Category: Starflight

All matters interstellar

Habitable planet around Proxima

How soon a New Earth?

No this is not a discovery announcement, so don’t stress. Instead Greg Laughlin has posted over on Systemic a discussion of what a positive detection of a habitable planet around a red dwarf star would require of current telescopic facilities. Proxima Centauri would be a surprisingly easy target and a logical one, since it is so close to our Sun allowing first class data to be gathered. Best of all it’s only 4.22 ly away – just 267,000 AU. Depressingly our fastest probes currently only do about 3 – 4 AU a year, so Proxima is still a long, long way off. Happily it’s also gradually getting closer, so by the year 27,000 it will be about 3.2 ly away. A few millennia shaved off the trip time.

In reality no one advocates a mere ~ 5 AU a year. The fastest seriously proposed vehicle is the Thousand AU (TAU) probe which can do a cracking 20 AU/yr. That’s a 10,000 year trip to Proxima at its closest. Of course there have been less serious designs for probes, the most detailed being the British Interplanetary Society’s “Daedalus”, which can do a mind boggling 0.122 lightspeed – almost 37,000 km/s – which puts Proxima a mere 35 years away, neglecting braking at the other end. With a magnetic-sail as a propellant-less brake a “Daedalus” probe could slow-down into orbit around Proxima, adding maybe 10 years to the trip time. A magnetic-sail would also allow the probe to visit all the planets in the system (Laughlin estimates anywhere between 2 and 5 terrestrial planets, as Jovian planets have already been ruled out.) Thus results from an orbital survey of a habitable planet could be beamed back to Earth some 50 years after launch.

Human Hibernation gets closer

Discover Magazine’s May 2007 edition has an amazing up date on work being conducted into induced hibernation in mammals. In recent years news arose of dogs and pigs being aroused from a death-like state after several hours, and of mice being induced to go into metabolic slow-down by breathing the right concentration of hydrogen sulphide. As the Discover article relates the work has gone further with dogs and pigs being put into hibernation by hydrogen sulphide and the amazing story of a Japanese man, Mitsutaka Uchikoshi, who went into a hypothermic torpor for 24 days after being knocked unconscious.

Naturally the first thought for the medicos working with this surprise mammalian ability is the preservation of a patient’s life when they’ve suffered major injury. If they can be suspended in a low metabolic state, then transfers from incident site to an ER becomes infinitely easier, giving the victim a much needed time-extension. Also many different major surgical procedures would become much less risky if a patient’s blood pressure, heart rate and so forth can be slowed to a crawl.

Of course, for a space-minded thinker, the next questions are:

  • just how long can the suspension last?
  • how much can physiological requirements be reduced?
  • how easy is it to induce and revive from?
  • does it reduce radiation damage?
  • could it be used as a step towards total biostasis?

    • In fiction there’s a long tradition of suspended animation, either through extreme metabolic reduction and/or cryopreservation. The new prospect of a natural suspended animation in all mammals is a rather exciting step forward for the credibility of the concept. Another curious prospect is based on the observation that mice which have had their metabolisms reduced, but not all the way into torpor, are able to breathe an atmosphere with a much lower oxygen content than what would otherwise kill them. Perhaps this provides a way to allow people to adapt to non-standard atmospheres?

    James Blish’s 1965 novel, Welcome to Mars, uses just such a plot device to allow the protagonist to breathe the thin Martian air – as thickened by being at the bottom of Hellas. Unfortunately for Blish’s story a few weeks later Mariner 4 showed the Martian atmosphere was less than 1/10th of the expected density. Still he did correctly predict that there would be plenty of impact craters and what “canals” (channels) that existed would be geomorphological in origin. He also predicted a frozen sea of ice, which Mars Express potentially has discovered.

Gl581c… habitable?

The big news from the ESO is the discovery of three planets around a red-dwarf star, Gliese 581, just 20.5 light years away – and one of the planets, Gl 581c, is “in” the star’s habitable zone and small enough to be (roughly) Earth-like.

