Can comets create a microclimate crater on Mars 30km deep with 0.7bars?

[darkdave3000]

TL;DR Summary: Can comets either one or many create simultaneously overlapping impact craters for higher air pressures to breed algae and plant life as a micro climate?

Is there a depth limit to crater creation on a planet? Let’s say Mars for a depth of 30km can it be done? Assuming the projectile is an icy comet. And we have the option to use more than one at whatever time interval. Also sequential and simultaneous clusters are an option too and we can give each rock angular momentum(spin).

Scale height of Mars is 11.1km and according to my calculations from -7km elevation on Mars a -37km should yield 0.7bars. Since trees can survive on mountains with similar pressures and it’s within the Armstrong limit I am thinking a win win scenario if a planet killer was deflected from Earth and toward Mars as it will also carry water ice to the bottom of the new Martian crater as liquid water (it can stay liquid since there it’s below the Armstrong limit) for plant and algae life to breed if we so choose to plant them there. The water can also be rocket fuel and oxygen source after electrolysis in addition to drinking water. In this case soda since it will mix with CO2.

A comet such as Haileys would have sufficient ice by it self to increase Martian atmospheric material, pressure and density by 1%. While negligible with just a single comet: with repeated engineered crashes up to a hundred we could double the Martian atmospheric density and pressures gradually reducing the required depth(by half from -30km to -15km ) of future micro climate semi habitable craters to terraform Martian atmosphere slowly with photosynthesis from CO2 to O2.

 

[snorkack]

Is there a depth limit to crater creation on a planet? Let’s say Mars for a depth of 30km can it be done?

Yes there is. And 30 km surely cannot be done.
Look at Hellas basin. Large impact basins on Moon, Mars et cetera do not get deeper.
The reason is the strength of rock. The rock shattered and softened by impact simply flows back and refills the basin to a roughly flat floor, with waves (single central peak or several rings).

 

[darkdave3000]

Yes there is. And 30 km surely cannot be done.
Look at Hellas basin. Large impact basins on Moon, Mars et cetera do not get deeper.
The reason is the strength of rock. The rock shattered and softened by impact simply flows back and refills the basin to a roughly flat floor, with waves (single central peak or several rings).

What can be done then? You say the rock shattered and softened flows back and refils the basin to rouhly flat floor. But how much of a percentage does it refil from the initial pre strike elevation? 100%? Or 90%? How do I calculate how much I can excavate each time I smash the same spot with a comet? If maximum has already been reached at Hellas because it's the bottom of a crater what about another similar elevation spot that is not part of a crater such as the Northern Lowlands? How do I confirmed mathematically that the lowest point has already been reached and that there's no more useful depth to extract for a planet? Can I cheat with cluster strikes? Can I splash up molten material into space with a 2nd comet seconds after impact of the first to excavate deeper? What would you do if aliens put a ray gun to your head or family and said u gotta figure out a way to do this or else?

 

[Baluncore]

How do I calculate how much I can excavate each time I smash the same spot with a comet?

Because the comet is ice and the planet is rock, you stand a snowflake's chance in Hell of making a deep hole with a comet. It would take a big iron meteorite to make a big wide hole.

Are you going to live at the bottom of a 30 km deep hole, or in the sunlight on the crater floor?

 

[darkdave3000]

Because the comet is ice and the planet is rock, you stand a snowflake's chance in Hell of making a deep hole with a comet. It would take a big iron meteorite to make a big wide hole.

Are you going to live at the bottom of a 30 km deep hole, or in the sunlight on the crater floor?

" Some of the biggest craters on Earth may come from comets."
https://www.seti.org/comets-path-earth

At the bottom of the crater.

 

[Baluncore]

At the bottom of the crater.

Do you mean on the crater floor, or at the bottom of the deep hole in the crater floor?

 

[Vanadium 50]

the comet is ice and the planet is rock, you stand a snowflake's chance in Hell

I saw what you did there.

 

[Vanadium 50]

Some of the biggest craters on Earth may come from comets

And how many of them are 30 km deep?

 

[berkeman]

Thread moved to the Sci-Fi forum, and may still be closed soon.

 

[Baluncore]

@darkdave3000
I suspect that you have not yet accepted that the hydrostatic pressure gained by going down a deep hole, will be lost again in the pipe rising back up to your habitation. The deep hole and the pipe are quite redundant, you will still have to place a solar-powered air compressor on the surface, close to your habitation.

