Sunday, November 7, 2010

Home Away From Home

The comment thread on my previous post about space patrols raised the issue of base stations for more prolonged missions, extending to years.

This has application far beyond military or quasi-military patrols. In fact it is fairly fundamental to any extensive, long term human presence in deep space. Whether or not we put permanent bases on the surface of Mars, Europa, or wherever, we will surely place permanent or semi-permanent stations in orbit around them. Particularly because the stations can be built in Earth space, where the industry is (at least initially), and flown out to where they will serve.

Hab structures intended for prolonged habitation should be fairly large, if only because if you are going to live for years in a can it should be at least be a roomy one. And they must be thoroughly shielded against radiation, much more than ships that you only spend a few months aboard every few years.

So let us play with some numbers. Make our spin hab a drum, 200 meters in diameter and 100 meters thick. Volume is thus about 3.14 million cubic meters. The ISS has about 1200 m3 of pressurized volume and a mass of some 300 tons, for an average density near 0.25, but the mass includes exterior structures such as keel and wings. Let average interior density be about 0.16, for a mass of 500,000 tons.

If we allow 100 cubic meters per person the onboard population (whether 'crew' or simply residents, or a mix) can be up to 30,000 people. This is about twice the density of a middle class American urban apartment complex. Given that much of the usable volume must be working areas, public spaces, and so forth, the actual crew or population might be more on the order of 10,000 people, equivalent to a decent sized small town or a fairly large university or military base. Thus the hab has 10 times the volume of an aircraft carrier and twice as many people.

Spin the hab at 3 rpm and you get almost exactly 1 g at the rim.

By my standard rule of thumb the cost of this hab is on order of $500 billion. That is a steep price tag, but on the other hand it is only five times the cost of the ISS, and you need very few of these unless you are engaged in outright colonization.

Now, shielding. The standard for indefinite habitation is about 5 tons per square meter of cross section. (Earth's atmosphere provides about 10 tons/m2.) Portions of the hab where people do not spend much time, and exterior to where they do spend time, can be counted toward the shielding allowance. So let us say that the outer 10 meters of the interior (about 35 percent of the volume) are used for storage, equipment rooms, and the like. This provides about 2 tons per square meter of shielding, 40 percent of the requirement.

The remaining 3 tons per square meter of exterior shielding must cover about 125,000 square meters of surface, so shielding mass is about 375,000 tons, adding 75 percent to the mass of the hab, now 875,000 tons. This shielding need not be 'armor.' As I recall, water provides pretty good shielding against GCRs, your biggest radiation problem, and water is so useful that having 375,000 tons of it on hand in a reservoir will never be amiss.

Moreover, to move the hab you can vent off the water (or pump it out) and not need to lug the mass, assuming you can replace it wherever you are going. The deep interior of the hab, more than 25 meters from the surface (about 28 percent of the volume) is still shielded by the rest of the hab structure, so the hab can carry a reduced population during the transfer.

You are still moving a half million ton payload, so don't expect to rush it unless you have a really badass drive bus handy. Habs being repositioned across the Solar System probably travel on Hohmann orbits, and have drive accelerations of a few dozen microgees, good for about 1 km/s per month of steady acceleration.

For a smaller hab structure, scale down the linear dimensions by half, to 100 meters diameter and 50 meters thick. Structural mass, volume, and capacity are all reduced by a factor of 8, to 400,000 cubic meters, 60,000 tons, and a crew / resident population of about 1500-4000. Our 'mini' hab is now broadly comparable in volume, mass, and crew to an aircraft carrier.

Surface area is only reduced, however, by a factor of four, to about 30,000 square meters. Moreover, the smaller hab provides less interior self-shielding. If we keep the same proportions our internal reserved zone is just 5 meters deep and provides only 20 percent of the needed protection, not 40 percent.

We now need about 120,000 tons of shielding - twice the unshielded mass of the hab. If we move the hab fully shielded our payload mass is 180,000 tons. Remove the shielding and payload mass is just 60,000 tons, but no part of the smaller interior is fully self-shielded, so any crew on board during a 'light' transfer must be relieved every few months. On the bright side, if you have a 100 gigawatt drive bus floating around, or about $100 billion to buy one, you can take a fast orbit and get there in a few months.