Maybe not so habitable. According to this article by Barrie Jones, Nick Sleep and David Underwood, the habitable zone of Gl 581 is from 0.113 AU to 0.224 AU – BUT the new planet in the news is at a radius of just 0.073 AU i.e. it’s more like Venus and less like Earth. Albeit a probably wet Venus – a scaldingly hot ocean under a massive atmosphere of water vapour. Not nice real estate until improved by a reflective soletta sending some heat back at the star. Call it an ETP… Easily Terraformable Planet.

It’s fellow star system mate, Gl 581 d, is at 0.25 AU and somewhat more likely to be habitable, but it masses about 8 Earths, so it’s a bit big. And, of course, from my cursory survey of comments pages at different fora… well the usual crap is being bandied around e.g.

  • So many stars, so many planets, there must be ETIs
  • Let’s not go and “pollute” this new world with our “impure” earthliness
  • Why use the money here on earth to help the poor before we go looking for new planets?

    • The usual truisms, humanity-hating, future-cringing and hopelessly idealistic BS.

    (a) ETIs might exist, or they might not. But if they do/did then why no visible signs? Astrophysical engineering is conspicuously absent in our Galaxy, plus many other Galaxies.

    (b) Life propagates Life wherever it goes. The “pollution” of human-life is in no way alien when you look at the behaviour of all large animal species – we change our environments. We might be so stupid as to foul our own nests BUT personally I see signs we’re getting better at cleaning up after ourselves. City-dwelling modern humans have a SMALLER ecological foot-print than their predecessors, a fact that Luddites and Tree-Huggers blithely ignore.

    (c) The last thing the poor of the world need is our money because they’re poor. They need us to buy stuff from them, not tie another pelican around their necks like so much foreign “aid” has been in the past. Community development is fantastically important, but it only helps if it builds up people to trade and compete with the rest of us. Mere hand-outs kill. And in the end they want to be like us – and part of being us is looking beyond this horizon. That’s why astronomy and space is so damned important. A “simple life” impoverishes all.

    My $0.02 worth.

Solar for a Mag-Beam

The Mag-Beam propulsion concept was developed by Robert Winglee and his fellow researchers as a means of propelling quick interplanetary shuttles without mucking around with rockets. Basically it’s a big plasma rocket turned into a big plasma gun, that fires a fast, hard ion-beam at a magnetically ensheathed shuttle vehicle. This gives the shuttle a big shove and sends it on its way – to be slowed down at the destination by another Mag-Beam, or a bit of adroit aerobraking if it’s slow enough.

Problem is the Mag-Beam wants a lot of juice to do the job – for example, a 20 ton shuttle being accelerated to 20 km/s needs a 300 MW Mag-Beam firing at it for about 4 hours, which is a lot of battery mass (3,000 tons at 400 W.hr/kg of battery.) Once the beam has fired the massive battery pack can be recharged via solar power over a few weeks or days before the Mag-Beam is needed again. But is the battery pack needed at all?

Geoffrey Landis designed a Solar Power Satellite that beams 1 GW to the ground @ 33% efficiency and it massed just 1,300 tons which means its in-space power output is 3 GW – ten times the power needed by the Mag-Beam. Thus an in-space SPS power source for a 300 MW Mag-Beam need only weigh 130 tons. That’s a mass that could be launched in one piece by an Energia or Saturn V class launcher – like the new Ares launchers for NASA’s Return to the Moon. To do everything the Mag-Beam is required to do the power has to be delivered to multiple Mag-Beam plasma-gun stations. What made Landis’ SPS so light was that it remained in an orbit perpetually pointed at the Sun, so there was no need for a rotating power transfer collar from the array to the rectenna, and the same reflectors used for power gathering act as rectennae for power transmission in the Landis design. To keep that simple design, and power Mag-Beam stations in multiple locations, Power Relay Satellites – really just microwave wave-guide horns for changing a beam’s direction – might be needed. Or we might just bite the bullet and have a rotating connection between array and station. Both add mass, but it’ll be a LOT less than 3,000 tons of batteries. Plus PRSs can point at rectennae beaming power up from the ground to transfer back down to the ground at another location, allowing a PRS to do multiple roles and make money transferring power to areas of peak demand on the ground.