“The only interesting answers are those which destroy the question”. —Susan Sontag

 

[darkdave3000]

Because the comet is ice and the planet is rock, you stand a snowflake's chance in Hell of making a deep hole with a comet. It would take a big iron meteorite to make a big wide hole.

Are you going to live at the bottom of a 30 km

@darkdave3000
I suspect that you have not yet accepted that the hydrostatic pressure gained by going down a deep hole, will be lost again in the pipe rising back up to your habitation. The deep hole and the pipe are quite redundant, you will still have to place a solar-powered air compressor on the surface, close to your habitation.

“The only interesting answers are those which destroy the question”. —Susan Sontag

There are no pipes or holes in the crater.

 

[russ_watters]

" Some of the biggest craters on Earth may come from comets."

You're googling the wrong thing. If you Google for the largest crater in the solar system you get this:

The largest confirmed impact basin in the Solar System is the South Pole-Aitken Basin, located on the Moon. This colossal impact structure is typically described as a broadly circular structure with a diameter of around 2,500 km (1,553 mi) and a maximum depth of around 13 km (8 miles).

 

[Rive]

Can comets either one or many create simultaneously overlapping impact craters for higher air pressures to breed algae and plant life as a micro climate?

Regarding impact craters, I think there are sufficient number of responses here, I would not beat that direction any further.

But, additionally you might need to consider the issue of sunlight in the bottom of that imagined enormous hole. The deeper the hole is, the shorter the 'day' there => what you get is actually an icy hole instead of a paradise.

 

[Vanadium 50]

Thread moved to the Sci-Fi forum

Do we want the SciFi section to becomes the dustbin for the rest of PF?

 

[berkeman]

Do we want the SciFi section to becomes the dustbin for the rest of PF?

Think of it as palliative care for a thread...

 

[Nik_2213]

Why would you drop icy comet(s) on Mars when 'a shower of rocks' would release ample volatiles locked in the ice-caps ??

Also, regardless of the initial crater depth, crust/mantle structure etc, the material would be sufficiently shattered to 'slump' rapidly. Perhaps aided by released volatiles...

FWIW, perhaps better to tunnel adits into canyon flanks, accessing water table / permafrost strata, and only needing modest air-locks plus flexible lining to contain atmosphere...

 

[Vanadium 50]

Think of it as palliative care for a thread...

Alternatively, there are three kinds of medical treatments: diagnostic, therapeutic and punitive.

The OP asked a question, which took us a little while to puzzle out. He got an answer, which he didn't like. Now it's science fiction?

I did look up the craters in the solar system. I was unable to find a single one as deep as the OP wants. That includes Mimas, which has one that nearly destroyed it, and the southern half of Vesta, which arguably did. You can buy a piece of Vesra through the mail these days.

I strongly suspect that there is no process that will make a hole like this without collapse. (Just as you can't pile rocks up arbitrarily high)

Now, Niven did invent a world, "Canyon" that was sort of like this (and I deserve a reward for re-railing this thread) but Niven's science was straight outy of the 60's and even so wasn't always 100% right. But Canyon!" is a poor rebuttal to "this won't work on Mars."

 

[darkdave3000]

Alternatively, there are three kinds of medical treatments: diagnostic, therapeutic and punitive.

The OP asked a question, which took us a little while to puzzle out. He got an answer, which he didn't like. Now it's science fiction?

I did look up the craters in the solar system. I was unable to find a single one as deep as the OP wants. That includes Mimas, which has one that nearly destroyed it, and the southern half of Vesta, which arguably did. You can buy a piece of Vesra through the mail these days.

I strongly suspect that there is no process that will make a hole like this without collapse. (Just as you can't pile rocks up arbitrarily high)

Now, Niven did invent a world, "Canyon" that was sort of like this (and I deserve a reward for re-railing this thread) but Niven's science was straight outy of the 60's and even so wasn't always 100% right. But Canyon!" is a poor rebuttal to "this won't work on Mars."

I would categorize Olympus Mons as arbitrrarilly high although it wasnt piled. Im just still struggling with the idea of the volatiles in the comet, if they can some how act as a coolant for the molten slag to harden prematurely before rising up to fill.

I've been beaten up before because of the color of my skin I can take intellectual bullying too! The sci fi is a nice hidden insult!