The image shows a drum-hab station ship with a spin hab of the full sized type described above, 200 meters in diameter by 100 meters thick, fitted with a heaviest class drive bus for transfer. I am delicately ignoring details of the connection between the spin drum and the hub structures.

The shuttles approximate the NASA Shuttle, as a visual size reference. The deep space ships docking up to it are large fast transports, 300 meters long, ten times heavier than the patrol ship discussed last post. The station ship itself is about 675 meters long by 450 meters across the outrigger docking bays.

In my image the station ship is no aesthetic triumph. Allowing for my limitations as an graphic artist (compare to commenter Elukka, from the last comment thread), the transport class ships don't look too bad, but the station ship merely looks tubby instead of grand. Some modest architectural improvements might yield a more impressive appearance with little change in overall configuration.

Of course the interior will matter immeasurably more to the people on board. Mostly, presumably, it will resemble the interior of a very large oceangoing ship, corridors and compartments, probably including some fairly imposing public spaces, comparable to the grand saloon of a 20th century ocean liner or even larger. It can be as elegant or as sterile as you like (or both, depending on deck and sector). The third popular choice, rundown industrial gothic, is constrained by how far you can go in that direction before the algae dies or the air starts leaking out.

So find yourself a cubby and make yourself at home. You might be here for quite a while.


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Tony said...


"How does it work for apogee/perigee changes, rather than plane changes? Is there a delta v cost difference between going into LEO and then a transfer orbit (to geosynch or wherever) versus boosting straight into the transfer orbit."

If there is a significant cost, we've been paying it forever. AFAICT, nobody launches direct to a transfer orbit. They always go into parking orbit first. In principle, going up or down, the delta-v changes are always either additive or subtactive anyway, depending on whether you're taking the up escalator or the down one.

"But in any case the real concern is propellant, not energy or even delta v..."

Well...look at it this way: If you bring up a high efficiency drive to take the shuttle up to a high orbit and bring it back down, the mass of the drive, its power supply and its fuel is subtacted from the payload. If you just bring up the fuel for a drive bus already in space, that invades payload margin less.

Thucydides said...

Thinking again about refurbishing, time etc., I can still see the need for some sort of minimal station in orbit. Consider:

In order to ensure all upgrades are done properly, it has been strongly suggested they be done as rebuilds at the factory.

The orbital mechanics suggests that even with high ISP drives there will be "shipping seasons" between various destinations

There will always be a limit to the amount of launching or lifting possible.

Given these conditions, the ship owners contracts with Boeing, HAL or Airbus to produce or remanufacture hab modules on the ground in their factory spaces. (This assumes habs are about the same size as the ISS modules today). Close to shipping season they get boosted to orbit and secured to the cage, so the available lift is distributed over a workable period of time. Boosters are also being booked to haul tanks of remass and fuel into orbit.

Ships arrive during shipping season, and habs are swapped out. the cage provides the workspace, houses various robot arms, tools and if necessary, astronauts to do EVA, as well as a control center on the spot to manage things.

After shipping season, old habs are inspected. Ones with enough service life remaining can be brought back to the factory for rebuilds, while time expired ones are sent to the breakers (which may also be on the planet's surface).

Fuel and remass tanks are treated the same way, and it is probably much more economical to use the cage as a fueling station (at least at first), given the somewhat dicey economics of ferrying habs back and forth.

Notice I am not advocating for old habs to be held in orbit as work and living space; if they need a rebuild after a space voyage to Mars or whatever then they will have certification issues even being used in LEO. This is not to say they might not end up being used that way (depending on whatever sort of legal regime exists in orbit), just I would not feel very confident about moving into a hab with lots of hard usage behind it already.

Raymond said...

Ferries and tugs, I can see using Rick's butterflies or Milo's fully-shielded nuke electrics. Cost would be relatively marginal compared to a full-out interplanetary craft, since it wouldn't have to have the same kind of mass nor performance envelope, and one or two could service a much greater number of larger craft if the timing is worked out. You can get by with smaller reactors/solar arrays, smaller engines, less-shielded habs, et cetera.

