A Stereo View of Lagrangia

Twin Spacecraft To Explore Gravitational ‘Parking Lots’ That May Hold Secret Of Moon’s Origin.

The L4/5 Lagrange Points (“Lagrangia”) are 60 degrees ahead and behind Earth’s orbit around the Sun, thus 150 million kilometres away and not well placed in the sky for observation because of their proximity to the Sun in the sky and the Zodiacal Light obscuring the view. As great big gravitational traps they might retain a few asteroids, leftovers from when the planets were forming. In fact they’re such effective space-traps that the impactor which ran into Earth and made the Moon, might have formed in them aeons ago.

Now the STEREO spacecraft are paying the L4/5 points a visit on opposite sides of the Sun, as part of their mission to watch the Sun’s storms from a better vantage point than Earth’s orbit. About 30 years ago an optical search was performed from Earth for asteroids in Lagrangia, with inconclusive results, but the justification of the search from a paper back then is interesting…

The search for asteroids in the L4 and L5 libration points in the earth-sun system

Abstract

The existence of Trojan-type aggregations of asteroids associated with earth and other planets aside from Jupiter is a problem which has not been fully solved either theoretically or observationally. In this paper, the dynamics of libration orbits in the earth-sun system are discussed, using both numerical integrations and simple theoretical models to outline problems of stability and effects of perturbations. It is found that the greatest potential disturbance to the stability of these orbits is due to the 13:8 synodic Venus perturbation, which resonates with the libration frequencies of most earth Trojan orbits. Problems of observation in searching for any existing asteroids include the distance, phase angle, and wide area of sky coverage corresponding to the possible orbits. The implications of the existence of earth Trojan asteroids for space industrialization based on nonterrestrial materials are discussed, with the conclusion that the short mission times and low required delta-V values to reach and retrieve them make them a potentially important resource.

That’s still true. Inspace bulk materials is a major supply issue for any future space-based industry. Mining the Moon isn’t enough because it’s deficient in light elements due to the temperature of the cloud it condensed out of after the “Big Whack”, so the asteroids – especially ones easy to get to – are vitally important. Lagrangia’s asteroids don’t move (much) relative to Earth and so are continually accessible. Getting there requires a “walking orbit” which doesn’t have to expend a lot of propellant if round-trip times a few months long are acceptable. What’s needed is the will and some forward thinking that thinks Big Enough for establishing a viable inspace materials industry.

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…

The origin of spiral structures of our Galaxy

[0904.4305] The origin of large peculiar motions of star-forming regions and spiral structures of our Galaxy.

Interesting paper by Baba et. al. which looks at how the current reconstructions of the Milky Way’s structure might be misled by certain assumptions. High resolution modelling of the Milky Way’s barred-spiral structure indicates it forms and reforms continually, rather than persisting as a static pattern. But our current reconstructions of its structure assume something more circular and less dynamic. There’s some very interesting graphics from the simulations which compare a synthesised image based on the old assumptions from the actual positions based on the dynamic model – the simulated reconstruction is very close to what I’ve seen in previous papers.

Have a look and a deeper read than me, if you want the full details. I’m a visual thinker and I tend to find explaining the eloquence of a picture difficult to put into words to my satisfaction. The simulated Milky Way just seems right, like a real barred spiral. The stately semi-circular spiral arms of traditional reconstructions are rarer in the Galactic Zoo.

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…

A Big Mercury around Gl 581

ESO – ESO 15/09 – Lightest exoplanet yet discovered.

The European Southern Observatory has used its Very Large Telescope (a bunch of telescopes ganged together to form a huge optical interferometer) to refine the orbits of the known planets around Gliese 581, b,c & d, and to discover a new planet, e, in a 3.15 day orbit. Planet e is surprisingly light-weight, just 1.9 Earth masses, but it’s way too hot for Life-As-We-Know-It. However after newly reining the planets’ orbits Planet d, massing just 7 Earth masses, is now on the outer edge of the star’s habitable zone. It would then be the nearest thing to a habitable planet yet discovered.