Aside from propelling shuttles to Mars and elsewhere a Mag-Beam can also boost a sub-orbital vehicle ( a modified Virgin Galactic SpaceShip, for example) to orbital speed. The power required maybe higher, but as a shorter burst. If the sub-orbital vehicle can boost to a horizontal speed of ~ 1 km/s, then another 6.8 km/s is needed for low orbit. At an acceleration of 20 m/s^2 that’s a boost for 340 seconds, just under 6 minutes. In energy terms it’s the equivalent of 120 minutes of a 300 MW beam. Perhaps the higher power can be supplied from the ground?

Outer Planet Mining

Self-fuelling an Enzmann starship

Jupiter was touted as the major source of He3 fusion fuel for Daedalus, some 30,000 tons of it, but Jupiter’s gravity well is HUGE and far beyond the abilities of solid-core fission rockets, stretching the capabilities of gas-core rockets in terms of thrust-to-weight ratios. So what about the other gas-giants?

Planet mass (Earths) radius (km) P-mag (sec) P-hydro (sec) Eq. velocity (km/s) Orbital vee (km/s) delta vee (km/s)
Jupiter 317.838 71492 35727.3 35618 12.573 42.098 29.525
Saturn 95.161 60268 38362.4 38196 9.871 25.088 15.217
Uranus 14.536 25559 62064 61704 2.588 15.057 12.469
Neptune 17.148 24764 57996 60120 2.683 16.614 13.931

As you can see Uranus has the most forgiving gravity field, but Neptune and Saturn aren’t out of reach either, and Saturn has proximity to the Sun and Jupiter, for gravity assists, in its favour as well as Titan, a moon with a dense atmosphere and decent gravity. The Daedalus study assumed floating factories at the 0.1 bar level in Jupiter’s atmosphere serviced by gas-core automated shuttles, but if there’s enough need for them volatiles from the big planet atmosphere can be scooped and shipped up for processing at an off-world base.

Scoop-ships could also allow starships to be self-fuelling. I have just received an issue of the October 1973 “Analog” – the one with a gorgeous Rick Sternbach cover of two Enzmann starships and the Cover article by G. Harry Stine, “A Program for Star Flight”. It’s quite a memorable article as Stine was arguing for a star flight program to begin c.1990, and the development of a massive in-space industrial base to support the effort. His initial phase would study the nearby stars with Lunar interferometers, then launch million-ton space-probes at 0.9c, and finally launch ten-ship fleets of Enzmann starships (roughly 12,000,000 tons each, mostly deuterium fuel.) Quite a major effort, but he optimistically costs it at $100 billion (in 1973$.)

A few problems arise – 0.9c from Orion-style pulse drives is a touch unlikely, even with mass-ratios over 1,000 – but over all the concept is sound. Magnetic sails might change the approach, but the basic idea of attaching starships to huge masses of propellant, rather than big tanks, is a good one. However I have read that hydrogen and deuterium ice are mechanically like Jello and thus utterly useless as envisaged. Lithium-6 is a fusion fuel and pretty strong at cryogenic temperatures, so it might be the fuel of choice. Either that or carbon nanotubes might allow very, very light weight tanks to keep deuterium Jello in. The 12,000,000 ton starships probably mass just 120,000 tons empty (the design needs BIG mass ratios for speed), but the size Stine quotes is all wrong. Deuterium’s density is 0.16 relative to water, yet the fuel sphere is described as 1000′ across meaning a density of ~ 0.8, some five times denser than deuterium Jello. A sphere 1,710′ across will do nicely.

