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In one of my universes, Humanity found a so-called "world anchor" on the Moon. They removed the anchor and returned to Earth. A few days later, the Moon exploded and a planet about the size of Mars appeared, just barely further than the Moon's orbit. The other planet, N'valdir, had a sentient species with technology barely better than modern Human technology. [EDIT- The N'valdir have technology equivalent to a K0.9 Civilization N'valdir, viewed from a 2d stellar map, would be pointed slightly towards Earth, at a rough 83 degree angle.]

1.) Would these two species be able to stop their planets from colliding? If so, with what technology?

2.) What effect would this have on the planets, and more so, to the species' on them?

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    $\begingroup$ Are the planets on a collision course in the first place? If not, the new planet might just sail past us like an asteroid, settle into a nearby orbit, or join up with earth as a double planet. It'll be highly dependent on N'valdir's incident trajectory and velocity. If it got from being undetectable in deep space to on our doorstep in a matter of days, it'll be moving really fast - unless it's aimed right at us, it'll just fly past, never to be seen again. If the planet was suddenly teleported into place with 0 velocity, it'll be a different story. How did it get here? $\endgroup$ – Nuclear Wang Nov 14 '19 at 19:16
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    $\begingroup$ Uh, when you say "the moon exploded", what exactly do you mean? 73 billion billion tonnes of stuff flying around in all directions, or being even partially converted to energy sounds like Really Bad News. Might be better for all concerned if it went to where N'valdir came from... $\endgroup$ – Starfish Prime Nov 14 '19 at 21:19
  • $\begingroup$ Given that your world anchor has the power to blow up a moon spontaneously maybe there's a hidden macguffin power source there? Nothing remotely realistic leaps to mind otherwise-- you'd need to be a type 2 civilization at least to move a planet. $\endgroup$ – Dugan Nov 14 '19 at 22:33
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the Moon exploded and a planet about the size of Mars appeared

If N'valdir is further away then the moon; but both the earth and N'valdir are caught in a gravity well with the moon. The moon then breaking apart due to tidal forces. That could have left both communities time to wonder if the result has a stable barycenter.

If stable then I would expect the remove/eat/enslave/communicate/mix/trade/relationships decision would depend a lot on social norms/need and physical/emotional appeal/compatibility between the races. But the need for both communities to survive the new 'seasons' due to the additional rotational center might require either, or both communities, to create biosphere homes if the results are not comfortable.

Otherwise, as @Halfthawed mentioned above, the best bet is likely be on a 'safe' part of the larger planet in a dwelling with its own biosphere or off planet in a craft to survive the collision. So possibly a battle for resources from both planets to build escape craft; if time allows.

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If the planets are on a collision course, they can't be stopped

Simple physics, really. A planet has mass, and it takes force to move that mass. The more mass, the more force. How much mass does the Earth have? 5.9 * 10^24 kg. Mars is slightly less, 6.4 * 10^23 kg. So if we were to move one of them, we'd have to move the Mars one. The problem is that we don't have anything near that amount of force. And even if they could, they'd face another problem - global extinction events.

See, you need to apply force to change the angular velocity. And applying force to planet would be problematic. Problematic in the sense that the energy would light the atmosphere on fire and kill everyone present. And planets colliding is like that, but worse because it's the full force of the planets. Both planets would smash like eggs, and everyone living on the surface would die.

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