Friday, February 3, 2017

Uranium on Mars: Where to Find It

NASA made some pretty neat posters. Share and enjoy.

So let's say, you've got some compelling reason to mine uranium on Mars. Perhaps a certain deranged Urth president has imposed crippling tariffs on your precious fission fuels, and your atomic rockets' fuel rods are steadily becoming depleted. You can scoop up propellant with a shovel and put it into the propellant tanks, but without the atomic pile to work its hot magic you're going nowhere. What do you do? Well, after rolling into a ball and crying a lot, you can dig out the geology maps for Mars because fortunately it's one of the few places in the solar system besides Earth that you can find anything more than the poorest-grade ores. People tend to assume that asteroids are full of everything from platinum to antimatter and rainbows - they're not.

In theory you can process almost any amount of a given rock and get something out of it - but processing a million tonnes of rock to get even one tonne of uranium is ridiculously uneconomical. Therefore, as with most minerals, you go looking for where it's concentrated.

There are two sorts of questions one has to ask about the possibility of uranium sources besides the Earth. The first question is, is the planet or asteroid core differentiated? That is, was it molten long enough for the elements to separate out into a heavy core, mantle and crust floating on top? The second is, was there some way for the uranium to be concentrated?

Before we talk about that, let's talk about what uranium is like - it's a "lithophile" element, like the rare earths and also thorium. Rare earths go by certain rules, which also sort of apply to U and Th. It makes them a useful geochemical marker, and you can often find uranium where you find rare earths, but that's a loose relationship. So when we talk about finding rare earths we're also talking about the fissile fuels, thorium and uranium. Rare earth tailings are often slightly radioactive because of the leftover U and Th.

But why is it so difficult to get a hold of the rare earths? Well, for a start you need some kind of separation process, and rare earths are notoriously difficult to concentrate in "veins" and so on. A magma sea allows molten minerals to precipitate out of solution and become concentrated. Generally, as magma cools other minerals and elements leave the magma party first by dissolving out as crystals. The rare earth elements are the hard partiers who you can't really get to leave. Eventually, by repeated heating and cooling, all that's left are the rare earths and you get some high purity stuff, along with tough crystals like garnet. But usually that's not the case and you get a mishmash of different minerals with some enrichment of rare earth elements. Generally, you know when a mineral is not going to hang out with rare earths because they are just incompatible due to electron valences etc. For example, olivine is always going to have very little in the way of rare earths.

I'm sure there's a Steven Universe joke in here somewhere.

Most asteroids are never heated to this degree or if they are, are not molten long enough for this to happen. So, they are a random collection of accreted space dirt. You won't find any ore concentration processes happening on them. No veins of uranium unless there happens to have been hit by a freak uranium-rich rock, which will probably turn out to be a derelict atomic rocket. Typically, the concentrations of U in asteroids are in the ranges of less than 1 part per million, which is pathetic as far as ores go. Marginally economic uranium ores on Earth are 100 times richer. This is also why you can't economically mine lunar rare earths - at best they are equivalent to low-grade ores on Earth. So forget it. Nothing short of desperation or some clever way to recover it as byproduct is going to make it worthwhile over simply ordering it from

Now, onto the next stage of concentration, which typically involves the use of liquid water - not something you'll ever find on an asteroid.

In general, uranium is leached out of rocks when water is oxidative, that is, has an excess of electrons to give away. Compounds like H20and the notorious perchlorates widely present on Mars can do this. The dissolved uranium then precipitates out under acidic conditions, as would be the case in the drying Martian seas. Then, as it is eroded to sand, it can be sorted by wind and wave action, for example. Processes like this are what makes uranium and rare earths start to part ways, especially the lighter rare earths like lanthanum. There are a large number of ways uranium can become concentrated. It can even be concentrated in plant matter, becoming coal.

Mars had both a differentiated core and the possibility of hydrothermal action. Heck, we know Mars was wet thanks to the various Mars rovers. And, as the planet became dryer, the water became more and more acidic: the right conditions for uranium to precipitate out into deposits.

Wind action, presumably when the atmosphere was more dense, would have worked with wave action to sift sands and gradually shuffle the heaviest stuff to the bottom. This is how mineral sands work. As Mars' seas gradually shrank, the stuff would wind up left behind on the dry seabed.

Martian thorium concentrations. The red zone is about 1ppm.

