Terraforming the Moon with comets

[pierre]

For context there was a discussion on this topic 10 years ago (http://forum.nasaspaceflight.com/index.php?topic=6827.0) but AFAICT there has been no dedicated thread since then.

There's an interesting article that proposes that ~100 Halley's-sized comets could add both volatiles and angular momentum to the Moon: http://www.slate.com/articles/technology/future_tense/2014/07/terraforming_the_moon_it_would_be_a_lot_like_florida.html

If they impact at the right angle and speed they could reduce the lunar day duration from 28 days to 60 hours, reducing temperature extremes. If desired they could also tilt a bit the axis of rotation of the Moon to create Earth-like seasons.

Comets would bring water and lots of other molecules containing oxygen, carbon and nitrogen. Even if pressure and temperature are Earth-like the atmosphere would be poisonous, similar to the Earth's primordial atmosphere. We'd need bacteria to decompose e.g. ammonia into nitrogen, plants to produce oxygen and a huge amount of other environmental fixes.

Small seas from the water in the comets would also reduce the swings in temperature.

Molecular oxygen would naturally trigger the formation of an ozone layer in the upper atmosphere.

An artificial magnetosphere could be created with satellites: http://forum.nasaspaceflight.com/index.php?topic=42438.0

The combination of faster rotation, lower maximum temperature, magnetosphere and an ozone layer would greatly increase the capability of the Moon to retain its atmosphere. The Moon's escape velocity is significantly higher than the average speed of oxygen and nitrogen molecules; it would still constantly lose gases to space like every planet does but the half life of the atmosphere would be at least thousands of years. Maintaining it in the long term is far from the biggest problem of this project.

Of course we don't know if 1/6 gravity is healthy in the long term for humans and animals but there are several other threads dedicated to this detail and possible solutions so I suggest to keep this bit out of this thread.

It's obviously a long term project that requires lots of existing space infrastructure in the solar system, but will we ever have wild rabbits living on the Moon's surface and how high will they jump?

[stefan r]

... it would still constantly lose gases to space like every planet does but the half life of the atmosphere would be at least thousands of years...

Based on what assumptions do you get "thousands of years"? 

If you have 1/6th gravity you need a lot more gas per square surface in order to get the same pressure.  Your air column would be much higher. Earth's thermosphere extends to 600 km.  At 3,600 km the moon's gravity is significantly lower.  Ions in the upper thermosphere would have a mean velocity over the orbital velocity.  That should blow off quite rapidly.

[Lar]

Maybe OP was thinking of the lowest partial pressure that is still livable? Helps some, yes? ..  but I think not orders of magnitude?

[pierre]

Your air column would be much higher. Earth's thermosphere extends to 600 km.  At 3,600 km the moon's gravity is significantly lower.  Ions in the upper thermosphere would have a mean velocity over the orbital velocity.  That should blow off quite rapidly.

90% of the Earth's atmosphere by mass is below 16 km and 99.99997% is below 100 km (https://en.wikipedia.org/wiki/Atmosphere_of_Earth). Even an atmosphere 6 times as tall would have the vast majority of its mass confined in a space significantly smaller than the Moon's radius.

Not to mention, only for the tiniest fraction of the atmosphere in the thermosphere or higher (>80 km on Earth) gravity matters at all, because molecules above that line follow mostly ballistic trajectories. Below that it's the thermal interaction between molecules that matters.

That's why Titan has been able to retain for billions of years an atmosphere denser than the Earth's.

Of course the Moon is hotter than Titan, the Slate article linked in the op mentions timescales of "tens of thousands of years", although if someone has a peer reviewed source it would be better of course.

[ThereIWas3]

Comets have a nucleus density less than that of water.   This makes them hard to push around.   (Earth's average density is about 10 times greater.)

[pierre]

Comets have a nucleus density less than that of water.   This makes them hard to push around.   (Earth's average density is about 10 times greater.)

A gravity tractor can solve this problem. The hardest part is where to get the energy for accelerating 10¹⁴ kg or so. That will probably require at least a fission reactor, if not fusion.

[stefan r]

90% of the Earth's atmosphere by mass is below 16 km and 99.99997% is below 100 km

So ballpark one part in a million is flying away at any given time. Would be like stabbing a pencil hole in the ISS.  Will not kill them right away.  But they would move quickly to plug it.  If they could not plug it they would need to abandon at least that module.

