White Dwarf Stars, Astro-Engineering and SETI

Earth-like planet around a White-Dwarf… How?

In 7.8 billion years, by current solar models, the unadulterated Sun will be a white-dwarf corpse. No longer fusing hydrogen or helium, the Sun will become a cooling mass of carbon/oxygen. In 2011 Eric Agol posited that white-dwarf stars might have Earth-like planets in their habitable zone for longer than the Sun will be hospitable to the Earth – up to 8 billion years or so. Since then several searches have been underway, or in preparation, and one such was discussed in the a recent “Centauri Dreams” blog, here: Life Around Dying Stars.

In the comments I noted:

The astro-engineering possibilities are worth exploring. For example, in Olaf Stapledon’s “Last and First Men”, there was talk of moving the planets inwards as the Sun was expected to cool after the catastropic brightening that forced the migration to Neptune. If a very efficient neutrino reaction and rocket could be developed, using some kind of inverse baryogenesis, then Earth (and other planets) could be sent in-spiralling towards the Sun after it begins its white dwarf phase.

According to Martin Beech’s astro-engineering work the Sun could have its useful lifespan extended many-fold by siphoning off “excess” mass. A necessity of the “easiest” scenario involves shifting the planets outwards as its luminosity increases slowly. Then, once a helium core develops, the Sun can be allowed to become a helium white-dwarf and the planets can spiral back inwards. The excess mass could be used to make low-mass companion stars, eventually creating a quintet of red-dwarfs. The various terraformed planets can be shared out between the new low-mass stars. Depending on the exact parameters chosen, the Earth could end up orbiting a quasi-terraformed gas-giant around one of the new stars.

How does one terraform Jupiter or Saturn, you might wonder. That’s a whole other story…

Martin Beech’s planet moving machinery was via asteroid-flyby or solar-sail, as my Bob Forward inspired neutrino rockets or Stapledon’s “sub-atomic energy beams” are based on speculative physics. Beech discusses engineering the Sun in his astrophysical work on “Blue Stragglers” and in his book “Rejuvenating the Sun and Avoiding Other Global Catastrophes” (Springer, 2008). To keep the Sun going while avoiding the runaway bloat of the Red Giant phase, the Sun needs to go on a diet – it has to lose mass. In one mass-loss scenario the Sun is shrunk to 11% of its current mass and extending its life to more than 12 times its normal Main Sequence lifespan (~10 Gyr, so 122 Gyr with mass-loss at the right rate.) Eventually the Sun, despite our best efforts, will form an unfusible Core, a White-Dwarf, and an inexorable decline, as its stock of heat trickles away, will begin.

Red-dwarf stars, made from the liberated mass from the Sun, might undergo a somewhat different fate. In the late 1990s, Fred Adams, Greg Laughlin and Peter Bodenheimer ventured into Deep Time – to compute the lives of the smallest stars. What they found was a multi-trillion year life-span quite different to that of the heavier stars, as related in their paper “The End of the Main Sequence”.

Evolution track of 0.1 Solar Mass Star

The diagram needs some exposition. The y-axis is the Luminosity in Log10(solar) units – so -2 is 1/100 and -3 is 1/1000 etc. The x-axis is the temperature in kelvin. The star’s development begins on the right, descending in luminosity along the Hayashi Track until fusion ignites in its core and it settles zags upwards on the Main Sequence. The initial point of the Main Sequence is the ZAMS, or Zero-Age Main Sequence, and forms a distinct change in the star. For most of its life the star is fully convective – the whole of its mass cycles through the fusion core, unlike the Sun which will only cycle 10% of its mass through the core on the Main Sequence. Eventually the core becomes radiative, like the Sun’s, and it rises in brightness at higher pace. As you can see the star gets as hot as the Sun (5800 K) but never as bright as 1/100th of the Sun. Thus why its Main Sequence lifespan is over 6 trillion years of hydrogen burning. Eventually all the hydrogen is fused and the star becomes a Helium Dwarf, which is a star only seen in the present day Universe due to extreme mass-loss processes. Such a star never becomes hot enough to initiate the Helium Main Sequence, which in the case of the Sun runs 50 times brighter than the Hydrogen Main Sequence and lasts 1% as long.

Slightly bigger stars live significantly brighter, shorter lives – the luminosity is roughly proportional to the 3rd power of the mass, thus a 0.2 solar mass star is about 8 times brighter than a 0.1 solar mass star, but with twice the fuel its life-span is 1/4 of the 0.1 solar mass star, about 1.5 trillion years or so. A 0.16 solar mass star approaches the brightness and hotness of the Sun for several billion years near the end of its Main Sequence – heavier stars show more of a trend towards forming Red Giants. Above ~0.25 solar masses and a proper Red Giant phase occurs.

