Category: Starflight

All matters interstellar

A Numerical Testbed for Hypotheses of Extraterrestrial Life and Intelligence

[0810.2222] A Numerical Testbed for Hypotheses of Extraterrestrial Life and Intelligence.

The study in question by Duncan Forgan. It uses pretty standard assumptions based on Panspermia, Habitable Zones (Stellar & Galactic), plus some stochastic guesstimation of the emergence and fate of ETIs. Even assuming they last as long as their stars once they spread to all the planets of their system, there aren’t too many in the Galaxy.

Of course the question then is: what happens if They proceed to colonize other systems?

At “The Habitable Zone” a Belgian amateur astronomer, Raoul Lannoy, frequently gives us space-optimists a space-pessimist take on things. Recently Raoul brought up a Space Review piece on mass colonization of space – i.e. billions leaving Earth to inhabit the rest of the Solar System and beyond. Mark Eby had this to say… Mark’s response …essentially that in human history small bands of intrepid wanderers have been the ones to spread far and wide, beyond the ancestral range.

Unless a star-system is being evacuated then mass colonization seems rather more difficult than it is worth. As independent New Beginnings for the human race, Star Colonies would have great practical and symbolic value, helping preserve humanity against system-scale collapses from whatever cause might destroy a system. Spread across the Galaxy our species would seem protected against the largest known energetic events in the Universe… except, so far spread, we wouldn’t remain the same species for very long in cosmic terms. We’d undergo divergent evolution, unless a deliberate policy of genetic mixing was pursued. Even more rapid divergence would occur if the pioneers were those posited small bands – the so-called Founder Effect
 would result in rapid fixation of any new alleles and genes while the effective population was small. Each new band settling into a different system would be a species divergence point unless more colonists followed.

Keeping those small intrepid bands in mind, once settled and grown strong, we have to ask if they will then spawn a new band of intrepid pioneers eager to venture further into the Galaxy. Geoff Landis modelled such a process as a discrete percolation process, which spread into the Galaxy not as an inexorable wave, settling every star system, but as dendritic filaments that left large unoccupied voids between them. This suggests our Galaxy could be well settled and we could still be left alone, as per the Fermi Paradox.

Alternatively They’re here and They’re watching us, but keeping quiet in any frequency we can scan. An old saying is that perfectly coded signals should be indistinguishable from noise…

SKYLON is on the Move!

Brian Wang reports on another small step towards a flying SKYLON SSTO…

SKYLON takes-off

…massing 275 tons fuelled and orbitting ~12 tons of payload, capable of ~ 200 flights, it would drop the LEO access cost dramatically – if it had sufficient business. One SKYLON can orbit 2400 tons over its life-span – what could we make with 30 SKYLONs? Some 72,000 tons to LEO…

There’s been lots of paper-studies of Solar Power Satellites – in the 1970s the assumed photovoltaic efficiency was 10-15% and they estimated SPS masses at about 10,000 tons per gigawatt. Concentrator systems, using space-capable inflatables, could get that down to ~1500-3000 tons per gigawatt. The latest SPS designs get rid of the big flat arrays and now use mirrors to reflect light onto an integrated PV converter/microwave rectenna system, to avoid mass penalties from wiring up the array and heavy slip-rings for power-flow from arrays to rectenna. The rectenna can also produce a tighter beam that can be transmitted over a range of angles thanks to improved “optics” that modern microwave arrays now have. Metamaterials could be used to overcome diffraction issues, thus allowing ground-receivers to be flexible in size.

I went to check the makers of SKYLON, Reaction Engines Limited, website and they’ve just completed a study on the economics of SPS using SKYLON for space-launch…

January 2009 News

…thus they’re thinking the same thing. Perhaps the times are pregnant with opportunity for ideas like SPS?

