The Medician League…

Jovian_Moons

The Medician Stars, as Galileo dubbed his discovery in orbit around Jupiter, reimagined as terraformed worlds by 1Wyrmshadow1, on his Deviant Art page. James Oberg discussed their terraforming in his (now classic) “New Earths” (1981), as has Martin Beech in his more recent “Terraforming: The Creating of Habitable Worlds” (2009) and Martyn Fogg in “Terraforming: Engineering Planetary Environments” (1995). The first, semi-serious discussion, was fictional – Robert Heinlein’s “Farmer in the Sky” (serialised as “Satellite Scout” in 1950, then novelised in 1953.) Heinlein’s tale began with Ganymede being given an oxygen atmosphere busted out of ice on the ground, via total-annihilation powered systems. Heinlein is vague on details – he knew the ingredients, not the details. So he had the moon being kept warm via “the Heat-Trap” and the ice being lysed by some energy delivery system. The story begins long after the process had progressed to a colony of 15,000 dirt-farmers slowly converting the place into living room for an over-flowing Earth.

Of course Heinlein had to fudge a few details. Even in 1950 Ganymede was suspected to be rather too low density to be as rocky as Heinlein describes. The real Ganymede is half ice. Not a total disaster. If only the outer-layers melted and the topography was kind, then water would pool in the lower parts and the meteoritic dust in the crust would form a layer of soil over it. Such a world would need to stay, on average, cool, but parts would be warm enough for water and vegetation. Ice warms slowly – witness the permafrost under the tundra of northern Europe. Given a low enough average temperature and enough salts in the ice and the stuff will remain stable. But beware TOO MUCH Global Warming…

Well this is disturbing…

Against_Guns

There are Lies, Damned Lies and Statistics. The worst lies are told with seemingly significant statistics. Though I’m not a statistician, I am an Australian, and the (ab)use of Australian crime statistics by the NRA in the USA is downright annoying. Less annoying, but more worrying, is how people swallow such crud holus bolus.

However, let’s flip this around. In the USA the homicide rate, from all causes, is about 5 times higher than in Australia, per 100,000 of population. And 2/3rds of homicides are via guns. I can understand how the right to self-defense with guns of one’s choice makes people feel more secure, whether they are or not. But the ease of access combined with the devastating collateral damage power of firearms makes the USA, overall, 500% deadlier than Australia – or even their neighbour, Canada. So I can understand the existential dread that drives people in the States to bear arms, but that fear seems to be more of a self-fulfilling prophecy.

My Mormon friend, Jen, pointed me to a very interesting web-site with all this gun homicide data. Worth a look…

UNODC Homicide Statistics

I am not asking my friends in the USA to give up their firearms – hell, I’d pack heat too – but they need to start asking questions about how their “freedom” affects everyone around them. Sure, bad people use guns, when good people don’t. But that Gun Culture comes with a price paid in blood. Denying that fact is suicidal ignorance.

Power from Heat… High-Efficiency Thermoelectronic Converters

The_Sun_medium
Turning a source of heat – such as concentrated sunlight – into useful power (say, electrical power) is not an easy proposition. There’s a dizzying array of options – thermal engines using different thermodynamic cycles, photovoltaic arrays, thermoelectrics and thermionic conversion. The last was used extensively in early space power generators using small reactors or radioisotope heat sources, but left behind by thermoelectrics and Stirling cycle free-piston systems in more recent work. Now a new approach to “thermionic” conversion, focussing on electrons (thus thermoelectronic), has shown promising behaviour in experiments and out-standing performance in theory.

