Interstellar Comparisons

By 2025 Elon Musk believes SpaceX can get us to Mars – a journey of about 500 million kilometres, needing a speed of over 100,000 km/h. By comparison travelling to the stars within a human lifetime via the known laws of physics requires energies millions of times more potent than that budget-price trip to Mars. In our energy hungry modern world the prospect seems fanciful, yet we are surrounded by energies and forces of comparable scale. By taming those forces we will be able to launch forth towards the stars, save our civilization and extend the reach of our biosphere.

How so? Consider the sunlight received every second by planet Earth, from the Sun. About 1.4 kilowatts of energy for every square metre directly facing the Sun – all 128 trillion of them – means a total power supply of 175,000 trillion watts (175 petawatts.) That’s 8,750 times more than the mere 20 terawatts human beings presently use. Earth itself receives a tiny fraction of the total available – the Sun radiates about 2.2 billion times more, a colossal 385 trillion trillion watts (385 yottawatts).

Just how much does a starship need?

Project Daedalus proposed a fusion propelled star-probe able to fly to nearby stars in 50 years. To do so it would fuse 50,000 tonnes of deuterium and helium-3, expelling them as a rocket exhaust with an effective jet speed of 10,000 km/s. A total useful energy of 2500 million trillion joules (2.5 zettajoules) – the actual fusion energy available in the fuel was about 10 times this, due to the inefficiency of the fusion rocket motor. However that gives us a useful benchmark. This is dwarfed by the energy from the Sun. A full Daedalus fuel-tank is equivalent to about 4 hours of Sunlight received by planet Earth.

Another design, the laser-sail, masses 2,500 metric tons and requires a laser power of 5 petawatts, which accelerates the laser-sail starship 1 gee for 190 days to achieve a cruise speed of half light-speed or 150,000 km/s. A laser-power equal to what Earth intercepts from the Sun, 175 petawatts, could launch ~67 laser-sail starships per year. Total energy required per sail is 8.24 yottajoules, equal to 5.45 days of Earth-sunlight.

What else could we do with power that can launch starships? Power on the scale of Worlds (i.e. 175 petawatts) allows the remaking of Worlds. Terraforming is the shaping of the dead worlds of the Solar System into more life-friendly environments. Mars, for example, is considered to be the most life-friendly nearby planet other than Earth, yet it lacks an oxygen atmosphere, a significant magnetic field, and is colder than Antarctica. To release Earth-levels of oxygen from its rocks, power an artificial magnetosphere to deflect away the potentially harmful solar-wind, add nitrogen to reduce the fire risk, and keep the planet warm, the energies required are similar to those required to launch starships.

Releasing oxygen from Martian rocks requires melting the rock, usually composed of about 30% oxygen, and breaking the chemical bonds. What results is a melt of mixed metals, like iron, and semi-metals, like silicon, and oxygen gas, plus hardy compounds like aluminum oxide. For every kilogram of oxygen released, about 30 megajoules of energy are needed. Earth-normal oxygen levels require a partial pressure of 20 kilopascals (20 kPa), which means a mass of 5.4 tons of oxygen for every square metre of Martian surface – 775 trillion tons in total. The total energy required is 10 yottajoules. Adding 80 kPa of nitrogen, like Earth’s atmosphere, requires mining the frozen nitrogen of Neptune’s moon Triton, doubling the total energy required. Pluto’s vast plains of convecting nitrogen ice is another possible source, though without the handy proximity of a big planet’s gravity well for getting a boost towards the Sun it might prove uneconomical in energy terms. Shipping it from Saturn’s moon, Titan, as Kim Stanley Robinson imagines in his “Mars Trilogy”, requires 8 times the energy of using Triton as a source, due Saturn’s less favourable gravity conditions. Warming Mars to Earth-like levels, via collecting more solar energy with a vast solar mirror array, means collecting and directing about 50 petawatts of solar energy (equal to about 10 laser-sail starships). Before we use that energy to gently warm Mars, it can be concentrated via a “lens” into a solar-torch able to burn oxygen out of Mars’s rocks. With 50 petawatts of useful energy the lens can liberate sufficient oxygen for breathing in a bit over 6 years.

