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Notes & Queries 10/15/13

Starship Century in London

Starship-Century-final-cover

A new Starship Century Symposium will be held at the Royal Astronomical Society, Piccadilly, UK on Monday October 21st. If I hadn’t exhausted my travel budget by September, I would definitely have this one on my agenda and follow it with a week or so in my favorite city. Here’s the information I have on the event from its organizers, James and Gregory Benford:

Starship Century addresses the challenges and opportunities for our long-term future in space, with possibilities envisioned by featured speaker Lord Martin Rees, Royal Astronomer, Ian Crawford, Birkbeck College, University of London, writer/scientist Stephen Baxter, James Benford, Microwave Sciences, and Gregory Benford, UC Irvine. Starship Century discusses the implications that these explorations might have upon our development as individuals and as a civilization.

Agenda

  • 10 am Starship Century: Toward the Grandest Horizon James & Gregory Benford
  • 10:30 Scientific Benefits of Starships Ian Crawford
  • 11:30 Contact at Alpha Centauri Stephen Baxter
  • 1 pm -Break for lunch-
  • 2 pm To the Ends of the Universe Lord Martin Rees
  • 3:30 Exploring Interstellar Space Panel with Lord Martin Rees, Ian Crawford, Stephen Baxter, Jim & Greg Benford, others TBA
  • 5 pm Symposium ends.

Proceeds from the sale of the Starship Century book go toward interstellar studies. Admission to the event is free.

Project Icarus Workshop

Yet another reason to get to the UK is the latest from Project Icarus, the continuing effort by Icarus Interstellar to produce a fusion starship design growing out of the original Project Daedalus work in the 1970s. Here’s what I have on this one from the Icarus team:

Project Icarus is presenting the results of our preliminary fusion-rocket based interstellar spacecraft at the British Interplanetary Society in London on October 21 and 22nd, 2013.

We are asking for support in raising $2000 to support our volunteer researchers and students attending the workshop. All donations made to Icarus Interstellar through October will be channeled to support the “2013 Project Icarus Design Competition”. FOUR (4) breakway teams are presenting variations of spacecraft designs and mission configurations which will be presented during an internal workshop on the 21st, followed by a public symposium on the 22nd of October.

In appreciation of your support, donors will receive:

  • $10 Your name listed in the acknowledgements of the final publication.
  • $20 Icarus Interstellar Lapel Badge (and above)
  • $50 Icarus Mug and t-Shirt (and above)
  • $100 Advance copies of the INTERNAL Spacecraft Design studies (and above)
  • $101 SPECIAL donations over $100 will receive an exclusive media pack containing HIGH RESOLUTION SPACECRAFT ENGINEERING DESIGNS rendered expertly by Adrian Mann (www.bisbos.com) (including all of the above!)

The exclusive content found in the INTERNAL Project Icarus Design Studies is the result of thousands of hours of research by our international volunteers. This is the FIRST TIME Icarus has shared full spacecraft designs. You can be among the exclusive few to explore our work first hand!

Icarus Interstellar the World’s Largest 501c(3) Deep Space and Interstellar Exploration Research Organization. All donations are tax exempt and deeply appreciated.

We are volunteers doing this work because it needs to be done.

Lets Build a Starship together!

Icarus Design Workshop

White Dwarf Planets

My interest in planets around small stars is probably evident from the amount of space I spend on M-dwarfs and their planetary systems. Only recently have white dwarfs come into the picture for me, but as we’ve seen, Mukremin Kilic (University of Oklahoma) and colleagues have been discussing how to re-purpose the wounded Kepler observatory to look for transiting planets around such stars. About a year ago we also looked at Luca Fossati (The Open University, UK) and his work on the possibility of habitable white dwarf planets in exceedingly fortuitous orbits.

Now I have an email from science fiction writer Randy Blackwell, who has been working on a novel which raises interesting questions about habitable planets around white dwarf stars. Blackwell was looking for more details about possible conditions on the surface of such a world and I didn’t have the answers, so I thought it best to float his questions past the Centauri Dreams readership, many of whom have solved previous astronomical questions raised here.