Thorium is often found in association with uranium, and a useful nuclear fuel in its own right. It has quite a strong gamma decay signal and so is fairly easy to pick up a gamma spectrometer in orbit. The Mars Odyssey orbiter picked up quite a strong concentration of the stuff in Chryse Planitia just where a shrinking ocean would have receded to.

These are probably monazite sands, which are eroded out of granitic rocks which have been subject to fractional crystallisation (that process mentioned above). This was probably fed by volcanic eruptions and lavas from Pavonis Mons etc. to the south-west. 

Along with monazite grains, fluorine-enriched minerals from Mars have been found, and even fluorapatite. This is nice because we can make uranium hexafluoride with the fluorine, an important part of the enrichment process (you can't just shovel uranium-rich sand into a nuclear reactor after all). Fluorine also makes a terrifyingly effective rocket fuel. And by terrifying, I mean the rocket engineers have not been able to make a fluorine-based rocket engine that didn't blow up. Fluorine is also necessary for a number of plastics, as well as teflon, used in liquid hydrogen tankage and plumbing and of course non-stick frying pans.

Fluorapatite - Ca5(PO4)3F. Like most minerals, usually occurs just as tiny crystals mixed in with other stuff.

Other potential sources include iron-oxide rich laterite clays, as well as certain carbonate rocks. Laterite clays are normally only found in tropical conditions on Earth, but are expected to be common on Mars. The nice thing about have sands as opposed to veins is that the sands are easy to mine. Just scoop them up - no rock crushing required, which requires extremely heavy duty machinery. Clays are also quite soft and require minimal processing. The rare earths, thorium and uranium can be chemically leached out of the rock with a series of hot acidic and alkaline solutions. It won't be the most efficient operation, but then again you are not trying to sell the stuff on the Earth market - possibly because the Urth-people have already nuked themselves into oblivion.

Perhaps the Asteroid Belters will come and trade rare heavy metals like platinum and rhenium for uranium and dysprosium (an important rare earth for high-strength electric motors).

This handy overview of Martian habitability for both human explorers and potential Martian life shows that the best locations with ice are mostly above or below 30 degrees latitude, which is also where solar power becomes less useful and temperatures drop. Fortunately the Th concentrated regions shown above in Chryse Planitia will have some subsurface ice to support the miners without being too far north as to expose them or their machinery to freezing long Martian winter nights.

So, there you have it. As far as uranium is concerned, there's no place like home (Earth) - but you can at least ask the Martians if they have some.

Monday, February 9, 2015

Generation Ships - Travelling Islands in Space

After the rather ironic theme of hibernation and a subsequent 3 year absence, I thought I'd come back with the old SF staple, the generation ship.

I'm not sure where the generation ship idea first came up but I do remember reading a novella by Robert Heinlein - Orphans of the Sky. The generation ship solves the problem of long travel time by basically dooming the crew to die on the ship and their eventual descendants to be the one that reaches the distant star.

The most obvious problem (beside the ethical hurdle of generations doomed to die between the stars) is you need a biiiiiiiiiiiiiiiig damn ship. At least, if you're a still reasonably human civilisation that cares about having a large enough community and so on. It's inevitable that the inhabitants of this ship are going to be as different from us we are from as the colonists on the Mayflower, coming from a civilization that regards a voyage to Mars as something like a transatlantic flight and the outer planets a place to go for cheap hostels and scuba diving in the liquid interiors of Europa and Enceladus.

So, estimates for a ship size range from a tiny vessel with one (female) crewmember impregnating herself to a vast city-ship carved out of an asteroid. On board, there is enough technical know-how, equipment and machinery to last for a flight time of hundreds, a thousand years even - plus all the creature comforts such as parks and soda fountains and a micro-Facebook. Or even a Fakebook with millions of simulated users posting photos of beer coming out of their noses. 

Desperate Housewives! In spaaaaaace!!!

The way the 21st century is panning out, living in space looks it will be significantly easier once you're up there. Technologies like 3D printing and genetic engineering would make it much easier to survive away from Earth once you have access to water, nitrogen and carbon plus metals. Smart materials and true nanotechnology could produce a ship that was literally a living creature, albeit one composed of alloys and exotic composites. VR technologies like the Occulus Rift, once refined, would take away a lot of the cabin fever. Primitive holodecks could even be constructed - certainly the demand for greater immersion in gaming is heading that way. 

