Random SF Science Questions

[austenandrews]

Here's a thread for random SF-related science questions, for the less scientifically challenged among us to hopefully answer. I'll start:

 

Can a solar sail tack against solar winds? I just read that they can't, but I'm unsure if that's true and why.

[gojira]

Re: Random SF Science Questions

 

Tack in this sense I think means "sail up wind."

 

Real sails work like an airplane's wings. Air flows across both surfaces of a sail (front and back). The air moving across the outer surface, the convex one, moves a longer distance than the air moving across the inner surface. This generates a lower preassure region on the outer surface, resulting in lift just like an airplane's wing.

 

Sails are therefore said to operate in pull-mode most of the time. They pull the boat along, they don't push it. Only when sailing almost directly away from the wind do sails operate in push mode.

 

I'm guessing that the solar wind is basically a stream of charged particles, not molecules and atoms like air. Being basically a vacuum, they can't generate lift, so there's no way to go towards the sun. Solar sails can only go away from the sun. They only operate in push mode.

[Savinien]

Re: Random SF Science Questions

 

What would the sky of a 'green' moon orbitting a gas giant look like? What effect would the planet have on the 'livable' satelite?

[Outsider]

Re: Random SF Science Questions

 

I may be out of date on my planetary & solar formation theories, but doesnt being far enough out from one's primary that it is cold enought to have a gas giant also mean that it is far enough out that its too cold to have an earthlike ecosystem on a smaller body?

 

T Tauri winds blasting the atmospheres from 'inner' planets and such...

[gojira]

Re: Random SF Science Questions

 

Some gas giants are close to being "brown dwarfs" and radiate more heat than they receive. I'm guessing the balance would have to be just right but it could happen that the moon of a gas giant was somehow earth-like.

[Cancer]

Re: Random SF Science Questions

 

The surfaces of the moons of the Jovian planets in our Solar System are too cold. The interiors of those moons may be something else.

 

Europa and Ganymede are suspected of having an extensive layer of liquid water under the surface ice (the ice may be hundreds of kilometers thick). Tidal heating keeps the interiors warm and the water layer liquid. Any biosphere in that ocean will have to be based on chemosynthesis rather than photosynthesis, both because it's hundreds of km deep and sunlight is unavailable, not to mention the energy density in sunlight out there is down from what it is here on Earth by a factor of roughly 25.

 

The interesting eruptions very recently discovered on the much smaller moon of Saturn Enceladus suggests there's at least intermittent liquid water in it, too.

 

EDIT: Also, many of the Jupiter-class planets known orbiting other stars are rather closer to their star than our Jovian planets are. If they have moons, they could be interesting places as well.

[Guest Schwarzwald]

Re: Random SF Science Questions

 

I may be out of date on my planetary & solar formation theories, but doesnt being far enough out from one's primary that it is cold enought to have a gas giant also mean that it is far enough out that its too cold to have an earthlike ecosystem on a smaller body?

 

T Tauri winds blasting the atmospheres from 'inner' planets and such...

Hmmm, maybe a habitable world orbiting a gas giant could be warmed by a 'greenhouse effect' if it's atmosphere was the right composition, or possible warmed by tidal forces from the giant causing the crust to shift, produging heat, along with earthquakes, volcanoes, etc....

[Tom Carman]

Re: Random SF Science Questions

 

Here's a thread for random SF-related science questions, for the less scientifically challenged among us to hopefully answer. I'll start:

 

Can a solar sail tack against solar winds? I just read that they can't, but I'm unsure if that's true and why.

Yes.

 

Solar sails are propelled by sunlight, not the particle stream of the solar wind. Remember that moving in space is an interplay of velocity and gravity. To sail from Earth to Mars, you would angle the sail to increase your velocity in the direction of your orbit around the Sun. Speeding up moves you "higher" above the Sun and closer to the orbit of Mars; calculate it right and you reach Mars' orbit when Mars is right there.

 

You can sail to a "lower" orbit closer to the Sun by angling the sail to reduce the velocity of your current solar orbit. Note that all this works best when you are following your own orbit around the Sun, ie. not in orbit of a planet. You can use a lightsail to accelerate to break planetary orbit, but it can be a time-consuming process with tricky maneuvers.

[Basil]

Re: Random SF Science Questions

 

Can a solar sail tack against solar winds? I just read that they can't' date=' but I'm unsure if that's true and why.[/quote']

Yes, though it's not "tacking" in the sense a sailboat tacks. The light sail can be set to reflect the incoming light "forward" (i.e., in the direction the vehicle is orbitting). This slows the vehicle, and the star's gravity brings the orbit closer.

 

BTW, "solar" sail is a poor term, as it makes it sound like the "solar wind" is the primary source of energy. In fact, the effect of the light is a few thousand times more important than that of the "solar wind." They are better referred to as "light sails" for this reason.

 

EDIT: gojira---light sails do not in any sense depend on the Bernoulli (spelling?) effect. They gain impulse from the momentum of the impacting and reflecting photons. In effect, they act like rockets where the power and propellent are decoupled from the vehicle itself. Thus, they can get closer to the star by the same way rockets do: they decelerate, and let the star's gravity pull them in.