It would also be much closer to that fuzzy dividing line between ship and station. Is a station still a station if it moves regularly from one parking orbit to another?


Solar-electric wings get more thrust per square meter than solar sails, but a) sails don't use propellant and b) sails' mass per unit area is an order of mag smaller (compare 1-2 m^2/kg solar panels to 3-12 g/m^2 for solar sails).

We also already have a solar-sail craft on its way to Venus, courtesy of JAXA: IKAROS. In a fun little bit, it has blocks of LCDs on the outer edges of the sail, to selectively block or allow the photostream for attitude control.

Luke said...

Re: radiation hazards of operational nuclear thrusters.

A single fission event releases about 200 MeV and, on average, about 2.5 neutrons. Each neutron has about 2 MeV of energy. In order for fission to proceed, you need at least one neutron to react with the fissile element. If we assume a fast reactor that is just critical and that all neutrons not involved in fission escape without interacting, we find that on average 3 MeV escapes from each 200 MeV fission event. This gives you 1.5% of the heat power of the reactor being radiated as neutrons. For convenience, let's just assume that scattering and inelastic collisions with nuclei reduce this to 1% of the thermal power.

Assume we have a 1 MWt reactor. This will be radiating 10 kW of neutrons. A human will present about half a square meter of cross sectional area for a full frontal full-body exposure. 20 to 30 cm of meat is pretty good at moderating neutrons, so pretty much all the neutrons incident on the person will deposit their kinetic energy in the person's tissues. I will neglect neutron capture reactions for this rough calculation (neutron capture releases gamma rays, and can activate nuclei so they later decay radioactively), and will assume the human has a mass of 75 kg. Furthermore, I assume the neutrons are radiating isotropically from the reactor. Under these assumptions, our human will be taking a dose of about 5 Gy per second divided by the square of the distance from the person to the reactor. 5 Gy is a lethal dose, 1 Gy will cause acute radiation sickness but will not necessarily kill. If we want our human to survive a 1 minute exposure (say), then he needs to be more than 17 meters away to reduce the dose to less than 1 Gy. At 100 m, our human will be taking a dose of 0.5 mGy/s, and you could expect acute radiation sickness at around half an hour of exposure.

The effects of low dose radiation is not well understood and controversial. It seems likely, however, although by no means certain, that doses of less than 10 mGy or so can heal without issue (or at least minimal risk of long term health effects). Using this criterion, at 100 m our target human will be essentially unharmed at less than about half a minute of exposure. He would probably not want to do this more than once a month or so to let his cells regenerate and recover.

A bit of shielding can go a long way. Since I just happen to have the data here - 3.8 cm of polyvinyl toluene doped to 4.4% boron-10 by weight will reduce the dose from fission spectrum neutrons by about 99.8%. This will extend the safe-time by a factor of 500. In practice, you would probably use boron-doped high density polyethylene (HDPE) rather than PVT (I am using PVT because it is a scintillator and I happen to have some simulation data for it). These plastics have a density of about 1 g/cm^3, so 4 cm thickness of the stuff would have a mass of about 40 kg per square meter. If our reactor is approximately cylindrical at 5 m long and 1 m diameter, this gives us a mass of about 700 kg to shield it. This is not negligible, but does not dominate the reactor mass by any means.

Once turned off, the reactor stops emitting fission neutrons. Beta-delayed neutrons will be emitted for about 10 more seconds, at about 1% of the reactor power.

I am neglecting the gamma ray dose, because gamma rays are best shielded by heavy elements, and the reactor core itself is made out of heavy elements giving it a lot of self-shielding. In addition, the steel and zircalloy of the containment and heat exchangers will act as a reasonably effective gamma ray shield. Without the gamma shielding, however, the dose will be approximately quadrupled from prompt fission gammas, and the radioactive fission products will continue to emit gamma rays for years after shut-down (although the dose will fall off rapidly with time).