Future spacecraft will probably resemble Farscape's Moya more than they do the USS Enterprise...

Herein lies the problem - if we can survive in space, the harshest of environments, why waste all that time and energy bumbling off to other star systems? The answer to that is somewhat paradoxical - if we can survive in space, we can choose pretty much any star system we choose - including close ones that have no habitable worlds. A mixture of asteroids and dwarf planets would be all a spacefaring civilisation needs to flourish. Being able to live with whatever the closest star systems throw at us is going to throw decades or centuries of travel time out of the equation.

Home sweet home for a shipload of travel-weary asteroid miners.

As with any space mission, mass constraints are always crucial. 3D printers, even in the future, may not make the highest precision or most durable replacement parts. The printers themselves may break down, software become corrupted and so on. Laptops sent to the ISS are basically unusable after 6 months in the comparatively benign radiation environment of low earth orbit. And keeping software code stable for hundreds of years will be a challenge - it is possible that hidden in transmissions sent from earth could be viruses tailored to attack the ships' networks. So, you will almost always have a finite stock of basic materials and parts.

This needs to be balanced against simply loading more fuel, thickening the debris shield and flying the mission faster. The problem with advanced technology is that it's harder to make it as robust as lower tech solutions.

Something is needed that can make a reliable replacement or jury-rigged part if the 3D printers go offline for some reason. Thus, 20th century machine shop technology will likely be used and practiced alongside cutting-edge printing technology.

All of this we will learn from building and operating space habitats. The most likely candidates for a generation ship would probably be a space habitat that got gatvol (Afrikaans for ass full) of the solar system and decided to make a star trek (another Afrikaans word) to where it's less crowded. They already know how to live in space, they all know each other and, if they lived in the asteroid belt or orbiting one of the gas giants, they wouldn't be bothered by slow internet surfing caused by the pesky lightspeed limit.

All they'd really need was to strip out all the heavy stuff from their habitat, gather enough He-3 and De, build a big fusion engine and light it. Although most popular SF depicts a single ship alone in the night, it's more likely that the generation ship will be one of many ships, some of them unmanned freighters launched years before with less velocity, all moving and sharing information. It would be a lot easier to talk with another generation ship only 1 light year away than with Earth, 10 light years away, even if you were heading to two different star systems. Once you start slowing down, you could match velocities with the slower freighters, loading fuel and resources. Maybe even using them as lifeboats if things have really gone bad, or crews doubling up on a ship when they only have a few years left to go. On arrival, there should be more than enough asteroids to supply the hungry factories, ready to build roomy space habitats after the generations of travel. Or maybe it's just an asteroid cottage for one veeeerry crazy cat lady.

Up next: artificial gravity.

Tuesday, July 5, 2011

Stopping an Interstellar Freight Train

So you're travelling to Alpha Centauri at 10% the speed of light and the two stars are shining out of your viewscreen. Congratulations, you avoided life support failure, the crew going crazy, or the ship's computer going crazy and killing the crew in their hibernation pods. Now how do you stop? I put this blog post up because I was inspired by the Icarus Project's post on the same subject, and I've been giving this a great deal of thought over the years.

The bad news is that it's going to take all the energy you put into getting up to your velocity into slowing down. You could use your rocket engines (assuming they still work) to slow down, but that means your initial fuel load goes up exponentially and the spacecraft at launch would have been the size of a mountain. (Launching this is going to make the Apollo project look like the Bobsy Twins and their kindly uncle building ships in their back yard*). The alternative is to simply cut your initial velocity in half and use the other half of your delta V (total velocity capacity) to brake at the other end. But it means getting there slowly, and slow means that taxpayers and politicians are less likely to fund such a venture (and the mission is already going to take decades).

Obviously, you would ditch as much of your unneeded payload as possible (but, light years from anywhere, every scrap of material is valuable). Radiation shielding, habitats, soil, empty fuel tanks, all jettisoned except for landers, science payload and your colony equipment. Oh, and colonists, all crammed into a tiny habitat, or camping out in the landers. The high-efficiency motor itself used to get up to this speed is likely to be massive; if one could drop it overboard if there was some other way to break, one probably would. Still, perhaps a fuel reserve could be used to slow the spacecraft a little bit before saying goodbye to the motor. A 10 or 20% reduction in velocity might make all the difference - this must be weighed up against the risk of the motor not working after all these years in vacuum and hard radiation, and already thousands of hours of operation.