Highly-Efficient Thermoelectronic Conversion of Solar Energy and Heat into Electric Power

Thermionics previously had efficiency limitations due to “space current” – build-ups of electrons mutually repelling each other and choking the flow of current – so the new system uses external electric or magnetic fields to get the electrons going in the right direction. The system promises a high fraction of the Carnot Limit can be converted directly into electrical power. The Carnot Limit is a measure of how much useful work can be extracted from a thermal cycle – if the heat source temperature is Tin and the heat-sink temperature is Tout, then the Carnot Limit is:

CL = (Tin-Tout)/Tin

…say the source is 2000 K and the sink is 500 K, then the Carnot Limit is (1500/2000) = 0.75. In practice realistic thermal engines achieve a fraction of the Limit and thermionics & thermoelectrics achieve a low fraction. Efficiencies of 5-10% are typical. The new thermoelectronic approach promises efficiencies in the high 40-50% range, achieving the latter by acting as a “topping cycle” to a lower temperature steam system. For example a coal furnace burns at ~1500 C (1773 K), but a steam turbine runs at 700 C (973 K) and outputs at 200 C (473 K). Thus there’s significant loss due to the mismatch between furnace and steam power-cycle. A thermoelectronic converter covering the 1773-973 K range will add significantly to the overall power extracted by the power-plant pushing its efficiency above 50%. In this case a 45% efficient coal plant can be pushed to 54%, thus increasing the power output for no additional fuel costs and NO MOVING PARTS.

Thermoelectronic Converter Test-rig
Thermoelectronic Converter Test-rig

Switching to solar-power applications, imagine a thermoelectronic converter at the centre of a concentrator system which focuses sunlight to 500 times its normal intensity (temp ~1900 K.) By using a Photon Enhanced Thermionic Emission (a cousin of the Photoelectric effect) the system can convert raw sunlight to electrical power at over 40% efficiency. While maintaining a hard vacuum around the emitter-collector system is difficult here on Earth (but easy enough given the right engineering) imagine such a system in space! Hard vacuum everywhere! Even the densest squall of the Solar-Wind is a harder vacuum than a Thermoelectronic system needs here on Earth. Concentrators have to remain pointed at the Sun, but this isn’t excessively onerous engineering either.

One problem is the trick of efficiently losing excess heat to maintain the temperature differential that drives the system, but even this isn’t intractable engineering in space. Given the right “heat-pipes” the whole system can be built without moving parts, eliminating the main failure-point for mechanical thermal-cycle converter systems that have been proposed in the past.

Wonder Material – 2

JPL_Sail

Using Carbon-NanoTube (CNT) sheets that we can make now, we might push towards ~2,200 km/s. Of course there will be structural mass and the payload reducing the top speed – thus we might hit ~1,800 km/s tops with CNT sheets, if made perfectly reflective. Even for lower reflectivity the speed will be about ~1000-1500 km/s.

How hard can we push it? A 1999 study by Dean Spieth, Robert Zubrin & Cindy Christensen for NASA’s Institute of Advanced Concepts (NIAC), which can be found here, examined using CNTs arranged in a spaced-out grid. One of the curiosities of optical theory is that, for a given range of wavelengths, the reflective material doesn’t have to be an unbroken sheet – it can be an open-grid.

CNT-Mesh

Computing the reflectivity of such things is difficult – best to make it and measure it – but estimates of how a CNT grid would perform suggests that a CNT sail might accelerate at ~18 m/s2 at 1 AU from the Sun, implying a final speed of 2,320 km/s. Dropping inwards and launching from 0.019 AU would mean a final speed of 16,835 km/s (0.056c), allowing a probe to reach Alpha Centauri in just 78 years, propelled by sunlight alone!

To send people, rather than rugged robots, a different approach will be needed – to be discussed in Part 3.

Wonder Material

Carbon is the material of the Future. Graphite, graphene, bucky-balls and nanotubes all have amazing properties. And then there’s diamond – which seems to come in several varieties, albeit rare and/or theoretical.

Making enough of any of the allotropes – different carbon forms – is rather tricky, aside from raw graphite, which can be mined. Diamonds fortunately can be made fairly easily these days – very pure diamond crystals can be (almost) made as large as one likes. Thus Jewel Diamonds, the kind De Beers sets the standard for, have to be slightly impure crystals, as they’re thus provably natural.