From these numbers, it looks like a shadow shield for the crew compartment and auxiliary thrusters for docking will be sufficient to keep the dose within acceptable limits.

Milo said...


"If you just bring up the fuel for a drive bus already in space, that invades payload margin less."

How about said bus being attached to your low-orbit space station?

Or even said "bus" being your interplanetary ship itself, that's docked to the low-orbit space station for shuttle access?

This shouldn't be an "if". Having high-efficiency engines already in orbit is the point of having separate shuttles and ships, whether you have a space station or not.


"Is a station still a station if it moves regularly from one parking orbit to another?"

Keep in mind, current "ferries" - you know, the mundane kind used to cross rivers and such - are usually quite localized, travelling a consistent route between two specific points. So "ferry" is a good name for something that stays in more or less the same place but is still mobile (and carries meaningful payload while moving).

Tony said...


"How about said bus being attached to your low-orbit space station?

Or even said "bus" being your interplanetary ship itself, that's docked to the low-orbit space station for shuttle access?

This shouldn't be an "if". Having high-efficiency engines already in orbit is the point of having separate shuttles and ships, whether you have a space station or not."

I tried to be done with the orbital station portion of the conversation, honest.

But, since you asked...

The discussion about the inter-orbit ferry has to do with the percieved safety requirement to park nuclear wessels in high orbits. The delta-v between a LEO parking orbit -- where the shuttles go first, before anything else happens -- and a high orbit in the 45000 km, is in the neighborhood of 3900 m/sec.

So I think ferry ships and ferry operations between LEO parking orbits and high altitude parking orbits can certainly be justified absent of any other considerations. It just doesn't make any sense to lift anything past LEO that doesn't need to go higher. Surface-orbit shuttles certainly fit in this category, dontcha think?

Jedidia said...

How does it work for apogee/perigee changes, rather than plane changes? Is there a delta v cost difference between going into LEO and then a transfer orbit (to geosynch or wherever) versus boosting straight into the transfer orbit.

the energy for getting from LEO to GEO is always the same, i.e. a ship from orbit will need exactly as much delta-v as a ship from the surface once it reaches the same orbit. So, dividing a GEO transfer into surface-LEO and LEO-GEO legs makes a lot of sense. Especially since launching from surface to GEO directly is one hell of a difficult undertaking if you don't want to take months to spiral out there with an ion drive...

The problem is of course, you still have to get all that Fuel to LEO. Might actually be cheapest having the ships refuel at GEO and shipping the fuel to there from the moon. If you can make the fuel on the moon cheap enough to make it really pay, that is. And if you can make the fuel there at all...

Sorry, but orbital mechanics says that's not a good idea. It's fundamentally easier to get to a given orbit from the ground than to go to another orbit before transfering to the target orbit.

And this is, of course, all too true... Didn't think it through enough.

Raymond said...

So, if we take nuke-electric interplanetary craft as too dangerous to park in LEO, requiring ferries (presumably still high-ISP drives, but possibly lower-performance than interplanetary versions) to move cargo and people to and from...

Well, then, we won't have a use for stations, methinks. Ferries, tugs, and maybe tenders (once we get to the point where certain repairs/services are worth doing in space). But at that point, I'd have to agree with Tony about the lack of uses.

Luke said...

For an orbital ferry for a planet with a strong magnetic field, you could use electrodynamic tethers to move cargo and passengers without expending propellant.

Anonymous said...

The thing about orbital stations is that they can be used for many different missions; these missions(scientific, industrial, commercial, logistics, etc), can be performed simutaniously and so stations need not depend on interplanetary spacecraft operations so much. And that's one orbiting Earth. One orbiting Callisto (for example), might be used for support of not only the exploration bases on the moon's surface, as well as on Europa and Ganymeade, and be the focal point for studying the radiation enviornment, magnetosphere, and Jupiter's atmosphere; a single point for cargo and personnel transfer from Earth, instead of half-a-dozen. So, yes, once you go beyond the narrow view of what a station could be used for, or what it should be used for, and look at all the reasons a station could be used for, then they become much more attractive.