The final task, creating an artificial magnetosphere, is puny by comparison. A superconducting magnetic loop, wrapped around the Martian equator, can be used, powered up to a magnetic field energy of ~620,000 trillion joules (620 petajoules), by about 12.4 seconds of energy from the solar-mirrors. This is sufficient to create a magnetosphere about 8 times the size of Mars, much like Earth’s.

Total one-time energy budget is 20 yottajoules – 8,000 “Daedalus” starprobes, or 243 laser-sail starships equivalent. The ongoing power-supply of 50 petawatts is enough to propel 10 laser-sail starships at a time.

To terraform the other suitable planets and moons of the Solar System requires similar energy and power levels. For example, if we used a solar-torch to break up the surface ice of Jupiter’s moon, Europa, into hydrogen and oxygen, then used it to ‘encourage’ the excess hydrogen to escape into space, the total energy would be about 8 yottajoules, surprisingly similar to what Mars requires. The nitrogen delivery cost is about 6 yottajoules, again similar to Mars. Ongoing energy supply would be 10 petawatts – two starships worth.

A less exotic location to terraform would be the Moon. One advantage, as well as proximity to Earth, is that it requires no extra input of energy from the Sun to stay warm. However, unlike Europa or Mars, water as well as atmosphere would need to be delivered, multiplying the energy required. If shallow seas are sufficient – an average of 100 metres of water over the whole surface – the energy to deliver ice and nitrogen from Triton, then make oxygen from lunar rocks, is 27 yottajoules.

The only solid planet with close to Earth gravity is Venus. To remake Venus is a vastly more challenging task, as it has three main features that make it un-Earthly: too much atmosphere, too much day-time and not enough water. Take away the atmosphere and the planet would cool rapidly, so while it is often likened to Hell, the comparison is temporary. The energy required to remove 1 kilogram from Venus to infinity is 53.7 megajoules. Venus has over a thousand tons of atmosphere for every square metre of surface – some 467,000 trillion tons of which is carbon dioxide. To remove it all requires 25,600 yottajoules, thus removal is far from being an economical option, even in a future age when yottajoule energy budgets are commonplace.

One option is to freeze the atmosphere by shading the planet totally. To do so would require placing a vast shade in an orbit between Venus and the Sun, about a million kilometres closer. In this position, the gravity of the Sun and Venus are balanced, thus allowing the shade to stay fixed in the sky of Venus. With a diameter about twice Venus’s 12,100 kilometres, the shade would allow Venus to cool down over a period of decades. Eventually the carbon dioxide would rain, then snow, covering the planet in dry-ice. Some form of insulation (foamed rock?) would then be spread over the carbon dioxide to keep it from bursting forth as gas again. Alternatively it might be pumped into natural cavities, once the sub-surface of Venus is better mapped. The energy cost of assembling such a vast shade, which would mass thousands of tonnes at least, would be far less than the cost of removing the carbon dioxide. So close to the Sun, the shade would intercept the equivalent of 8 times what Earth receives from the Sun – 1,400 petawatts in total, sufficient to propel 280 laser-sail starships, or power the terraforming of the other planets. Or both.

The next desirable for Venus is the addition of water. If 100 metres depth is required the total energy to ship it from Triton is 144 yottajoules. Using 50 petawatts of power, the time to export the water is about 122 years, with a 30 year travel time for ice falling Sunwards from Neptune. The total energy of creating an artificial magnetosphere similar in size to Earth’s would be 6 exajoules (6 million trillion joules) – a tiny fraction of the energy budget.

Further afield than the Inner System and the Outer Planets (including IX, X, XI…) is the Oort Cloud, a spherical swarm of comets thousand to ten thousand times the Earth-Sun distance. According to current planet formation theories there were once thousands of objects, ranging in size from Pluto to Earth’s Moon, which formed out of the primordial disk of gas and dust surrounding the infant Sun. Most coalesced via collisions to form the cores of the big planets, but a significant fraction were slung outwards by gravitational interactions with their bigger siblings, into orbits far from the Sun. One estimate by astronomer Louis Strigari and colleagues hints at 100,000 such objects for every star.

