Category: Starflight

All matters interstellar

Catalytic Nuclear Ramjet

Catalytic Nuclear Ramjet(application/pdf Object).

Robert Bussard first proposed the Nuclear-Fusion Interstellar Ramjet in 1960 and it caught the imagination of researchers (like Carl Sagan) and fiction writers (like Larry Niven & Poul Anderson) alike. Basically Bussard proposed to scoop up the interstellar medium and fuse it for propulsion, thus allowing a rocket to refuel for its entire journey. A 1,000 ton rocket could theoretically scoop propellant and fly at 1 g ‘forever’ – at least until drag became equal to its thrust.

A problem arose – hydrogen is very hard to fuse all by itself. The reaction rate of proton-proton fusion at “low” (i.e. an achieveable 100 million degrees) temperatures is essentially negligible and only powers the stars because they’re so gigantic. The Sun’s energy production rate is a bit more than 10 Watts per cubic metre of the fusion part of its core, which is far less than the power packed into a battery, for example. Unlike a battery, of course, that energy can trickle out for billions of years – but that’s no good for propelling a starship.

How do we make the reaction go faster? Physicist Daniel Whitmire proposed we burn the hydrogen via the well-known CNO Bi-Cycle. Basically a hydrogen fuses to a carbon-12, then another is fused to it to make nitrogen-14, then two more to make oxygen-16, which is then highly ‘excited’ and it spits out a helium nucleus (He-4) to return the nitrogen-14 back to carbon-12. Since the carbon-12 isn’t consumed it’s called a “catalytic” cycle, but it’s not chemical catalysis as we know it. Call it “nuclear chemistry”.

The CNO-cycle was first proposed by Hans Bethe as the means by which the Sun makes heat & light. It’s one means by which the Sun does so, but proton-proton fusion is dominant at the lower temperatures in the Sun and lower mass stars. Slightly bigger stars are predominantly powered by the CNO cycle and as the Sun evolves and its core contracts as helium builds up, it too will burn mostly via the CNO cycle.

Whitmire’s paper gives a rough guide to how well the CNO cycle improves a ramjet’s power-levels and shows, reasonably, that a Catalytic Nuclear Ramjet could propel a ship to the stars…

…the rest is an exercise for the student.

What the Hell is a Polytrope?

polytrop.pdf (application/pdf Object).

This little pdf file covers some interesting properties of polytropes – but what’s a polytrope? Basically it’s a sphere of gas, or some other matter, governed by a particular equation of how the pressure and the density are related.

P = K.(rho)1+1/n

…(rho) being density, P is pressure, K is a constant, and n is the polytropic index.

This equation is used to create an expression for the structure of the sphere that is converted into a differential equation, the Lane-Embden equation, which then can be integrated. Polytropes of n = 3/2 are used to model brown dwarfs and planets, for example, while polytropes of n = 3 are used to model stars like the Sun. Both need to be computed numerically as closed form solutions only exist for n = 0, 1 & 5.

The paper referenced above derives an expression for the gravitational binding energy of a polytrope of arbitary index. And it’s surprisingly easy…

(Omega) = -[3/(5-n)]*GM2/R

…thus a sphere of constant density (n=0) is -(3/5)*GM2/R,
n=3/2 case is -(6/7)*GM2/R,
and n=3 case is -(3/2)*GM2/R. What that means is that the Sun has squeezed into it about 5/2 times the potential energy that you’d expect from the Kelvin-Helmholtz solar model. If its energy derived from gravitational contraction then it has about 50 million years stored up inside it in its current configuration.

A puzzle of stellar structure, prior to the breakthrough that was relativity and quantum mechanics, was what was stopping a star from collapsing forever? Nothing seemed strong enough to hold back the inexorable squeeze of a star’s own gravity.

Rydberg Matter… Invisible Hazard?

Rydberg Matter in Space.

