Whatever Happened to Black Holes as Star-drives?

Black Hole Hawking Radiation Power & Thrust
Black Hole Hawking Radiation Power & Thrust

Back in 1979 Robert Freitas, in his massive now-classic study “Xenology” first discussed Black Holes and their Hawking radiation as a possible propulsion system for interstellar flight (section 17.3.5.) Since then the concept has remained relatively ignored, since black holes are hard to make and hard to handle. By the late 2000’s, as our confidence in the existence of Hawking radiation had grown, the idea was revisited by Louis Crane, with his colleague Shawn Westmoreland, in a 2009 preprint.

In the above Table I’ve set out black-holes of various masses and have computed their self-thrust, with results similar to Freitas’s. Since the holes mass millions of tonnes, any associated starship should likewise mass similar amounts, so the acceleration can be ~halved. The very smallest black holes might prove difficult to feed at the indicated rates, since they’re smaller than protons, so Crane & Westmoreland suggested using the black hole as a “battery” – a finite store of energy – and letting it push self and payload until just before its final explosive last few seconds. One problem is that the hole becomes very energetic indeed as it loses mass, so just when the appropriate time to EJECT is an interesting question. For every 10-fold decrease in mass, the self-acceleration increases 1000-fold, so a crewed starship would need either acceleration mitigation or would need to eject once the black-hole was under one million tonnes.

For some background, several good introductions to Hawking radiation exist – Andrew Hamilton’s and the Think Quest discussions are the ones I’ve found most helpful. And, of course, there’s the paper by Crane & Westmoreland.

Since then, however, further exploration of the concept has been pursued by Jeff Lee, under the resonant name Black Hole Kugelblitz – though with less than interstellar results: Acceleration of a Schwarzschild Kugelblitz Starship The main problem is that the known particle spectrum of the Standard Model of particle physics causes much lower purely energy outputs, producing mostly a spray of near useless short-lived particles. Worse, the gamma radiation also produced is near impossible to redirect and can only be partially absorbed by a huge hemisphere of titanium (a good gamma absorber), thus making a poor Photon Rocket, which uses just a fraction of its power to produce directional thrust.

In conclusion the concept needs considerable work before it can be considered an interstellar drive option. The radiation intensities that need to be handled boggle the mind. However coupling our particle theories to black holes is not without problems – quantum gravity may well alter the intensity once the hole is small enough and we have no clear idea of the fate of the multitudinous particles produced. Does a super dense ball of quagma result, “stuck” to the ball by gluons dragged out of the vacuum of space? The related idea, of quark matter, might present the option of embedding a Kugelblitz inside a quark nugget. A more developed understanding of the quantum chromodynamic (QCD) vacuum and quantum gravity needs developing.

For now, like the original Photon Rocket, this idea goes back on the shelf, until our physics catches up.

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.

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…

SpaceX to Mars! Here’s How…

Following on from my Mars Anthology there’s now more Mars Society material available online…

Trans-Orbital Railroad to Mars

…a Picasa Web-Album of slides from the weekend elaborating on Robert Zubrin’s latest concept, which I have discussed previously. I was particularly struck by this one, which I first saw on “The Space Review”…

…from Jeff Foust’s evocative article on the Transorbital Railway concept. I didn’t realise when I first read Jeff’s essay that Zubrin had given a talk on the concept. Here’s the slide…

…which explains how the two-person crew have a quite roomy inflatable habitat attached to the Dragon to fly to Mars in. Provocatively Zubrin notes that some astronauts have already experienced cosmic-radiation exposures equivalent to the full mission to Mars, with no ill-effects noted thus far…

…seems the ISS (and MIR before it) have done a great service to the Mars effort by slaying this particular bogey. The serious issue of Solar high-energy events remains – otherwise known as solar-flares – but these can be mitigated by relatively simple shielding. Ultimately we’ll have magnetic deflectors for the particle stuff, but the x-rays will need careful shielding even then.

Once the First Expedition arrives, here’s how “Mars Base One” will look…

…ready to expand into a fully operational Base, if the nations of the world are willing to help. The Falcon Heavy can send significant payloads to Mars, not just people. Industrial machinery can be sent, able to begin utilization of local resources on Mars. Additive manufacturing technology can be used to make small components and, with sufficient incentive, can lend itself to larger manufactured items. Mars has plentiful carbon, oxygen & hydrogen to make plastics and polymers, and doubtless it has minerals of all kinds.

Of course the first two products should be propellant for the rockets and power-cells (solar or other.) Proper reusable Mars ferries will allow transfer of returning crews to waiting ERVs, eliminating the need to send separate MAVs. With sufficiently proven life-sustaining resources (material, technical and personnel) the Base can start receiving one-way arrivals – true Colonists. That’s something I’d like to see all nations, who are willing, to contribute towards. Let Mars be the true melting pot to alloy something wonderful out of all of Old Earth’s children.

