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I think rather than trying to lift the bag we really need to be able to creat it insitu as the BFR or the ITS will require 6 ship tankers to get just the one beast loaded with payload out of orbit for the 100 ton that they could possibly do.
Then again using the insitu resources of the moon and lauching from there is a hugh boost in payloads to anywhere.,
I agree with the process to narrow down the possible targets for such an under taking that was in post #10
The bag approach is assuming that most are loose rubble at least on the outside and possibly not much better with void areas with in them.
The basalt fiber glass is also a insitu material to wrap the asteriod with in as it is desired to have a high tensile strength. Also as you bore into the core of it we could re-enforce its shaft entrance with more fiberglass as well. One we are deep enough to hollow it out once more line the would be floor with more fiber glass to seal the chamber before making an atmospher.
I would leave a false ceiling of the core with strut leggs to it so we can colorize it to project sunrise and sun sets as we spin the asteriod up for artificial gravity.
Just some thought keep or toss out...
Basalt fibre would appear to be an excellent ISRU material. Reading some of the material that you have referenced, suggests that low iron basalt would be ideal (I believe stony asteroids are mostly olivine?) and melting point is typically 1500C. We would need an electric furnace to melt the material and gravity to allow the melt to settle, allowing it to be drawn through dies.
Maybe the easiest solution in a limited space, would be to produce some woven, tensile strips and then filament wind the entire asteroid. The tensile strength of basalt fibres is about 2GPa. Using the thin walled pressure vessel equation, I estimate that some 16,000 tonnes of woven basalt fibre would be needed to enclose Apophis.
Spinning the asteroid does not appear to be practical. If we were to spin it to produce 0.3g at its long ends, we would need longitudinal filaments capable of resisting the weight of the entire asteroid in an average gravity of 0.15g. By my calculations, that implies a longitudinal force of 40GN, requiring some 140,000 tonnes of fibre for a 450m long asteroid, assuming a safety factor of 6. That is almost an order of magnitude increase. Maybe I have gone wrong somewhere.
Thanks chaps, some intriguing answers. I will answer in more detail tomorrow; I have been away from my computer today.
For Calliban re #12 ...
Bravo! The Lewis was the one I was thinking about, and agree that Zubrin is a great resource as well.
Before I go much further here, I'd like to confirm that you are actually in a position to bring about changes in the real world (solar system in this case), and that you are psychologically prepared to make the sacrifices that would be required. You can look at Musk and Bezos to see what top tier leadership looks like.
(th)
Not sure what you mean. I find the ideas interesting and would happily spend some time developing them. But in terms of making this happen in the real world, that is the realm of governments and billionaires. I'm not sure that I would want to make asteroid mining into a crusade that defines the rest of my life. I am an engineer (mostly mechanical) and like to dabble with interesting concepts.
Hello tahanson,
Mining the Sky, by John Lewis and Case for Mars (Zubrin) are two of my favourite books. Aside from that, I have various electronic resources and have access to Science Direct, through which I can access scientific publications. There is a lot of general information on the internet now, which wasn't available 10-15 years ago.
I have been looking at minor planet listings for suitable candidates, with diameter 30-100m and suitable orbital characteristics (aphelion and perihelion both ~1AU). The most desirable candidates are in the Aten asteroid group. Earth coorbitals would appear to require the least delta-V. There are surprisingly few suitable candidates.
https://en.wikipedia.org/wiki/List_of_Aten_asteroids
Here are a couple, all at the lower end of the size range:
https://en.wikipedia.org/wiki/367943_Duende
https://en.wikipedia.org/wiki/2013_BS45
A 30m diameter restraining bag would weigh 2.6 tonnes. Maybe all of the equipment needed can be launched to LEO with a single ITS launch.
This could be a stretch case at 160–360 meters:
https://en.wikipedia.org/wiki/(164207)_2004_GU9
One way of doing this would be to start with a bag capable of providing a 1KPa restraining pressure for an asteroid 360m in diameter and dig tunnels at a depth of say 100m and limit the density of tunnels to ensure that averaged pressure at the surface does not exceed 1KPa. But that would mean digging an initial shaft 100m deep. As the colony grew, additional restraining strips could be added to the surface, allowing the number of tunnels to increase.