Here’s Blackwell’s message about the scenario depicted in his novel:

The main characters find themselves on another planet. It is a water surface planet. The planet orbits a white dwarf. My complication is figuring out what the sky would look like by day and by night from the planet’s surface. Would there be an aura due to the great magnetism of the white dwarf? Would a dust trail be visible? If the desirable distance and size is 1 million miles at same size as Earth (both planet and dwarf) then what size would it look like in comparison to our Sun in the sky of the planet. Would it look closer or further? Would it be darker or more light with a 10 hour rotation?

I’m curious to see any answers to the questions Blackwell raises.

WhiteDwarf

Image: A white dwarf as compared with the Earth. Credit: Ohio State University/Richard Pogge.

Deep Time in Aeon Magazine

A quick nod to Colin Dickey and his fine essay in Aeon Magazine on the Svalbard Global Seed Vault, which I actually encountered first in a post on the Long Now Foundation’s site. Looking at the lessons of geology and deep time, Dickey refuses to play the catastrophe card as a driver for long-haul projects like these. Calling the lessons of Svalbard “more complex than the simple, immediate apocalypse intimated by the hype surrounding the seed vault,” he goes on to say:

This recognition of the work of seed banks like the one in Svalbard is quotidian, bordering on the banal, and it can help to refocus an attitude towards the environment that sometimes verges on the self-important. A proper relationship to nature must involve a sense of stewardship, to be sure, and a willingness to work for a better tomorrow. But it might also do well to be stripped of a histrionic sense of perpetual catastrophe. Places such as Svalbard can help us to think on a much longer, deeper scale — one in which we are peripheral characters in a drama taking aeons to unfold.

In other words, a proper relationship between ourselves and the natural world is thoughtfully long-term whether or not global catastrophe ever forces us to rely on such repositories.

tzf_img_post

Comments on this entry are closed.

  • steven rappolee October 15, 2013, 12:09

    what would be the stars the Roche limit?

    tidal locking?

    radiation it seems would argue for life under the surface of this worlds ocean, so the world ocean is a good pick here!

    is this a warm greenhouse world ? if so we might not have a troposphere but thunder clouds dozens of miles high, but then again its these that protect us from this angry star,
    the warm green house will evaporate the ocean down to surface mountains where the warm green house might be “arrested”we need to find that paper on this subject that made an appearance here a while back

  • steven rappolee October 15, 2013, 12:24

    its not clear how I can donate over $100 but less then $1000 on the starship congress website and to specify the donation is for the 21 October event

  • Paul Gilster October 15, 2013, 13:21

    steven, I see your point. I checked at:

    http://www.icarusinterstellar.org/icarus-fusion-spacecraft-takes-shape/

    and it does appear the choice jumps from $100 to $1000 at the next level. I’ll see if I can get one of the Icarus team to check this.

  • Paul Gilster October 15, 2013, 16:14

    steven, just heard back from Icarus. I’m told you can select ‘other’ from the options on their site and then, after hitting ‘donate,’ you’ll be given the ability to enter an amount. But my source warns that there may be a browser issue here if it doesn’t work. If it doesn’t, let me know.

  • Adam October 15, 2013, 17:06

    In answer to Randy’s questions, firstly the apparent size of a star giving the same energy input (insolation) as the Sun depends on its temperature – more strictly the inverse square of the temperature. Thus a star half as hot will appear 4 times as wide, while a star that is twice as hot will appear a quarter the size. To avoid excessive UV a habitable planet hosting white dwarf should be about as hot as the Sun – somewhere between 6000 and 4000 K. This is also the temperature of the oldest white dwarfs.

    Due to the extreme proximity to a white dwarf the orbit of a planet will be circular if it is habitable – eliptical orbiting will mean extreme tidal forces and quick circularisation, while undergoing extreme vulcanism. Note: extreme.

