Friday, May 07, 2004
Modern Laplacian Theory
Forming planets might seem like a solved problem. With all the whizz-bang results of
computer modelling that hits the science news magazines that would be an easy mistake, but a number of puzzles aren’t answered by the
popular accretion models – the biggest is how the planets got most of the solar system’s angular momentum.
If the Sun and the planets formed from the same cloud of gas and dust, then why does the Sun spin so slow? How did it lose angular momentum to the planets?
One theory – a peer amongst many – is Andrew J.R. Prentice’s Modern Laplacian Theory. He has a large list of articles, which can be found by searching NASA’s abstract service here…
http://adsabs.harvard.edu/ads_abstracts.html
…which is a website well worth getting to know for astrophysics papers, especially old ones.
If you do a search you get big hypertext links and so doing your own search is best. When you see
the search form you will understand, but all you have to plug in is the name and tick subject boxes, astronomy and geology.
Basically the MLT revives Laplace’s original idea of a collapsing proto-Sun throwing off rings
of gas and dust to lose angular momentum. The rings become planets through accretion
and the Sun ends up as a slow rotator. Prentice updates Laplace’s chemistry and physics
and provides a means for the proto-Sun to collapse as a rigid disk-like nebula. He
originally imagined needle-like filaments of high speed gas rising and falling within the
proto-Sun to help it rotate rigidly – spokes for the wheel. However recent simulations are
suggesting that mere convection could do the job, with gas/dust rising to the rim slowly, and
then falling quickly – supersonically – in narrow channels once cool.
Prentice’s theory allows for the various molecules that make up each ring to form out of the
initial gas at increasing temperatures as the Sun collapses. This allows him to make precise
predictions about chemistry throughout the Solar System – and usually he is right. When
he is wrong the discrepancy is often due to poor initial assumptions or poor data on the chemistry
involved. The MLT itself has survived every test, but the basic mechanism is still in question.
Michael Woolfson has also pointed out a few flaws in the MLT, but check out the results and decide for yourself.
Posted at 11:50 pm by Adam
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Plasma Magnet II…
Solar-sailing is a smart way to travel between the planets because it uses energy that is available for free – the power of the Sun. But the light-pressure of the Sun, near Earth, is very low. Hence solar-sails are immense for decent accelerations and sail-material incredibly thin. so Robert Winglee, and colleagues proposed using the Solar Wind, and riding it. But the initial design wasn’t quite right, hence John Slough proposes a new system here…
http://www.niac.usra.edu/studies/study_template.jsp?id=860&cp_num=02-02&phase=I&last=Slough&first=John&middle=null&title=The+Plasma+Magnet&topic=Advanced+Propulsion&sub_topic=null&organization=University+of+Washington&active=false&begin_date=Oct+1%2C+2003&end_date=Mar+31%2C+2004
…that’s a big link, but the main bit to watch for is NIAC, from which you can find the file in question. Basically Slough is proposing using a fluctuating plasma to generate a large magnetic field, that is ultimately pushed on and shaped by the solar wind, or the magnetosphere of a planet. The chief advantage of this system is that the plasma bubble created changes size as the wind pushing against it weakens, growing to compensate for the lessening wind and so providing constant thrust.
Such intense plasma fields can be generated – Slough describes an actual experiment in a vacuum chamber with a plasma source providing the “wind” analogue. What remains unknown is whether the plasma field then transfers the forces acting on it back to the plasma source [i.e. the spaceship.] If so then all sorts of possibilities open up. Sufficiently powerful plasma generators can ride the solar wind up to its maximum speed ~ 300 – 800 km/s.
What I want to know is if the plasma can act as a magnetic sail under an even stronger plasma wind – a relativistic particle beam stream. If so then it enables interstellar travel – firstly, it provides a way to accelerate vehicles to very high-speed (~ 0.3-0.5 c) and secondly, it can act as an interstellar brake and intra-system drive at the stellar destination. Particle accelerators are far more efficient than lasers, but the beam diverges rapidly – which isn’t a problem for the plasma magnet.