Rick said...

What is the radiation picture for prolonged exposure? For example, supposing two ships with 100 MW drives, how much distance must they maintain in formation during a 3 month mission? With the drive shut down, how long can it be parked alongside another ship or station.

On ferries, human passengers, etc., may be taken up by chemfuel (which could mean simply refueling the shuttle, or launching on a bigger booster).

A fast trip, instead of spiraling up or down, gets passengers through the Van Allen belts much more quickly, and also means they can make the trip in airliner style seats. The slow spiral requires a higher habitability level.

On using semi-retired interplanetary habs in orbit, my premise is that interplanetary missions require a really high reliability margin, no failures requiring any material support whatever for a year and upwards. But failures that would endanger a mission a year from Earth may be trivial if you are two weeks from a replacement part.

Practical engineering considerations may rule this out, but it has a cousin, ships that are simply obsolescent, particularly the drive bus.

If you have an older generation hab, still in parameters, with an obsolescent drive bus, it may not be worth mating to a new bus for interplanetary service, but still available for orbital service if you have use for it.

The underlying question is whether you have any orbital work force that needs hotel accommodations. If everyone in orbital space can conveniently bunk at night aboard a shuttle, ferry, or deep space ship in parking orbit, you don't need a hab pod as an orbital hotel. But if this does not fit your work cycle, you will need something.

Luke said...

For prolonged exposure, you need to take into account the fact that some types of radiation are more effective at causing chronic symptoms than others. Correcting for this gives you a dose in units of Sv - for neutrons this is assumed to mean multiplying by 10. The galactic cosmic ray dose in space varies from about 400 mSv/year to about 900 mSv/year. This corresponds to 40 mGy/year to 90 mGy/year. For a 100 MW unshielded reactor, my previous assumptions give a dose of 500/r^2 Gy/s, where r is the distance from the source to the target. From this we can solve for the distance that will reduce the neutron dose to about the same as the cosmic ray dose (let's pick 500 mSv/year), and this turns out to be 178 km. So a few hundred km separation will allow two craft to fly in formation safely using their cosmic ray shielding. If the reactors are shielded, this distance can be decreased. 8 cm of borated HDPE, for example, would allow them to safely approach within a few hundred meters.

The radiation from a shut-down reactor depends on how much time has passed since it has been shut down and for how long it had been running. A very rough estimate would put the gamma radiation after 10 days of inactivity from a reactor that had been running for a month at about one billionth of the original power level and the neutron radiation as negligible. For a 100 MW reactor in which we neglect any shielding 9including self-shielding) at 10 meters distance, you are looking at something on the order of a microgray per second, or about 100 times the galactic cosmic ray dose rate. If the reactor had been operating for several years without refueling, you might be getting an order of magnitude or two larger dose rate. Again, shielding will reduce the dose.

Tony said...


"So, if we take nuke-electric interplanetary craft as too dangerous to park in LEO, requiring ferries (presumably still high-ISP drives, but possibly lower-performance than interplanetary versions) to move cargo and people to and from..."

It's not just the nuclear risk that might motivate parking in high orbit. There's also the question of propulsion optimization. Even with something like VASIMR, performance optimization is still possible. So in very much the same manner as staging during Earth to orbit launch offers the opportunity to use upper stage engines optimized for the vacuum operation, parking in high orbit keeps the interplanetary spacecraft out where its engines can be optimized more towards the interplanetary environment. The inter-orbit ferries would have engines optimized for their task as well. Might be worth doing for that reason alone.

Raymond said...


I'm not exactly sure how you'd optimize an engine like VASIMR, given its ability to essentially self-optimize, but I can certainly see the powerplant being tailored to orbital service. Say, Rick's solar butterflies or Milo's fully-shielded nukes. Maybe even nuke thermal (if the rads aren't that bad) or solar sails (if the tricky orbits can be reliably used).

(Now that I think about it, I can see nuke thermal being employed for cargo on slower orbits, given it fits a little better with the disposable stage paradigm. Solar sails may also be better suited for interplanetary cargo, at least on the outbound leg.)