    Another factor to consider is that any asteroid impacts will be very energetic due to the high orbital velocity. Thus expect some epic cratering.

    Look for the original white dwarf habitable zone papers by Eric Agol for more details.

  • Eniac October 15, 2013, 21:10

    If the desirable distance and size is 1 million miles at same size as Earth (both planet and dwarf) then what size would it look like in comparison to our Sun in the sky of the planet. Would it look closer or further? Would it be darker or more light with a 10 hour rotation?

    All of this depends on the surface temperature of the star. This ranges widely for white dwarfs between much hotter than the sun and quite a bit cooler. It stands to reason that the hottest stars would cool too quickly to leave enough time for life to develop, so we would have to look at the cooler ones.

    A star with the same temperature as the sun (6000K) would look the same color, and also the same size if viewed at the distance where it gives the same light, i.e. in the habitable zone. A hotter star would look whiter and smaller, a cooler one would be redder and bigger. The light level would be about the same as on Earth. All in all, I think it will be hard to find any difference between white dwarfs and normal stars in this respect.

    However, the orbit would be a LOT faster, and tidal forces would be far greater, most likely leading to a significant deviation from the spherical shape for the orbiting planet. Tidal locking is inevitable if that is the case. Also, the orbit would have to be very circular, otherwiset he planet would undergo periodic deformation leading to violent quakes and excessive heating of the crust. Magnetic fields may or may not be noticably different. Note that the aurora on Earth is caused by Earth’s magnetic field, not the sun’s.

  • Scott Gordon October 15, 2013, 21:11

    A white dwarf with radius 5800 km at 1,000,000 miles (1,609,000 km) fills an angle in the sky of 2*arcsin(5800/1609000) = about 0.41 degrees. By comparison the Sun (and coincidentally also the moon) appears as about 0.53 degrees on average from Earth. Hence the white dwarf will appear about 20% smaller in diameter than the Sun (or moon) does from Earth.

    Presumably it has to be brighter by about 66% to produce the same warming effect as Earth receives due to smaller apparent size in the sky (depending properties of the planet of course) – or the planet needs a fairly effective greenhouse effect.

    Doing the calculation in reverse : the white dwarf would need to be 120 times closer to the planet than Earth is to the Sun, to appear the same size in the sky (works out to be about 1,250,000km or 0.78 million miles).

  • Eniac October 15, 2013, 22:13

    steven rappolee:

    radiation it seems would argue for life under the surface of this worlds ocean, so the world ocean is a good pick here!

    is this a warm greenhouse world ? if so we might not have a troposphere but thunder clouds dozens of miles high, but then again its these that protect us from this angry star,

    As I understand it, white dwarfs have fairly pure blackbody radiation output, making them no more or less “angry” at HZ distance than any other star. Less, perhaps, because apparently there is no convection going on within them, as there is in the sun, presumably leading to much less surfae activity. Even if there was excessive radiation, a normally dense atmosphere would care of this completely, water is not really necessary.

  • Eniac October 15, 2013, 22:22

    For space travellers, it would seem the steep gravity well would pose a huge hurdle when trying to travel into the outer system or into interstellar space.

    However, there is also an opportunity from the star’s enormous escape velocity. It would enhance the opportunity for “sundiver” style light-sail missions, where you would fly by close around the star to use its intense light, and perhaps also a rocket engine, to accelerate deep within the gravity well, taking advantage of the Oberth effect to reach many times the end velocity you otherwise could. Perhaps this would be enough to make up for the above penalty, perhaps not.

  • Michael October 16, 2013, 2:33

    I would expect a light show on the dark side of the planet due to significant amounts of dust that normally surrounds White Dwarf stars. The Sun facing side would be exposed to a lot of U.V with a white light. I am not sure how scattering effects would affect the sky colour though, most likely a deep blue. The day and night would be exposed to a large amount of metoers, the night side would be quite a sight to watch.