Most of the candidates are too large for this scheme to be practicable. For example, Bennu is a promising candidate in terms of composition and its orbit would appear to be workable. But its diameter is almost 500m, requiring a restraining bag weighing about 10,000tonnes for a 50KPa restraining pressure or 2000 tonnes for a 10KPa restraining pressure and just 200 tonnes for 1KPa.
https://en.wikipedia.org/wiki/101955_Bennu
If Musk successfully develops the ITS launcher and meets his projected launch cost of $200/kg to LEO in reusable mode, then perhaps more ambitious projects are achievable. A 4200 tonne restraining bag could be launched in LEO in 14 sections of 300te each. When bolted together, these would provide a restraining bag sufficient for a 350m diameter asteroid. If a mass driver tug could deliver them to the asteroid from LEO that is.
I begin to wonder, given that the market for any mined material is Earth or Earth orbit, if it would be more efficient to simply capture a small NEO and deliver it into Earth orbit using some of its material as reaction mass. That way, processing equipment can be pooled in Earth orbit rather than shipping it out to numerous distant targets. Not quite such an exciting idea, but possibly a more profitable one. Asteroids in the size range of 30m in diameter, would mass about 30,000 tonnes. We could use restraining bags to allow material to be mined from within them and then fed to mass drivers.
Could we shift that into high Earth orbit using mass driver engines within a few years? How much power would we need say, if 1/3rd of the asteroid is used as reaction mass? We are probably talking a delta-V of at least a 3km/s from an Aten asteroid to high Earth orbit. Any thoughts?
Thanks Spacenut. Plenty to think about here and I will give the topics a read.
From what I have read elsewhere, the shell world concept involves using the gravity of the body to balance internal pressure, through the weight of a thick layer of rock balancing the pressure within. In near Earth space, only a few of the largest asteroids have sufficient gravity to allow this at any atmospheric pressure useful to humans. Eros is one; Ganymed another. Phobos is probably massive enough.
ISRU fibreglass would allow us to bootstrap; starting with a small asteroid and building the fibreglass bag needed to hollow out a much larger one, perhaps a kilometre in diameter. This would truly be large enough to house a million people if we hollowed it out, although disposing of waste heat would then be a problem.
In terms of location, it would be most productive to consider near term achievable targets. That means near Earth objects with aphelion and perihelion not too far from 1AU; and as small as possible delta-V between the asteroid and low Earth orbit.
Thank you for a good lead off to a topic Calliban, and welcome to NewMars.
We have lots of targets to which this could be done with but it still a lifting problem from earth to get critiacal mass of items that we need to achieve the building to ocupying steps.
Is there a favored target for teraforming?
Have we studied the possible resources that will be needed to support man?
What are the limits to the colony size?
Are there plans for this colony to be a deep space port to leverage from?
I raised the topic here precisely to explore these sorts of questions and to expose any flaws in the concept.
The purpose of wrapping the asteroid in a strong polymer bag is to allow mining to take place within a pressurised environment with at least some spin gravity (note: because of micro meteorite impacts, the bag itself would not remain airtight, but would provide a restraining force that allows tunnels in the asteroid to be pressurised). The concept is analogous to a pre-stressed concrete pressure vessel, in which steel cables on the outside of a hollow concrete block provide a compressive force that balances the pressure inside. As more material is excavated, more living space becomes available. The asteroid evolves from a small outpost to a human colony and industrial centre incrementally, as living space expands and more equipment is delivered.
Whilst in the long-term, it would make more sense to construct purpose built habitats, as Terraformer suggests, the bag idea would appear to allow a group of colonists to arrive at an asteroid with a modest payload and rapidly convert it into a shirt-sleeve environment without the need to construct a large habitat using refined metals. As soon as the bag is deployed, tunnelling machines would get to work excavating materials that would be processed and shipped back to near Earth space. Human living areas would be set up in the empty tunnels left behind. The practicality of the idea depends upon our ability to create the restraining bag using high specific strength materials. In the initial example, a restraining bag weighing only 98 tonnes, allows access to an asteroid weighing 1.3million tonnes. That is a ratio of 10,000:1.