Carbon nanotubes are proving easier to make and to make into useful forms. One application caught my eye:

Carbon Nanotube Sheets

…which have the rather amazing property of being strong and yet massing just ~27 milligrams per square metre. If we can dope it (add a sprinkling of other elements) to make it more reflective, then it makes rather impressive solar-sail material. Sunlight’s pressure – as felt by a reflective surface facing flat to the Sun – is about 1/650 th of the sun’s gravity, so creating lift against the Sun’s gravity requires very large, light sheets. And doped CNT sheets – if 100% reflective – would experience a lift factor (ratio of light-pressure to the sail’s own weight) of 57 (!)

In theory that means a suitably steered solar-sail made of CNT sheet could send itself away from Earth’s orbit and reach a final speed of 42*sqrt(57-1) km/s ~ 315 km/s. If it swooped past Jupiter then swung in hard for the Sun, scooting past at 0.019 AU, then it would recede at ~2,200 km/s.

We’ll ponder that some more next time.

The Unknown Solar System

Kuiper-Belt

Just beyond Neptune is the Kuiper Belt, a torus of comet-like objects, which includes a few dwarf-planets like the Pluto-Charon dual-planet system. Despite being lumped together under one monicker, the Belt is composed of several different families of objects, which have quite different orbital properties. Some are locked in place by the gravity of the big planets, mostly Neptune, while others are destined to head in towards the Sun, while some show signs of being scattered into the vastness beyond. Patryk Lykawka is a one researcher who has puzzled over this dark, lonely region, and has tried to model exactly how it has become the way it is today. Over the last two decades there has been a slow revolution in our understanding of how the Big Planets, the Gas Giants, formed. They almost certainly did not begin life in their present orbits – instead they migrated outwards from a formation region closer to the Sun. To do so millions of planetoids on near-misses with the Gas Giants tugged them gently outwards over millions of years. We know what happened to the Gas Giants, but what of the planetoids? A fraction today form the Kuiper Belt and the Oort Cloud beyond it (how many Plutos exist out there?) But a mystery remains, which Lykawka convincingly solves in his latest monograph via an additional “Super-Planetoid”, a planet between 0.3-0.7 Earth masses, now orbiting somewhere just beyond the Belt.

Noaa_ganymede

Such an object would be a sample of the objects that formed the Gas Giants, a so-called “Planetary Embryo”. Based on the ice and silicate mix present in the moons of the Gas Giants, the object would probably be half ice, half silicates/metals, like a giant version of Ganymede. However such an object would also have gained a significant atmosphere, unlike smaller bodies, and being cast so far from the Sun, it would have retained it even if it was composed of the primordial hydrogen/helium mix of the Gas Giants themselves. This has two potentially very interesting consequences. David Stephenson, in 1998, speculated on interstellar planets with thick hydrogen atmospheres able to keep a liquid-water ocean warm from geophysical heat-sources alone. Work by Eric Gaidos and Raymond Pierrehumbert suggests hydrogen greenhouse planets are a viable option in any system once past about ~2.0 AU. A precondition that obtains for Lykawka’s hypothetical Super Trans-Neptunian Object.

So instead of a giant Ganymede the object is more like Kainui, from Hal Clement’s last novel, “Noise”. Kainui is a “hot Ganymede”, a water planet with sufficiently low gravity that the global ocean hasn’t been compressed into Ice VII in its very depths. Kainui’s ocean is in a continual state of violent agitation, lethal to humans without special noise-proof suits, but Lykawka’s Super-TNO would probably be wet beneath its dense atmosphere, warmed by a trickle of heat from its core and the distant Sun.