  • Adam October 16, 2013, 7:02

    An Eric Agol paper to look at: Finding habitable earths around white dwarfs with a robotic telescope transit survey which has some useful summarising of the main details of white dwarfs with Earth-friendly conditions.

  • andy October 16, 2013, 15:35

    The star would probably appear white even when it is cooler than the Sun, though the size/surface brightness would depend on temperature. You have to get to pretty cold stars (probably equivalent of a mid-L dwarf) before it stops putting out enough blue light for your eyes to compensate, and I’m not sure that white dwarfs, other than extremely low-mass ones produced by mass transfer in binary systems have had time to cool that far.

    Consider: how hot is a filament lightbulb, or burning magnesium?

  • Eniac October 16, 2013, 20:35

    andy:

    The star would probably appear white even when it is cooler than the Sun,

    How is this possible? Are you saying the spectrum is not blackbody? Why not?

  • Adam October 16, 2013, 21:20

    Eniac, it’s due to the limitations of the human visual system – everything hotter than about 2000 K is “white hot” to our eyes.

  • Alex Tolley October 17, 2013, 10:07

    @Adam I am not clear on this temperature point either.

    Om the one hand, what you say is confirmed by the color temperature of LED bulbs, where 2500K gives a “soft white” color, rather than a red. On the other hand, we see red giants like Betelgeuse as red. Is this something to do with intensity, rather than the emission spectrum?

  • andy October 17, 2013, 13:55

    Betelgeuse isn’t sufficiently bright that your brain is able to adjust everything to compensate for its redness.

    And in any case, vision is a tricky thing. Do you think you are seeing the world as it is, or as how you think it should be?

  • Eniac October 17, 2013, 22:59

    Andy:

    Eniac, it’s due to the limitations of the human visual system – everything hotter than about 2000 K is “white hot” to our eyes.

    That should go for the sun, then, too, which is much hotter (6000K), and yet we call it yellow.

    Anyway, we should probably not argue about the semantics of subjective color perception. The fact remains that a 2000K star, “white” dwarf or not, would appear considerably more yellow than our sun. Much like a light bulb. If it were the only source of light, our vision might compensate, but at most that would lead to equal appearance. The “white” dwarf could never look whiter than our sun, unless it were hotter. In which case it could probably not be old enough to have inhabited planets around it.

  • Eniac October 17, 2013, 23:35

    From the paper by Eric Agol’s group that Adam cites we learn at least two interesting things: 1) The temperature distribution of white dwarfs happens to peak just below the temerature of the sun, meaning their sky appearance is likely to be very similar. 2) The HZ planets will be quite close to the Roche limit. Presumably this implies a huge deviation from spherical, to the extent that it would make a real difference in cartography and other areas of geography or geology. I am not sure how much, if any, effect this would have on surface gravity. Gravity might be a lot lower at the “poles” than at the “equator”. I am drawing the polar axis here radially such that the poles are towards and away from the sun, and the equator would coincide with the “twilight zone”, where eternal day meets eternal night.

  • andy October 18, 2013, 14:29

    That should go for the sun, then, too, which is much hotter (6000K), and yet we call it yellow.

    Personally the midday sun appears to me as white, though often people think of it as yellow. There’s definitely a cultural element to colour perception, e.g. the concept of blue seems to be a comparatively late development in various different cultures.

    And if you find yourself in a world with bronze skies over wine-dark seas, are you on an alien planet or are you in the Iliad?

  • Adam October 18, 2013, 18:00

    Eniac, you’ll note the orbital period is also similar to Earth’s diurnal period so Coriolis and similar forces will be akin. A planetary mass won’t retain tidal bulges over geological time periods unless the object is very cold and stiff. Planets are more like balls of fluid than solid objects.

  • Eniac October 19, 2013, 12:25

    Adam:

    A planetary mass won’t retain tidal bulges over geological time periods unless the object is very cold and stiff. Planets are more like balls of fluid than solid objects.