GEO SPS could be eventually superceded by Criswell arrays on the Moon. And a SKYLON system can support Moon operations easily too. The ESA’s predecessor, ELDO, designed a two-stage Moon-capable OTV system that each component massed under 12.5 tons and could deliver 7 tons to the Moon. This could be used to build up equipment needed for cooking arrays straight out of the regolith, eventually wrapping the Moon in an equatorial power network.

Either way, once power is feasibly seen as coming from space it might be a driver for further serious investment in space-access and eventual commercial flights to the rest of the Solar System. With large space-power available we can use beamed power to fly a lot faster than via rockets. Robert Winglee & crew have proposed the Mag-Beam for an Earth-Mars transport system which is eminently feasible and able to get round-trip times down to ~3 months, for a beam-power of 300 MW. Not quite quick enough for a short holiday, but looking good.

Eventually robotics will enable self-replicating SPS factories to be constructed which, 10 years ago now, Gerald Nordley pointed out would enable large-scale interstellar travel. How so? Imagine a 1000 ton starship being propelled to 0.86c – or a 200,000 ton freighter pushed to 0.1c – which would require 180,000 exajoules of energy input. If it accelerates to that speed over 5 weeks the power level is 60 petawatts – or 4,000 times present World Power usage. But it represents but a fraction of the Sun’s 385 million exawatts of power. And that power could be tapped by an SPS array – if we could build it big enough. How big? Say we put the array at 0.1 AU where it’s exposed to 100 times the solar energy flux that Earth feels. Assuming 50% conversion efficiency the array needs to be a 936 km square, or 876,000 km2, some ~0.172% of the Earth’s surface area. If we built SPS at a pace sufficient to keep pace with our growing demand for energy ~2.6% it’d take 320 years. That’s a long wait, which some have accepted.

But I’m impatient. I want interstellar travel, at least in embryo, by 2100. So what if our self-replicating SPS starts off at 1 GW and doubles once a year. How many doublings do we need? Just 26. If we began in 2064 we’d be in time for the deadline. If we waited for a few more doublings then by the year 38 we’d be able to launch 30,000 of those starships per year. If we enshrouded 95% of the Sun, then we’d be able to fling the equivalent of 34 trillion tons of shipping to 0.87 c using the same efficiency I’d assumed before. Just another 20 years of SPS self-replication…

Kind of makes the Fermi Paradox more glaring, doesn’t it?

Except… Robert Freitas discussed interstellar trade in his “Xenology” book and a piece on “Galactic Empires”, basically concluding that if cargo pods can be sent and their kinetic energy recovered during decceleration, then no problem. It can be done for a lot cheaper than we currently imagine. You just have to be patient :o) Maybe ETIs get by with a lot less energy expenditure than my exuberant imaginary interstellar humanity.

NB – I’ve corrected the single ship power level from 60 exawatts to 60 petawatts, adjusting the resulting figures accordingly and I owe a debt of inspiration to Gerald Nordley for the modern day revival of the mass-beam concept.

Centauri Dreams » Blog Archive » A Science Fictional Take on Being There

Centauri Dreams » Blog Archive » A Science Fictional Take on Being There.

Paul Gilster, el Supremo of “Centauri Dreams”, discusses Robert Metzger’s description of very subtle high-resolution means for ETIs to spy – in detail – on the whole of Earth’s biosphere… gazillions of nanoprobes, indistinguishable from dust, riding piggy-back on everything interesting about Terra’s children, reporting back covertly to a network of larger probes that beam the data Home. If that’s not sufficiently paranoid, the ETIs might spy us via reading the space-time vibrations every atom of our world sends out through the “fabric of space-time” like a vast, taut tapestry. And they might do so from back Home…

we’ll never know, if we never go.

Sedna’s All Alone…

[0901.4173] A Search for Distant Solar System Bodies in the Region of Sedna.

A study by the discoverer of Eris, Mike Brown, and colleagues, finds that Sedna, the putative inner Oort Cloud object, seems to be rather lonesome out there. There aren’t very many Sednas to be found – less than scores, more like a dozen or so. But the sensitivity to Sedna-sized objects (about 1800 km across) is out to about 100 AU, with larger bodies discernible against background stars out to 1,000 AU. There could be a whole bunch of smaller bodies, but they’re too dim for current scopes. Much further afield and the scope has to stare at the same patch of sky to see objects move for a lot longer than this study did.