NOISE

Gravitational perturbation studies of planetary orbits by Lorenzo Iorio constrain the orbital distance of such a body to roughly where Lykawka suggests it should be. A Mars-mass object (0.1 Earth-masses) would exist between 150-250 AU, while a 0.7 Earth-mass body would be between 250-450 AU. If we place it at ~300 AU, then its equilibrium temperature, based on sunlight alone, would be somewhere below 16 K. That’s close to the triple-point of hydrogen (13.84 K @ 0.0704 bar), suggesting a frozen planet would result. However geophysical heat, from radioactive decay of potassium, uranium and thorium, could elevate the equilibrium temperature to over ~20.4 K, hydrogen’s boiling point at 1 atm pressure. Thus a thick hydrogen atmosphere should stay gaseous.

To keep liquid water warm enough (~273 K) at the surface, the surface pressure will need to be ~1,000 bar, the equivalent of the bottom of Earth’s oceans. An ammonia-water eutectic mixture would be liquid at ~100 bars and 176 K. With a higher rock fraction and higher radioactive isotope levels (as seen in comets, for example), liquid water might be possible at ~300 bars. Such a warm ocean would seem enticingly accessible since a variety of submarines and ROVs operate in the ocean at such pressures regularly. While the prospects for life seem dim, the variety of chemosynthetic life-styles amongst bacteria suggest we shouldn’t be too hasty about dismissing the possibility.

A primordial atmosphere also invites thoughts of mining the helium for that rare isotope, helium-3. At 0.3 Earth masses and 1:3 ratio of ice to rock, such a body has 75% Earth’s radius and just 40% the gravitational potential at its surface – even less at the top of the atmosphere. Such a planet would be incredibly straight-forward to mine and condensing helium-3 out of the mix would be made even easier by the ~30-40 K temperature at the 1 bar pressure level. There’s no simple relationship between the size of a planet and its spin rate, but assuming Earth’s early spin rate of 12 hours, then the synchronous orbital radius is just 2 Earth radii above the operating altitude of a mining platform. A space-elevator system would be straight-forward to implement, unlike the Gas Giants or even Earth.

Travelling to 300 AU is a non-trivial task, ten-times the distance to Neptune. A minimum-energy Hohmann trajectory would take 923 years, while a parabolic orbit would do the trip in 390 years. Voyager’s 15 km/s interstellar cruise speed would mean a trip of 95 years. A nuclear saltwater rocket, with an exhaust velocity of 4,725 km/s, could be used to accelerate to 3,000 km/s, then flip and brake at the destination. The trip would take six months, which is speedy by comparison.

2312: Terraforming the Solar System, Terraforming the Earth

Kim Stanley Robinson’s latest book “2312″ is set in that titular year in a Solar System alive with busy humans and thousands of artificial habitats carved from asteroids. Earth is a crowded mess, home to eleven billion humans, but no longer the home of thousands of species, now only preserved, flourishing in fact, in the habitats. Spacers, those living in space, are long-lived, thanks to being artificially made “bisexual” (male & female) and some are living even longer by virtue of small size. Humans live from the Vulcanoids – a belt of asteroids just 0.1 AU from the Sun – out to Pluto, where a quartet of starships are being built for a 1,000 year flight to GJ 581. Mars has been terraformed, via Paul Birch’s process of burning an atmosphere out of the crust to make canals, while Venus is snowing carbon dioxide (another Birch idea.) The larger moons of Jupiter and Saturn are extensively inhabited and debating their terraforming options.

On Mercury Stan introduces us to the moving city Terminator, which runs along rails powered entirely via thermal expansion of the rails as they conduct heat from Mercurian day and radiate it away in the Mercurian night. Mercury is a planet of art museums and installations of art carved out of the periodically broiled and frozen landscape. Sunwalkers walk forever away from the Sunrise, braving the occasional glimpse of the naked Sun, which can kill with an unpredictable x-ray blast from a solar flare.