Rydberg atoms are atoms orbitted by electrons in an excited state which leaves them barely attached to their nucleii, often for surprisingly long time periods. Rydberg Matter is basically clusters of Rydberg atoms attracted to each other and forming stable small n clusters. These small clusters typically form “atomic snowflakes”, but these in turn link up in long chains. Collected together in space Rydberg matter composed of interstellar hydrogen is largely transparent to radio, visible and IR light, thus it’s almost impossible to see… and possibly a billion times denser than the 106 protons per m3 that makes up most of the ISM. That’s bad news for interstellar travel as running into the stuff at high-speed (~0.1 c or more) would cause incredible meteoric levels of heating ~25,000 K.

An explanation for Fermi’s Paradox? Unsure. The chief researcher on Rydberg Matter seems kind of solitary in his research, perhaps indicating it’s dubious nature. More data from other researchers would be helpful.

Dense Deuterium for Nuclear Fusion

News detail – University of Gothenburg, Sweden.

A Swedish physical chemist, Leif Homlid, is producing ultra-dense deuterium, though the news-bite is a bit vague on how much and how stable it is. However it’s 130,000 times denser than water and would make a fantastic fusion fuel in an ICR fusion system. Might even make deuterium rockets a whole lot easier than Friedwardt Winterberg has imagined. Sure would be a less bulky way of storing the stuff if it could be made in bulk.

New Nanocrystals Show Potential for Cheap Lasers, New Lighting : University of Rochester News

New Nanocrystals Show Potential for Cheap Lasers, New Lighting : University of Rochester News.

Lasers are one of the backbones of interstellar travel via large reflective sails. If they can be made sufficiently powerful and sufficiently efficient that is… and sufficiently cheap. This research might address the latter point.

As the saying goes: “No Bucks, No Buck Rogers”

Warp Drive 6.0

[0712.1649v6] Warp Drive: A New Approach.

A New Approach to Warp-Drive

December 2007 should be remembered as the date that a viable warp-drive burst forth from multi-dimensional obscurity… well no, but the blogosphere did notice the paper by Richard Obousy and Gerald Cleaver which tackled the warp-drive concept via LEDs (Large Extra-Dimensions) and gave an estimate of how much energy it would take – a 1000 m3 volume needs 1045 joules of cosmological constant energy. But that’s for that whole volume to be filled by a sufficiently strong cosmological constant for the vehicle to be propelled at lightspeed. If it was confined to a thinner shell the energy requirement would be much less. Say it was as thick as the postulated LED (10-6 metres) then the volume is reduced by ~3.2 million fold to ~3×1038 joules… still pretty “yipes!” but now only Ceres mass in size, not half a Jupiter.

Alternatively there’s the “Pocket Universe” design suggested by van de Broek, which had the ship in a mini-space connected to our own by the warp-bubble itself. In this case the minimum volume is probably ~(10-6)3 metres3 and the energy required is just ~1024 joules. That’s just 10,000 tons of mass-energy converted into the cosmological constant of the warp-bubble… if we can figure out just how do the trick of converting energy into cosmological constant, of course. I think the LED size-limit is probably as small as a tamed bit of space-time can get, so we’ve quite a task in mass-energy manipulation to fling ships around in warp-bubbles. Eventually experiment will tell us more about the LEDs – if they exist, for starters – and perhaps give us a big hint on how to make a “pocket Universe” wrapped in a cosmological constant…

Where Are They?

It’s said that Enrico Fermi wondered why we didn’t see signs of aliens since nuclear energy made interstellar travel possible. It’ s a worthwhile question and one which has been answered in a dizzying number of ways. Here’s my $0.02 worth…

Personally I think there’s two likely equilibrium states that answer the Fermi Paradox. Either we’re in the pre-Colonization era, before anyone ventures forth and colonizes the lot, due to Life starting late in the Universe’s life because of some recently changed astrophysical process.
Or the Galaxy has been colonized, in which case two sub-divisions seem reasonable:

(1) We’re an undeveloped patch missed by the last few waves of colonization as per Geoff Landis’s Percolation Theory.
(2) Or we haven’t been missed and we’re colonized, but just not how we usually imagine.