The technology to send a laser beam to a starship accelerating to half light-speed over thousands of Earth-Sun distances opens up that vast new territory we’re only just beginning to discover. A laser able to send 5 petawatts to a laser-sail at 1,000 times the Earth-Sun distance, would be able to warm a Pluto-sized planet to Earth-like temperatures at a distance of a light-year. Powering starships will thus anable the spread of the Earth’s biosphere to thousands of worlds which would otherwise remain lifeless. Life on Earth spread out in abundance, aeons ago, once it learnt the trick of harnessing the Sun’s energy via photosynthesis to make food from lifeless chemicals. Humankind can do the same, on a vastly greater scale – it’s the natural thing to do.

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.

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…

Fermions & the Fermi Paradox

R.J.Spivey writes a provocative essay for the arXiv…

From Fermions to the Fermi Paradox: A Fertile Cosmos Fit for Life?

…basically Spivey suggests we’re jumping to conclusions too soon about Life in the Cosmos, that the real party is after our current Stelliferous Era, when Life exists in a multitude of planets formed from supernova remnants, powered by neutrino annihilation in pressurized iron. Spivey is also disinclined to include us as that “Life” – we might yet attain that level of advancement, but for now our Future fate is for us to create. We might fail to advance to the level of Galactic Colonists, able to adapt to Ocean planets under ice, living off the thin trickle of energy from neutrinos (via the reverse photo-neutrino effect) for 100 billion trillion years. He suggests that the efforts to make artificial life will fail and that we’ll need to hone our bioengineering skills to remodel an ecosystem fit for the Ocean planets of the distant future.

Making the Ringworld

Natural planets capture a minute fraction of the life-giving energy of their stars. Earth’s cross-sectional area facing the Sun is 0.45 billionths of the total area at its orbital distance. What can be done to reduce the wastage? Freeman Dyson originally proposed civilized beings might build an immense spherical cloud of habitats to maximize the sunlight captured. Some presentations of his idea, perhaps incorrectly, represented him as proposing a solid shell surrounding the Sun.

Given nuclear strength materials a civilization can make such immense structures, immensely bigger than planets in area – Dyson Spheres, Niven Ringworlds, Alderson Disks and Banks Orbitals, all of which more efficiently capture the energy of the central star. I’ll discuss them all in turn, but let’s look at the Niven Ringworld, so named because Larry Niven’s novel “Ringworld” (1970) first presented the concept to a wide audience in fictional form.

The basic idea is that a continuous ring around a star is rotated to provide centrifugal gravity on its inside face. The speed of the Ringworld’s spinning has to be incredibly high – a Ringworld at Earth-like insolation from a Sun-like star would be spinning at 1,438 km/s to produce Earth-like gravity. Niven uses that to good effect in his tale, but the engineering practicalities boggle the mind.

First let’s look at the strength required. Assuming the outward centrifugal force on each unit area of the Ringworld is what is stressing the structure, we can compute the Hoop Stress, s, as…

s = P.r/t

…where P is the outward pressure, r the hoop radius and t the thickness of the material. The radius is somewhat larger than Earth’s orbital radius – a Ringworld experiences day/night cycles via “Shadow Squares” which shade the ring, but orbit closer in, thus allowing 24 hour night/dark cycles. This means the heat experienced is somewhat more, on average, so there needs to be an adjustment to compensate. I estimate a distance of about 1.4 AU is optimal, thus r = 2.11E+11 metres.

A method proposed to supply the mass needed, and extend the life of the Sun, is called “Star-lifting” which would provide 1E+30 kilograms of mass to play with. Thus a Ringworld 2.11E+11 metres in radius and 1E+9 metres ribbon-width would have an areal density of ~7.5E+8 kilograms/kg^2 and experience an outward pressure of about 7.4E+9 N. That means the Ringworld material needs to be strong enough to withstand a P.r stress of 1.56E+21 per metre of its thickness. Alexander Bolonkin estimates the strength of nuclear matter to be roughly ~1.6E+32 N/m^2, thus a thickness of 2E-11 metres is enough to provide the x2 safety factor for the mass loading implied above. The Ring will definitely be strong enough. In fact it can probably be made with significantly less nuclear strength material.

If we want 100 metre thicknesses of water or soil (50/50 split in area) then the total outward pressure of that would be about ~1.96E+6 N/m^2 and would require ~4.14E+17 N/m of stress to be supported by the nuclear matter. But we have to factor in the mass of the nuclear matter as well. After a bit of algebra we can compute the nuclear matter layer is just ~5 femtometres thick, with an areal density of ~2,500 kg/m^2 (assuming density of 5E+17 kg/m^3.) Thus the total mass of the Ringworld is just 2.6E+26 kg – about twice the mass of Neptune. A lot less than the half-a-Sun we had to play with.