Interstellar travel – and even Out-Planet missions – could take years, with massive requirements of closed-cycle biospheres and the like. So naturally enough many sci-fi writers use the device of suspended animation. Now it seems its actualisation isn’t far-off, but is on offer as a medical procedure, at least partly. Here’s a Wired article on the latest biomedical process for suspended animation of organs on offer…
http://www.wired.com/wired/archive/12.05/biotime.html?tw=wn_tophead_6
…here’s a copy of the article (already!) on rense…
http://www.rense.com/general52/feeze.htm
…the company offering the service, BioTime, is currently putting patients (albeit, animal models, though close to humans) into a deep hypothermia, to basically suspend biological processes and stop blood-flow. Next move is using a cryoprotectant to freeze animals, brain-dead organ donors and, maybe, live patients once their technique is verified and the FDA lose their jitters.
No surprises that the company’s directors are Cryonics believers – seems their efforts might be starting to pay off. Personally I doubt the current corpsicles awaiting resurrection will ever be recovered, as ice crystals will have mashed their neural structures, but if the cryoprotectant pans out that will change. Then will we see a run of Cryonauts trying for a fast-forward?
Posted at 11:31 pm by Adam
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Thursday, May 06, 2004
Oort Cloud and Beyond…
Far out in space is an ellipsoidal “cloud” of comets that orbits our Sun – far, far beyond the visible range of telescopes and yet somehow we “know” it is there. How? Chiefly by what falls Sunwards from it – long-period comets. Any comet on an orbit that takes more than ~ 200 years is classified as a long period comet. In the late 1940s Jan Oort, and others, noticed that most long-period comets have orbits that cluster on a certain range of semi-major axes, just like they had fallen from that “height” after being drawn out of orbit by passing stars or tidal forces from the Galaxy itself. Hence Oort postulated a diffuse Cloud of comets around the Sun, ranging from 50,000 AU to 110,000 AU [1 AU being the Earth-Sun distance.]
http://www.ifa.hawaii.edu/~jewitt/oort.html
Further studies of long-period comets and simulations of their orbital evolution has determined the Cloud is a bit closer in [out to about 44,000 AU] and perhaps extending inwards as a flattened disk. So where did they come from? Oort, and more recently astronomer Tom van Flandern believed that both comets and asteroids come from an exploded planet, or planets, but no one has thought of a convincing way to blow up a planet. Good simulations of formation of the big planets, Jupiter and Saturn suggest these flung huge amounts of unaccreted material out as far as the Oort Cloud, and passing stars perturbed this debris into circular orbits – the reverse of delivering comets back to the Sun.
http://www.ifa.hawaii.edu/~jewitt/kb/migrate.html
Uranus and Neptune are suggested as the driving force for another reservoir for comets – the Kuiper Belt – which has been in the news lately since very large (~ 1000 km diameter) cometoids have been found orbitting in it. And beyond, since the very latest find – Sedna – is believed to be from the inner Oort disk we mentioned. On Sedna’s current orbit most of its orbital revolution is spent far beyond the Sun. If other “Sednas” exist then there will be more invisible than visible to our current telescopes. If Sednas range between 50 AU and 1000 AU, and are visible to ~ 100 AU, then about 30 times as many are invisible. That’s a lot of potential mini-planets waiting to be found.
This is even more so for long period comets. Most spend about ~ 3.3 years this side of Jupiter where they become visible. An average orbit out to 44,000 AU takes ~ 3.3 million years. Hence long-period comets are visible for about ~ 1/1,000,000 th of their orbit. If comets are spread at even time intervals along their orbits [an assumption of “randomness”] then about a million invisible comets exist for every visible one – and that’s on Sunwards orbits.
What happens to comets over the long term? The International Astronomical Union maintains various lists of minor-planets and comets. One particular list of interest here is “miscellaneous objects” – minor planets on odd orbits that no one has an agreed category for. Here it is…
http://cfa-www.harvard.edu/iau/lists/Others.html
… if you browse the list you will notice a lot have an aphelion, Q, [maximum distance from the Sun]value between 3 – 5.2 AU – i.e. this side of Jupiter’s orbit. This is the sort of orbit Jupiter captures long-period comets in to. Alternatively, comets can be boosted by Jupiter into escape orbits, never to see the Sun again. Hence long-period comets are either lost or they become “Jupiter-family” comets, which eventually wither away as the Sun sublimates off their ices.
An inactive comet – one with no exposed ices – would look a lot like the “miscellaneous” minor planets listed by the IAU. What remains is the rocky component of the comet, but also ices protected by a crust of rocky debris mixed with radiation-processed organic matter, or “tar”.
http://www.ifa.hawaii.edu/faculty/jewitt/irradiation.html
…this tarry dust coat is actually a lot like asphalt. Hence dead comets are a potential source of crude-oil like material, plus the ices they wrap.
