Reality Check: Rogue Planet Flyby Scenarios and Survival Feasibility

[cassius]

Hello everyone,

I’m looking for some feedback on the scientific realism of a specific catastrophic scenario for a story I’m working on. Here are the parameters:

• The Event: A Mercury-sized rogue planet passes Earth at a speed of 200 km/s.
• Closest Approach (Perigee): 70,000 km from Earth's surface.

The proposed consequences in the story:

1. Tidal Forces: Severe enough to rupture the Earth's crust, causing massive magma outflows.
2. Atmospheric Effects: The atmosphere heats up significantly due to the planet's passage. A portion of the atmosphere is "siphoned off" toward the rogue planet, leading to a dramatic drop in air pressure (making survival impossible without a pressure suit).
3. Surface Devastation: The superheated atmosphere causes all vegetation to ignite.
4. Orbital Shift: Earth’s orbit is deflected closer to the Sun, leading to the eventual "baking" of any remaining life.
5. The Survival Attempt: The protagonist wants to witness the flyby face-to-face. He prepares a bunker anchored into solid bedrock (to avoid being swept away) in a vegetation-free zone. During the perigee, he steps outside wearing a fire-resistant suit over an old pressurized spacesuit with a filtered air supply. He survives just long enough to see the surface of the rogue planet before dying.

Is this at least approximately realistic?
I’d appreciate any insights into the physics or orbital mechanics of this scenario!

 

[Bandersnatch]

So, back of an envelope, that's approx. 250 times the tidal forces of the Moon, at its closest approach, which would rapidly decrease on either side of that point.
The effects of the tidal forces are much harder to ascertain than their strength, but - fwiw - my intuition suggests that'd result in some nasty tidal waves and winds, maybe a bunch of earthquakes, but nothing of the apocalyptic magnitude you desire.
Other than that, I don't see a plausible mechanism for superheating or siphoning off the atmosphere. The latter maybe at a much, much closer approach.

You can get the feel for the magnitudes involved by remembering that
1- tidal forces scale linearly with mass, but with the third power of the inverse of the distance.
2- Lunar tides affect the crust on the order of centimetres, the seas - metres, the atmosphere - kilometres.

So you can sort of see that from the combination of mass and distance from the planet as compared to the Moon, you get the aforementioned 250 times stronger tides, and that these tides should cause maybe a couple metres of deformation of the crust - on the scale of the entire globe, mind you - maybe a hundred metres of global sea surface deformation, and a few hundred kilometres-high bulge in the atmosphere.

As for the orbital deflection - this a very small planet. Some 5-6 percent of the mass of the Earth. Rapid, one-off passage. Even in ideal flyby conditions you wouldn't get much of a change in the orbit. I'd be much more likely to mess up the Moon, though.
What I'd do, is I'd put the scenario into an n-body simulator, and see what happens. It'd let you adjust the mass and distance parameters until you get what you want out of the flyby.

 

[Hornbein]

I'm not going to do any calculations but feel that's not nearly enough to have such a large effect.