How much energy is required to boost it up to speed? Lots. Roughly 23,000 years worth of the Sun’s output, which is a truly immense amount. But if we use rockets to do the job, powered by fusion, then we need only about 10% of the mass of the Ring (Vex ~0.05 c.) That’s surprisingly not a big burden on a project of this scale and relatively reasonable. Of course a mass of rockets boosting such a Ring up to speed would produce a brilliant display of energy that should be visible as a massive X-ray flare… which makes one wonder just how many Red-dwarf “flare-stars” are really undergoing massive natural flares and not fine-tuning bursts from their Ringworld motors?

ISV “Venture Star”

ISV VentureStar

For his gigadollar earning movie “Avatar” James Cameron wanted as plausible a design as possible for the interstellar vehicle featured, the ISV “Venture Star”. Advised by his good friend Charles Pellegrino, Cameron has given us, in film, a starship design that could really work – or close to. Check-out Winchell Chung’s detailed description and discussion at his “Project Rho: Atomic Rockets” site…

ISV Venture Star at “Atomic Rockets”

Mystery at the Core

What’s eating the stars out of our galaxy’s heart? – 15 September 2010 – New Scientist.

The Galactic Centre is home to a massive black-hole, estimated at ~4 million solar masses, meaning it’s event horizon is 12 million kilometres in radius. Not the biggest – there are multi-billion solar mass black-holes in the cores of other Galaxies – but big enough. And now there’s this mystery of the Missing Stars. The astrophysicists are modelling natural explanations, as that’s a reasonable assumption, but I’m with Greg Benford – the Core is full of mass and energy. If advanced civilizations are astroengineering on a large-scale, then the Core is where it’s at.

One possibility is that the black-hole is surrounded by collapsed remnants of stars, like stellar black-holes and neutron stars. In my mind if there’s any chance a natural black-hole can be made into a wormhole, then that’d be the place to do it. That ETIs might be shepherding the stars in that region, could also explain the odd lack of visible stars near the Core. Carl Sagan’s old image (in the 1985 “Contact” novel, not the 1997 movie) of a Galactic Grand Central Station for wormholes might actually be true!

Human/Bacteria Hybrid Emortals

Found in nature’s freezer, the secret of living to 140 | Mail Online.

Certain bacteria and/or their by-products when injected into mice & flies (standard lab-animal models for such studies) extend their healthy life-spans by almost double. Thus the headline about humans heading to 100-140 years of age as normal. Rapid advancements in reprogramming the body’s natural stem-cells to become fully pluripotent and this newly discovered bacterial longevity, combined with tissue engineering of organs (eg. lungs made on demand, as recently demonstrated), suggests that human longevity might be feasibly pushed to +150-200 years.

Where would that get us in the Galaxy? Speed is of the essence for distant targets, but nearby stars become accessible if 100 year trip-times are acceptable for a crew. Assuming 0.1-0.3c are reasonable cruising speeds, then targets 10-30 light years distant come within reach. That’s literally hundreds of possible destinations, as the RECONS and SolStation websites describe in some detail.

Another recent find is the possibly inducible suspended animation that some people have experienced to survive extreme circumstances, most spectacularly the case of a Japanese man who lay on a hillside in the cold for 23 days without food or drink. Oxygen restriction, at a cellular level, seems to cause cells to switch into the “slow state” and survive periods of extended low metabolism. Could this be used to extend the effective lifespans of starship crews? My mind boggles at the possibilities, but the need for very brave test subjects to explore the fringe between life and death sobers one’s thoughts. Eventually we might see starships full of hardy emortal (“extended mortal”) colonists venturing into the Deep Cold between the stars, their ships seeking out a new life in the warmth huddled close to the stars.

But the Deep Dark Cold between the stars might be worth inhabiting too. Recent modelling of the formation of the Oort Cloud implies many comets are born, then thrown loose from the stellar nurseries where stars are formed. The Galaxy might have an Common Comet Swarm surrounding all the stars, providing potential habitats for those unafraid of the Cosmic Dark. While they might get energy from fusion, collecting starlight with huge soap-bubble thin reflectors might provide enough energy for sustaining life indefinitely.