Refining out 12,000,000 tons of deuterium from a gas giant will be quite a task. Since deuterium is about 1/2000th of the abundance of protium in Jupiter some 24 BILLION tons of hydrogen will be sifted through to collect the fuel. Bit of a tall order, but inevitable when you’re trying to fit 2,000 people on a starship that’s over 2,000′ long and push it through a delta-vee of 0.3 c.

Actually I don’t know the empty mass of the Enzmann starship is 120,000 tons which is a touch frustrating since I’m paying attention to details here. What Stine does say is that the probes will hit 0.9 c with a mass ratio of ~ 1,000, and the starships will hit ~ 0.3 c (thus a delta-vee of ~0.6 c.) A bit of non-relativistic maths, and assuming the same exhaust velocity, means the starships have a mass-ratio of 100 (=1000^(0.6/0.9).) Now fusion reactions don’t make enough particles with sufficient energy to get that sort of exhaust velocity (about 0.13 c), nor are Orion pulse-drives 100% efficient (~25%?) More realistically an Enzmann starship will hit 0.08-0.15 c which is respectable for a fusion-drive.

Alpha Centauri Rising

Tonight the Two Pointers and the Southern Cross were rising not long after sunset – have missed them in the sky. I always look wistfully at Alpha Centauri – the Brightest Pointer – and think it’s only a cosmic stone’s throw away… all 40 trillion kilometres of a throw *sigh*

Back on Topic

Multi-bounce laser-sails

Crowlspace is chiefly about spaceflight and cosmic things, but the Tomb stuff has popped up, so I felt compelled to write. Back to the central topic now… a new idea has arisen for pushing things around with lasers – why not recycle the laser beam itself? This idea was recently discussed at Centauri Dreams and references a paper by Robert Metzger and Geoff Landis, which looks at an Earth-Mars mission using multiple-reflection.

Apparently ultra-reflective surfaces for a given frequency are now feasible – we’re talking 99.99995% of all the energy of a beam being bounced off the surface. That’s enough to allow many thousands of bounces of the beam itself, between laser and target, thus boosting the effective momentum transferred by the beam’s energy. It takes 150 MJ to give a 1 Newton impulse to a perfect mirror, but if the beam could be bounced 20,000 times, losing a bit of energy as heat each time, then the effective force multiplies about 10,000-fold. Of course the effective path-length goes up 20,000-fold, so the acceleration distance has to be pretty short too before the diffraction limit spreads the beam out too much.

In the Metzger/Landis example the boost is assumed to be 1,000-fold and the diffraction-limit gives a boost range of 210,000 km for acceleration at 0.33 m/s^2, boosting the laser-sail/payload of 20 tons to 12 km/s. That’s enough for an Earth-Mars transit in 96 days, which is pretty respectable. The energy cost works out at $240/kg, supplied by a couple of reactors at Earth and Mars. There’s no reason why nuclear reactors are required as Landis himself has posited designs for 3 GW (in-space power) Solar Power Satellites that mass a mere 1,300 tons. With 70% efficient concentrator solar cells the mass is halved, and for just a GW of power the mass drops to a mere 200 tons. Lighter than a GW reactor, unless it’s an open-cycle gas-core reactor, which by itself would make a good rocket.

A 10 ton payload may not sound like much, but it’s 1/3 what the Shuttle laboriously hauls into orbit when fully loaded. A lot of payloads launched fast would make the system worthwhile, and that’s the main point. Also payloads boosted into near-Hohmann transfer orbits requiring a mere 4 km/s could mass 90 tons over the same boost distance and that’s serious payload on an interplanetary trajectory.

Star-Brake II

Mag-sails acting as interstellar brakes follow the equation:

V = Vo/(1 + k.Vo^(1/3).t)^3

…and as we discovered the distance the mag-sail travels while deccelerating from Vo to V during a fixed time, t, is:

s = Vo.t/2 * [ (V/Vo)^(1/3).(1 + (V/Vo)^(1/3)) ]

But the behaviour of a starship isn’t obvious from an equation alone. So here’s a table. What you’re seeing is the velocity broken down into short steps of 0.05 c, except the last, and the time taken for each step, plus the cumulative total. As you can see the decceleration is high – 5.6 gee at the start – and rapidly declining. The final skid from 0.05 c to just 0.0054 c takes a whopping 522 days, while the whole 0.9 c delta-vee prior takes a mere 297 days.