Not to mention, only for the tiniest fraction of the atmosphere in the thermosphere or higher (>80 km on Earth) gravity matters at all, because molecules above that line follow mostly ballistic trajectories. Below that it's the thermal interaction between molecules that matters.

In the ballistic trajectory gravity matters (also velocity).  The molecules crashing down on top of the atmosphere give it a pressure.  If the molecules go into orbit then you lose the pressure. 

Titan is much colder than earth/moon. 

Another angle is to look at the mass of what you are talking about.  You need something like 6 kg per cm² (90 pounds per square inch) of lunar surface.  Glass, iron, and aluminum is very abundant on the moon.  Compare the energy required to change the orbit of 6 kg comet material vs the energy to process and handle 6 kg of aluminum and glass.  In most cases changing the orbit takes more energy.  You can build vacuum safe domes with a small fraction of that mass. 

You can build pieces of the dome(s) independently.  Doming the surface lets you build custom environments.  Weather problems do not spread or build up momentum.  Ask someone in Huston what they think of the atmosphere this week. 

Also consider launching the aluminum and glass to the comet.  Then you have lots of space to expand into.  Gravity problems are negligible in comparison.  You can spin the living space to 1 g without having to tilt an apparatus.  Crashing the comet is going the wrong direction.

An atmosphere is painful for a space economy.  When there is no air friction the cars on orbital rings can merge onto high speed rail lines.  Might be worthwhile figuring out how to trap air at the Moon's poles so that it can be removed.  Leaked oxygen could be a major pollutant. 

[pierre]

90% of the Earth's atmosphere by mass is below 16 km and 99.99997% is below 100 km

So ballpark one part in a million is flying away at any given time.

The top of the mesosphere (i.e. the top of 99.99% of Earth's atmosphere) is as cold during the day as Antartica during the polar winter. At that temperature air molecules move at an average speed of 370 m/s. Even at room temperature they move at 464 m/s. The Moon's escape velocity is 4.5 times as high, even at hundreds of km from the surface.

The main way for a molecule to get to such anomalous speed is to acquire energy either via high-energy UV photons (which would be blocked by ozone at the very top of the atmosphere) or via the solar wind (which can be redirected with an artificial magnetosphere).

Said molecule would also need to be accelerated in the right direction and would need to avoid any further collision with other, slower, molecules. Even at the altitude of the ISS there's still enough air that molecules on average collide every km. Escaping this soup requires extraordinary luck. The thing would leak, just like the Earth leaks, but it would be slow.

That the Moon is able to hold an Earth-like atmosphere for at least thousands of years is a very commonly cited figure, see the old thread linked in the op and the Slate article. Admittedly I'd love to see a peer reviewed article on this.

You can build vacuum safe domes with a small fraction of that mass.

Terraforming a world is not the most energy-efficient or quicker way of doing anything at all.

If cheap habitable space is the goal we can get it in a much simpler way with swarms of orbital habitats.

The point of terraforming is that a civilization that's long-lived enough, advanced enough and rich enough that the project is even possible at all, may want to do it not for purely utilitarian reasons but to see a blue sky, green forests, clouds, rain, snow, rainbows, rivers, seas and wild animals living freely on the surface of the Moon.

If we solve potential health issues due to low gravity (or if they turn out not to be an issue at 1/6g), a billion people can comfortably live on the Moon. Might as well give them a great panorama.

[KelvinZero]

If we solve potential health issues due to low gravity (or if they turn out not to be an issue at 1/6g), a billion people can comfortably live on the Moon. Might as well give them a great panorama.

Way off topic, but I sometimes wonder about the best way to fake a great panorama, eg an optical trick to make a curved interior appear straight inside a spinning habitat. For example sections could be divided with opaque walls and doors say 2m high, but above them could be glass walls that deflect any light though them by just the right angle. Im not sure if such a refraction or reflection could be designed.

[savuporo]

That the Moon is able to hold an Earth-like atmosphere for at least thousands of years is a very commonly cited figure, see the old thread linked in the op and the Slate article. Admittedly I'd love to see a peer reviewed article on this.