Consider: There’s so much room in the Edgeworth-Kuiper Belt, or the Opik-Oort Cloud, and if ETIs exist as star-travellers then that’s where they are. While the inner system seems warm and inviting, perhaps they avoid it because of coronal mass ejections, UV or solar flares or some such. Remember they’re used to living in space in their own habitats, not on wild, native planets.

Perhaps there’s a whole lot of deuterium lying around ready to be scooped up from the surfaces of cold Plutoes or Marses that might be Out There. Imagine a Mars mass planet that has chilled to ambient temperatures and all the hydrogen has frozen out – its atmosphere would be helium! Makes getting at the He3 easier. Assuming it formed with a primordial atmosphere that is, which is a fair assumption for the Outer Solar System.

Even if they only use fusion for propulsion and they use solar for life-processes, there’s sufficient materials for constructing huge solar collecting mirrors (“sollectors”) in most average sized cometoids. A sufficiently efficient closed-loop ecology can be sustained on a lot less sunshine than we experienced here on Earth.

Just how efficient is the Earth’s biosphere? About 100 billion tons of carbon is fixed by primary production in the land and sea biospheres – about 250 billion tons of carbohydrates – per year. If the energy content is ~480 kJ/mole and a mole is about 0.03 kg, then the total energy stored by the biosphere is ~4,000 exajoules per year. Earth absorbs from the Sun about 3,850,000 exajoules per year, so the biosphere captures and stores just ~0.11% of that energy torrent. So, in theory, we could get by on less energy than what drenches the Earth currently.

Of course a biosphere would have to stay warm, but with sufficient insulation the heat-loss can be brought into line with the biosphere’s own waste-heat, so the habitat is not radiating as a black-body at the biosphere’s preferred temperature, but at some lower value in line with its actual heat use. But I’ve covered similar ground in a different post. The main thing is that ETIs can lay-low in the OOC, if they so wish. And we’re not the cosmic backwater that Fermi scepticism might lead us to think…

Direct contact

ScienceDirect – Planetary and Space Science : Direct contact among galactic civilizations by relativistic interstellar spaceflight.

Carl Sagan caused some academic excitement in 1963 [i.e. the academics blasted him] by modestly proposing that interstellar travel was a reasonable way for civilizations throughout the Galaxy to pay each other a visit every few thousand years, and that at some point in history (the last 10,000 years) we had actually been visited – or at least it was worth checking myths and legends for a sign of such a visit. Robert Bussard’s Ramjet paper inspired Sagan’s optimism – here was a seemingly reasonable design which could give a technological civilization the Keys to the Cosmos.

A lot of arguing over interstellar travel, alien life and the Fermi Paradox has happened since then. Can we conclude anything from all the arguments? One positive thing is that interstellar travel can be achieved at relativistic speeds even if interstellar ramjets can’t be made to work. All sorts of beamed-energy designs mean that it’s an unreasonable objection to visits by aliens to claim interstellar travel is impossible. It’s not.

But could it be made even easier than we imagine? One technology that would enable easy interstellar travel – in so far as packing a closed-loop environment or a lot of frozen meals is “easy” – would be total annihilation drives. Frank Tipler’s current formulation of the Omega Point Theory requires the invention of macroscopic sphaleron generating… somethings to annihilate matter and in one version he proposes the conversion of baryons into lots of neutrinos. This would allow the drive to be operated without melting down the local topography with terawatts of gamma-rays – in otherwords it could launch from the surface of a planet, even your own backyard.

Once you’re in space what else is liable to impede one’s progress? Interstellar matter. So turn a problem into a virtue and Bussard scoop the lot into one’s mass annihilator. Thus a ravening proton-storm becomes one’s neutrino-beam to the stars. The Galaxy is yours.

Except… well there is the travel-time issue. The basic equations are well known for constant acceleration flight:

let me introduce B & g (actually a beta and a gamma.) B is the ratio, v/c, where v is your velocity and c is lightspeed, then g = (1 – B²)-1/2.