 

[Vanadium 50]

A. The fact that Olympus Mons is x meters tall does not mean you can dig a hole x meters deep. Both have limits, but not the same limit.
B. Mount Olympus isn't 30 km tall anyway.

This remionds me of the old saw "How many legs does a horse have if you call a tail a leg?" "Four. Calling a leg a tail doesn't make it one."

 

[Rive]

the volatiles in the comet, if they can some how act as a coolant for the molten slag to harden prematurely before rising up to fill.

That just does not works like that. The 'molten slag' takes up only a small portion of the explosion (and even that'll scatter, won't participate directly in the crater formation): most of the energy goes to the gravel (creation and move).

The depth/width ratio of the crater mostly depends on the depth of penetration of the incoming object.
The depth of penetration (with a given, rocky surface) is ~ about the structural strength.
A 'dirty snowball' (comet) does not have much of that. Most likely it would leave a wide but shallow crater only.

 

[Drakkith]

Im just still struggling with the idea of the volatiles in the comet, if they can some how act as a coolant for the molten slag to harden prematurely before rising up to fill.

At 20+ km/s the entire impactor is going to be superheated and vaporized, no matter what its made out of. There will not be any 'coolant' anywhere. Also remember that the further down you go, the greater the subsequent uplifting as the material attempts to reach gravitational equilibrium. It is this that ultimately limits the depth of holes on any planetary body. What that depth is on Mars, I don't know, but 30 km is probably fairly close.

I've been beaten up before because of the color of my skin I can take intellectual bullying too! The sci fi is a nice hidden insult!

No one is bullying or insulting you. I suspect Berkeman is actually being nice by moving your thread here, as the rules are somewhat more relaxed here in the sci-fi forums than the main forums.

 

[Drakkith]

How do I confirmed mathematically that the lowest point has already been reached and that there's no more useful depth to extract for a planet?

I doubt anyone on PF knows how to do this except via a rough back-of-the-envelope calculation. That's probably one of the main reasons you haven't gotten any strict yes/no answers.

 

[Vanadium 50]

Well, it's probably calcuable numerically, but I sure wouldn't want to propose it. I can hear the laughter now.

We can look at impacts in the solar system and we have our answer.

• No impact has left a crater this deep
• The OP doesn't just want "deep", he wants "small" (or "narrow") and deep craters are large - and get a lot larger and only a little deeper.
• We have evidence of impacts that have severely disrupted the target body - Mmas, Vesta - and they don't leave craters this big. Increase the impactor's energy and you don't get a bigger crater; you get more asteroids.

 

[darkdave3000]

Thanks for this useful answer. So would raining down smaller ice rocks at slower speeds say a cluster of 20 x 1km diameter at 8km/s encourage the water ice to act as a coolant? Im thinking like a civil engineer trying to get the structure I want using comets as a sort of a "tool". Im hoping to find the magic formula between speed, mass, size, angular spin, strike angle, cluster size etc. I guess at this point it doesn't look like I'll get any encouraging answers. But thought I throw this out in the forum just in case there is some brilliant person out there watching that can help me out if he can see an angle.

 

[jrmichler]

A good rule of thumb is that the kinetic energy of an object moving 10,000 feet/sec (3 km/sec) is equal to the energy of an equal mass of high explosive. Then keep in mind that kinetic energy is proportional to the square of the speed.

So the kinetic energy of an object at 8 km/sec is 7 times the explosive energy of an equal mass of high explosive. The relative cooling effect of water vs rock completely disappears. In fact, as a first order approximation, the damage of an object at 8 km/sec is almost the same if it's ice or high explosive.

 

[darkdave3000]

A good rule of thumb is that the kinetic energy of an object moving 10,000 feet/sec (3 km/sec) is equal to the energy of an equal mass of high explosive. Then keep in mind that kinetic energy is proportional to the square of the speed.

So the kinetic energy of an object at 8 km/sec is 7 times the explosive energy of an equal mass of high explosive. The relative cooling effect of water vs rock completely disappears. In fact, as a first order approximation, the damage of an object at 8 km/sec is almost the same if it's ice or high explosive.

Thanks for this! So what if I moved an ice rock from the asteroid belt say a 20km diameter one and crashed it into Mars as slow as possible say 2km/s? The Asteroid Belt does not have as much potential difference in gravitational energy as compared to kuiper belt objects as you know so we should be able to manage a lower delta V in this case. What do you predict would happen in such a scenario?

