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I have not had much success finding out what the effects of bombarding a planet with comets or asteroids could have on the temperature of that planet.

I expect it would add some heat, and I understand it would be temporary. I understand that if it was a very large asteroid or a moon or something it could heat up the planet so much as to make it unlivable for millions of years. I guess I am more interested in the kinds of bombardments that could be done on a planet that already has inhabitants, at least living in enclosed habitats. I am sure there could be all sorts of effects depending on the composition of the object bombarding the planet, and the angle and location of bombardment. Maybe it could even kick up a bunch of dust or cause volcanic activity that would temporarily cool the planet?

I am mostly interested in the possibilities of adding heat to a planet via bombardment and I am wondering how much heat could be added compared to how devastating the impact would be and how long the heat would last. (Aside from the effects of whatever was in the comet being added to the atmosphere.)

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  • $\begingroup$ Just the hit, or the ensuing wildfires and soot $\endgroup$ – JSCoder says Reinstate Monica Aug 21 '18 at 2:20
  • $\begingroup$ I would be interesting to see the entire sky covered in comet trails. Enough small comets that burn up in the atmosphere so it doesn't impact the inhabitants, but add enough energy via burning up to increase the temperature. $\endgroup$ – Shadowzee Aug 21 '18 at 2:34
  • $\begingroup$ You have the 'terraforming' tag here, so should we be assuming the objective of this exercise is take a cold planet and heat it up to make it more suitable for human life? $\endgroup$ – Morris The Cat Aug 21 '18 at 2:57
  • $\begingroup$ @MorrisTheCat Yes. $\endgroup$ – Moss Aug 21 '18 at 3:28
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As you've mentioned, you've out some very important pieces of information to give a definitive answer, but perhaps this link would be a good start. Select the attributes most closely representing your world and then click on "Data View" to see the amount of kinetic energy imparted. From this you can get a pretty good idea on the largest heat/energy component in your system.

This of course ignores parts of the equation such as the heating created by the body entering your planet's atmosphere/ejecta removing energy, but it's a pretty good start.

The nice thing about the impact calculator is it gives a good visual on crater impacts and the potential for fireballs.

The thermodynamics behind how 'long' the heat lasts would vary greatly depending on a great number of factors, but suffice it to say, with energy being added in the Chicxulub crater event, it will be around for quite a while...

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  • $\begingroup$ That's a handy calculator. And the comparison to different energy levels on the Wikipedia page is very useful too. This is probably as good an answer as I'll get. But I'm still not sure what to make of the amount of heat vs. destruction cause by an impact. It looks like it would be a fairly large asteroid just to get energy equivalent to a day's worth of sunlight on Earth. I suppose if a planet was bombarded repeatedly with these moderately destructive asteroids it could warm things up significantly. But would most of the heat just end up in the atmosphere and dissipate quickly? $\endgroup$ – Moss Aug 22 '18 at 11:35
  • $\begingroup$ Maybe a better approach would be to break up large asteroids before they hit the atmosphere so that the bits could burn up before hitting the ground? Would a long rain of meteors be a way to heat up a planet? I suspect there are better ways. $\endgroup$ – Moss Aug 22 '18 at 11:37
  • $\begingroup$ But people talk about hitting Mars with comets to add water and atmosphere. It seems you would need to hit it with a LOT of comets to make a significant contribution to Martian oceans or atmosphere. So, after so many bombardments (done in a relatively safe/gentle way), would the energy from the impacts alone (or burning up in the atmosphere) raise the temperature significantly, and for how long? $\endgroup$ – Moss Aug 22 '18 at 11:42
  • $\begingroup$ Basically you are describing the creation of Earth. Yes, it takes a long time by known living being standards, but galactically it was a 'drop in the bucket'. $\endgroup$ – ColonelPanic Aug 22 '18 at 13:15
  • $\begingroup$ The problem with bombarding an inhabited planet, either with one big thing or many little things, is, well, the bombarding part. The amount of energy imparted is going to be about the same. The only real way to prevent this is to disperse some of that energy far enough away from your planet to effectively make it meaningless. This means slowing down any body you plan on having hit the planet to control the amount of energy impacted. Or you can spread it out over a much longer time to make it survivable to the planet's inhabitants. $\endgroup$ – ColonelPanic Aug 22 '18 at 13:18
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Atmosphere Thickening Retain's Heat

Instead of impacting the comets into the planet why not have them do aero-braking maneuvers through the upper reaches of its thin atmosphere? Comets are made from water, rock, and CO2 so if you did enough maneuvers with enough comets you could noticeably thicken the planets atmosphere. A thicker atmosphere retains more of the heat that the planet is already getting. Its basically using global warming and the greenhouse effect.

Other Options To Consider

If your planet is geologically active enough it will have geological out-gassing that raises the atmospheric pressure, but maybe your planet's magnetosphere isn't enough to retain the atmosphere so its being stripped away by the solar wind. This is what happened to mars. The thin Martian atmosphere is actually only held in limbo where it is at by geological out-gassing. If a large di-pole magnet were placed in an orbit between Mars and the sun it would only be about 20 to 50 years before mars had an atmosphere again. The temperature differentials between light and dark would stabilize, and liquid water would begin to pool. Within 100 years the icecaps would melt and we'd even begin to see small shallow oceans on mars again. Maybe your people could utilize a similar method to regulate their own atmospheric pressure. Even Earth has solar stripping of atmosphere, if we were to apply a shield similar to the theorized Mars one we would see atmospheric pressure rise with a subsequent increase in global average temperature.

We're talking a really, REALLY powerful electromagnet though. Minimally it would require a large fusion power source and would be using more energy per day than all of humanity puts out per year currently. But its scifi, stuff is supposed to be big. Plus, according to NASA it would actually work (we just have no idea how to power it yet or how to build and put an object that large into orbit.)

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  • $\begingroup$ I have heard about the things you are talking about, however I am specifically NOT asking about thickening a planet's atmosphere. Is that really true what you are saying about the amount of out-gassing on Mars? Do you have any sources to back that up? $\endgroup$ – Moss Aug 23 '18 at 2:38
  • $\begingroup$ phys.org/news/2017-03-nasa-magnetic-shield-mars-atmosphere.html $\endgroup$ – TCAT117 Aug 23 '18 at 3:45
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Bombardment can indeed cause heating, both directly and indirectly. The mare on the Moon are believed to be lava flows created when the Moon formed for instance. This heat quickly radiates away. At least quickly on geological timescales. More long-term heating is also possible though. The bombardment would also deposit large amounts of radioactive elements, which as they decay generate heat. It is believed that that is a significant reason why Earth still has a molten core.

But if there is already a significant atmosphere, the bombardment can cause long-term cooling. The soot and dust blasted into the upper atmosphere will block a large amount of sunlight.

Which effect wins out depends on a lot of factors. Dense asteroids that don't break up before they hit the ground will cause a lot more dust to be launched than asteroids which explode in the air. If the planet is far from its star or the star is dim, there isn't much light to be blocked anyways, so dust won't matter as much. How strong the weather on the planet is will also affect cooling. Stronger winds will mean dust will stay in the air longer, meaning the cooling effect will be stronger and last longer. Big enough asteroids will actually cause secondary impact events as debris is blasted in space and falls back down. It is believed that the asteroid that killed the dinosaurs did this, sparking forest fires all over the world.

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