V1 V2 factor time (days) time (cumulative)
0.95 0.9 0.0184994 3.239800003 3.239800003
0.9 0.85 0.019923023 3.489119075 6.728919078
0.85 0.8 0.021550153 3.774078302 10.50299738
0.8 0.75 0.023425071 4.102432734 14.60543011
0.75 0.7 0.025605464 4.484284936 19.08971505
0.7 0.65 0.028167793 4.933025557 24.02274061
0.65 0.6 0.031215428 5.466757946 29.48949855
0.6 0.55 0.034891343 6.110520895 35.60001945
0.55 0.5 0.039398605 6.899877768 42.49989721
0.5 0.45 0.04503483 7.886949865 50.38684708
0.45 0.4 0.052252928 9.151055264 59.53790234
0.4 0.35 0.061774604 10.81858634 70.35648869
0.35 0.3 0.07481817 13.10290615 83.45939483
0.3 0.25 0.09359947 16.39207514 99.85146998
0.25 0.2 0.122574895 21.46654131 121.3180113
0.2 0.15 0.172096111 30.13919193 151.4572032
0.15 0.1 0.272362632 47.69886779 199.156071
0.1 0.05 0.559982927 98.06980993 297.2258809
0.05 0.0054 2.985502206 522.8510013 820.0768822

If you switched to rockets at 0.1 c then the mag-sail braking would last a mere 200 days. But on interstellar flights what matters is fuel expenditure and so with multiyear trip-times, a few hundred days isn’t a big reason for wasting reaction mass. But the above assumes that the mass density of the Inter-Stellar Medium (ISM) is a constant for the whole trip. This isn’t necessarily so. Larry Niven’s classic description of a ramjet in flight (in A Gift From Earth) uses the increased density of the ISM around the UV Ceti star system to provide extra braking for a rapid flight to Tau Ceti. Maps of the ISM will prove vital to future starship captains, for many reasons.

Magnetic Sail as a Star-Brake

No-fuel decceleration

Robert Zubrin and Dana Andrews invented the concept of the Magnetic Sail, or Mag-Sail, which is basically a large superconducting ring which generates a large magnetic field, which mimics a planet’s magnetosphere and interacts with the Sun’s plasma-wind. Within a solar system a Mag-Sail provides unlimited range, so long as coolant lasts for the superconductors. But the most interesting use for a Mag-Sail is providing a reaction-mass free way of slowing an interstellar space vehicle. According to a NIAC study by Zubrin et. al. a Mag-Sail can potentially slow a vehicle from 0.95c to just 0.0054c in just 820 days – with no reaction mass required.

To the mathematically inclined quoting a few figures doesn’t tell one much, and so I went a little further in my reading of Zubrin and developed his equations further. His various works state that the final velocity, V, for a deccelerating vehicle goes like so…

V = Vo / (1 + k.Vo^(1/3).t)^3, where Vo is the initial velocity, k the decceleration constant, and t the decceleration time.

…with a bit of rearrangement the time can be worked out from the velocities…

t = (1/k).[ V^(-1/3) – Vo^(-1/3) ]

…i.e. inverse of the decceleration constant times the difference of the inverse of the cube-root of the final velocity and the inverse of the cube-root of the initial velocity. Which is easier to understand in symbols, if you ask me.

Let’s take a step back to the first equation. Call (1+ k.Vo^(1/3).t) a separate name, B, thus we get…

V = Vo/B^3

…and the acceleration, dV/dt, is now…

a = -3k.Vo^(4/3)/B^4

…and the initial acceleration, a(0), is -3.k.Vo^(4/3). The minus sign indicates the vehicle is slowing down.

