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.

 

[cassius]

Thanks for the answers! So, the distance needs to be reduced (probably below 5,000 km) for the effects to remain the same. Except for the deflection of the planet towards the Sun... for that, a more massive planet would be needed. How massive exactly? At least half the size of Earth? Or would it have to be much more massive than Earth?

 

[Filip Larsen]

You may want to consider that the premise of a massive (natural) body of nearly same mass as the Earth passing so close, will be a very, very unlikely event (1) thus it will as such probably not make your story that more believable to those "in-the-know" anyway. So perhaps you are better off with a setup that are consistent for your story you want to tell and just leave the details hazy enough that noone will be the wiser.

(1) I would say it is extremely unlikely because
a) The presence of interstellar bodies that massive are (as far as I know) believed to be extremely rare (e.g. borderline non-existing) in the first place since the planetary body must have accreted around a star (which require a somewhat stable orbit over millions of years) and then sudden gotten ejected by something more massive (which also have to have been accreted around same star for even longer).
b) The likelihood of collisions between two massive objects in the galaxy, each on their own "random" trajectory, is astronomically unlikely. To give a scale of the likelihood of (near) collision consider that (if I recall correctly) in order for a 50% chance of at least two stars to have physically collided during the lifetime of our universe you would need to include around 15 billion galaxies in that measure.
c) The event takes place right in the timeline of your story, i.e. it singles out a very narrow point in time for the above events to happen in.
So, if you multiply the chances in a), b) and c) you get immeasurable close to zero for that event.

Even if you somehow involve aliens in the setup (we are in the sci-fi part of PF after all) I would still think it would require high-caliber magic to pull this off.

 

[cassius]

What is more likely? That we will travel through time, or that two massive bodies will collide? And yet, how many wonderful stories have been written about these two improbabilities?

Even greater magic would be required for two advanced civilizations to meet—considering the sheer size of the galaxy and the 'lifespan' of intelligent life, it’s a matter of extreme magic. Yet, you yourself mention the involvement of aliens :)

But perhaps it wasn't aliens who caused it, but God. At that point, we reach a probability of zero... and yet, the greatest bestseller in history was written about Him.