Posted at 11:53 am by Adam
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Thursday, April 29, 2004
Slowboat to LEO
Seems Edgar Alan Poe’s idea about slowly climbing to the Moon might work. Partly. JP Aerospace have
detailed concepts for a slowly ascending airship that uses the residual
atmosphere above 140,000 feet (~ 43 km) for lift and electric propulsion to
get to orbital speed over 5 days. They presented the concept at Space Access
2004, as seen here…
http://rocketforge.org/twiki/bin/view/Main/SpaceAccessPictures
…there are links to the individual pics at the bottom of the page. What
makes JP believeable is their extensive experience with high-altitude
vehicles – the USAF believe them enough to fund full-scale airships, as seen
here…
http://www.jpaerospace.com/index.html
…the USAF are pursuing “mesosphere” vehicles and statites
(“aerosatellites” or aerosats) – a “gentler” alternative to barging into
orbit on a multi-gee fire-cracker. William Gibson in one old short-story
posited using aerosats as an alternative to GEO-based solar power sats and
had people launching to LEO from there. Maybe he was too conservative?
Is there more to Poe’s original idea? Poe’s story, “The Unparalleled Adventures of Hans Pfall”, has the narrator ride to the Moon in a balloon – he artfully argues for a medium between the Earth and Moon, even if it is very diffuse, based on observations of comet tails i.e. the Solar Wind. While a regular balloon won’t work a plasma balloon will, by catching the Wind…
http://www.space.com/businesstechnology/technology/m2p2_winglee_010621.html
So how cool is that?
Posted at 6:14 pm by Adam
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Friday, April 23, 2004
Habitable planets…
Darren Williams has a job that I envy – he gets to study habitable planets around other stars. Or at least he gets to study carefully constructed imaginary ones, since no Earth-mass planets are known around other Main Sequence stars [yet!] He has written extensively on a variety of alien worlds – specifically…
1. Planets with high obliquities – more tilt to their rotational axis than Earth. Older climate modelling suggested that such planets would have very nasty climate swings, but recent modelling using full Global Circulation Models suggests something radically different to our expectations – low obliquities are actually nastier. What happens is the cold and nastiness of the polar regions actually reaches down to lower latitudes because seasonal variation is what keeps the atmospheric heat flow pumping.
2. Planets with high eccentricities – Earth’s orbit around the Sun is nearly circular, but it needn’t be. More elliptical orbits are possible, and seen on many of the known planets around other stars. Traditionally eccentricity was considered to be a bad thing, but once realistic climate modelling was applied to the situation that perception has changed. Large oceans actually act as climate regulators keeping conditions reasonable up to ~ 0.4 eccentricity. For Earth that would mean ranging from 0.6 AU out to 1.4 AU – from inside Venus’ orbit out almost to Mars. For very extreme eccentricity, 0.7 in the simulations, the planet was only bareable if the Sun was cooler – the equivalent of Earth being moved beyond Mars. Around the poles are fairly large region was habitable.
3. Earth-like moons – many of the exosolar planets are Jupiter-like but much closer to their stars. So would their moons be habitable? The main constraints are related to the mass of the moon – can it hold its air? At about 20% bigger than Mars this was easy, but potentially the planet would die once its plate tectonics quit, like on Mars. But using Jupiter’s moons as analogues we know that vulcanism can continue for aeons on small moons if they “pump” each other’s eccentricities – like Europa and Ganymede do to Io. Hence a moon with maybe ~ 0.07 Earth’s mass could remain viable for aeons, though it might need to be ~ 0.12 Earth’s mass to keep its nitrogen.
.Another question being actively researched is the width of the Habitable Zone – the minimum and maximum orbits for habitable planets. For Earth-like planets this might be really tight since a planet gets chilly without lots of carbon dioxide, and can get boiled if too close to the Sun. Maybe.
But research by Mark Bullock and David Grinspoon is suggesting that Venus retained its oceans until relatively recently – perhaps a mere billion years ago, some two billion longer than most researchers have previously thought. So back then the habitable zone was a LOT closer to the Sun. Since then the Sun has brightened by ~ 10%, which means the inner orbit is now ~ 0.75 AU.
But how far out can a habitable planet then go? If we relax the low CO2 constraint the outer edge is roughly 1.4 AU – past that and the CO2 alone won’t keep the planet warm. How much is present? Thanks to the silicate-weathering cycle about ~ 2.1 bars of CO2, which would be lethal to us, but perhaps liveable with the right physiology?
