Using comets at Mars L1 point, allowing the tail to constantly add to the atmosphere in both volatiles and increased temperature from the infalling matter, as well as blocking the solar wind which comets are known to slow and divert. Then taking small 1 to 5 meter chunks (or larger) of the comets and dropping multiple a day in the Hellas Planitia, (lowest point of Mars), starting at the very lowest point there is a 10 km crater in the north central part of Hellas Planitia. Eventially creating lakes, capping the 10 km crater for colonization, dropping more comet rocks on Hellas Planitia (softer landings not hard).

The hope is to create a moderately tolerable climate in the 10 km (eventually) capped crater, 0.4 atm. And to create a micro climate in the lower elevation of the upper and western region of Hellas Planitia and eventually all of HP to grow foods, etc. (HP is very low compared to the surrounding terrian, and we may need to seal off some escape routes for the atmosphere to reduce strong wind tunnels forming for the higher pressure, though I do expect constant loss I want to minimize it.) Humans would walk freely under the eventually capped crater, but need oxygen and radiation protection in the HP areas (after the pressure has risen enough.) What issues do you see with this from a science point of view?

  • $\begingroup$ without wading too deeply into hypothetical waters, the same impact-plume and fallout that would product a 'nuclear winter' effect on Earth would do at least as much for mars, blocking much of the already spare sunlight from the surface. The thinner atmosphere might let the dust settle sooner, but Martian dust storms have been famously powerful since the 1970's (or earlier). $\endgroup$
    – Joe
    Sep 21, 2017 at 21:01
  • $\begingroup$ Why Lagrange 1? $\endgroup$
    – Karl
    Sep 21, 2017 at 21:17
  • $\begingroup$ The point of the softer landings is to reduce those negative effects of the impacts. If smaller or slower entering chunks need to be used, that can be done, slower using possible even parachutes. $\endgroup$ Sep 21, 2017 at 23:16
  • $\begingroup$ Lagrange 1 puts the comets directly between Mars and the Sun and I'm using the comet's coma to block the solar wind and protect Mars's atmosphere. As a bonus the tail they form will steadily 'rain' volatiles onto the planet. $\endgroup$ Sep 21, 2017 at 23:18
  • $\begingroup$ Like others mentioned, there are lots of gaps in this approach. The easiest way to build a colony right now would be probably to build one underground. It would be easier to build and air-tight it and it would safeguard humans from radiation and dust storms. $\endgroup$
    – Sachin
    Apr 7, 2021 at 6:37

2 Answers 2


From a planetary perspective, one comet will add a negligible amount to the atmosphere as the gas content of the comet is just too small compared with the atmosphere of the entire planet.

If you can somehow catch your comet and drag it to Mars (no mean feat) you will have a mixed bag of materials such as water carbon dioxide, ammonia, hydrocarbons, dust and other stuff, but keeping this dusty gassy snowball stationed at the L1 point will be difficult as gases venting in all directions will be pushing it off station. It will be more like trying to herd cats than move a large object. It’s likely to disintegrate if you start trying to move it around and you may well end up with a ring-like structure of gas and debris. The Mars-Sun L1 and L2 Lagrange points would also probably be unstable due to the presence of Phobos and Demos.

Capping a structure to withstand even 0.5 atm pressure over a 5km radius dome would be extremely challenging, to say the least. The whole structure would have to be totally airtight or all of the contents would leak out and distribute themselves around Mars.

Think of a cylinder 5m high (0.5atm pressure at this depth) and 10 km across filled with sea water. How would you lift it? That’s the magnitude of the forces you are dealing with except in your case you have the reverse issue of how to prevent that size of force from blowing the top of the dome off. Any flaws or faults rips or tears and the consequences could be unfortunate.

1-5 meter chunks dropped one per day would take a very very long time to have any effect. You would need a much faster rate for any reasonable timescale. These chunks of the comet are going to be highly unstable and frictional heating would cause them to disintegrate on contact with the Martian atmosphere if you were planning on slowing them and just letting them fall to the surface by gravity. I’m not sure how you might soft land these chunks on the surface in a practical fashion?

Assuming that the cometary material arrives on the surface you will then have a lot of water but also a lot of pollution from ammonia to hydrocarbon tars to deal with as well as methane and carbon dioxide and monoxide which will have to be processed into something more useful or removed. Not a show stopper but it might be a bit smelly down there for some time!