    Tidal bulges exist because planets are fluid. As long as the tidal forces are there, the bulges will be, too. Unless the object is small, cold, stiff and was originally spherical for some reason.

    A planet near the Roche limit would look very much like a giant egg. Surface gravity would vary considerably: You’d be lightest at the tips and heaviest near the twilight zone. At the Roche limit, you’d be weightless at the inner tip, which is pretty much what the Roche limit means, IIRC.

  • Adam October 21, 2013, 7:51

    Eniac, have you run the numbers on the tidal forces? At 0.01 AU around a 1 sol mass star, the radial force is the same as the Earth’s equatorial centrifugal force, while the lateral compressive force is ~twice that. Not very “egg-shaped” really.

  • Eniac October 21, 2013, 11:13

    Adam: No I have not run numbers. I read part of the Agol paper, which says HZ planets can be close to the Roche limit, and conjectured the rest from that. See the Wikipedia entry on “Roche limit” for formulas and pictures on the deformation: Apparently the maximum deformation just at the Roche limit is almost 2-to-1, ie the major axis is almost twice as long as the minor one.

    My guess would be that your calculation is correct, but that the 0.01 AU and solar mass assumption is far from the Roche limit. Perhaps those are for a much hotter star than Agol is talking about? Tidal forces do fall off steeply with distance….

  • andy October 21, 2013, 15:30

    Incidentally, the WD mass distribution peaks around 0.6 solar masses. Sirius B is atypically massive.

    Nearby WD with more typical mass and that has cooled down to the point where habitability could be considered is van Maanen’s star (Gliese 35). Incidentally there is evidence of metal pollution for this star, spectral type is DZ8.

    I strongly suspect that any habitable planets around a WD would have to be “third generation”: i.e. produced when a binary companion also goes through its own red giant phase.

    Generation 1 = planets formed at the start of the system’s lifetime
    Generation 2 = planets formed from material ejected from star A
    Generation 3 = planets formed from material ejected from star B

    This allows the planet to skip the luminous and extremely hot early stages of the white dwarf’s evolution that would do a good job of boiling off volatile material around close-orbiting planets. Whether material ejected from a red giant star is good for building planets, let alone habitable planets, is another matter.

    You might also be able to do something with a generation 1 giant planet/brown dwarf that migrated through the red giant envelope into a close-in orbit, then gets tidally destroyed at a later stage to form planets in an “excretion disc”. Again, the composition of the resulting planets might be somewhat odd.

  • Rob Henry October 21, 2013, 19:24

    I don’t understand the results given by andy and Eniac here. Being of simple mind, I would do my calculations direct from Kepler, who stated in 1609 that the square of orbital period is proportional to cube of ‘a’. to my mind this tidally-locked planet at 0.01 AU around a one solar mass WD should spin just as fast as Jupiter of Ceres, and be more oblate (here we also must add the change in gravitational potential to those of rotation) – except that oblateness should be torpedo shaped not Frisbee shaped. Given the former example this should be even more obvious to the naked eye… shouldn’t it?

  • John G October 22, 2013, 8:00

    Was at the Starship Century symposium yesterday (21 October). It was my first time at a conference for astronomy/stellar based topics, I fitted it right up against an open-source computer hardware/software (un)conference in Liverpool on the weekend (OggCamp if you’re curious). I enjoyed myself and felt honored to be in the presence of so many great and wise SF names and interstellar/planetary research fellows.

    (website link is to my blog post commenting on some of my thoughts, though if you’re reading it in the next couple of days, you might find its a bit bare still/not so well formatted, as I have updates to make & photos to link to)

  • andy October 22, 2013, 16:52

    Just to clarify here, I am not Adam. :)

  • Adam October 26, 2013, 3:20

    …and I am not andy.

