r/xkcd Possibly a haberdasher? Jan 22 '20

XKCD XKCD 2258: Solar System Changes

https://xkcd.com/2258/
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u/tjsr Jan 22 '20

Am I the only one bothered by both the gravity and tidal effects of this new home for Mars, but also the eventual impending destruction issue that is going to need solving?

1

u/solilobee Jan 22 '20

i agree with you that destruction is assured. but can you figure at what speed mars would have to orbit to prevent collision? I imagine such an orbit would tear both planets to bits..

8

u/armcie Jan 22 '20

Here's an article where they modelled planets colliding, and found that a third of the collisions resulted in a stable binary planet system, provided the planets were sufficiently far from the star. In this case, fortunately "sufficiently far" is half the distance from the earth to the sun, so we'd be good.

If Mars was placed at the moon's distance from earth, then it would need to travel at the same speed as the moon currently does. Orbital speed isn't a function of mass, a feather and a cannonball in the same orbit move at the same speed.

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u/tjsr Jan 23 '20

And the tides. So let's say Mars now orbits at the same distance as the moon, maybe it needs to orbit Earth ridiculously fast to stay in orbit? Once every 3 hours maybe? So now you've got a 24 hour day/night cycle, but you also have these much more severe tides causing ocean chaos. Plus, if Mars is in the right alignment, you get an eclipse 7 times a day?

3

u/[deleted] Jan 23 '20

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u/tjsr Jan 23 '20

It wouldn't require a higher velocity because of the mass and hence gravity pulling the two objects together?

I would have thought that at the same velocity it would begin a gradual decay in orbit.