  • $\begingroup$ I intend to use multiple comets, starting with those in the 3 to 5 AU range, and to use them for the small crater and then only Hellas P. (Though of course they'll be leakage to the rest of the planet, the constant inflow will hopefully keep pressures and temperatures up.) I agree there's a good chance maintaining L1 will be challenging, 1.5 AU is barely within the tail forming region so I hope to keep excessive degassing down. My understanding is Phobos and deimos are significantly closer and very small bodies to be effecting L1 stability. (Which will be challenge enough!) $\endgroup$ Sep 21, 2017 at 23:51
  • $\begingroup$ The pressure difference is 0.0124 bar outside vs. 0.5 bar inside. (Unless the external has already been raised some ahead of time.) Also a leak will not cause a catastrophic loss of the structure, there will be leaks but even a bullet hole will only cause an increase in the leakage. (Others have covered this better than me.) I agree on the once daily chunks, it'll need to be many more. Parachutes should work to slow the impact. Agreed on the other volatiles that'll need processing and or consideration. $\endgroup$ Sep 22, 2017 at 0:05
  • $\begingroup$ I once did the math for this. If a comet were composed of 100% water and other needed volatiles, and the average comet is 6 miles on diameter, it would take several hundred comets to provide a minimal atmosphere to Mars. Accelerating the comet at 0.01 G to change its orbit would require several thousand thrusters (and volatiles comparable in mass to the comet). $\endgroup$
    – pojo-guy
    Sep 22, 2017 at 1:57
  • $\begingroup$ The dome is definitely too ambitious, with those pressures and the real key being size it would be massively heavy. So more likely 0.4 bar inside the habitats and greenhouses, 0.08 bar inside the dome, and hopefully raising the Hellas P area directly near the site to 0.02 or 0.03 bar. The intention is to mostly affect one area of Mars, not get the entire atmosphere built up, the difference in elevation/pressure is quite extreme for Mars. Hellas Planitia's basin floor is 23,400 feet below the rim, which is why I'm trying for a microclimate within. $\endgroup$ Sep 22, 2017 at 12:18
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    $\begingroup$ @Brooks Nelson L1 points are unstable, but you are correct to say the effects of Phobos and Demos would be small (and probably insignificant compared to out gassing effects). Wrapping the comet is an interesting concept and might work. Parachutes are only deployed at relatively low speeds after most of the orbital kinetic energy has been dissipated via a heat shield. The problem is that you will need to manufacture a parachute and heat shield for each block of ice or launch them back to orbit for reuse. $\endgroup$
    – Slarty
    Sep 22, 2017 at 15:19

Capturing, encapsulating, and positioning a comet above the planet in Mars synchronous orbit above Hellas Planitia, then draw off "the good stuff" in orbit and run a rocket or a hose down to the lowest point on Mars? Sure, the encapsulating film would have to be reflective to avoid off-gassing. Collecting and separating the gases and freezing them back into balls to blast out habitat from orbit could also be a go. We need a "small" deep hole in Hellas Planitia that can be covered over airtight. Of course, if the crater of one of the other 40,000 + craters on Mars was made by a metallic meteor impact, t'would be good for mining metals, perhaps useful to build a new city?

Encapsulation of a comet would be easier than on earth since light gravity would allow much thinner film thicknesses. If the comet sublimates partially or even entirely, then some device to stratify the mutually miscible gases would need employment. The various gases condense at different temperatures, but reaction products may be taken off first. We cleanse gases all the time here on Earth, no reason not to do so above Mars or on the surface of Mars.
Dropping a huge bag of oxygen or CO2 into Hellas Planitia? We would need to engineer the bag to hit the right spot, or even arrange for multiple deliveries from the Oort cloud somehow. The comets will remain mostly intact and frozen and might be steered into impact in Hellas P. with chemical rockets, might even excavate on Mars with them, killing two birds with one comet, so to speak--at least for a while.

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    $\begingroup$ Welcome to Worldbuilding! Would you be able to explain how light gravity would allow for a thinner film thickness? There's a rough thermodynamic approximation for the distribution of a gas in a gravitational potential which is $\propto e^{-\beta mgz}$ at a height $z$, so for a weaker gravitational field (smaller $g$), the thickness is increased (or am I misunderstanding what you mean by 'film thickness'?) $\endgroup$ Jul 30, 2018 at 21:23
  • $\begingroup$ A thinner thickness of plastic films like polyethylene can be attained in weightless or near weightless environments. It is relatively easy to produce such films in the vastness of space, even robotically, onsite. This is something I read in a NASA paper awhile back. $\endgroup$ Sep 27, 2018 at 8:52

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