However this implies Mars was never warm enough for liquid water – yet as the NASA Rovers have dramatically shown this is incorrect. So how did Mars keep warm???
Posted at 11:24 pm by Adam
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Recent musings…
Semester I, 2004 has meant I haven’t been blogging as much, but I have been reading aplenty. Some is course work – lots of physics stuff. I made the mistake of thinking my Physics subject’s textbook was a push-over but it has proven to be a deeper treatment of the concepts involved in classical physics than I initially thought. That’s cool.
Then there’s Cosmogony – big word, which really means “how it came to be”, though in Astrophysics it means how the planets and stars were made. The current major players are accretion theories and disk-instability theories, but an earlier idea by James Jeans in the 1920s and 30s had a revival in the 1960s – the Capture theory. Basically the Sun disrupted a passing protostar in their common birth nebula, drew off a long filament of material, which collapsed into protoplanets. Some were captured by the Sun and became the major planets, while others escaped the Sun and became free-floating masses.
But what of the terrestrial planets? The protostar disruption placed a disk of gas and dust around the Sun, which acted as a brake on the major planets as they fell into orbit around their new primary. This caused them to round off their initially massively elliptical orbits, and also caused fierce precession. Two planets had a near-miss which flipped one of them onto its side [Uranus], while the two inner-most planets collided. One was severed into two main masses and debris, while the other was ejected from the Solar System. The debris became asteroids and comets, while the core material circularised and formed two rock/metal planets – Earth and Venus.
A quick summary…
http://www.planetarium.btinternet.co.uk/Society/solar_system_model.htm
A recent paper on prevalence of planets formed via the theory…
http://ast.star.rl.ac.uk/symp202/posters/woolfson.html
And a glossy version (in PDF) that summarises the theory…
http://www.oso.chalmers.se/~michael/astrobiologi-2002/j.1468-4004.2000.00012.x.pdf
Another topic has been terraforming Venus. A much tougher proposition than originally believed at the start of the modern debate – Carl Sagan’s 1960 paper that was the first to explicitly tackle the hot Venus we know today. However at the time the atmosphere was thought to be only a couple of bars and the clouds pure steam, with the surface about ~ 650 K. Now we know the real Venus has 92 bars of atmosphere (~ about 100 times ours, allowing for gravity) and the clouds are hydrous sulphuric acid, while it’s even hotter at 730 K.
http://nssdc.gsfc.nasa.gov/planetary/factsheet/venusfact.html
Because Venus is so dry seeding its clouds with algae won’t work – photosynthesis needs water to turn carbon dioxide into oxygen and sugar. And there’s too much CO2, which would make too much oxygen. Venus’ real problem is not a lack of oxygen. When its oceans – yes it had them – boiled and the Sun’s UV destroyed the water, thus allowing hydrogen to escape, Venus had maybe a hundred bars of the stuff. What it really needs is the missing hydrogen. But where from?
Posted at 4:08 pm by Adam
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Sunday, April 04, 2004
Notable Nearby Stars
SolStation is an excellent information site on other star-systems and their planets – present or possible.
http://www.solstation.com/stars.htm
…this sub-page covers a good range of near-by stars and their systems. With the re-release of Gregory Benford’s “Across the Sea of Stars” some of the target stars of the Lancer mission can be found here…
http://www.solstation.com/stars/ross128.htm [home of “Pocks”]
…and here…
http://www.solstation.com/stars/la21185.htm [home of Ra and Isis]
…interstellar travel, even with a 1-gee ramscoop-drive like the Lancer’s, is a long-duration exercise. Lalande 21185 is ten years away, while Ross 128 – the next target system – is eight years more. Even relativity [“time squeeze”] doesn’t bring those figures down much for the crew [four-and-a-half years to Isis and four years to Pocks.]
Lancer itself is a huge hollowed asteroid starship, fitted with magnetic ramscoops and a huge catalytic fusion drive. To sustained a 1 gee fusion burn, with an exhaust velocity of ~ 0.05c – about 15,000 km/s – requires an immense power level per kilogram of starship – some 73.5 megawatts, running continuously for year after year. If the Lancer masses ~ 500 kilotons, that’s 36,750 terawatts of power
– which is only 2450 times the whole human race’s current energy useage.
It’s a huge task to get between the stars using rockets and similar reaction drives. Is there anything better?