The other consideration is that orbital ferries are also well-suited for Lunar travel (at least to Lunar orbit, anyways). Their very existence may substantially reduce the cost of a Lunar base. If we already have the transport infrastructure pretty well in place, would a Lunar base be worth the (now lessened) marginal cost?

Rick said...

Yes. From a delta v point of view, everything from geosynch to translunar space and lunar orbit is one fare zone, so to speak. From LEO, assuming Oberth boot, it is 3.2 km/s to Earth escape, 3.9 km/s to low lunar orbit, and 4.1 km/s to geosynch.

High ISP drives are nominally less efficient in this envelope because their low acceleration precludes Oberth boot. However, their fuel economy is so much better that they are preferable, except perhaps for human travel. (Van Allen belts and seat time.)

Nuclear thermal may between stools. The specific impulse is 2-3x chemfuel, and acceleration is sufficient for Oberth boot. But you need a reactor with heavy shielding for human travel, and you cannot use aerobraking for the return to LEO - nuclear meteors in the upper atmosphere are a VERY BAD idea.

Rick said...

Nuclear thermal may fall between stools.

Raymond said...

Nuke thermal may make a useful ferry tech and/or slow interplanetary cargo tech, though. It's at least an order of mag smaller than nuke electric by specific power - no radiators, generators or ion engines, and the shielding can be lessened due to the exposure times being measured in hours instead of months. It could also be used bimodal, allowing a small ion engine to slowly return to low orbit (with substantially lessened power levels, and thus radiation). I think it might even be a preferred crew ferry drive tech for the above.

Tony said...


"I'm not exactly sure how you'd optimize an engine like VASIMR..."

I'm not either, but, in principle, any engineering artifact that can be more narrowly optimized towards a performance regime can be made more efficient.

Thucydides said...

Humans are rather time sensitive, so for short trips (like to the Moon), high thrust, low ISP drives are probably the way to go. After all, chemical rockets took us to the Moon and back using 1960's technology with a trip time of three days, and it ony took that long because the Moon's gravitational field is pretty weak and a fast flyby would need a much more powerful engine and more fuel to slow down and enter Lunar orbit.

Once we make the decision to head out to deep space (NEO's, Mars, the gas giants), then low thrust, high ISP drives become more useful since they can get you there much faster (VASMIR to Mars in 39 days, and IEC fusion [if it can be made to work] to the outer planets in about 2 months according to Robert Brussard). Flocks of solar sails or solar panels attached to ion drives will also work out to the asteroid belt at least.

Low tech, low cost "homes" can be made out of tire shaped bladders filled with water (and water seems to be everywhere, even on the Moon, which seems a bit weird after being told the moon was stripped of volatiles during formation. Clearly we have a lot to learn). The Neofuel site has the basic outline, a 100m dia. donut can be made with a 12 ton bladder and 8000 tons of water (houses 100) while a larger 215 m dia ship can hold 1900 people and weighs in at 40,000 tons). Moving a beast like that would probably take a low ISP high thrust drive for breaking orbit, terminal manouevres and a low thrust, high ISP drive for "cruise" to get anywhere in a reasonable time frame.

Since the drive machinery would be in the middle of the donut hole, you also have your reactor/power supply with you at all times, or could swap it out/ditch it if needed, or were settling down as a space station.

Tony said...


"The Neofuel site..."

Is nonsense.

If our propulsion technology is nuclear-thermal with water remass, then cometary ice is essentially unreachable. The energy to rendezvous an ice mining/refining operation with a comet, then slow any mined/refined ice back down to manageable orbital velocities is beyond that technology's parctical capabilities.

And no, I don't believe in plentiful enough lunar water ice to mkae a difference, so let's forget that part of the discussion right now.

Scott said...

"A courtroom. Not primarily for criminal cases, but a lot of civil things. Mining claims, lawsuits, and the like. It may be more informal than our current system- more along the lines of a Judge Judy at first."

Nah. Captain's Mast. put a green tablecloth down and read someone the riot act, bust them back a couple paygrades, and then cost them a couple month's pay.