HR Diagram for the evolution of Low Mass Stars

Stars of the same mass and metallicity (fraction of elements heavier than helium) as the Sun have ~10 Gyr Main Sequence life-spans. Lower metallicity levels mean significantly shorter lifespans, with such a star brighter than the Sun at its ZAMS entry to the Main Sequence. Increase the mass slightly and the stars also have significantly shorter life-spans, living brighter and hotter lives than the Sun. Yet such stars might be ideal candidates for life-bearing planets. With these two factors in mind this suggests there have been significant numbers of habitable planets that have been faced with the prospect of a Red Giant Sun.

How might a civilization adapt? Astro-engineering via mass-loss requires engineering over billions of years, something that inhabitants of a shorter-lived star don’t have. What are the options then?

Greg Matloff, interstellar solar-sailing Guru, has examined this question in several papers. His NIDS Essay, “The Re-enchantment of the Solar System” is a provocative classic, as it posits the ETIs escaping such stars might be in our outer Solar System, with observable consequences. In “Red Giants and Solar Sails” he more formally looks at the boost in final speed that a Red Giant Sun can give a Star-Sail – up to 2-3 times what would be expected from the Sun. In “Giant/Red-Dwarf Binaries: New SETI Targets and Implications for Interstellar Migration”, he discusses wide binaries of a higher-mass star with a red-dwarf. An F star might last ~5 billion years, while a 0.2 solar mass red-dwarf will last over 1 trillion years. In a wide-binary separated by ~100s of AU, the red-dwarf would be a very attractive target for refugees from the Red Giant. Even in a post-Red Giant binary, where a red-dwarf circles a White-Dwarf, there might still be Star-Sail activity and observable radio-traffic. Interestingly, for “Star Trek” fans, the star Keid (40 Eridani), host to the planet Vulcan, is a trinary featuring a K-dwarf and a white-dwarf/red-dwarf pair. Perhaps added reason for SETI surveys of this already interesting near-by star-system.

Methone – an unusually smooth tiny moon of Saturn. Could it be a Matloff-style World-Ship?

Inspiration Mars… and Beyond.

Mars_Capsule_220213.m

Dennis Tito, space-tourist and uber-rich dude, is planning a Mars Fly-by. The 2018 Launch promises a 501 day Free-Return trajectory – a great big loop past Mars (ain’t stopping) and return to Earth for a high-speed re-entry. Such a mission – if successful – will be a proof of concept that Mars flights aren’t impossible for humans. Actual landing flights will take ~180-130 days in orbital trajectories, either way, so they will require even less zero-gee exposure (if spin-gravity isn’t used) than Tito’s proposal. People have stayed in orbit for much longer than that, and have soaked up even more cosmic-rays than that. So, as Robert Zubrin has noted more than once, there is no real space-medicine case against flights to Mars.

Mars Fly-by in 2018?

Dennis Tito, Space Tourist, and colleagues are holding a press-conference and a quick search of talks at the IEEE Aerospace Conference in Montana brings up Tito’s “8.0105 Feasibility Analysis for a Manned Mars Free Return Mission in 2018″, which discusses (probably) a 501 day Free-Return mission to fly-by Mars, launching in 2018. Of course the usual nay-sayers and craven know-it-alls have predicted probably insanity if a crew of two are cramped up in a SpaceX “Dragon” Capsule for 501 days, but I am sure the wannabe interplanetary astro-tourists are well aware of the challenges ahead. Such daring requires a certain monomaniacal insanity to even contemplate, so I have no doubts that it can be achieved by sufficiently willing and mentally tough individuals.

Of course the supplies and living quarters will need something more like this, than a basic “Dragon”:

First Mars Expedition in Parking Orbit
First Mars Expedition in Parking Orbit

The expandable Habitat isn’t basic SpaceX equipment, but a Bigelow Aerospace derived concept. The ISS is due to get such an expandable “Trans-Hab” like extension and Bigelow Aerospace have orbited mini-space stations based on their designs, as well as having full-scale mock-ups tested here on Earth.

Just to be clear, the current proposal does not seem to be a landing mission. In 2011 Robert Zubrin proposed a bare-bones mission to Mars, to land and live for ~18 months, which I discussed here: SpaceX to Mars! Here’s How… . One cute feature, perhaps a nod to the Australian Mars Society’s effort, was the rolled out solar-arrays to power the MAV fuelling system. That’s only more and more viable as the ability to make flexible solar-cells increases all the time.

Mars Base One
July 20, 2019?

There’s nothing stopping, but will and perceived feasibility, the launch of a Mars Base One in time for the 50th Anniversary of the First Moon Landing and certainly no excuse not to do so before the 50th Anniversary of the Last Moon Landing. Humanity’s Nations need an ambition far more creative and positive than being the Top Dog in Geo-Political Arena. If the Nations don’t do it, as an aspirational goal for their people, then I hope some billionaires will step to the challenge. A Trail-Blazer effort – a successful one – like a fly-by and teleoperated activities on Mars, will change the risk-assessment. People, flesh and blood, CAN do this.

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.