The two main protagonists are Swan, an Androgyn resident of Mercury, a renowed designer of space-habitats whose mother, Alex, has just died; and Wahram, a Wombman resident of Titan, who is negotiating access to solar energy for the terraforming of his home world. Due to a freak “accident” the two must journey through the emergency tunnels underneath Mercury’s Day-side, an experience which draws them together inspite of being literally worlds apart in personality and home-planets.

There’s a lot going on in 2312 and Stan only shows us a slivver. Plots to reshape the worlds and plots to overthroe the hegemony of humankind. But for our two interplanetary lovers such forces can’t keep them apart.

Of course, I’m not here to review the book. This being Crowlspace, I’m looking at the technicalities. Minor points of fact have a way of annoying me when they’re wrong. For example, Stan mentions Venus wanting to import nitrogen from Titan, which is rather ridiculous. The atmosphere of Venus is 3.5% nitrogen by volume, which works out as the equivalent of 2.25 bars partial pressure. Or about 3 times what’s on Earth. So importing nitrogen would be the equivalent of the Inuit importing ice.

Stan is critical of interstellar travel being portrayed as “easy” in Science-fiction. He mentions a fleet of habitats being sent out on a 1,000 year voyage to a star 20 light-years away – given the uncertainties of these things and the size of habitats, that’s not an unreasonable cruise speed. Yet he describes it as being “a truly fantastic speed for a human craft.” But at one point he mentions that a trip to Pluto from Venus takes 3 weeks, an unremarkable trip seemingly, yet that requires a top-speed of 0.022c – significantly higher than the starships!

He’s a bit vague about the pace of travel in the Solar System via “Aldrin cycles” – cycling orbits between destinations, timed to repeat. Buzz Aldrin developed the concept for easy transport to Mars – have a space-station with all the life-support in the right orbit and you only have to fly the passengers to the station, rather than all their supplies. The station either recycles everything or is resupplied by much slower automated freighters using electric propulsion. Stan’s mobile habitats do the former, with some small topping-up. But such Cyclers are slow. Stan mentions a Mercury-Vesta Cycler trip taking 8 days. Not possible for any Cycler orbit that’s bound to the Sun (i.e. cycling) – a straight-line parabolic orbit would take a minimum of 88.8 days. A proper Cycler needs to be on an orbit that can be shaped via the gravity of the planets to return it to the planets it is linking together, else too much fuel will be expended to reshape the orbit. Preferably an orbit that isn’t too elliptical else the shuttle fuel bill is too high. A minimum-energy Hohmann orbit would take 285 days to link Mercury and Vesta.

These are quibbling points. The real meat of the book is the optimistic future – a dazzlingly diverse one – that is basically plausible. Enticingly possible, in fact. Yet the optimism is tempered by the fact that not everyone is living in a wise, open society. Earth, even in 2312, remains a home to suffering masses, their plight made worse by the greenhouse effect’s flooding of low-lying parts of the Globe, and the Sixth Great Extinction’s erasure of most large animals from the planet (fortunately kept alive or genetically revived in the mobile habitats.) New York is mostly flooded, becoming a city of canal-streets, something I can imagine New Yorkers adapting to with aplomb.

The real challenge of the 24th Century, in Stan’s view, is the terraforming of the Earth, remaking a biosphere that we’ve ruined in our rush to industrialise. Perhaps. We certainly have many challenges ahead over the next 300 years…

Speed Kills?

Father & Son team, William and Arthur Edelstein discuss one of the dangers of near lightspeed travel in their paper published just last month: Speed kills: Highly relativistic spaceflight would be fatal for passengers and instruments [citation: Edelstein, W. and Edelstein, A. (2012) Speed kills: Highly relativistic spaceflight would be fatal for passengers and instruments. Natural Science, 4, 749-754.doi: 10.4236/ns.2012.410099.] They highlight the lethality of the high-energy proton head-wind that the Interstellar Medium (ISM) becomes when moving at near light-speed, which they define as above about ~0.9c.

