As “Dawn” crawls towards Ceres, the camera views become clearer and clearer. Hints of craters and that Spot! What revelations are coming?
As “Dawn” crawls towards Ceres, the camera views become clearer and clearer. Hints of craters and that Spot! What revelations are coming?
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.
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.
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.
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.
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.
s – David Nyeland gave a us BIG hint on how to launch a Starship in 100 years… reach out to EVERYONE.
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.
Two recent arXiv preprints combined make for an interesting idea. Here’s the most recent Science headline maker…
…which handily has the arXiv link…
…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…
…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…
…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]
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…
…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?