The good news is that there are ways to slow down, albeit each with their own limitations. The most viable way to slow down would be with a Magsail. This uses the interstellar medium, which although is a huge vacuum, actually has one or two ions per cubic centimetre. Travelling at such high velocities, this actually translates into a very, very thin mass flow. Running a current through a huge superconducting loop hundreds of kilometres in diameter would tug on these stray ions with a magnetic field, transferring the ship's momentum to the them and slowing the ship down. A very elegant solution which works better when decelerating from higher velocities. Perhaps it would be better to hang onto the engine and only fire it when the ship's velocity got down to ~0.001c, or in the region of a few hundred kilometres per second.

The second no-fuel alternative is to use a solar sail. A BIG solar sail. But to slow down from 0.1c, even a mere 50 tonne payload would require a sail 1000km in diameter, and using an efficient inflated sail made of beryllium diving right in. Making the sail out of super-high strength, high temperature carbon nanotube technology doesn't help that much. Where it does get better though is that when the slower clip you can come in at, the less of a problem it is. The diameter appears to scale with the incoming velocity; so at 0.01c the sail diameter is only 94km. And of course, the sail mass is proportional to the inverse square of the diameter, so a 94km sail only weighs 1% of what a 1000km sail would be. Interested readers should consult Pat Galea's article on this. Alpha Centauri, however, does offer the chance to use two suns to slow down - albeit not in line with one another, making it a lot more difficult to brake at one and then brake at the other. A gravitational slingshot might help, though - calculated to shave off velocity instead of boosting it.

Aerobraking is how planetary landers shed all their nasty orbital velocity, but could we do that to shave off a few percent off of our incoming velocity? Planetary atmospheric deceleration is basically impossible for even a tiny fraction of lightspeed; the probe would almost surely incinerate. At very high velocities, radiative flux heating rather than conductive heating dominates. The radiative flux from plasma sheath surrounding the probe would also incinerate the sidewalls. Also, since this is a huge spacecraft, aerobraking is problematic even for regular atmospheric entry, due to surface area to volume constraints. Heatshields are heavy. Aerobraking also requires a spacecraft to be moving at a speed that is still slow enough to allow the planet's gravity to pull it close in an arc, getting more braking effect. Decelerating from even ~0.001c would create something like a Tunguska explosion on the surface of the planet. Even then, a survivable deceleration spike (say a couple of hundred G's if everybody's vitrified) would only be for a few seconds and shave off a few dozen kps, because, travelling in a straight line at even low sublight means you only dip into the atmosphere for a very short distance. Space shuttles and capsules can afford to decelerate slowly because they travel thousands of kilometres because they are travelling slow enough that they are curving around the planet as they do so. At such high speeds, a planet's gravity wouldn't curve the spacecraft's trajectory at all.

So aerobraking is a no-go. Our spacecraft would disappear in a Tunguska like explosion. So what now? We've got all this mass sitting at (basically) zero velocity for us to slow down, but we're going too fast and planet atmospheres are too small. I considered trying to punch out a "corridor" of atmosphere with projectiles, but I think that would dissipate too fast. Plus, it would be rather catastrophic for the planet's environment, doing some rather drastic remodeling. Not what you want if you want to study it, especially if there's the possibility of life.

This is a somewhat insane proposal that involves using your environment and whatever you have lying about to your advantage. The idea is precisely that your spacecraft is a flying bomb; a snotty tissue tossed out the window at a passing planet would obliterate a city when travelling at even these "low" sublight speeds.

Towards the end of main engine deceleration, the spacecraft dispenses a number of small, independent solar sails. This zip on towards the target star while the main craft continues to large behind. Having no payload, they can decelerate to lower velocities by braking at the star. The main craft then arrives in the system, ditches its main engine and deploys its own solar sail. It brakes hard at the target star, bleeding off a large chunk of its remaining velocity and carries on past the star, heading towards the now near-stationary sails. The next phase is tricky and smacks of insanity. As it approaches each sail, a small railgun on board fires a pellet at each mini-sail in front, blowing it up. The explosion is like a small nuclear detonation, and the main craft's sail catches the debris, slowing it down almost like a parachute. Alternatively, the sails decelerate completely (if the closing velocity is slow enough) because they are so light, and then shoot back out towards the main craft at the same speed, getting a huge energy boost from the sun. If depending on thousands of mini-sails is too problematic, then one big sail could be released and then dole out its energy in the form of small pellets of ice, which the main craft could then ram into. The disadvantage is that not all of the available mass would be used up in the ramming. The mini-sails' total mass would be totally used up in the ramming explosions, whereas the mass of a larger sail wouldn't be used at all - it would just carry on out back towards Sol.

