Reason I'm asking, it's seemed to me that the best way to create an atmosphere on Mars is by moving an icy planetoid/comet/Kuiper Belt object and basically smashing it into mars, though I think a low orbit, perhaps near the Roche Limit where it breaks apart pretty easily would be more practical than a crash landing.

Roughly speaking, comets are mostly too small to be that interesting. To create a 1 ATM atmosphere you'd need something in the range of 120 KM in radius - figuring some of it would be water, some of it would be dust and rock, some of it would be CO2 that you'd want to turn into plant life.

In terms of moving the object, you could do a gravity assist around Neptune and using Neptune, you could adjust the trajectory either towards mars or towards another planet for another gravity assist. The object could be moved by large mirrors, melting some of the ices on the surface creating a propulsion or by explosions. I think that would be the easiest way.

And once in orbit, the process is rather simple, the sun would melt some of the lower freezing point ices, and in a near roche limit orbit, it could be broken apart fairly easily given the tidal forces already in play.

Figuring a Kuiper Belt object is made up of a combination of dust, frozen H2O, CO2, CH4, NH3 some frozen N2, a bit of CO, maybe some NO, some O2, a sprinkling of more complex molecules, a little Argon, Hydrogen and Helium, it's not something you'd want to breath, but it would create atmospheric pressure, perhaps similar to earth. (120 KM radius, figure 1.5 density, mass would be about 10 billion billion KG - about twice the mass of the earths' atmosphere, but if half of it is rock, dust and water-ice - then you've got about what you need for 1 ATM on mars).

So, while this would by no means be easy or quick, it certainly seems possible - given a thousand years or so, and we'd want to be careful not to miscalculate and send a 100 KM radius object into a path where it might hit earth - as that would be a bad day, but, with care, this should be doable - so you have an object in orbit around mars that's gradually breaking up and it gives the planet an atmosphere and water.

Is there a way to turn this quantity of NH3 and CH4 as well as CO and NO and other things we maybe wouldn't want to breath, into N2 and CO2 and O2? again in a relatively short (1,000 years or so) span of time, or would that be even more difficult and take a lot longer than getting the object where we wanted in the first place?

In short, the question is, can icy space objects be used to create planetary atmosphere for terraforming? It's always seemed so, to me, but I'm curious if my assumption is incorrect.

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    $\begingroup$ The mirror idea might be problematic. 50 AU from the Sun the light is only 1/2500 of that on Earth. I think one needs nuclear power out there, or maybe in situ chemical fuel production. $\endgroup$
    – LocalFluff
    Mar 23, 2015 at 12:24
  • $\begingroup$ Why stop at 1 Kuiper Belt object? Scale it up! Earth, and Earthlings probably have the math, and the computational ability to use gravity to execute a 'cascade' effect. We steal momentum every time Oberth manoeuvre is used. Do it the other way around with time in your favour. A few lifetimes down the line there could be N KB objects barging into Mars' orbit - all having started from a smaller body. Of course, to say the least, we would need a fairly detailed map (mass+orbit) of KB objects to begin with! $\endgroup$
    – Everyone
    Mar 27, 2015 at 7:38
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    $\begingroup$ If Phobos has ices internally, how about pushing it towards Mars? @SF. $\endgroup$
    – LocalFluff
    Dec 16, 2015 at 9:23
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    $\begingroup$ @userLTK: Instead of single big, you should be looking at thousands of smaller ones. And a lot of them contain ice. $\endgroup$
    – SF.
    Dec 26, 2015 at 11:32
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    $\begingroup$ The biggest problem I see with this is not actually moving the object but the logistics of the timing of doing it. If you are going to crash something into the planet, especially of the sizes you are talking about, you are going to want to do it before the planet is populated but who's going to finance such a thing and wait such a long time if their aren't already people there? If there are already people there then even much smaller objects could be potentially catastrophic if off by even a little bit. Seems like you'd need to capture something in orbit for this to work. $\endgroup$ Aug 28, 2017 at 18:38

1 Answer 1


Here Robert Zubrin and Christopher McKay talk about terraforming Mars. Have your browser find "Moving Ammonia Asteroids" and it will take you to the relevant section about 3/4 down the page.