Sunlight to electricity – direct!

Solar power without solar cells: A hidden magnetic effect of light could make it possible

(PhysOrg.com) — A dramatic and surprising magnetic effect of light discovered by University of Michigan researchers could lead to solar power without traditional semiconductor-based solar cells.

…of course the efficiency is currently below 10%, but improvements are likely. The problem with solar isn’t turning it into useful energy, but storing it! If you could do that easily and cheaply, then you can change the world.

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.

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!

Nuclear Power Forever*

*for more years than you can count at least.

Interesting discussion of nuclear power as the climate change solution… Nuclear FAQ

The point I want to look at is the claim that there’s 220 million years worth of Uranium & Thorium for a 10 billion terrestrial population at US energy usage levels, in the top 4 Km of crust. Consider the erosion rate of the continental crust – some 7 gigatons per year. In 220 megayears some 1540 quadrillion tonnes of rock will have eroded into the sea – 570 quadrillion cubic metres at an average density of 2.7 tonnes/cu.metre, which on 150 trillion square metres of surface is a layer 3800 metres thick.

Thus, to get at the Uranium/Thorium we don’t have to mine anything but the oceans.

Space Based Solar Power II

Ground-based and space-based solar power, in spite of certain misconceptions, shouldn’t need to compete because they’re optimal for two different energy markets. Consider ground-based solar – its peak output is during the hottest part of the day, when the Sun is bright and high in the sky. But it’s at its worst when low in the sky (and at night, of course) and during the colder seasons. Space-based is good all-day round, all-year round… BUT it’s hard to up the output. That’s especially needed when the Sun is high in the sky and the ground-based solar is working hardest.

Couldn’t ground-based cover both by more collectors being on the ground to make up for low-light and night conditions? Let’s look at that proposition in some detail. How is power stored? If we assume batteries then we’re faced with some difficulties – firstly a top lead-acid battery, with a supercapacitor to help, is only able to store 75 W.hr/kg. Say a household needs 3,000 W averaged over its whole day – and we’re assuming the car is being charged by the power system too. That’s 40 kg of battery per hour of supply, some 960 kg of battery for the 24 hours in the day. And batteries have a discharge efficiency of 80-85% – meaning we need a total of 1,200 kg of batteries to store the day’s energy needs.

But how much day have we got to collect the energy from the Sun in? At best it’s about 0.25 of the day – thus 3,000 W continuous supply needs to be fed by 15,000 W of solar collectors (remember that battery inefficiency.) Thus at $0.5/W our “Total Power” solar-power system costs ~$7500 of solar array plus all those batteries (~100 total, costing ~$15,000, plus the power handling system ~$5,000.) Thus our system costs $27,500 on the optimistic side of costing, since there aren’t any $0.5/W solar collectors on sale. The system would recover costs in ~5.5 years at ~$0.2/kW.hr from the energy retailers. Assuming its output didn’t decline over time that is…

But what about space based solar? Firstly it doesn’t need storage – it can supply all the time. Of course not everything is being used “all the time” but with a decent amount of homes with some level of power storage and the excess could charge those, drawing back when it’s peak time. Honest retailers might even reduce the “consumers'” bills in exchange for the storage service. So no storage. No battery costs. However the collectors need to be ~1/6th of what’s needed for the same continuous output on Earth. And concentrators, rather than flat panels, can be used all the time. Expensive, ultra-efficient cells can get 500-1000 times the sunlight of their terrestrial counterparts and convert at 40-65% efficiency…

Towards nanowire solar cells with a 65-percent efficiency

Published on: 16 June, 2010

TU/e researchers want to develop solar cells with an efficiency of over 65 percent by means of nanotechnology. In Southern Europe and North Africa these new solar cells can generate a substantial portion of the European demand for electricity. The Dutch government reserves EUR 1.2 million for the research.

…thus immense savings in materials and array mass. My rough BoTE computations suggest a 1 GW SPS can mass 1,400-720 tons, with huge savings in launch costs. Assuming we’re launching via Falcon 9 Heavies that’s 72-36 launches. Elon Musk would love us!

So instead of spending ~$7,500 on 15 kW of array per household, only ~$1,250 is needed. And no batteries. Thus, potentially, a BIG saving overall. But some extra panels for peak power usage – in the bright, hot part of day – and there’s a neat synergy between the two power supply sources. Of course that’s all evened out over ~1 GW/3000 W households (~333,000) as individual supply isn’t so easy to do with a SPS.

However initial demonstration SPS units might only provide a few MW, enough for small communities. In theory there’s no reason why a bunch of smaller sub-units can’t eventually be ganged together in a common structure and modern phase arrays used for sending out power-beams to several different rectennas on the ground. If 720 tons is 1 GW of supply, then a single launch demo SPS massing ~20 tons at GEO might supply ~30 MW to the ground. That’s enough for a 10,000 household township in a remote area. Worth considering.