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 – 2


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.


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.

The Unknown Solar System


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.


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.


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.

Life in the Year 100 billion trillion – Part I

If our Universe is open, either flat or hyperbolic in geometry, then it will expand forever… or at least until space-time’s warranty expires and a new vacuum is born from some quantum flip. Prior to that, most likely immensely distant, event the regular stars will go out and different sources of energy will be needed by Life in the Universe. A possible source is from the annihilation of dark matter, which might be its own anti-particle, thus self-annihilating when it collides. One possibility is that neutrinos will turn out to be dark matter and at a sufficiently low neutrino temperature, neutrinos will add energy to the electrons of atoms of iron and nickel by their annihilation. This is the energy source theorised by Robin Spivey (A Biotic Cosmos Demystified) to allow ice-covered Ocean Planets to remain hospitable for 10 billion trillion (1023) years.

Presently planets are relatively rare, just a few per star. In about 10 trillion years, or so, according to Spivey’s research, Type Ia supernova will scatter into space sufficient heavy elements to make about ~0.5 million Ocean Planets per supernova, eventually quite efficiently converting most of the baryon matter of the Galaxies into Ocean Planets. A typical Ocean Planet will mass about 5×1024 kg, be 12,200 km in diameter with 100 km deep Ocean, capped in ice, but heated by ~0.1 W/m2 of neutrino annihilation energy, for a planet total of ~50 trillion watts. Enough for an efficient ecosystem to live comfortably – our own biosphere traps a tiny 0.1% of the sunlight falling upon it, by comparison. In the Milky Way alone some 3,000 trillion (3×1015) Ocean Planets will ultimately be available for colonization. Such a cornucopia of worlds will be unavailable for trillions of years. The patience of would-be Galactic Colonists is incomprehensible to a young, barely evolved species like ours.

We’ll discuss the implications further in Part II.

Futures of the Earth

James Lovelock once estimated Earth’s biosphere would crash in about 100 million years when carbon dioxide levels dropped too low. James Kasting and Ken Caldeira updated the model to include a different photosynthetic cycle amongst land plants, pushing back Doomsday to about 900 million years in the Future. Those “900 million years” before Earth overheats is based on a certain model of Earth’s response to the Sun’s gradual rise in luminosity. That particular model assumes everything else will remain the same, but that’s unlikely. If the partial pressure of nitrogen declines, then the greenhouse effect from carbon dioxide will decline and the Earth could remain habitable to life for another 2.3 billion years. Alternatively because the greenhouse instability of the Earth is driven largely by the thermal response of the oceans, if Earth became a desert planet then it would remain habitable until the Sun reaches ~1.7 times its present output. Combined with a reduced atmospheric pressure, it means Earth might remain habitable until the end of the Sun’s Main Sequence in 5.5 billion years.

But this all assumes no technological intervention. Several scenarios are possible – a variably reflective shell engulfing the Earth is the simplest. Planet moving and Solar engineering are more dramatic possibilities. Given sufficient thrust a leisurely spiral of the Earth outwards from the Sun would compensate for the brightening, though the pace of travel would need to be rather rapid for a 6 billion trillion ton planet to escape the more dramatic stages of the Sun’s Red Giant Branch (RGB).

Once the Sun hits the Horizontal Branch/Helium Main Sequence, the habitable zone will be roughly where Jupiter will be – as the Sun’s mass loss during the RGB will cause all the orbits to expand by ~30%. The HB offers just 110 million years of stability before the Sun begins a series of dying spasms known as the Asymptotic Giant Branch. Not healthy for any of the planets. If the RGB’s mass-loss can be tweaked a bit, then the Sun won’t hit the HB at all and will slowly decline into being a helium white dwarf. Earth can remain in the white dwarf Sun’s habitable zone then for billions more years, more if it spirals inwards as it cools.

Post 100 YSS… First, Fast Thoughts

As a fan I can tell you it was an SF-Fan’s dream come true to meet, in the flesh, so many SF-writers and so many Icarii, as well as the Heart & Mind of the TZF. People I met, for the first time, but have corresponded with for a while…

(1) Paul Gilster & Marc Millis, the guys who set the train in motion some years ago
(2) The Icarus Interstellar Board
(3) wide Team Icarus
(4) The Benford Twins
(5) my co-author, Gerald Nordley, and perhaps the best ultra-hard SF writer I know.
(6) Athena Andreadis, molecular biologist and SF thinker
(7) John Cramer, author of “Analog’s” ‘The Alternate View’ and physicist
(8) Jack Sarfatti, the Showman of Speculative Physics

Others I met/heard who maybe aren’t so well-known, but may prove influential in times to come. Such as Young K. Bae, laser propulsion research and inventor of the Photonic Thruster (a very clever multi-bounce photon-propulsion system.) Mark Edwards, of Green Independence, who might have a way of feeding Starship Crews and the whole of Starship Earth.