In terms of resources, it would be ideal if we could find an asteroid with some inventory of water, carbon and nitrogen, as these are essential to life and it would be costly to ship these from Earth. Given that the bag is functioning as a pressure vessel, its mass will be proportional to the volume enclosed, assuming that pressure stays the same. Enclosing a 200m diameter body would require a bag weighing almost 800 tonnes, which is far beyond the lift capacity of near term heavy lift vehicles. A 1km diameter asteroid would require a 98,000 tonne restraining bag, which is far beyond near term achievability. On the other hand, enclosing a 50m diameter asteroid, would require a bag weighing just 12.25 tonnes. This idea works best for small asteroids in the near term.
We would probably want to leave a layer of rock at least 2m thick between the bag and the tunnels closest to the surface. This allows the a shell of rock to act as cosmic radiation shielding. For very small bodies, this would waste a lot of material and there would be insufficient internal volume to be useful for habitats or equipment. So I believe that near term practicality of the concept is for spheroid bodies some 30-100m in diameter.
I think the market for excavated material would be Earth itself for platinum groups metals and near Earth space for all other materials. Bulk silicate might make useful reaction mass for transporting other more valuable materials to Earth orbit. Transport will be via mass driver tugs.
Greetings! I am new to the board and have been interested in space travel and space colonisation for as long as I can remember. I thought as my first post here, I would share an idea that I have been working on.
The idea of building human settlements in pressurised spaces in hollow asteroids is quite an old one and goes back to the work of Dandridge Cole in the 1960s. The idea has a number of problems, chief amongst them being that asteroids are not rigid structures and unless internal pressure can be counteracted by the force of gravity, the asteroid is likely to fail catastrophically and the air, along with human occupants, would be blown out into space.
Small near Earth asteroids are one of the best options for near term space colonisation, due to their proximity to Earth, their abundance of rare metals, which could be mined and returned to Earth and their low escape velocity. Burrowing into the asteroid allows miners to protect themselves from micro meteorites, space radiation and thermal extremes.
I want to explore the idea of structurally reinforcing small asteroids with high strength polymers and carbon fibres to allow them to withstand internal pressure. My base case is a 100m diameter asteroid. If density is typical of a stony asteroid, it would mass 1.3 million tonnes. Upon arriving at the asteroid, we would wrap it in a fibre reinforced polymer bag, with airlocks fitted into it. The bag would then be tightened around the asteroid to provide prestressing. We would them enter through airlocks and mine out the interior of the asteroid and pressurise the tunnels created. The tunnels would probably have to be lined with a polyethylene wall paper to prevent air from leaking out.
The required mass of the bag would depend upon its specific strength, the pressure in the internal tunnels and the proportion of the asteroid hollowed out. I used the thin walled pressure vessel equation to estimate the required thickness of the bag. My assumption is that the bag is made from woven zylon fibres with an epoxy binder. The break strength of zylon is 5.8GPa. With the epoxy binder taking up 50% of volume, breaking strength would be reduced to about 3GPa. For a 100m diameter bag, rated to a pressure of 50KPa, the minimum required thickness would be 0.42mm. Applying a safety factor of 6 allows tolerance for micro meteorite damage and increases thickness to 2.5mm. Assuming a density of 1250kg/m3 for a zylon-epoxy composite, total mass of the bag would be 98 tonnes. This is light enough to be launched by near term heavy lift vehicles.
We could even spin the asteroid to generate internal gravity in the tunnels, although the bag would need to be stronger and heavier to accommodate this. Natural light could be allowed to enter the interior, by cutting pits into the exterior of the asteroid, assuming that the bag can be made transparent.
If some 50% of the volume of the asteroid were excavated, then the asteroid would have enough internal space to house a thousand or so people, along with industries and manufacturing.