Zubrin and McKay talk about using a gravity assist. But using Saturn's gravity to toss down a 2.8 kilometer Centaur, not Neptune's gravity to toss down a 240 kilometer KBO. They figure .3 km/s would suffice to nudge an ice-teroid close enough to Saturn to throw it Mars-ward.

They also suggest vaporizing the asteroid's volatiles for the reaction mass. To accomplish this they propose a "quartet of 5000 MW nuclear thermal rocket engines". That's 20 giga-watts. The Palo Verde power plant is the largest nuclear power plant in the U.S. It is about 3.3 giga-watts.

So they're proposing sending the equivalent of 6 Palo Verde nuclear power plants out past Saturn. Not very plausible, in my opinion.

You seem to be talking about an object about a million times more massive than Zubrin and McKay's pebble. Let's say we could send it to Mars by nudging it near Neptune. For around .3 km/s, like Z & M's scheme. You'd need 20 million giga-watts. Sunlight's power density out among the KBOs is about 1/1000 of what we enjoy. You would need a very large mirror.

Then what happens at impact? An object falling from 30 A.U. would strike Mars at about 10 km/s (at least). It'd have a kinetic energy of about 50 mega-joules per kilogram. I believe most of the cometary volatiles would escape Mars. The impact would also probably blast off all Mars' CO2 atmosphere above the tangent plane at impact.

Terraforming Mars via volatile rich asteroids pre-supposes we could send massive power sources to the asteroids and establish extensive onsite infra-structure.

If we had this ability, why not just use the asteroidal volatiles where they sit? Developing the small bodies would give us far more real estate and natural resources than the outer surface of Mars. I make this argument in more detail at Terraforming Mars vs Orbital Habs.

  • $\begingroup$ Thank you for that. Great answer. I do want to point out, that, while it would be enormously difficult, a 120 mile radius icy space rock could create 1 ATM on mars, and you'd gain much more real estate than you'd give up. You lose real estate when you move many smaller objects to create the atmosphere but 1 big one and you come out significantly ahead. Still, I enjoyed your articles very much. Some very good points. Also, I think, the goal with a big one would be a near miss and close orbit around where the ice-object would break apart, not a crash land, but that's a minor point. $\endgroup$
    – userLTK
    Mar 23, 2015 at 15:52
  • $\begingroup$ Might be a better idea to use mass drivers to lauch small chunks of a Jovian moon, small enough not to leave major craters when they hit. (Assumes you have workable nuclear fusion as a power source, and cheap self-replicating machinery.) As for the drawbacks of in situ use: 1) Claustrophobia - would you want to spend your life shut up in a tin can? 2) Difficulty of creating a sustainable biosphere in that small a space; 3) Leaks. $\endgroup$
    – jamesqf
    Mar 23, 2015 at 19:11
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    $\begingroup$ @userLTK For planetary surfaces heat and pressure are obstacles that prevent burrowing too deep. This is no obstacle for asteroids, we could burrow right through the middle. The entire volume is accessible, not just the outer surface. I believe this is true even for Vesta. Ceres' center might have too much heat and pressure, but much of Ceres' volume would still be accessible. $\endgroup$
    – HopDavid
    Mar 23, 2015 at 19:49
  • $\begingroup$ All good points. The one thing Mars could give us (well, maybe, maybe not), that an asteroid or icy object can't is stuff like a sky, waterfalls, forests, wild animals, open space. We like those things, at least, I like those things. Though, probably smaller settlements come first. They're certainly easier. $\endgroup$
    – userLTK
    Mar 24, 2015 at 4:21
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    $\begingroup$ @BrooksNelson "3100 meter ice-teroid... OP mentioned 120 km would suffice, so about 40 more should do it." No, mass scales with $r^3$. You'd need $40^3$ more or about 64000. And even a 120 km asteroid is insufficient I suspect. $\endgroup$
    – HopDavid
    Aug 24, 2017 at 4:06

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