entry temperatures on Space Exploration Stack Exchange. The space shuttle was designed for 1500C but other landers have gotten hotter.

The lander drops below the speed of sound 5 minutes and 25 miles from the landing point, and hits the ground at ~220 mph (ouch!). Better have your seat belts buckled.

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  • $\begingroup$ Thanks kingledion :) I suppose I'm figuring a scenario where the "vehicle" is a portion of a larger modular vehicle (say a marooned colony vessel), that has separated as "a last ditch attempt to escape catastrophe". To up the drama, I'm de-powering the vehicle, except for where a fudge is needed to ensure I don't end up in a completely unrealistic scenario. I'm not sure to what extent the vehicle is designed as an emergency lander, but I'm allowing for materials science to be sophisticated enough to keep the crew mostly protected through heating and ordinary deceleration. (cont/) $\endgroup$ – scicurious Apr 3 '17 at 11:06
  • $\begingroup$ (cont/) I'm trying to figure out what I need to do to get the survivors off (some kind of personal bail-out device is the fudge for now), before the vehicle breaks up and then impacts. Thanks again for your reply :) $\endgroup$ – scicurious Apr 3 '17 at 11:07
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It's the 'unintentional' that is a stickler. With the shuttle entries I think they had a 2 degree window on the angle of their trajectory. Shallower than that, and you skip like stone off the top of atmosphere. Deeper than that and you hit thick atmosphere before you've shed enough speed, and you crisp.

The second factor is that an unintentional landing puts you down in a random place. On this planet random gives you a big chance of going 'splash' Early space flights deliberately did wet landings because they couldn't maneuver. A carrier can maneuver.

On LEO orbits, the change in velocity isn't big. 1/2 to 1 km/sec? Rest is done by the atmosphere. On something like the shuttle loss of power would doom the ship. It was fly by wire with a lot of computing happening between the joystick and the controls. I don't know if a fully manual landing was even possible.

The shuttle goes through several regimes -- initially there's not enough air for controls to bite, so it's done with thrusters. Then there is a hypersonic phase where it's at many multiples of the speed of sound. Then supersonic, then subsonic. The controls react differently at each stage. Without power you don't have instruments.

On a blunt re-entry body, I don't know how much it matters, as long as the load is balanced. There were reasons the early astronauts make jokes about being 'spam in a can'

The blunt capsules required parachutes to land.


The effect of a jovian mass passing the earth at a distance that didn't destroy the earth with tides I don't think would have much effect on an Earth orbiter, but I'd have to spend couple of days playing with an orbital simulator to convince myself one way or the other.

However: Our jupiter has an intense magnetic field, whcih traps lots of protons from the solar wind. They are lethal. So your re-entry isn't planned, but it is intentional. Someone realizes that there is a cloud of high energy particles and you have to scramble to get to the re-entery vehicle. The magnetic field can fry the main ships electronics. The initial descent scenario is blast. Shut down all electronics. Wait to swing around to the back side of earth. start the electronics, and make a landing.

In passing a two body encounter cannot make a capture. Baring collision they do a hyperbolic orbit around each other. If there is a third body involved, then it's possible.

This is relevant only if you are planning on the jovian body grabbing the earth and taking it with it. A far more likely event is that the earth will be thrown in a far more elliptic orbit, or just ejected from the system.

Gravitational energy is a conservative field. No friction. A body approaches, speeds up, passes, slows down. The slowing is equal to the speeding. The flypast is symmetrical. The total of gravitational potential energy and kinetic energy remains constant.

https://physics.stackexchange.com/questions/134819/how-can-a-planet-gravitationally-capture-objects

This one is a bit clearer:

https://en.wikipedia.org/wiki/Gravity_assist

However real captures are extremely improbable. But we've had 4-5 billion years to play cosmic pool with our solar system. You're seeing the pool sharks. The chumps and marks ended up in Jupitar, the sun, or cast into the outer darkness where there is much wailing and gnashing of teeth.

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  • $\begingroup$ Hi Sherwood Botsford, thanks for your reply. Can you expand on what you mean by "two body encounter" and "capture". Given this scenario requires some kind of bail-out before landing I'm trying to figure out if that is at all workable. See my response to kingledion above, if you're interested. Thanks again :) $\endgroup$ – scicurious Apr 3 '17 at 11:10

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