I think this is the 'thousands of years' quoted source

Vondrak, R. R., "Creation of an Artificial Lunar Atmosphere," Nature 248, 657-659, 1974

http://www.nature.com/nature/journal/v248/n5450/abs/248657a0.html?foxtrotcallback=true

Here are the referring works

https://scholar.google.com/scholar?cites=16168342061603676328&as_sdt=2005&sciodt=0,5&hl=en


More recent take, 1992

http://adsabs.harvard.edu/full/1992lbsa.conf..347B

[savuporo]

So the biggest issue with this idea is energy management.
Geoff Landis briefly looks at what it would take to cook oxygen out of local rock, a small cube of regolith of 50 km an edge ..

http://www.geoffreylandis.com/moonair.html

Conclusion: insane levels of energy required, beyond even fusion. So what if we bring comets from Oort cloud instead ? The result will be about 3% of the total mass of volatiles being retained from cometary impacts, according to current
 simulations ...

cfpl.ae.utexas.edu/wp-content/uploads/2014/07/Benedicte-Icarus-2011.pdf

In order to follow the water from the near field of the impact to the full planetary induced atmosphere, the 3D parallel DSMC code used a collision limiting scheme and an unsteady multi- domain approach. 3D results for the 45° oblique impact of a 2 km in diameter comet on the surface of the Moon at 30 km/s are presented. Most of the cometary water is lost due to escape just after impact and only  3% of the cometary water is initially retained on the Moon. Early downrange focusing of the water vapor plume is observed but the later material that is moving more slowly takes on a more symmetric shape with time. Several locations for the point of impact were investigated and final retention rates of  0.1% of the comet mass were observed.

Here is a fairly new paper comparing natural cometary bombardment retention rates on Moon and Mercury:
https://www.hou.usra.edu/meetings/lpsc2015/pdf/1680.pdf

It'll be a long bombing run ..

EDIT: of course all analysis has been done on natural impacts, so these are rookie numbers, we gotta get those numbers up..

Maybe with careful maneuvering or colliding objects on orbit or somehow slow boiling them on low or it it's could work a bit better.

[stefan r]

That the Moon is able to hold an Earth-like atmosphere for at least thousands of years is a very commonly cited figure, see the old thread linked in the op and the Slate article. Admittedly I'd love to see a peer reviewed article on this.

I think this is the 'thousands of years' quoted source

Vondrak, R. R., "Creation of an Artificial Lunar Atmosphere," Nature 248, 657-659, 1974

http://www.nature.com/nature/journal/v248/n5450/abs/248657a0.html?foxtrotcallback=true

Here are the referring works

https://scholar.google.com/scholar?cites=16168342061603676328&as_sdt=2005&sciodt=0,5&hl=en


More recent take, 1992

http://adsabs.harvard.edu/full/1992lbsa.conf..347B

I could not open the nature article without spending $$.  The abstract says "hundreds of years".  About the time scale of comet orbits. 

The NASA Johnson space center article is clearly talking about the length of time pollution will linger.  They are looking at how many kilometers away from a mining site you need to place the telescopes.  Also drawing the conclusion that a few hundred kilometers away the pollution would be a non-issue.  There is no atmosphere. 

EDIT: of course all analysis has been done on natural impacts, so these are rookie numbers, we gotta get those numbers up..

Maybe with careful maneuvering or colliding objects on orbit or somehow slow boiling them on low or it it's could work a bit better.

Should be able to do much less than 30km/s.  A 2 km wide comet can be broken up years before impact. 

The top of the mesosphere (i.e. the top of 99.99% of Earth's atmosphere) is as cold during the day as Antartica during the polar winter. At that temperature air molecules move at an average speed of 370 m/s. Even at room temperature they move at 464 m/s. The Moon's escape velocity is 4.5 times as high, even at hundreds of km from the surface.

The moon's escape velocity is decreasing with altitude (usually called "radius").  Earth does that too but the decrease in gravity per kilometer altitude is much lower because earth is much bigger. 

The main way for a molecule to get to such anomalous speed is to acquire energy either via high-energy UV photons (which would be blocked by ozone at the very top of the atmosphere) or via the solar wind (which can be redirected with an artificial magnetosphere).

I am talking about the molecules at the top.  If there is ozone above it then it is not the molecule on top. 