[Basil]

Re: Random SF Science Questions

 

What would the sky of a 'green' moon orbitting a gas giant look like? What effect would the planet have on the 'livable' satelite?

Assuming the "gas giant" (aka GG) is a brown dwarf, or a "hot Jovian" (vide posts by gojira and Cancer respectively), a "green moon" would still have to be much larger than Ganymede, or it would not be able to hold an atmosphere. Thus, an Earth-sized moon is necessary.

 

Such a moon is probably gravitationally locked (i.e., one face points toward the GG at all times). If the GG is a brown dwarf, that face will be warmer than the opposite, though by how much is a tricky question. Note that even a non-brown-dwarf GG would reflect enough light to warm one side of the moon.

 

As well, whether the GG is a brown dwarf or not, the moon's period around the GG is likely to be several days long (it would have to be a fair distance out from the GG to form as so [relatively] large a body). Thus, the "day" (the period of the star's apparant period) would be days long, which would lead to ferocious weather, and thus difficult to live there.

 

If the moon is not gravitationally locked, the suplimentary heating of the GG would be distributed evenly, but the problem of the extremely long "day" would remain.

[gojira]

Re: Random SF Science Questions

 

I was going to debate this, but Wikipedia says otherwise. In fact, they claim that radiation preassure is about 100 times greater than the impulse from charged particles, which surprised me greatly.

 

Looks like I was mistaken. Given that one is able to reflect the light in a given direction, you should be able to speed up and slow down ("tack") as Tom describes.

 

It's also very interesting that the Wikipedia article says that radiation pressure is already be used by NASA to correct orbits and conserve fuel, something I didn't know either. Good call, Tom and Basil.

[Cancer]

Re: Random SF Science Questions

 

Assuming the "gas giant" (aka GG) is a brown dwarf' date=' or a "hot Jovian" ([i']vide[/i] posts by gojira and Cancer respectively), a "green moon" would still have to be much larger than Ganymede, or it would not be able to hold an atmosphere. Thus, an Earth-sized moon is necessary.

 

Such a moon is probably gravitationally locked (i.e., one face points toward the GG at all times)...

 

As well, whether the GG is a brown dwarf or not, the moon's period around the GG is likely to be several days long (it would have to be a fair distance out from the GG to form as so [relatively] large a body). Thus, the "day" (the period of the star's apparant period) would be days long, which would lead to ferocious weather, and thus difficult to live there.

I agree with everything you have here, except that I don't understand why long "days" make for severe weather. An earth-mass moon in orbit around Jupiter-mass planet with an orbital period (and hence "day", assuming the moon orbit and planet orbit are approximately coplanar) of about a week (so this is a Callisto-size orbit) sounds like a plausible system to me, and if the planet's orbit around its star is about 1 AU in size, you've got a decent hypothetical system there. The "day" is a bit long, but you need the moon to be far enough out from the planet so as to avoid the worst of the GG's magnetosphere and radiation belts.

[keithcurtis]

Re: Random SF Science Questions

 

An earth mass is not necessary in order to hold an atmosphere. Just ask Titan, 1.6 times our atmospheric pressure and only .22 the mass.

Closer to the sun and a big planet, it would help if the planet/moon had a nickel/iron core to give it a good magnetic field.

 

As for solar sails, gojira covered it very well, including noting that "tacking" is not the right term at all. If propulsion came from the particles and not light, I suppose you could make a rudder, though I have no idea how such a thing could be made.

 

Keith "Great topic, austenandrews" Curtis

[Cancer]

Re: Random SF Science Questions

 

An earth mass is not necessary in order to hold an atmosphere. Just ask Titan' date=' 1.6 times our atmospheric pressure and only .22 the mass.[/quote']

Minor comment ... you slipped a decimal place there (Titan is 0.022 Earth masses), but correcting that error actually strengthens your argument.

 

At 1 AU, I expect you do need close to an Earth mass. Titan, being further out, has about half the surface temperature (in Kelvin) of Earth, maybe somewhat less. Atmosphere retention is a contest between the thermal velocity of the atmosphere molecules (which scales as square root of T) versus gravity. Mars, at about a tenth the mass of Earth and only slightly cooler, has clearly lost a lot of gas.

 

Closer to the sun and a big planet' date=' it would help if the planet/moon had a nickel/iron core to give it a good magnetic field.[/quote']

No argument here. Because Jovian planets have such hugely strong magnetic fields, I think a strong geodynamo in the "moon" would be essential for any life that might get going there, even if it is outside the principal radiation belts of the gas giant planet it was orbiting.

[keithcurtis]

Re: Random SF Science Questions

 

Minor comment ... you slipped a decimal place there (Titan is 0.022 Earth masses)' date=' but correcting that error actually strengthens your argument.[/quote']Oops. Stupid scientific notation.