It's worked for about 100 years here on earth, even in the submarine force. It also doesn't happen very often.

Once you get to the point of needing to define mining claims, then you'd need someone (poor soul) dedicated to the job. It's not like the US Bureau of Land Management is going to be out there!

Scott said...

"What is the radiation picture for prolonged exposure? For example, supposing two ships with 100 MW drives, how much distance must they maintain in formation during a 3 month mission? With the drive shut down, how long can it be parked alongside another ship or station."

Water is your friend. Water stops neutrons, and the water-density plastic tanking stops x/gamma. Plus you have decent distances for that part of the rad-health equation (time-distance-shielding).

A US nuclear-powered submarine uses (according to wikipedia) a reactor in the ~150MW power range. The reactor itself sits in about 25m^3 of water. Then there's a couple water tanks, each about 3m thick (yes, it's heavy, but water is too useful to not have *lots* onboard ship).

Total dose from living ~200 feet from an operating nuclear reactor for several months? Zero, if everything works the way it's supposed to.

You want lots of water for life support, shielding, and catastrophic-emergency remass.

Raymond said...


"If our propulsion technology is nuclear-thermal with water remass, then cometary ice is essentially unreachable. The energy to rendezvous an ice mining/refining operation with a comet, then slow any mined/refined ice back down to manageable orbital velocities is beyond that technology's parctical capabilities."

Not necessarily. There are NEO objects which may contain useful amounts of water within the delta-v of chemfuel, much less nuke-thermal. But we already have ion engines of a couple different types, and VASIMR's looking good. no reason to assume we're limited by nuke-thermal delta-v.

I still don't think habitats made from large amounts of water lying about in space are feasible or sensible in the next century or two, so I'm not really disagreeing with you on this one. By the time such an operation is economical, wouldn't we have proven by definition that we don't really need it? (I mean, it could be useful for propellant you don't have to bring up the well, but whole habs?)

Tony said...


"There are NEO objects which may contain useful amounts of water..."

I include those in the same category of the unproved unproved water on the Moon. Even if the water is there, it isn't all that much. One estimate that gets tossed around a lot is a billion gallons in the crater where they dumped LCROSS. That's only 3,000 acre-feet. Which would supply 12,000 households for a year, even by stricter Desert Southwest conservation standards. And, oh yeah...that's after you land thousands of tons of collection and refining equipent to make use of it.

That's the problem with lunar/NEO/cometary water -- the snake oil salesmen who try to convince you it will work just handwave away all of the real world engineering. Well, the ones who know any engineering -- most of them are simply ignorant idealists.

Raymond said...

Unproven, not impossible. There are 85 near-earth comets as of June 1 this year (NASA stat link), and the asteroid categories include both rocks and old comets whose volatiles have evaporated (the latter may still be largely ice). More detailed composition data is required before we could say one way or the other. (Incidentally, this is one of the reasons I'm excited by the prospect of more NEO missions in the near future.)

I share your skepticism in the near-term, though. We really shouldn't plan on getting any materials from NEOs in any real quantities. If we find a few iceballs within easy reach, great, but it's better for the near term to be pessimistic about it.

Tony said...


"...the asteroid categories include both rocks and old comets whose volatiles have evaporated (the latter may still be largely ice)."

Ummm...water is a volatile chemical for planetological purposes. If the volatiles have evaporated, there ain't no ice left.

Raymond said...


Surface volatiles, I should say. There are still dormant comets like 14827 Hypnos, which are possibly comets whose nuclei have been covered in a few cm of other materials, and thus don't outgas at present (but still may be mostly ice).

I say we wait until we have a few more core samples from various NEOs before dismissing it entirely.

Anonymous said...

Since we seem to have decided that the only difference between a ship and a station is the lack of a bus; a movable base for exploration of NEO's could be useful; the base/ship could be a (relatively)cheap hab module with an attached lander, solar panels, support module, and a cheap drive. Think of it as a wenibago fitted for space exploration; trade out the crew every few months and move it to a new location at the end of each mission. The whole thing could be launched in small sections, perhaps even inflatable with water, and assimbled in orbit. The ISS would gain another mission; assimbling, servicing, and coordianting these wandering base camps...