I hadn’t realised the Edelsteins finally published their work until a Facebook friend, Jay Real, sent me a link. Of course these issues have been discussed in the literature for years so their discussion is nothing new – but welcome nonetheless as an explicit statement of the problem. High relativistic speeds are difficult to achieve, so most vehicles would probably stay below ~0.9c unless something exotic appeared, like an easy way of making one of Sonny’s warp-drive fields for rapid sub-light travel. In our part of the Galaxy the proton flux is much lower than the 1.8 protons/cc assumed by the Edelsteins. Some hot bubbles in the Local ISM go down to ~0.01-0.05 protons/cc and the local clouds are ~0.1-0.2/cc. This doesn’t change the results very much, but does lessen the local applicability.

Their analysis focuses chiefly on mass-shielding – big enough chunks of material to absorb the incoming flux. Magnetic shielding is mentioned dismissively, but I think that’s premature. Workable designs using known materials exist which can deflect 10 GeV cosmic rays, the equivalent of flying at 0.995c. Advanced superconductors, which will be needed for antimatter containment, plasma nozzles, magnetic-sails, will allow even higher protection levels. Thus I submit the Edelsteins’ negativity is premature.

The energy flux of interstellar matter hitting the ship can cause a lot of heating. If the ISM is just 100,000 atoms per cubic meter the flux is equivalent to 536 K temperature at 0.866 c. Peak temperature during re-entry is 2700 K for a moonflight – that level is reached at about 0.997c. Of course a starship wouldn’t just absorb that heat on its forward surfaces. A magnetic deflector would channel most of it away- but deflecting particles makes them lose momentum as high energy photons (x-rays) which would need to be shielded against. And the shield would get HOT! Fast starships would need to be long and narrow to minimise the energy absorbed. An x-ray reflective diamond coating could be used, but will need to be keep highly reflective while operating. Maintenance will be tricky!

As an example of the kinds of particle energies we can handle the Large Hadron Collider regularly bends a high energy stream of particles into a circle – the protons in the beam have a speed of 0.99999999c when it’s at full power. Cosmic-rays can reach much higher energies and need protection against. However the very highest energy cosmic rays are very rare, so only lower energy particles need deflecting in a crew habitat. The ones of biological concern, due to their numbers, are in the 1-10 GeV range. If we can deflect 10 GeV protons coming at us from our motion through space, then cosmic rays aren’t an issue.

Aberration comes into play at such high-speeds – the direction of origin of incoming particles and photons starts piling up directly in front of the starship. I would suggest the best protection at very high speed might be a “diffuser” – a high intensity magnet held far forward of the starship’s main hull which deflects the charged particles and creates a “shadow cone” behind it. The faster we go, for the same magnetic intensity, the further forward we put the diffuser. We fly, in safety, in its shadow thanks to aberration concentrating all the radiation to directly in front of us.

If we can deflect particles up to LHC energies, then how far can we accelerate at 1 gee? The acceleration distance required to increment the time-distortion/gamma factor (call it the TDF) by 1 is about 1 light year at 1 gee. At 0.99 c the TDF is about 7. So it takes about 6 light-years (because we start with TDF = 1) to get to 0.99c. To reach 0.9999c (TDF = 70) takes about 69 light years. Thanks to the time distortion, on ship the trip-time is much less. Remember a light-year is a distance, but as we’re flying so close to light-speed the ship is seen to take about 70 years to travel 69 light-years. A speed of 0.999999c (TDF = 700) takes 700 years Earth-time and 699 light-years of distance, but on the ship only just over 7 years have passed. If we decide to stop, then another 7 years ship time, 700 Earth-time, and 699 light years is needed – meaning we’ve flown 1398 light years in 14 years ship-time. But let’s push on. We’re pushing to TDF = 7,000 (0.99999999c) so the distance is 6,999 light-years, 7,000 years Earth-time, about 9.5 years onboard ship. Thus we could travel 13,998 light years and stop, in 19 years of our time, if can protect against proton energies equal to the LHC.