A brief understanding of this concept is that the energy for deceleration is only available at the target star for a short time, which gets even shorter the faster the probe is moving and thus requires much bigger sails. Using mini sails captures this energy and the main craft gathers it up in chunks (by basically ramming into them).

*I have no idea who these characters are, I only know Jerry Pournelle mentioned them. i.e. Something from before my time. I assume it's like the Hardy Brothers (which I also never read).

Friday, July 1, 2011

The Science of Mass Effect

What's a computer game and a propellantless drive doing in a blog about realistic interstellar flight with solar sails, fusion drives and the scuzzy facts of cosmic radiation? Well, I was thinking back about having played Mass Effect, and looking back at some discussion with some (smarter than me) folks who discuss and experiment with propellantless propulsion and advanced physics, I thought I'd try and show how some of this is cropping up in mainstream entertainment (what do you mean you never heard of Mass Effect?).

First off, Mass Effect, and its sequels. I'm sure a lot of you will have at least heard of it, and some played it. It's a combination of shooting game and role playing. While it's a fantastically well crafted and seriously fun game with some great acting, it's also really meaty in its science. Unlike Star Trek's "Particle of The Week" technobabble, the creators of Mass Effect carefully pondered what they could do with cutting-edge physics and technology. Ships, for example, have to have radiators or else they'll cook from their own internal heat in the vacuum of space. Weapons are variants on mass drivers and need to either cool down or deposit their waste heat into disposable heat sinks. Soldiers and everyday civilians are enhanced with cybernetic implants and nanotechnology.

Other topics are touched on, such as what happens when AI evolves and computing progress leads to Singularities. This is reminiscent of Sci-Fi authors like Peter F. Hamilton and Charles Stross. In fact, the game's fluff specifically mentions a couple of people I've had lively discussions with - space wargame designer Ken Burnside and the creator of the Atomic Rocket website, Winchell Chung. You can see a pair of recruits with their names being chewed out (with some mild profanity) here.

The most interesting aspect for me is the titular Mass Effect. In the game, the Mass Effect is generated by applying electrical charge to a chunk of Element Zero (just your average plot Unobtanium). There's a bit of waffle about how this Element Zero manipulates dark energy, the odd force causing the universe to expand. This Mass Effect is used to generate gravity, allow ships to accelerate at stupendous velocities, travel faster than light and to create force fields. In this video, everybody's favourite pop scientist, Michio Kaku, discusses the tech of Mass Effect.

What struck me is that the Mass Effect is an inertia-modifying effect. Which is exactly what the Mach Effect is, just without the Element Zero. It can be achieved by common or garden variety capacitors, so the theory goes, or anything else that fluctuates in internal energy quickly enough. I'll spare the long discussion, but it's really a logical outgrowth of Einstein's General Relativity to explain inertia - basically it's caused by the gravity of the rest of the universe pulling on an on object. Wiggle the object in the right way, and you can get those rubbery strings of gravity to work for you (note, nothing to do with String Theory). A bit like how a vibrating table can make an otherwise heavy object easy to push. Basically, the object's inertia is being lowered for a microsecond, and if you time the shove right, you can push it with less force. Interestingly, it appears that it would great gravity fields around it (because of all those stretched or relaxed gravity "strings"). Those gravity fields could give us artificial gravity generators, force fields, tractor beams and maybe even faster than light travel. Just like the Mass Effect universe.

The scientists (notably Dr. Woodward), engineers and Joe Averages (i.e. yours truly) who discuss the Mach Effect were talking about how to raise awareness of it, so instead of making a Doritos-and-Mountain Dew-fuelled looong email trying to explain all of this, I thought I'd put it here in the public eye, so to speak. A lot of promising physics concepts are familiar in Sci-Fi, or are otherwise making their way into the public consciousness thanks to the general curiosity of people surfing the net. Of course, Joe Average might look at you and go what? But those of us who watch Big Bang and have a vague clue about what Sheldon spouts may know. And maybe all that's needed to get promising revolutionary technologies off the experimental bench and into spacecraft...