 

At 1 AU' date=' I expect you do need close to an Earth mass. Titan, being further out, has about half the surface temperature (in Kelvin) of Earth, maybe somewhat less. Atmosphere retention is a contest between the thermal velocity of the atmosphere molecules (which scales as square root of T) versus gravity. Mars, at about a tenth the mass of Earth and only slightly cooler, has clearly lost a lot of gas.[/quote']

I had thought that the solar wind played a part in atmosphere loss as well, which is one of the reasons I mentioned the EM field. Oops again.

 

Keith ":)" Curtis

[Cancer]

Re: Random SF Science Questions

 

I had thought that the solar wind played a part in atmosphere loss as well' date=' which is one of the reasons I mentioned the EM field. Oops again.[/quote']

I think it does, but the impression I have is that it's important only if your planet is at the edge (low gravity) anyway.

 

The strength of the stellar wind is certainly dependent upon age of the star (it's largest for young stars). There's also an effect due to stellar mass, but I can't recall what it is now, at least in the regime of interest.

 

Planetary dynamos die out as time passes as the internal heat reservoir is exhausted, with low-mass planets dying out first.

 

So there's two effects there which work toward habitable planets having nice liveable atmospheres: more mass gives more gravity to hold the molecules in, and more mass keeps the magnetic field going longer to protect from solar wind stripping.

 

It would be really, really nice to know what Venus looked like a billion years back. It'd settle a lot of speculations.

[Curufea]

Re: Random SF Science Questions

 

I would think it very difficult to use solar sails withing a solar system - primarily because of the currents and wind shear of the solar/stellar winds.

Photons have much smaller mass than the various gas particles that make up the solar winds - and solar sails are photon powered.

[keithcurtis]

Re: Random SF Science Questions

 

Could you elaborate?

 

Keith "currents and wind shear?" Curtis

[Nyrath]

Re: Random SF Science Questions

 

http://solarsails.jpl.nasa.gov/introduction/how-sails-work.html

http://sail.quarkweb.com/light.htm

http://scientium.com/specials/sotm/starship/sailships.htm

[Curufea]

Re: Random SF Science Questions

 

Basically just that if you imagine the sun creating ripples - and imagine what happens when the planets move through those ripples and create their own reflections.

But in three dimensions - so the most turbulant solar winds would be in the orbital plane. The eddies would dimish towards the poles of the system.

[oberon]

Re: Random SF Science Questions

 

For anyone who is interested, the Coyote books (Coyote, Coyote Rising and Coyote Frontier) by Allen M. Steele involve the creation of the first extrasolar colony in just such a situation.

I'm currently reading the first book, which has been written around the setup of the colony.

Coyote is an Earth-sized moon orbiting a GG that sits just outside the habitable zone of it's primary. However, reflected radiation, etc, from the GG are enough to make the moon habitable, although with pretty extreme seasons.

A good read so far.

 

oberon

[keithcurtis]

Re: Random SF Science Questions

 

Basically just that if you imagine the sun creating ripples - and imagine what happens when the planets move through those ripples and create their own reflections.

But in three dimensions - so the most turbulant solar winds would be in the orbital plane. The eddies would dimish towards the poles of the system.

???

Ripples of what? Solar sail act by light pressure. Very low light pressure at that. any variation in light pressure will be negligible. Yes, planets would reflect sunlight, but you're comparing that to the overwhelming light coming directly from the sun is like comparing the north wind to a house fan.

If you are talking about ripples in the stream of charged particles, that has already been addressed above. Also negligible.

If it something else entirely, I'm not getting your meaning.

 

Keith "perplexed" Curtis

[Savinien]

Re: Random SF Science Questions

 

So, I believe we have established that a moon (if earth-sized) orbitting a gg could be 'green'. Does it need to be gravitationally locked? Regardless, what would the weather and such be like? If a large volcano, or an asteroid crashed into it, how long would the 'dust/debris' be in the air and what affect would that have on civilizations?

 

What would the sky look like? (pre and post cataclysm)

[Cancer]

Re: Random SF Science Questions

 

Tidal locking into synchonous rotation is just about guaranteed for any satellite of a gas giant. The only exceptions are moons that are very far away, and moons which are in states of chaotic rotation due to repeated tidal interactions with other moons in the system. Examples of both types exist in the Solar System. The chaotic rotators are unlikely to be large moons, and if they were large, they'd probably turn out to be hellishly uninhabitable, like Io is now. Tidal interactions like that generate way too much heat in the planet's interior.

[Savinien]

Re: Random SF Science Questions

 

Tidal locking into synchonous rotation is just about guaranteed for any satellite of a gas giant. The only exceptions are moons that are very far away' date=' and moons which are in states of chaotic rotation due to repeated tidal interactions with other moons in the system. Examples of both types exist in the Solar System. The chaotic rotators are unlikely to be large moons, and if they were large, they'd probably turn out to be hellishly uninhabitable, like Io is now. Tidal interactions like that generate way too much heat in the planet's interior.[/quote']

 

So, for my nefarious purposes that means... what? (In dumbspeak, please)