 

[darkdave3000]

@darkdave3000
I suspect that you have not yet accepted that the hydrostatic pressure gained by going down a deep hole, will be lost again in the pipe rising back up to your habitation. The deep hole and the pipe are quite redundant, you will still have to place a solar-powered air compressor on the surface, close to your habitation.

“The only interesting answers are those which destroy the question”. —Susan Sontag

There's no pipe here in my hypothetical scenario.

 

[darkdave3000]

At 20+ km/s the entire impactor is going to be superheated and vaporized, no matter what its made out of. There will not be any 'coolant' anywhere. Also remember that the further down you go, the greater the subsequent uplifting as the material attempts to reach gravitational equilibrium. It is this that ultimately limits the depth of holes on any planetary body. What that depth is on Mars, I don't know, but 30 km is probably fairly close.No one is bullying or insulting you. I suspect Berkeman is actually being nice by moving your thread here, as the rules are somewhat more relaxed here in the sci-fi forums than the main forums.

Thank you Berkman!

 

[Frabjous]

You are doing this the wrong way. You have postulated that a 30km hole will produce an acceptable environment (including structural stability). You need to be working towards proving this. These “how to make it“ discussions are a distraction.

 

[darkdave3000]

You are doing this the wrong way. You have postulated that a 30km hole will produce an acceptable environment (including structural stability). You need to be working towards proving this. These “how to make it“ discussions are a distraction.

Hi sorry, I thought I already mentioned Mars has a 11.1km scale height. Do you know how scale heights work?

 

[Frabjous]

Hi sorry, I thought I already mentioned Mars has a 11.1km scale height. Do you know how scale heights work?

A back of an envelope calculation is not proof.

 

[darkdave3000]

A back of an envelope calculation is not proof.

I actually did the calculations on a spreadsheet already but it's really simple. You don't even need spreadsheet to work out that 11.1km equates to 30km depth yielding roughly 0.7 bars and 50km 1 bar, that is from the lowest point of Mars. Do you follow me so far?

 

[Frabjous]

I understand the calculation. This is one issue of many of being at the bottom of a 30 km hole. You need to systematically identify the issues and address them.

 

[darkdave3000]

I understand the calculation. This is one issue of many of being at the bottom of a 30 km hole. You need to systematically identify the issues and address them.

Ok, I think the problem will be that CO2 is still the gas type even though we are now within the Armstrong Limit. That means carbon dioxide poisoning, so the solution is we still wear suits but they dont need to be pressurized suits. That's probably the 2nd major problem apart from creating the crater it self which in my book is the biggest challenge. However if we can create the crater this deep, the benefits far out weigh the new problem it presents that I mentioned above. Benefits such as being able to have liquid water (which will taste like soda) and having air pressure that will be high enough so that we dont need airlocks anymore. Also third benefit is now we can terraform the planet from the crater with photosynthesis by introducing algae and plants (within glass houses)

 

[Frabjous]

How wide does it need to be?
Is it structurally stable?
What is the temperature?
Does it mix well with the general atmosphere?
and many more…
It is a long list of questions.
Until you identify the “what,” “how” is not a high priority question.

 

[darkdave3000]

How wide does it need to be?
Is it structurally stable?
What is the temperature?
Does it mix well with the general atmosphere?
and many more…
It is a long list of questions.
Until you identify the “what,” “how” is not a high priority question.

I'm not God so I need your help to figure these things out. We are all in this together right? We all want to be able to live on Mars within our life time. I think it needs to be wide enough so the rim of the crater is not a threat the the settlement in the center of the crater. The temperature increases as you dig deeper into Mars so this should not be as big as a problem as the CO2 poinsoning, but I will get on my spreadsheet and give you a figure later. I have been ill this holiday season so not my self. Maybe you can help me come up with some of these answers that you have presented your self as im unwell.

 

[Frabjous]

I'm not God so I need your help to figure these things out. We are all in this together right? We all want to be able to live on Mars within our life time. I think it needs to be wide enough so the rim of the crater is not a threat the the settlement in the center of the crater. The temperature increases as you dig deeper into Mars so this should not be as big as a problem as the CO2 poinsoning, but I will get on my spreadsheet and give you a figure later. I have been ill this holiday season so not my self. Maybe you can help me come up with some of these answers that you have presented your self as im unwell.

There is much more HARD work required on YOUR part before people will begin to be drawn to you vision. Most of us already have Muses that we are following.