    Just to show the figures, the tidal force is computed as G.M*/R^3, times the radius of the body undergoing the tidal force. For a solar mass white dwarf, G.M* is 1.327E+20 – but 0.01 AU cubed is 3.37E+27… so the tidal force per metre of the object is ~4E-8 s^(-2). It’s actually just 0.0256 gee – which is ~7.4 times more than the centrifugal force at Earth’s equator, so the bulge will be somewhat larger than I initially indicated. The lateral force will squeeze the Earth with twice that so it’ll be a prolate spheroid – i.e. kind of football shaped. But not by very much, maybe 10%.

  • Rob Henry October 26, 2013, 16:09

    Oops Adam tidal force is exactly twice what you stated, and about 0.0512 gee sounds about spot on, or about fifteen times that centrifugal force of Earth. Of cause here the spin is faster at once every 0.365 days, and that would give a magically same shaped Earth a surface centrifugal force seven and a half times higher here – there is your 0.256 gee – but for a completely different reason. And sorry andy – I got confused.

  • RJ November 4, 2013, 11:55

    (Warning: I am not an expert, I’ve learned all this through independent curiosity)

    Two formulas for tidal locking time are here:
    http://en.wikipedia.org/wiki/Tidal_locking#Timescale

    Note that there may be an issue with the “simplified” formula–not sure if it should be multiplied by 10^10 years or 10^9. Its only good for a first approximation, but if your star system is 5 billion years old and the tidal locking time comes in at 300 million years, its a safe bet that the planet is locked. I would guess that a planet orbiting so close to a white dwarf would be extremely likely to be tidally locked.

    Another problem would be the evolution of the star system that leads to the configuration you have. White dwarfs are supposed to be stellar remnants, leftovers from a red giant that sheds its outer layers. I’m not sure how violent the shedding process is, but a red giant is quite large and any planet very close to a white dwarf would probably have been inside the giant at some point. For comparison, when the Sun becomes a red giant it will likely have a radius of ~1AU (93 million miles), equal to the Earth’s orbit. So the orbital distance is very important. On top of that, a red giant gives off quite a lot of energy despite having a low temperature–the sheer size of its radiating surface raises the luminosity quite a bit, which in turn means that the surface temperature of any orbiting planets will be higher during the red giant phase.

    So, your planet orbiting the white dwarf star will have to be far enough out to not be engulfed by the earlier red giant phase, will have been fried by the red giant’s output if its outside but still too close, and is probably tidal locked (depending on the exact distance of its orbit and age of the system). If it was fried by the red giant, then it probably lost its atmosphere and oceans. This means that if its tidal locked, the heat on the sun facing side can’t be distributed by a thick atmosphere, so you get a situation like on Mercury with big temperature extremes (note: Mercury is actually 3:2 spin locked, but its close enough to 1:1 serve as an example). Additionally, in order to survive near the red giant, it has to have been either a rocky world or a gas giant with a rocky core, in which case it is now just rock as everything else was stripped off.

    I can think of two possible scenarios for how an ocean world ~1 million miles from a white dwarf came to be. (1) Someone put it there. (2) Highly unlikely orbital shift: The planet started out as an icy world in the outer system, something like Pluto or the Galilean moons with a rocky core and enclosed in ice, possibly with a liquid ocean sandwiched in between. The system went through its red giant phase but this planet was far enough out to not be affected much. Sometime after the star progressed to a red dwarf there was a chance encounter with an interstellar wanderer (possibly a brown dwarf) that knocked the planet out of its orbit and into an elliptical one toward the inner system. Not sure how long it would take to circularize such an elliptical orbit, and I suspect that another encounter in the inner system may be necessary to push it into a circular, close in orbit. Vulcanism, including cryo-vulcanism, would have been quite prominent during this time, providing material for the atmosphere.

    Since the planet started out with lots of ice, that ice would melt and turn into the surface ocean with an atmosphere. Additionally, the subsurface ocean could have harbored some single celled life while in the outer system, providing a boost to the evolutionary process once it warms up nearer the star. Something to watch out for is the mass of the object, as a low mass world would tend to lose its atmosphere faster, giving the planet a shorter “living” period compared to Earth. http://en.wikipedia.org/wiki/Ocean_planet