Fast thoughts – David Nyeland gave a us BIG hint on how to launch a Starship in 100 years… reach out to EVERYONE.

Orlando is Awesome!

Too much to tell on the very aggressive schedule here, so a detailed report will need to wait, but I met a FAN! You know who you are. Thanks for the encouragement and I promise more content – I have some actual journal paper ideas gestating and I will need input from my audience, I suspect. One is a paper on Virga-style mega-habitats and Dysonian SETI, to use a new idea from Milan Cirkovic. The other looks at exoplanets and Earth-like versus the astrobiology term of “habitable” – the two are not the same and the consequences are sobering. The recent paper by Traub (go look on the arXiv) which estimates 1/3 of FGK stars has a terrestrial planet in the habitable zone does NOT mean there’s Earths everywhere. What it does mean and how HZ can be improved as a concept is what I want to discuss.

More later. I have my talk to review and get straight in my head – no hand notes, though I have practiced it – plus I want something helpful to say to Gerald Nordley, mass-beam Guru, on the paper he graciously added me as a co-author. Also I will summarize my talk and direct interested readers to the new web-site from John Hunt, MD, on the interstellar ESCAPE plan.

Black Holes older than Time?

Two recent arXiv preprints combined make for an interesting idea. Here’s the most recent Science headline maker…

Some black holes may be older than time

…which handily has the arXiv link…

Persistence of black holes through a cosmological bounce

…Carr & Coley pose the idea that some black holes get through a cosmological Bounce (a Crunchy Big Bounce) relatively unscathed. George Zebrowski used something like that idea in his “Macrolife” novel (1979), in which Intelligent life from previous Big Crunchy Bounces survived in the Cosmic Ergosphere. Poul Anderson did it earlier in “Tau Zero” (1970), but the problem with both is that the mass of the Universe, even if it has a net spin, probably won’t form a black-hole style ergosphere when it contracts inside its own event horizon. The topology is all wrong for regular cosmology and it’s doubtful whether a white-hole style cosmos expanding in a precosmic void would ever go Big Crunch. However they might’ve been partly right, thanks to this intriguing preprint…

Is There Life Inside Black Holes?

…in which Vyacheslav I. Dokuchaev speculates that Life might orbit within supermassive black hole event horizons because it can and it might use the emissions of the Cauchy Horizon and massive time dilation for technological purposes. If Life can live inside a Black Hole, and Black Holes can survive the Crunchy Big Bounce, then might not Life survive too? Or am I speculating over a data-void on too many planks of inference? Perhaps only a dive into a Black Hole will ever tell us for sure, though whether we can ever send the news home is debatable. According to Igor Novikov we might be able to access the regions inside via a wormhole specifically dropped in…

Developments in General Relativity: Black Hole Singularity and Beyond

…which might provide a means to reach the aliens inside from past Cosmic Cycles. Perhaps that’s exactly what they want or are hoping for. Of course such vastly old entities – if they’ve survived – might be so utterly foreign to us cosmic youths that we might be unwittingly unleashing “Elder Gods” of Lovecraftian style moral indifference. Or perhaps we’d find them to be akin because of the daring that sent them across the Event Horizon in the first place? Cosmic Extreme Sports, anyone?

[found Under a Gibbous Moon]

Hydrogen Greenhouse Worlds…

The first planets to form probably attracted a primary atmosphere of H/He from the solar Nebula. In our Solar System these were driven off from the four Inner Planets and retained by the Outer Giants, but in theory smaller planets can retain such a mixture. I’ve speculated about such worlds on these blog pages before and now there’s a new arXiv piece discussing the greenhouse abilities of H/He…

Hydrogen Greenhouse Planets Beyond the Habitable Zone

…the summary conclusion being that 40 bars of H2 can keep the surface at 280 K out to 10 AU around a G type star and 1.5 AU around an M star. Thus planets with oceans of water can exist at Saturn-like orbital distances given enough primary atmosphere. Super-Earths are the most likely to retain their H/He primary atmospheres due to their higher gravity, as young stars put out a LOT of EUV light which energizes the hydrogen and strips it away in a billion years or so, if the planet is too close. Out past ~2 AU for a G-star and that effect isn’t so dramatic, thus a Super-Earth where the Asteroid Belt is today would’ve retained its primary atmosphere and probably be warm & wet.

Such a “habitable planet” is only barely defineable as habitable because it has liquid water, but is unlikely to remain warm/wet habitable if the hydrogen is exploited/depleted by methanogens making methane out of it with carbon dioxide, nor oxygenic photosynthesisers making O2, via CO2+H2O->CH2O+O2, which then reacts rapidly with hydrogen. Could another kind of photosynthesis evolve to restore the hydrogen lost? Hydrogen makers exist on Earth, so it’s not unknown in biochemical terms, but I wonder what other compound they need to release net hydrogen from methane/sugars/water?