A habitable planet? It’s likely too small to be much of a gas giant. It’s probably an Ocean Planet, which would imply – paradoxically – that it’s too wet for much of a biosphere. Maybe. We don’t yet know everything Life can do, but planets wrapped in thousands of kilometres of high-pressure ice (“red hot ice”) aren’t likely to have life-sustaining geology.

But it’s a step in the right direction…

H-Bombs, A-Bombs and a Different tack

The Holocaust Bomb: A Question of Time.

Nuclear weapons are commonly divided into A-Bombs and H-Bombs – “atomic” and “hydrogen” bombs – but aside from that fact, most of us have very little idea about the mechanics of the bombs, and their consequences. As a kid I was fascinated with the concept and was delighted to find some descriptive diagrams in an old “Time-Life” book on “Energy”. So for years I “knew” that ‘H-Bombs’ actually used lithium deuteride as their fusion fuels – specifically deuterium, “heavy hydrogen”, with the lithium being a fission/fusion metal (Lithium-6 + a neutron becomes deuterium and tritium, which can then fusion) to make a solid hydride out of. Lithium can also fuse by itself, but in this case it doesn’t. After learning that ‘secret’ I never thought much about it.

As a teen – especially after “The Day After” – I had dreams about nuclear war. It was a fear that I lived with all through my later childhood, but not one I entertain much ever since the old “Cold War” ended in 1991. But its a possibility that we all live with as Islamic radicalism makes gains in a nuclear nation like Pakistan, or as old Soviet arsenals rot in newly Islamic breakaway states.

Could we make the world a safer place by making the nuclear “secrets” actually public knowledge, thus making the whole nuclear exercise pointless as a means of technological superiority? Howard Morland, who gave us the “H-Bomb Secret” back in 1979, thought so and still thinks so. One point from the little we know is that such bombs are not easily made. They require state-level stability that groups like the Taleban and Al-Qaida are unlikely to sustain. The real nuclear threat comes from those who presently have the damn things, not those who want them.

Finally, Friedwardt Winterberg has mentioned the deuterium bomb as an example of a doable fusion reaction of immediate usefulness to spaceflight. He’s right – it’s a good reaction in a rocket, but damned horrible in a bomb…

All-Seeing Eye…

Optics and Materials Considerations for a Laser-propelled Lightsail.

A particularly striking statement…

As a note in passing, the 1000 km lens proposed by Forward, with the pointing accuracy he requires, would make an extremely high magnification telescope [12]. Clearly, a lens focusing to a spot of 100 km at a distance of 4 ly would allow not only planets of nearby stars to be detected, but they could be mapped with a resolution of 100 km. This was the original intention of O’Meara in proposing the paralens. The diffraction-limited beam spread of a 100 km lens for visible light (say 500 nanometer wavelength) would be 0.5 nanoradians. With this resolution, it should be possible to detect Earth-sized planets at a distance of thousands of light years, and Jupiter-sized planets virtually anywhere in the galaxy.

…thus if you can launched a laser-sail at the stars you can also see where the sail is going in some detail too.

The Crowded Earth

Inderscience Publishers – Article.

Just how many people can Earth be made a home to? If environmentalists and “Greenpeace” are to be believed, you might think “less than what we’ve got now”, which based on current extractive industry and hit-and-miss agriculture would be true. But what if our industry and agriculture weren’t so messy and inefficient? The abstract linked to above estimates a population between 300 million million to 4,000 million million (i.e. 300 trillion to 4 quadrillion) is sustainable albeit with herculean heat-dumping systems throwing excess heat into space in the extreme case.

One wonders just how much room there would be for other life on Earth. But it’s worth remembering that the current biosphere only captures and uses a tiny fraction of the 120,000 terawatts Earth gets from sunlight. With Life covering every bit of the planet in super-abundance, then the extreme Biosphere could be much larger, even if it was only humans and their symbiotic organisms. That “wildness” would be of necessity banished from such a world is one factor to ponder when considering human options for the future.