[0901.4235] High Velocity Dust Collisions: Forming Planetesimals in a Fragmentation Cascade with Final Accretion

[0901.4235] High Velocity Dust Collisions: Forming Planetesimals in a Fragmentation Cascade with Final Accretion.

Forming planets is something of a puzzle – we can’t presently observe them directly in their early days, just the stars they orbit and the dust they’re cocooned in. We have a pretty good idea of how stars form and we can figure out how dust forms, but going from dust to planets has a few obscurities. Or rather from dust to small planet bits about 1-10 km across is obscure. Once there are billions of bite-sized planet bits they quickly accrete into planets according to all the computer simulations. That bit is easy and it forms planets much like the ones we see.

And, in recent years, the path from dust to centimetre size dust bunnies is looking nearly solved too. Zero-gravity experiments with dust on the Space Shuttle formed fractal fluffy aggregates that gradually smush together and get denser. But between a centimetre and a kilometre things get very tricky – the dust balls start seriously feeling the drag of the thin gas around them (the protoplanetary nebula) and this gentle drag is enough to plunge the growing dust-balls into the central star in 100-1000 years. So how do they survive?

Well once they’re about a kilometre across they’re safe, but growing 100,000-fold in size and a quadrillion-fold in mass is tricky. Presumably they butt into each other and ‘stick’, but just how has been doubtful. Now this study appears which answers some of the questions about dust-ball collisions by actually colliding dust-balls at the expected range of collision speeds. Surprisingly they tend to stay together, held together by the cohesion forces of dry dust (the stickiness of space itself we call “van der Waals forces”.) What bits fly off tend to be as big as the bits that collided, and so things seem to only get bigger.

So some of the process is now less obscure, but there’s still a ways to go. They’ll make planetesimals yet!

Microwave Power Beaming paper, Please

Microwave Power Beaming Infrastructure for Manned Lightcraft Operations.

Looking for a PDF copy of this paper – if any of you have it or know Prof. Leik Myrabo well enough to ask, can you send it my way? Budget is too tight to shell-out the ~$250AU needed for the Conference proceedings.

Expect Helium Showers This Aeon

Helium rains inside Jovian planets.

Extra heat from falling helium rain… who’d’ve thunk it?

Actually it’s been postulated for a few decades, but now the high-pressure experimental physical chemistry has caught up with the theorising and finally it seems to work. Saturn is most affected as Jupiter is still too hot from residual heat for the condensation to occur in a large fraction of the planet’s insides.

The HD 40307 Planetary System: Super-Earths or Mini-Neptunes?

[0901.1698] The HD 40307 Planetary System: Super-Earths or Mini-Neptunes?.

A clever use of orbital tidal evolution to constrain the possible structure of the 3 sub-Neptune mass planets around HD 40307. Fast-forwarding the evolution of the system shows it becomes unstable if the planets are too efficient at turning tidal deformations into heat. A terrestrial planet is estimated to turn roughly 1.0%-0.5% of tidal flexing into heat, which doesn’t sound like much, but is enough to liquefy much of Io’s interior, for example. For terrestrial planets very close to their star the situation would be even more dramatic and they’d be largely volcanic. Worse than Io, if you can imagine such a thing.

Ocean planets, for real, which are fluid down to the core, if not beyond, are much more flexible and less able to turn flex into heat. Typically a Jovian or Ice-Giant is estimated to turn 1/20,000th or less of the tidal flex into frictional heating and so the HD 40307 planets, if they were mini-Neptunes, wouldn’t evolve in their orbits as quick.

Sounds like a long chain of inference, I know, but much of the geophysical figuring involved comes from hard-data from the planets we know best, including our own. Nature, however, might know something we don’t…