How far does the vehicle travel while deccelerating from Vo to V? It’s a straightforward integration – much to my surprise – and boils down to…

s = Vo/(2k.Vo^(1/3))*[(B^2 – 1)/B^2]

…which, after some algebra, becomes…

s = Vo.t/2*[(1 + B)/B^2]

…which isn’t so intimidating. Alternatively we can express the decceleration constant, k, as an expression…

k = 1/t * [V^(-1/3) – Vo^(-1/3)]

and substitute into our equation for the displacement to get…

s = Vo.t/2 * [(V/Vo)^(1/3).(1 + (V/Vo)^(1/3))]

which may seem a bit strange, but even the regular equation for displacement under constant acceleration can be rearranged into a similar form…

s = Vo.t/2 * [1 + (V/Vo)]

but what does that all mean? Part II coming soon.

Long term habitats

Red dwarfs… trillion year real estate

Red dwarf stars are the slow living members of the galactic community. Just why sheds light on some odd denizens of the galaxy. The maximum sustainable temperature in the core of a star is dependent on its mass, and the rate of fusion in that core is dependent on the temperature to about the 7th power – thus a star that’s half the Sun’s core temperature of 15.7 million K is burning protons some 2^7 (128) times slower in a lower mass core. Slowly burning its hydrogen a minimum mass red dwarf (0.08 solar masses) will last for about 13 trillion years before going out, almost literally as if switched off. Unlike heavier stars there’s no red giant stage, just a gradual brightening until its final decline a few billion years before the end of hydrogen fusing. Greg Laughlin, Fred Adams and Peter Bodenheimer modelled the long-term evolution of low mass Main Sequence stars… The End of the Main Sequence …and for stars in the 0.08 – 0.2 solar mass range the picture is much the same, trillions of years of slow brightening and an end that’s a bit brighter, then a ‘quick’ decline. In the 0.2 – 0.25 solar mass range the stars end more like red giants and less like helium-rich dwarf stars. Another feature of low mass stars is they convect almost all their material and thus end up fusing 98% of their hydrogen – unlike our Sun which ends up fusing only about 8% of its hydrogen on the Main Sequence. Why the convection trick, which seems so unfair? A cooler core and a denser star. Consider, a 0.1 solar mass star has a radius of 0.125 of the Sun, which means it’s 51 times denser than the Sun, with a gravity that’s 6.4 times higher too.

Below a few million degrees in the star’s core and the fusion reaction rate is ridiculously slow and essentially doesn’t happen, so the “star” is actually a brown dwarf, which only shines by virtue of gravitational collapse. The heavier the brown dwarf, the more it can collapse too, so lighter brown dwarfs run out of gravitational energy much quicker than heavier ones.

Brown dwarfs are, oddly enough, closely related to white dwarfs – star corpses – which also only shine via gravitational collapse, and then by the slow trickle of their massive internal heat capacity. White dwarf collapse is halted via electron repulsion – the so-called degeneracy pressure caused by electrons being crowded too much and gaining Heisenberg Uncertainty energy. Without that repulsive force the white dwarfs would collapse into black holes after radiating away a few trillion years worth of heat. According to work on Supersymmetry (SUSY) by L. Clavelli a white dwarf can ‘catalyse’ the transition from our broken SUSY world to a world of exact SUSY in which there’s no degeneracy pressure to stop the star from collapsing into a black hole. Such a transition is sudden and would produce a massive release of all its gravitational energy as gamma-rays – highly collimated gamma-rays just like a humungous laser, or gamma-ray burst.

But what if the energy release is slowed down? If the collapsing star radiated at the Sun’s current output – but a much higher temperature because it is so much smaller – then as its mass trickled away as radiation (4.5 million tons a second is a ‘trickle’ to a star) then it would last some 14 trillion years if it massed the same as the Sun. This is about 1,150 times the Sun’s expected life-time as a fusion-powered star, a vast improvement for intelligent life in our system. The question is: how do we control the rate of collapse so our Sun doesn’t become a massive laser? We only have a few billion years to find out.