 

[darkdave3000]

How wide does it need to be?
Is it structurally stable?
What is the temperature?
Does it mix well with the general atmosphere?
and many more…
It is a long list of questions.
Until you identify the “what,” “how” is not a high priority question.

I found these two sources about the temperature on Mars, seems to me we want the impact crater to be close to the Equator and the crater should at least be temporarilly hotter than usual as soon as it's form. I expect it will cool off gradually over time.

"Surface temperatures may reach a high of about 20 °C (293 K; 68 °F) at noon, at the equator, and a low of about −153 °C (120 K; −243 °F) at the poles.[25] Actual temperature measurements at the Viking landers' site range from −17.2 °C (256.0 K; 1.0 °F) to −107 °C (166 K; −161 °F). The warmest soil temperature estimated by the Viking Orbiter was 27 °C (300 K; 81 °F).[26] The Spirit rover recorded a maximum daytime air temperature in the shade of 35 °C (308 K; 95 °F), and regularly recorded temperatures well above 0 °C (273 K; 32 °F), except in winter.[27]"

https://en.wikipedia.org/wiki/Climate_of_Mars

"If we look at Mars' possible geothermal gradient (see Earth's) which is about 25 °C per km. Using the low estimate of Mars's gradient to be 1/4 that of Earth's Source, that's a bit over 6° C per km. so 55 km, 330° C. Added that to Mars' average surface temperature of -55 C, you're talking 275° C or 527° F at 55 km underground, and that's a low estimate."

https://astronomy.stackexchange.com/questions/14875/what-is-the-temperature-55-km-beneath-the-surface-of-mars#:~:text=Using the low estimate of,and that's a low estimate.

 

[Vanadium 50]

This sounds like an invitation, in the words of Sheldon Cooper, "[for us] noble, semi-skilled laborers to execute the vision of those who think and dream."

But in any event, if you have the technology to move comets around willy-nilly, why do you you want them to dig impossible holes? Wouldn't it be simpler to have them provide the atmosphere in the first place?

 

[Rive]

So what if I moved an ice rock from the asteroid belt say a 20km diameter one and crashed it into Mars as slow as possible say 2km/s?

Comparing the difficulty of landing a 20km diameter asteroid at around half of the escape velocity and digging that hole - well, just digging it would be a lot less stressful.

For us too

 

[Drakkith]

Comparing the difficulty of landing a 20km diameter asteroid at around half of the escape velocity and digging that hole - well, just digging it would be a lot less stressful.

Dear god, the fuel requirements to move a 20km diameter comet are staggering.
That's about 4x10¹⁵ kg of mass. Just to get 1 m/s of delta-V using hydrolox (isp of roughly 400) you'd need 10¹² kg of fuel. That's a billion metric tons! And that's before calculating the fuel required to get that fuel to the comet! That would take roughly 40x more fuel!

Assuming you got the comet waaaaaay out in the outer solar system where it is moving slowly, you might only need 1000 m/s of delta-V or less to get it to where you want. That amount of delta-V would require roughly 1.2x10¹⁵ kg of fuel. That's 1000x more delta-V for more than 1200x more fuel (the tyranny of the rocket equation!)

I'm not sure how much delta-V you'd need to adjust a comet's orbit, as it depends on the orbital details of both the comet and the target, but let's say that you'd need a mass of fuel of AT LEAST 30% of the comet's mass. That'll give you about 1000 m/s, which is probably about the bare minimum you'd need unless you want to go waaay out past Neptune and Pluto to snag a comet. And then wait a few hundred years for it to get to your target.

 

[Rive]

Dear god, the fuel requirements to move a 20km diameter comet are staggering.

Any Sci-Fi plot (on the 'hard' side) involving planetary rearrangement of such calibre would use some other forms of engines than chemical: fusion drive or such, at the very least.

Still: once they have the tool they would rather carve that hole from above, than all that mess with smashing comets onto the surface and cleaning up all the rubble later on...

The whole topic (as seemingly intending to be serious) is just plain ridiculous.

 

[Vanadium 50]

the fuel requirements to move a 20km diameter comet are staggering.

Oh, you naysayers with your facts and numbers and equations. Bah!

 

[Drakkith]

So what if I moved an ice rock from the asteroid belt say a 20km diameter one and crashed it into Mars as slow as possible say 2km/s? The Asteroid Belt does not have as much potential difference in gravitational energy as compared to kuiper belt objects as you know so we should be able to manage a lower delta V in this case.

