Such a handy summary that I couldn’t not post it.
Such a handy summary that I couldn’t not post it.
Somewhere between 100-200 times Astronomical Units (AU) from the Sun, planets get COLD enough for hydrogen to begin condensing, as the surface temperature drops below the hydrogen critical point (~33.2 K, 13 atm) and then triple point (~20.6 K, 0.07 atm). Raw hydrogen is unlikely to be present without helium – both were ~99% of the mass of the Solar Nebula that the planets formed out of. Helium, in the form of its stable isotopes helium-4 and helium-3, doesn’t condense until much, much further from the Sun and inside the Galaxy might be too hot for it to condense at all.
The recent study, blogged here, which computed primordial H2/He atmospheres would resist being eroded from Super-Earths by their stars’ early high levels of soft x-rays (“XUV”), also computed significant H2/He atmospheres would be captured by planets from Mars-size and upwards. If such an object was driven out of the Inner System, or from near the Gas Giants, then it might’ve retained its primordial atmosphere and found orbital stability in the region beyond Neptune. Several such objects may exist, for there’s reason to think the Gas Giants formed from Mars-size “planetary embryos”, but such would be undetectable by astronomers due to their slow orbits and dimness… all, except the inner-most, which may explain the curious orbits of objects like Sedna and 2012 VP133, as reported recently:
Of course, being Crowlspace, our take is the implications of a planet with a condense hydrogen surface (oceans?) and an atmosphere of high-purity helium. Helium-3 is an advanced fusion fuel and, once we have reactors up to the task, highly desirable. With a solar abundance of 4E-4 (i.e. 1/2500) relative to Helium-4, that might sound a bit scarce, but it’s immensely more than what we can scrape together here on Earth. A “Super-Mars” (~0.5 Earth masses) with a primordial atmosphere might have captured anything from 4.7 bars to 772 bars of atmosphere according to this reference:
Let’s assume a neat 100 bars. A 0.5 Earth-mass planet, of Earth composition, would have a radius of (0.5)^0.3 ~ 0.81 times Earth and thus a surface gravity of (0.5)^0.4 ~ 0.76 times Earth, with a column mass of ~1,350 tonnes, of which 75% is hydrogen. Thus 336.4 tonnes of helium for every square metre of planet and ~100 kg of that would be helium-3. Total supply would be ~3.8E+16 kg. Interestingly the primordial deuterium/hydrogen ratio is 1/40,000, meaning that the condensed hydrogen would supply 50 kg/square metre of deuterium. A stoichometric mixture of D/He3 for fusion would be 50/75 or 125 kg/square metre of planet – 4.2E+16 kg or 42 trillion tonnes. One could fuel up 840 million “Daedalus” class starships. For the same fuel mass, 50,000 tonnes per ship, one could send 50,000 tonnes of payload (a small space-colony) to the stars at 0.012 c and slow it down at the destination. With a mass allotment of ~50 tonnes per person, the total travelling population would be ~840 billion people…
We’ve discussed the potential boost that quarter-wave sails made of Carbon Nano-Tubes (CNTs) might achieve from being pushed by the Sun. Dropping to 0.019 AU, the final velocity is 0.056c – dropping to 0.00465 AU (skimming the photosphere) would allow a speed of over 0.11c, but the material might not be up to the beating. Crewed vehicles would not endure the extreme acceleration – 84,000 gee at peak – so the speeds that might be achieved by solar-sailing star-travellers would be limited to 1,000 year flights to Alpha Centauri, with just 17 gee peak acceleration (as described in papers by Matloff).
Yet there is another option. Given a supply of small sails, carefully aimed or with some guidance, then why not use them as the momentum transfer system for a crewed starship? Greg Matloff explored the “macron beam” option in his discussion of non-nuclear starship propulsion in the early 1980s, but AFAIK didn’t suggest using mini-sails to accelerate larger starships. Jordin Kare proposed micro-sails pushed by laser to use as a momentum beam, but what I am suggesting is using very rugged solar-sails for boosting manned vehicles to higher speeds than their ‘natural’ acceleration limits would allow.
If CNT quarter-wave sails prove as agile as Christensen, Zubrin & Spieth have described, able to accelerate at 18 m/s2 at Earth’s orbit, thus having a thrust/mass ratio of ~3,000, then they could form the basis of a naturally energised “Sail-Beam” or “Macron Beam”. The most energy efficient ratio of macron-particle to space-vehicle velocity is 2:1, which allows a macron beam total mass of 1/2 the space-vehicle to be used. If the peak speed is limited to 0.056 c, then the most efficient starship speed is 0.028 c. But we can go faster if we have plenty of sails, approaching the macron beam speed asymptotically. In theory a Macron Beam of mini-sails could push an Icarus Probe, with a payload of 150 tonnes, to the preferred mission speed of ~0.045 c.