Posted at 10:56 pm by Adam
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Saturday, March 27, 2004
Deep Future II
Deep time is a theme in countless science-fiction stories – most of which have been made quaint by advances in our understanding of cosmology, stellar physics, biology and so forth. In H.G.Well’s classic the Time Traveller voyages thirty million years hence to see a bloated, red sun over a chilly Earth. An updated vision of the end would be over twenty times further away in time and hotter than the present day. Or would it?
The Sun will evolve away from stability once its Core exhausts its fusion fuel – but an additional 100 billion years fuel supply will still reside in the rest of the Sun. However the Sun’s Core doesn’t mix very well – if at all – with the upper 80% or so of the Sun’s mass. But what if humans could change that? We could use immense magnetic fields or gravity beams of some sort to induce mixing and perhaps give the Sun a new lease on stable life.
Easier said than done, but in the billions of years ahead perhaps advances in knowledge and control over matter will reach astrophysical levels. If so what about right Now? Are there extraterrestrial intelligences [ETIs] billions of years ahead of us? Have they remodelled the stars and galaxies themselves?
According to Charles Lineweaver’s research the average Earth-like planet is some 1.8 billion years older than our Earth…
http://au.arxiv.org/abs/astro-ph/0012399
…implying that ETIs are on average older than us.
However – as we have noted – there’s little indicating they’ve visited the Solar System. But can their activities be discerned in the night-sky? If they’ve wrapped their stars in light-absorbing habitats we can see the infra-red leaking from them. If they’ve wrapped a Galaxy of stars in such, then we could see that too.
So what’s stopping ETIs from doing so? Here’s one possibility…
http://au.arxiv.org/abs/astro-ph/9901322
…massive stellar explosions called Gamma-Ray Bursts [GRBs] could conceivably decimate large parts of a Galaxy. Their current frequency appears similar to the time-span for ETIs to have evolved and expanded to fill the Galaxy – but this might only be a recent occurrence, a phase-transition from a lonely Galaxy to one filled with life. Might be.
Posted at 10:08 pm by Adam
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Tuesday, March 23, 2004
Orbit Equations – Good links
Brief note…
For conics here…
http://www.gdnordley.com/Kepler.doc
For low-thrust spiralling orbits here…
http://www.rmc.ca/other/usn/ionsupp/ionsupp.htm
…which gives the basics and some very useful graphs. For ultra-low thrust (1/10,000 local gee) the dV is just the difference in orbital velocity between start and finish orbits. To escape that means dV = Vo, your initial orbital velocity.
Posted at 12:13 pm by Adam
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Kuiper Belt II: the Oort Cloud
Recently a mini-planet, Sedna [2003 VB12, until officially named by the IAU], has been found on a big orbit unlike any other Kuiper Belt denizen known…
http://www.ifa.hawaii.edu/faculty/jewitt/kb/sedna.html [Dave Jewitt on Sedna]
http://www.gps.caltech.edu/~mbrown/sedna/ [Mike Brown, co-discoverer]
… contrary to some media bites Sedna is not an Oort Cloud comet in a strict sense, but its position puts it between the Kuiper Belt/s [classical and scattered disk] and the Oort Cloud proper. It could be an Inner Oort Cloud Object. Its closest approach to the Sun is 76 AU, almost twice the average distance of Pluto, and about 30 AU beyond the classical KBOs. The Scattered Disk objects reach out as far as Sedna’s maximum distance of 942 AU, but have far more orbital energy. Sedna, and future objects like it, is a long way out – probably scattered by the orbital evolution of Uranus and Neptune like the other comets out in the Deep Dark…
The IAU’s Minor Planet Centre is a cool source for all known comets, asteroids, KBOs and odds and ends…
http://cfa-www.harvard.edu/cfa/ps/mpc.html
…the KBOs, Centaurs and Scattered Disk are here…
http://cfa-www.harvard.edu/iau/lists/TNOs.html [TNOs… or KBOs]
http://cfa-www.harvard.edu/iau/lists/Centaurs.html [Centaurs and SDOs… Sedna isn’t really an SDO but it fits nowhere else currently]
So what is Sedna like? If we assume a density like Pluto ~ 2.1 and a radius of ~ 1500 km, then its gravity is a mere 4.5% of Earth’s and its escape velocity just 813 m/s. With albedo of 0.25 at its current position its global average temperature is a mere 27.3 K, and at the sub-solar point it’s just 38.65 K.
Sedna is just the first deep space object – many, many will be found. Stay tuned!
Posted at 11:54 am by Adam
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