When playing Mass Effect 2, I took a trip down to the engine room and saw the Mass Effect core vibrating... much like the way the Mach Effect devices would work (although you wouldn't necessarily *see* the vibrations, which would be in the mega to gigahertz range...). 3D artists, game programmers and designers are a smart bunch, and you often see unexpected references to some really intriguing ideas wrapped in a game or movie. I wonder if the inspiration for the Mass Effect was indeed the Mach Effect?

Saturday, January 29, 2011

Interstellar Flight: The Generation Ship

I talked about hibernation previously - now let's talk about simply slogging it to the stars the hard way, no sleeping on the job. Many SF writers talk about generation ships as a way to get human colonists to a distant star. But there's a couple of problems with generation ships which don't make them so attractive.

First off is the sheer size of these things. In order to get a minimum viable population without the hazards of inbreeding, you'll need something like 180 individuals. Small groups have made it to colonise islands, perhaps 50 or less breeding members. A single female even could simply impregnate with frozen herself and raise her daughter to fly the ship (not that her daughter would likely be grateful for the thankless life she's born into).

The trouble is, we need a lot of people to cover all the scientific and engineering disciplines. Small armies of technicians, engineers and scientists service the space shuttle, for example. The 7 or so people on board really just push the buttons, as competent as they are. Fixing complex problems like those that would arise on a spacecraft require lots of smart brains. And there's no guarantee that subsequent generations would be as smart or motivated as their astronaut parents. The automated systems on the ship had better be pretty reliable or else extremely easy to fix. Possibly even to the point of having an AI or expert system and be almost self-repairing (can you say HAL 9000?) One might envision a generation ship with primitive humans, having lost the skills of their ancestors, worshipping the benevolent computer-deity literally controlling their world. This, plus the possibility of disease or accidents wiping out large chunks of crew, points to the need to have as large a crew as possible. And more humans equals more mass.

Mass requirements for keeping a human alive in space, and fed with soyburger and supplied with toilet paper, range from 100 tonnes to 1000s of tonnes. Biotechnology can really help here; improving crop yields and increasing efficiency of recycling systems. Certain tools, chemicals and medicines could also be grown in bioreactors. The entire ship (or at least the habitat) could be constructed of organic materials. This pretty good from a radiation shielding standpoint, the abundant hydrogen atoms in organics and plastics are great for stopping cosmic radiation and preventing the lethal backscatter of secondary radiation that occurs when a speeding iron ion smashes into structural aluminium.

In addition to the mass requirement, there's also volume. NASA studies estimate that 100 cubic metres is enough for a single human for an indefinite period. I rather think the ship's crew may grow up a bit nutty... anyway, with the life support requirements, rather more volume than that is likely. Inflatables seem the current best technology, but just how safe will they be after decades of hard radiation exposure? Perhaps they will need some extra reinforcement for a more permanent solution, but at the moment they look like a good bet.

SF is replete with stories of hollowed-out asteroids as generation ships, but the truth is that they're terrible spaceship hulls. For a start, they're weak - asteroids being composed of rubble, and would need the rock to be fused. The rock would still be fundamentally very weak for its mass. And that mass would weigh in the millions of tonnes for a generation ship a kilometre or so across. Furthermore, the rock is not such a good radiation shield. Plain old plastic, water or wood is better for cosmic radiation. And radiation shielding wants minimum volume to be used most effectively - generation ships are anything but minimum volume.

Hollowed-out comets or ice asteroids seem to be a somewhat better option - although frigging cold, they would provide water, oxygen and reaction mass. A layer of insulation could allow for an inner shell lined with water, and aquaculture. Everybody living in boats and stilt houses - how very appropriate for the island-in-space theme! Of course, the best option is still to purpose-build an actual hull for the job. And that's going to be heavy.

Assuming the minimum case of 180 people (at any one time), the ship needs to weigh at least 18 000 tonnes if 100 tonnes of life support infrastructure are necessary to keep things going - that's a WWII battlecruiser. Being more conservative, that could mean 500 000 tonnes of ship for 500 people and 1000 tonnes of life support. To match the current mass of the ISS at 400 tonnes, we would need something like 8 tonnes of life support for 50 people (and no idea of how they're going to go down to the surface). That's a pretty miserable cramped existence, eating algae glop for hundreds of years, living naked and escaping to VR all the time. But it might be possible.