You are correct. Let's look as some numbers.

Let's first assume the icy object is in a near-circular orbit at 3.0 AU. Orbital velocity at this distance from the Sun is 17.2 km/s. To bring the periapsis down to intersect Mars' orbit, 1.66 AU or less, requires slowing the object down from 17.2 km/s to 14.5 km/s, for a delta V of 2700 m/s. Upon reaching Mar's orbit the object's velocity has increased to 26.1 km/s, compared to Mars' 22 km/s, a difference of about 4 km/s. Now, upon reaching Mars' sphere of influence the object will begin accelerating towards Mars, increasing its speed relative to Mars even more.

A slow moving object falling from the edge of Mars' sphere of influence gains about 4 km/s delta V. But our object is already moving at 4 km/s, so we wouldn't gain quite that much. Let's say 3 km/s. So now you have to slow your impactor down from 7 km/s to 2 km/s, a delta V of 5 km/s.

Added together, we need a total delta V of 2.7 + 7 - 2 = 7.7 km/s.

Now let's grab a comet from way out in the outer solar system. If you grab one whose periapsis is already within 1.66 AU, then you're in luck. You don't need to spend the delta V to slow it to bring its orbit down to meet Mars'. But let's look at that anyways. For an object in a near-circular orbit at 30 AU (Right around Neptune's orbit):
Speed: 5.44 km/s
Once we slow it down so the periapsis is at 1.66 AU:
Speed at apoapsis: 1.76 km/s
Speed at periapsis: 31.8 km/s
Mars' speed at apoapsis (where we want to intercept it): 22 km/s

So we need to spend about 3.68 km/s delta V to slow it down and our intercept velocity is much higher. Assuming our object is whipping around and catching up to Mars from behind, that's a difference in velocity of almost 10 km/s. Add in another 2 km/s at least to account for when the object 'falls' to Mars inside the sphere of influence, and we're looking at 3.68+10+2 - 2 = 13.68 km/s of delta V if you want to crash a 20 km diameter icy object into Mars at only 2 km/s.

How much fuel would we need? Let's assume a 'perfect' rocket of negligible mass compared to the mass of the fuel and payload.

Case 1
Object from asteroid belt, mass 4x10¹⁵ kg:
Required delta V: 7.7 km/s.
Engine ISP: 450 (liquid hydrogen-oxygen engine)
Fuel mass: 19x10¹⁵ kg (19 petatons, or 19 million gigatons)

Engine ISP: 20,000 (DS4G ion engine)
Fuel mass: 160x10¹² kg (160 gigatons)

Case 2
Object from 30 AU, same mass as above:
Required delta V: 13.68 km/s
Engine ISP: 450
Fuel Mass: 84x10¹⁵ kg (84 petatons, 84 million gigatons)

Engine ISP: 20,000
Fuel Mass: 290x10¹² kg (290 gigatons)

Well, on the plus side, an ion engine cuts your fuel usage from 18x the mass of the object to about 0.05x the mass of the object.

*Note: I used apoapsis and periapsis instead of aphelion and perihelion because that's what I'm used to seeing, as I've played a lot of Kerbal Space Program and that's what they use. The -apsis words are generic and apply no matter what body you're orbiting.

I used this Delta-V calculator for all the delta V calculations and Universe Sandbox for the object's various velocities.

 

[russ_watters]

Any Sci-Fi plot (on the 'hard' side) involving planetary rearrangement of such calibre would use some other forms of engines than chemical: fusion drive or such, at the very least.

Still: once they have the tool they would rather carve that hole from above...

That's the plot of Star Trek.

No, the movie.

No, the other one.

 

[256bits]

The whole topic (as seemingly intending to be serious) is just plain ridiculous.

I thought @snorkack gave the answer in post #2?

 

[jrmichler]

So would raining down smaller ice rocks at slower speeds say a cluster of 20 x 1km diameter at 8km/s

This idea was used by Robert L. Forward in his SF novel Martian Rainbow. Since he specialized in hard science fiction, you might find that book a good read.

 

[DaveC426913]

As long as we're putting forward reading material for Areoforming, might I suggest Kim Stanley Robinson's Mars Trilogy?

He made a pit on Mars, but he simply dug it with autonomous excavation equipment. (A couple of big plusses with this method are: 1] technology is less advanced than moving comets around, and 2] that you don't have to wait centuries for the crater to cool and solidify.)