I’ve derived a proof of the most efficient ratio, using an equation originally derived by Matloff – the proof is available here: Maximum Mass-Beam Efficiency
The basic equation is:
dV/V = 2.e.(Mp/Ms)/[1 + 2.e.(Mp/Ms)]
…where dV is the velocity change, V the macron beam speed, e the momentum transfer efficiency, Mp the total particle mass, and Ms the vehicle mass. If we rearrange it to find the ratio Mp/Ms, we get:
Mp/Ms = (dV/V)/[2.e.(1-dV/V)]
If we assume a perfect reflection (e = 1), then double the mass of the vehicle in Macron Beam mini-sails is needed to get to 0.045 c, with a Macron Beam speed of 0.056 c. As the mini-sail approaches the ship it’s zapped by a laser tuned to a frequency at which it absorbs strongly, quickly blasting it into plasma. Alternatively it is heated by smashing into the vehicle’s magnetic field at a high relative speed. Then the plasma is reflected from a magnetic mirror arrangement on the starship. Some will stream forward at the centre of the magnetic mirror, reducing the reflection efficiency slightly.
If the starship accelerates at 1 m/s2 then it’ll need a final mass-flow of ~0.03 kg/s to push a 200 tonne starship. This doesn’t seem onerous. To reach 0.045 c will need 13,500,000 seconds – just over 156 days. The real trick is keeping the sails on course over ~600 AU when the acceleration finishes.
A fortuitous sample from the Transition Zone in the Mantle has demonstrated the long suspected presence of Ringwoodite and its water-rich properties, confirming the idea of buried oceans deep within the Earth. Crustal slabs are believed to carry water into the mantle as they subduct. Such water returns to the crust as water-enabled melts like the granite ‘balloons’ (batholiths) which float up through ‘solid’ rock and lift up the terrain. This new evidence confirms that there’s water down there already, possibly several oceans worth.
The “Nature” paper is: Hydrous mantle transition zone indicated by ringwoodite included within diamond
The SF implications of all that water have not remained unexplored – Stephen Baxter flooded the Earth with mantle water in his tale “Flood” (one-word titles are a favourite of Baxter’s). Of course, Flood-believers posit that at least some of the waters of the Noachic Flood came from the mantle and then returned, but – unlike SF-writers – they’re obliged to explain the mechanism aside from a wave of God’s magic-wand.
The news is that every M dwarf probably has at least 1 planet, if not more, which is good news when looking for cosmic real estate. The bad news – as blogged here previously – is that exoplanets bigger than Earth, or even Earth-sized and unlucky, are unable to rid themselves of the primordial H/He atmosphere they capture while forming. Too big means the planet becomes a mini-Neptune.
One of the systems identified is a two-planet system around the star GJ 682. The planets are Super-Earths, though the uncertainty range goes as low as 2 Earth-masses for the habitable zone planet. Being radial velocity measurements, the odds are it’s a Sub-Neptune massing ~4 Earths. Intriguingly GJ 682 is only 16.6 ly away, so we should probably add the planet to the list of “Nearby Habitable Zone Planets” even if it is a sub-Neptune.
An atmosphere of H/He can be an asset for an anoxic (oxygen-less) biosphere for planets further out from their stars, though too much primordial atmosphere means a very large greenhouse effect trapping geothermal heat in. For example, Earth emits ~0.08 W per square metre, on average. Thus, with no stellar input, its equilibrium temperature is ~34.5 K. With an adiabatic atmosphere holding the heat in, and a photospheric temperature of 34.5 K, then the critical point of water (647.3 K) is exceeded at a surface pressure of ~5742 bar. A convenient measure of total H/He is “Earth Ocean Equivalents (Hydrogen)” (EOEH) – in Earth’s case there’s 1.5E+20 kg of hydrogen in our oceans, the equivalent of 29.4 bar surface pressure. Thus >200 EOEHs would mean no liquid water is possible. Bigger planets would have higher radiogenic and cosmogonic heat-fluxes, roughly increasing with the 1/2 root of the mass. A 10 Earth-mass planet would have a photospheric temperature of ~46 K, but it’d also have a higher gravity, but a higher surface area to spread a given mass of atmosphere over too. For Super-Earths the surface pressure produced by a given atmosphere mass at the surface of the solid core decreases slowly with planet mass (a ~-0.1 power, roughly). So >90 EOEHs would broil a 10 Earth-mass Super-Earth.
Sounds like a lot, but even 200 EOEHs is only 0.5% the mass of the Earth. Not a lot of hydrogen, by proportion, will mean death for an otherwise Earth-like planet.