The point is, at what point is it simpler to put more fuel up to go faster? With a solar sail, you've no choice - you're limited to 0.001c with a scorching approach. But fusion-powered craft might just prefer to burn more fuel and get there faster rather than build a big expensive habitat and risk the crew dancing around fires playing bongo drums when they should be getting in their landers to go the surface. Reducing the mass from 18 000 tonnes to just 180 yields a 100x jump in mass ratio, which can be cashed in for a 2.4x increase in speed over just having a 10x mass ratio (rough approximation). Instead of getting to Alpha Centauri in 200 years, a trip of 83 years becomes possible. The crew might be old duffers by the time they get there, but they can limit their numbers because they don't have to keep a society going on board. Or they could prolong their lives with life extension drugs.

Life extension also poses its own problems. While it's useful for long voyages where you want to actually live to see the target star and use your expertise instead of teaching it to your children and hoping they teach it to the next generation, it's a problem on a generation ship. Even with people sticking to two children per couple (or one child to one parent in polyamorous Heinlein-esque communal love-fests), room's going to run out real fast. Great-great-grandpa and grandma may have to be euthanised.... or their children only allowed to breed when their parents die. Which is a problem is females can only safely breed up to about 40. There better be some serious mojo in those pills if that's going to be the case.

Speaking of kids... can you imagine what a 2 year old would do in a delicate, tightly enclosed environment? Or a sulky teenager? Best just feed the crew contraceptives and boosterspice* til they get to their new Eden...

*anti-aging drug in Larry Niven's known space novels

Saturday, January 15, 2011

Interstellar Exploration - Two Motivations

Chinese junks, similar to what Zheng He's fleet may have looked like. Credit: Wikimedia

Exploration is rarely about the pure, unfettered pursuit of knowledge. Scientists want to prove a theory. Politicians want to see high-tech industry stimulated and their national prestige elevated. Private citizens want to go "because it's there." History has shown that there are two broad kinds of exploration - with their own separate outcomes. The first is the show of force. The second is sustained interest.

1405 saw the launch of Zheng He's expedition of exploration from China. A massive flotilla of 317 ships and 27 000 crewmembers, with 44 huge treasure ships measuring 120-150 metres in length. Or, accounting for typical historical exaggeration, probably half that as 100 metres is the limits of what is possible with wooden ships (note that no remains of these craft have ever been found).

Christopher Columbus had 3 ships, 23 metres long, and 270 men (before the usual diseases started wiping them out). The only thing he had over Zhang He was the fact that he didn't sing soprano (Zhang He was a eunuch).

The Chinese weren't interested in colonisation or trade. They wanted to show off, impress people with their bling (hence the treasure ships) or else kick them around a bit. They wanted to remind people that China, just like every other Empire in the world, was the centre of the world. It was the Apollo project of the era, in more ways than one. Because after a flurry of these expeditions, the whole thing was called off due to escalating military conflicts and finally a big damn wall to keep the Mongols out.

Christopher Columbus of course, had different motivations. He wanted money by opening up an alternate trade route to Asia. And he didn't make such a big investment - three ships, perhaps a big thing in Medieval Europe. Although everybody thinks of him as a big success because he found this place called America, he failed to find the trade route. He wasn't even out to prove the world was round, everybody knew that.

Now, how did the New World wind up colonised? Why didn't the Chinese do it? Europe was a dirty little backwater, although the first glimmerings of scientific enquiry were beginning. Simple: China wasn't interested. China could have easily afforded to colonise and conquer these new lands. The Europeans were broke, and constantly fighting each other. But there was land to exploit, and savages to convert! There was also somewhere to run away to if you didn't fancy being oppressed!

So, we have two scenarios with which to place interstellar colonisation in. The first is the massive show of force, done mainly to impress and sustained by government interest. The second is a trickle which eventually becomes a flood, sustained by continued private interest (Hey Sven! Could I interest you in a place called Greenland!). This can tell us something about what kind of colonies we can expect, and who's going to found them.

Thursday, January 13, 2011

Interstellar Flight: Hibernation

So let's say we've discovered a nice habitable planet around those nice, nearby sun-like and well-behaved stars, Alpha Centauri A or B. How are we going to get there? As it stands, there are three possible ways of getting there. Obviously it's going to take a long time.