The travel time, onboard ship, is then…

t = (2c/a)*arcosh(g)

…a is acceleration and arcosh(g) is the inverse hyperbolic cosine (“cosh”) of g, which is ln[g + (g²-1)1/2]. Thus, at large g values, it simplifies to good approximation to:

t = (2c/a)*ln(2g)

…but one variable is missing: distance, S. It’s a part of g i.e. g = (1 + aS/2c²) …notice I’m computing a trip starting and finishing at zero relative velocity to the starting point.

So what does that mean for the people at home? Well Planet-time – as measured by observers at rest with respect to one’s starting point – is computed pretty straightforwardly as T = 2v.g/a. Of course you can expand v and g out in terms of S and a to get…

T = [4S/a + S²]1/2 …or…

T = 2[(S/a)(1 + aS/4c²)]1/2

…or even purely in terms of g & a…

T = (2c/a)(g²-1)1/2

…which shows us that the ratio of the two, t/T, is…

ln[g + (g²-1)1/2]/[g²-1]1/2

which simplifies in the high g limit to…

ln(2g)/g

…so to get one’s experienced trip-time down for a given distance, then you have to hit very high g factors. Now at an acceleration of 1-gee (Earth gravity) the distance travelled to increase g by 1 is c²/a ~0.97 light-years.

To get to high g factors in a short time requires high acceleration. Some fictional examples…

  • Robert Forward illustrated this quite memorably in his book “Timemaster”, wherein the hero has to accelerate at 30 gees for 5 weeks to get to a g factor of 10. Naturally he’s floating in a gee-tank.
  • Stanislaw Lem propels his heros in “Fiasco” to the planet Quinta, some five light-years from the mothership, in 3 months of subjective flight-time at 20 gees.

    • Other examples, as meticulous, are harder to find. Alastair Reynolds is an SF-writing astrophysicist (like Greg Benford before him) and probably is as careful with the time-factors – I’m yet to finish one of his books, so I’ll let you know.

How far does the water’s edge go?

Looking For Extraterrestrial Life In All The Right Places.

Ocean planets are an old idea in SF, but relatively new in the burgeoning field of observational exoplanetology. As the masses of discovered exoplanets goes down, the planets themselves are becoming less like gas-giants and ice-giants and more like bigger, wetter versions of Earth. Scott Gaudi, and colleagues, have studied just what the new “Ocean Planets” are like, with some interesting results.

In this abstract, like the news piece linked above, the interesting finding is just how far out an open liquid ocean can be maintained. If I’m reading the figures correctly, then between 3-5 AU (Asteroid Belt to Jupiter’s orbit) an ocean can persist under the right mix of a hydrogen/helium/methane atmosphere. That’s quite impressive, with all sorts of interesting implications for aqueous life, but not as we know it.

An AGU abstract list expands a bit further… Ocean-bearing planets near the ice line: How far does the water’s edge go? …session P13C-1333 specifically. It discusses denser atmospheres which won’t have liquid oceans – they’d be too hot past ~200 bar because of the efficient retention of internal heat.

Another AGU paper (here) finds that salt, specifically NaCl, will change the melting curves of Ice VI and Ice VII, high pressure polymorphs expected in the icy mantles of large moons and ocean planets. Planets with large water fractions (10-50%) will have large high-pressure Ice mantles, but – as all chemistry students know – the properties of solvents change in the presence of solutes. Salt in water/ice means it will remain liquid at lower temperatures, even under high pressures. With other solutes thrown in, there might be all sorts of weird convection and the like keeping the “ice” from being a static monolithic mass. That’s good news for life on all Ocean Planets.

Especially since statistical analysis of the exoplanets found so far indicates ~30% of Sun-like stars will have “Super-Earths” – likely Ocean Planets… (from here)

What is the frequency of Neptune or rocky planets orbiting
G and K dwarfs? A first estimate based on the HARPS
high-precision survey suggests a frequency of 30 +/- 10% in the
narrow range of periods shorter than 50 days.