Super-Earths might be Dead from Gas…
…rather surprising study of hydrogen-capture and loss from planets near Earth’s mass and orbit. Planets bigger than 0.1 Earth masses (i.e. Mars size and up) will capture the hydrogen-helium gas (H2/He) from the nebula that forms around young stars. The XUV (“soft x-rays”) light from young stars is enough to drive the primordial atmosphere away – but critical to future life on such planets, enough must escape to allow a secondary atmosphere, of heavier gases, to form. Below about 1.5 Earth masses, the planets can lose the primordial H2/He. Above that mass not enough is driven away to produce an “Earth-like” planet. Instead a “mini-Neptune” forms, with a deep H2/He atmosphere over a hot, rocky core.
But it’s not all bad news – small planets that were ejected from close to their stars by migrating gas giants, might retain sufficient H2/He to remain warm enough for liquid water, far from their original orbit. Such objects might be sprinkled through interstellar space, awaiting discovery.
Marshall Eubanks has posited the presence of million tonne masses of stable quark matter inside solar system objects – potentially both matter and antimatter forms of it, with the antimatter version protected from annihilation by a 100 MeV Colour-Force potential well.
While pure antimatter/matter propulsion promises high exhaust velocities (~c) the difficulties of achieving that ultimate performance are considerable. But what if we use something else for reaction mass and use antimatter to energise that? And, instead of using it in a rocket, we use a magnetic scoop to draw in reaction mass from the interstellar medium? This is the Ram-Augmented Interstellar ‘Rocket’ – though technically a rocket carries all its reaction mass – and it promises high performance without all the disadvantages of exponentially rising mass-ratios. Mixing 1% antimatter into the matter flow could, in theory, produce an exhaust velocity of ~0.2 c. Scooping and energising the equivalent mass of ~100 times the mass of the starship would allow a top-speed of 0.999999996 c to be achieved, before braking to a halt using half that mass. This would allow, at 1 gee acceleration, a journey of ~20,000 light-years. The nearby stars would be accessible at a much lower antimatter budget.
Very Rapid Rotating asteroids might be held together by the additional gravity of a mm-sized million tonne quark nugget.
Such quark nuggets would be made in the Big Bang potentially, if antimatter is squirrelled away in such a form, the explanation of the observed lack of free-antimatter in the Universe. The abundance of such ultra-dense tiny specks, to be compatible with microlensing observations, would be in the ‘interesting’ mass-range suggested by the Solar System evidence.
Sonny presented at the Icarus Interstellar “Starship Congress 2013″ in Texas. His talk starts at 13:20 and ends 58:00 at Icarus Interstellar’s YouTube video of the event: Day Three of Starship Congress 2013. Note the interesting discussion of Q-Thrusters for interstellar missions (30 years to Alpha Centauri) and the “warp-drive” illustrations.
Sonny’s discussion of Metric Engineering began with a 2003 paper on the implications of the Alcubierre Metric, here in abstract: A Discussion of Space-Time Metric Engineering. A search on Google will find it at the publisher’s web-site for about $40 US, but a reformatted reprint (White 2003) might be found.
A citation search of the above paper presents two interesting derivative papers:
Artificial gravity field Which is a discussion of how one might use Metric Engineering to generate SF-style artificial gravity. Not easy, but intriguing nonetheless.
Conformal Gravity and the Alcubierre Warp Drive Metric explores an alternative formulation of gravity’s implications for the Alcubierre Metric and the feasibility of warp-drive. Conformal gravity offers the tantalising possibility of warp-drive without the need for NET amounts of negative energy/pressure.
Harold also collaborated with Eric Davis, an exotic propulsion Guru, to discuss the Higher-Dimensional version of the Alcubierre Metric: The Alcubierre Warp Drive in Higher Dimensional Spacetime. The work Harold did for this paper fed into the more recent series of papers on optimising the Warp-Drive’s properties to minimise the energy required. See the previous Crowlspace blog-post for details.
Wikipedia covers the Interferometer Test for space warps in the lab here: White–Juday warp-field interferometer. Present status of the experiment is encouraging, but not conclusive. Sources of noise need to be pinned down and minimised or analysed and removed via post-data analysis.
Recently (last month, almost in time for Christmas) this intriguing paper discussing Sonny’s 2003 paper appeared: The Alcubierre Warp Drive using Lorentz Boosts according to the Harold White Spacetime Metric potential ?. Fernando Loup and Daniel Rocha are warp-drive enthusiasts, not mainstream Relativists, but have produced interesting bodies of work over time. Best read critically.
