Monthly Archives: January 2013

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.