Will they be habitable? With such short periods, they’ll be hot around G stars, but late K stars are getting kind of dim. And the HARPS data for planets a bit further out might reveal even more planets in the right place…

Pan-STARRS and the OOC

Normally astronomy discovers new Outer Solar System objects pretty slowly – a telescope stares at a patch of sky for a while, a few days in a row, and anything that moves relative to the “fixed” stars is further analysed. Is it something we already know? is the first question. Then its orbit is further refined… and the telescope has to move on to other things. If what you’ve spotted is something really excited, then you might be able to book some time on the Keck or the HST, but that’s really expensive.

But now something different is watching the sky – ALL of it. At least the bit that can be watched from Hawaii. A large, dedicated telescope is taking all-sky images and monitoring the whole sky over several years. They’re hoping to spot all sorts of odd things, as well as Earth-threatening comets and asteroids. The observation program is called Pan-STARRS and it will, by 2012, have 4 1.8 metre telescopes ganged together to form a single 3.6 metre aperture. From its Hawaiian locale Pan-STARRS will observe about 30,000 square degrees of sky. (there’s 41,253 square degrees total, thus Pan-STARRS covers ~73% of the total sky.) Because it uses a specially designed CCD, of 4800×4800 pixels, it is able to record a relatively large patch of sky down to about magnitude 24, and a whole-sky observation can be made in about 40 hours observing time (typically 4 nights.) The astronomers are hoping to make whole sky observations about 4 times a month.

Here’s the homepage… Pan-STARRS

A consortium of Universities and the like will run the first scope… PS1 Science Consortium …but we’ll have to wait until 2012 for all four to be up and running to full strength.

Being such a new kind of high magnitude observing there’s the potential for a whole bunch of new phenomena to be observed – variables of new varieties, optical transients of all sorts, and who knows what else. From an Outer Solar System perspective the system will discover thousands upon thousands of Trojans, comets, KBOs, Plutoids, Centaurs and various odds and ends. The catalogues of such objects will swell enormously.

But what else? Just how far out is Pan-STARRS going to let us see at a limiting magnitude of 24?

Well “New Scientist” asks “Is there a Planet X?” Pan-STARRS might track it down.

Ian O’Neill asks a similar question at astroENGINE… Where is Planet X? Where is Nemesis?

Ian pointed out a paper by uber-Kuiper Belt guru, Dave Jewitt, from 2004 which gives limits for Pan-STARRS observations of various objects. Interestingly Plutos will be detected down to the M = 24 limit out to 300 AU.

TABLE III
Detectability of distant planets
Planet V (1 AU) R (M=24) (AU) Rgrav (AU)
Earth -3.9 620 50
Jupiter -9.3 2140 340
Neptune -6.9 1230 130
Pluto -1.0 320 N/A

V (1 AU) is the apparent magnitude of the object as seen at 1 AU; R(M=24) is the radius at which the object is seen at magnitude 24; and Rgrav is the radius at which the objects gravity would already be noticed by its perturbations. As you can see Pan-STARRS will cover a huge volume around the Sun, picking up any Jupiter-sized body out to 0.034 light-years.

Lorenzo Iorio has updated the gravitational minimum distance limits based on the motions of the Inner Planets, for which we have the best orbital data. Mars-mass, Earth-mass, Jupiter-mass and Sun-mass objects can orbit around the Sun at radii of 62 AU, 430 AU, 886 AU and 8995 AU respectively, without being noticed in current orbital data. Of course we would SEE a Sun-like star at 8,995 AU as a -1 magnitude star, so it’s definitely ruled out, but red-dwarfs or brown-dwarfs would be much harder to see.

Of course that’s just the Inner Opik-Oort Cloud (Inner OOC) covered for large bodies. Any Earths or Plutos in the OOC will be missed if they’re too dark – Earth and Pluto reflect about 30% and 60% of the incident light, but if they were as dark as some KBOs, down to albedos of just 0.1-0.06, then they’ll be largely invisible. However once Pan-STARRS proves the technology of combining the images from 4 scopes, then virtual apertures of arbitarily large sizes will be possible for anyone who wants to fund such a large scale search in the future. A similar set-up on the Farside of the Moon, or the moons of Jupiter, will eventually allow a very comprehensive survey out into the depths of OOC. Nothing of decent size will be able to hide from prying eyes…