Finally I should mention the EMDrive controversy and its possible relevance to the Q-Thruster. A patent lawyer, Robert Shawyer, has developed and promoted a propellantless drive based on the behaviour of microwaves in a convergent reflective cavity. He believes it provides thrust without propellant being expelled – its claimed thrust level is much too high for it to be a radio “photon-rocket” (unlike some supposedly propellantless drives that have appeared over the years). Shawyer has claimed interest and experimental validation from Chinese researchers, even though many mainstream physicists (including Greg Egan, the Australian SF writer who is a mathematician by training) have computed the relevant fields of the microwave cavity to demonstrate NO net thrust.
However, there may be a theoretical “out”, which might apply further to other proposed Thrusters. Fernando Minotti discusses one version of gravitation theory in which such thrusts might be produced: Scalar-tensor theories and asymmetric resonant cavities. While the EMDrive probably doesn’t work, another Thruster concept, developed by Cannae, might. Unfortunately their web-site is down so I can’t direct you to their rather interesting material. Whether it’ll produce real results in proper testing conditions *might* come from work at Sonny’s Eagleworks Lab.
NETS 2012 presentation: http://www.lpi.usra.edu/meetings/nets2012/pdf/3082.pdf
Space Times 2009 write-up, starts page 8: http://www.astronautical.org/sites/default/files/spacetimes/spacetimes_48-6.pdf
STAIF 2007 Presentation: http://forum.nasaspaceflight.com/index.php?action=dlattach;topic=13020.0;attach=173105
Harold White’s papers at the NASA Technical Reports Server:
Eagleworks Laboratories: Advanced Propulsion Physics Research
100 YSS 2011 Paper: Warp Field Mechanics 101
100 YSS 2012 Paper: Warp Field Mechanics 102: Energy Optimization
Mainstream NASA Papers:
Spacecraft Applications for Aneutronic Fusion and Direct Energy Conversion
Technology Area Roadmap for In Space Propulsion Technologies (1)
Technology Area Roadmap for In-Space Propulsion Technologies (2)
Roadmap for In-Space Propulsion Technology
Earth as we know it today, is transient. The atmosphere has changed significantly since the earliest days. Soon after formation a dense atmosphere of carbon dioxide and water is suspected, though fortunately Earth was cool enough for the oceans to condense. After nitrogen levels rose and the carbon dioxide was mostly buried, the Earth was without free oxygen. The Sun was 25% less luminous, thus some sort of greenhouse gas kept Earth warm enough for liquid water rather than frozen oceans. Carbon dioxide, methane and hydrogen are suspected.
But beyond those what is the range of the possible?
William Bains discusses the issues and describes a possible silicon biosphere here: Many Chemistries Could Be Used to Build Living Systems. He discusses this in more detail on his web-page: The nature of life. Engagingly, doing polysilanol chemistry in liquid nitrogen sounds like fun, in a chilly, frost-bite prone way…
The National Academy of Science produced this book about 6 years ago, which discusses the issues of alternative atmospheres: The Limits of Organic Life in Planetary Systems
Also there’s this paper by Johnson Haas which discusses a biosphere based on halides as the active gases: The potential feasibility of chlorinic photosynthesis on exoplanets. While chlorine is “rare” in cosmic terms, there’s enough in our oceans to replace oxygen has the active gas in our atmosphere, thus “rare” should not be confused with “available”. Chlorine is definitely available.
A much older discussion, though still pertinent, is John Campbell’s discussion of hydrogen breathing life on Jupiter, from the 1930s: Other Eyes Watching. While our model of Jupiter has changed, there has been much discussion of biospheres on hydrogen rich planets in recent years – even Earth is suspected of quite high hydrogen partial pressures in the past. Hydrogen greenhouse planets could provide liquid-water conditions for photosynthetic life all the way out to Saturn. Past that point, the Rayleigh scattering of light makes photosynthesis too hard for life to pursue, so liquid water biospheres further out would need to run on more exotic energy sources. In the right parts of the Galaxy, capture of dark matter and its possible self annihilation could warm planets to provide more clement conditions for life.
Ammonia is often touted as a replacement for water as a biological fluid in cold conditions – there’s at least one astrobiology group experimenting with precursors to biomolecules in ammonia as the replacement solvent. Under pressure, the temperature range for ammonia becomes wider, so ammonia-based life need not be “cold” life.
While ammonia is analogous to water, as they’re both polar molecules, non-polar liquids like carbon dioxide and methane have been discussed as homes for some kind of life. Super-critical carbon dioxide – i.e. warmer than 31 C – has also been discussed as a medium of interesting chemistry relevant to life, but our ignorance of the limits of chemistry hobble our imaginations.
Stephen Baxter, the British SF writer, made the interesting suggestion of metallic life arising from even more exotic liquid environments – oceans made of iron carbonyl, which decomposes at relatively low temperatures into iron metal and carbon monoxide. His “robotic” aliens, the Gaijin (“alien” in Japanese), are initially believed to be artificial, but instead evolved on such an exotic world, in his novel “Manifold: Space”.