Scott said...

You guys missed my point. Water is so valuable for life-support purposes and radshielding that it's worth hauling up out of the grav-well, even at ~$100m/lb. If you can find it from NEOs, that's great, less to haul up from the surface, but I expect to see megatons of water going up with a serious space presence.

Thucydides said...

Lifting the quantities of water needed from Earth would require hundreds of Saturn V class launches for each assembled spaceship, a practical and economic impossibility.

Getting that amount of water can only be done by extracting it from a NEO. Since many NEO's can be reached with less delta V than needed to get to the Moon and back, current technology should be able to do the job. NERVA type NTR's or Solar Moth's would be better in terms of getting more mass to the target (and VASMIR might be better still), but there seems to be no conceptual reason that we could not do this, if we were willing to devote the resources to the project.

zlionsfan said...

FWIW, I think common household pets - at least the types that are common today - might not work for a station of 10,000 or so. If the environment permits pets at about the same ratio as on Earth (which is unlikely, given the supplies you'd need to sustain them), that'd mean somewhere between 6000 and 7000 cats. (A similar number of dogs, as well, but I'm a cat person, so I'm not as familiar with what's required to support dogs.)

That sounds like a lot, but I don't think it is, not in genetic terms. If they were all, say, Domestic Shorthairs or Domestic Longhairs, you'd probably be fine. Any purebred cats would likely be the last of their line on the station: there would be so few others of their breed that they'd have to mate with other breeds; several breeds have recessive genes responsible for some nasty traits. In a huge population, it's trivial to breed them to another line in that breed; on a space station, it's practically impossible.

Of course that's assuming that the cats are not spayed/neutered before being brought aboard, and also that growth is monitored closely to keep the population reasonable. (That itself is not a trivial task.) If they are fixed, then the only source for pets is the PetCo Shuttle, and it seems unlikely that cargo space on plausible mid-future shuttles would be used to transport pets. (I guess it would be life-support space rather than cargo space, but space nonetheless ... and you would have to provide supplies for them like you would for human travelers. No big deal for a seven-day trip from surface to orbit, more than a small consideration for a month-long trip.)

Speaking of supplies, it's true that you'd have to have appropriate food (and water) supplies for the cats ... if they're eating, say, mice, then you have to have supplies and such for the mice as well, all the way down the line. Even if you try to make your own cat food, it's still going to be meat-based, and it seems like a lot would be required to support a few thousand cats.

Medical care would be a mixed bag: from what I understand, vets study some of the same subjects that doctors do, branching off once they get the basics down. (It's mostly the same pieces, just put together differently.) It's possible/likely that you could have one or more doctors with vet knowledge, or failing that, vets with enough doctor knowledge to serve as backups (so they're not totally focused on pets).

Medicine would be the same as for people (sometimes literally, just in smaller doses): some medicine would come from the same sources as medicine for humans (made on board, if that's the goal, or arriving on the five-year shuttle or whatever), and some you probably just wouldn't stock. There might even be value in not treating certain types of genetic diseases, because you might be able to breed those out of your Spacefaring Shorthairs.

I feel like there are other parts of the pet-support business that I'm missing (physical maintenance to keep claws and fur from damaging the station) ... but on the other hand, there's a pretty significant psychological boost to having pets around, and given that this is mid-future, psychology itself could be enough of a reason to try to make this happen.

As for zero-gravity environments, well, cats probably wouldn't react well at first (see the cat-in-zero-gravity video), but then if you put the appropriate substance on the "ceiling" they wouldn't have any trouble getting "up" there, would they? (The worst part for cats would no doubt be the realization that without an actual "up", there would be no high places from which they could survey their surroundings.)

Rick said...

Cat-keeping in space was so much easier in old Heinlein stories!

I didn't even know there was a cat in zero gee video. Interesting, not to mention bizarre. That puss was really confused!

Assuming that cats do adjust, I wonder if other cues would serve to identify rock outcroppings for them to watch the cabin from.

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