Why to you prefer ozone to silica?  Or low density polyethylene?

Said molecule would also need to be accelerated in the right direction and would need to avoid any further collision with other, slower, molecules. Even at the altitude of the ISS there's still enough air that molecules on average collide every km. Escaping this soup requires extraordinary luck. The thing would leak, just like the Earth leaks, but it would be slow.

You can leave orbit in any direction.

The rate of leakage is an exponential function. 

You can call it "slow".  The rate that the surface recovers from comet impacts could be called "slow".  Subjecting rabbits to this environment could be called "cruel". 

[savuporo]

I could not open the nature article without spending $$.  The abstract says "hundreds of years".  About the time scale of comet orbits. 

Saved you some $$. It says :

One conclusion is that if the density of the lunar atmosphere is increased, a point can be reached where loss occurs so slowly that itis negligible over human time scales (that is, exponential decay lifetimes are greater than hundreds of years).

But the point wasn't mr Vondrak's old paper so much, the point is that the question has been looked at and modeled multiple times using more modern data and tools, easily found by following the referring papers.

[KelvinZero]

An artificial magnetosphere could help slow atmosphere leakage.

The thickness that a physical barrier would have to be to restrain the atmosphere would also drop to zero the higher you made it. Something as ephemeral as lunch wrap could probably hold in the atmosphere at sufficient height. I could imagine a sort of self healing shield that spacecraft could just fly straight through and ignore.

[Paul451]

The problem with terraforming (and this applies to Mars) is that it's pretty much useless effort until it's close to finished. Indeed, it may make things more difficult for surface activity.

Paraterraforming seems like a better option. You get maximum utility out of each section-area enclosed per unit of input.

[Patchouli]

The problem with terraforming (and this applies to Mars) is that it's pretty much useless effort until it's close to finished. Indeed, it may make things more difficult for surface activity.

Paraterraforming seems like a better option. You get maximum utility out of each section-area enclosed per unit of input.

Paraterraforming would be easier on the moon due to it's low gravity.
But adding a thin atmosphere still might be desirable to allow spacecraft to aerocapture into orbit,help keep LLO clean though the mascons tend to deorbit things, and burn up smaller meteorites.

[stefan r]

Paraterraforming would be easier on the moon due to it's low gravity.
But adding a thin atmosphere still might be desirable to allow spacecraft to aerocapture into orbit,help keep LLO clean though the mascons tend to deorbit things, and burn up smaller meteorites.

You can aero capture into the earth-moon system by skipping off earths atmosphere. You can tether drop to lunar surface.  Or land on a maglev train car.  With momentum exchange the energy form incoming craft because usable. 

[savuporo]

Which brings the question. Given that the biggest problem with bringing comets to moon is the high energy of impacts and resulting minuscule retention rate of volatiles, is there some orbital momentum exchange dance that could be done to 'soften the blows' ?

[savuporo]

I've had a penchant for the scientific method lately.  Yes, Halley's comet is only one of a handful of objects on a highly elliptical orbit in our solar system.  However, there is very little proof that the Oort Cloud really exists.  I am very proud to say that I reaally am one of those that deny the existence of the Oort Cloud.  If the Oort Cloud really existed, then there would be some evidence by now using space telescopes.

At the risk of derailing this thread ..

Not sure if you aware or not, but denying Oort cloud being the source of comets and its existence is a creationist claim.

And no, we don't have telescopes that can see a tiny cold chunks of ice lightyears away, unless they bother to swing by.

[Paul451]

Which brings the question. Given that the biggest problem with bringing comets to moon is the high energy of impacts and resulting minuscule retention rate of volatiles, is there some orbital momentum exchange dance that could be done to 'soften the blows' ?

Honestly, the level of engineering required to bring sufficient mass from beyond ice-line to the lunar surface to create an entire atmosphere (and presumably seas/etc) means you have a level of industrialisation in space that easily allows for things like lunar space elevators, rotovators or orbital rings. In which case, you wouldn't use impactors. (Indeed, the industries operating on the moon would be very upset with you if you did.)

Until then, a glass dome on the moon with some air at reasonable pressure sounds doable.

Domes make bad pressure vessels. While it's a common trope in SF and space-art futurism, it's not a practical idea.