The simplest in engineering terms is hibernation (aka "suspended animation"). This has the very important engineering advantage of not needing all the mass of a big habitat or a closed ecology. The ship can go faster or be smaller (and cheaper) as a result. Throw the humans in the hibernation chambers and let them sleep the journey away. Plus they won't go stir crazy or forget how to do their jobs. However, there are significant challenges: we don't really have the first clue as how to induce hibernation in humans.

Short-term hibernation is possible through induced (or accidental) hypothermia. Unfortunately it's dangerous - Swedish radiologist Anna BĂ„genholm was dunked under ice with a core body temp of 13.7°C. A drop below 28°C is often fatal. However - this hypothermia-induced hibernation can be extended for long periods. Mitsutaka Uchikoshi, a Japanese skiier, went missing and was recovered 24 days later with his organs shut down and his body temperature at 22°C. What was particularly amazing was how he survived for so long without any fluid intake.

Hypothermia can also be induced for medical reasons (with much better prospects of surival as it's controlled): English and Japanese doctors pioneered the technique of using deliberate hypothermia for heart surgery with packs of ice, and the Russians perfected the technique (the normal solution these days is to use a heart-lung machine). Basically the brain chilled to about 16°C (60°F) and the body to 24°C (75°F). This gives the doctors a 30 to 60 minute window where the heart stops and they can tinker with it, then whip the body back up to full temperature with no lasting brain damage. A recent discovery with mice showed that hydrogen sulfide combined with hypothermia could be used to induce hibernation in mice - but only mice. It probably won't be as simple with humans, but the US military is certainly interested in it for trauma applications. Extreme hypoxia might also be a trigger.

Unfortunately, humans just aren't built for hibernation; this technique must be done correctly otherwise there is severe risk of cardiac arrest. More so is the problem of simply lying inert for years on end. Wouldn't the body degenerate? What crucial processes might be impaired? It might be necessary to wake the sleepers up every year or so to let them recover, before climbing back in the freezer again - with significant impacts on life support requirements.

For really long term flights, it might be necessary to go all the way and use cryogenic preservation - ie freezing the astronaut solid and then thawing him/her out. Unfortunately, this is kind of problematic - small embryos and organs can be successfully quick-frozen, but damage from expanding ice crystals (especially during re-warming) causes all kinds of havoc. A way around this is vitrification - basically pumping the body with antifreeze, turning the body into a block of glass at -135°C. This unfortunately creates further problems in the form of the toxic antifreeze now flushing the body.

A vitrified rabbit kidney being thawed - notice the lack of frost and ice.

Although reviving the patients is tricky for both cryogenic preservation and hibernation, perhaps AI will have advanced enough for it to handle the process without having to have someone on hand. Continual waking and thawing may not be a good idea - so perhaps some brave souls will volunteer to spend a significant chunk of their lives in extreme boredom watching over their crewmates. It may also be possible to grow whole new bodies around the vitrified brains, with the ship becoming more like the mass (and cost) of an unmanned probe. Once it arrives in the system, it mines sufficient material to build a habitat and create nutrients to grow the crews' bodies. However, this is getting to the real bleeding edge of the possible - FTL may be possible before then.

The most likely scenario - hibernation through some combination of induced hypothermia, hypoxia and drugs, seems feasible enough that we could conceive of an interstellar mission if the propulsion technology is likewise developed (which seems to be the far more difficult problem). Since hibernation has many more applications than just long-duration spaceflight, it'll probably be developed sooner rather than later.

The hibernation "pods" would probably be some small climate-controlled chambers with a comfy bed, IV drip and some sensors, hardly weighing anything at all. More like the cheap-looking hibernation pods in the original Planet of the Apes than big bulky cryo-coffins. They'd be located right in the heart of the ship, surrounded by as much radiation shielding as possible, and with a bit of rotation to keep fluids behaving properly. The quarters for the awake crew would probably be right next to it, to take advantage of the proximity to the sleep chambers' radiation shielding. Upon arrival, the sleep chambers could be used as bunks for the crew.

Reliability of the hibernation mechanisms would be a big issue - springs, one of the most reliable components, rarely last beyond 60 years. New, highly fatigue tolerant materials need to be designed, along with fault tolerant systems - more so than with a ship where everybody's awake and able to fix things. This in itself is a significant challenge, but the aerospace industry tries to reduce maintenance and extend aircraft life to lower operating costs, so this is another area where industry may make the development anyway.