Astronomy/cosmology questions...

[ColdWizard]

So conservation of angular momentum is a variation of voodoo magic. Got it.

[Rusty Edge]

I'll watch the video and comment when I get home from work...

I lied. Anywho, the problem with Mars is that it's dry and cold and has a thin wisp of an atmosphere. The bad news is there's no good way to directly combat either the dryness or the coldness.

If you melt the ice caps or bombard the planet with icy comets or setup an interstellar bucket brigade, the extremely low pressure means any liquid water more or less instantly boils away.

You also can't really increase the temperature because, well, what is it you're heating up? Not Mars itself--it's way too large. You just want a higher surface temperature, but there's barely anything on the surface to have a temperature.

The good news is both these problems can be solved by introducing an atmosphere. There is dry ice (frozen CO2) in the polar caps and probably some other greenhouse gases stuck in rocks. And the solar system is full of ammonia and CO2 and hydrocarbons that could be used to trap heat. Throw a whole lot of atmosphere onto the planet, trap heat, warm it up, melt ice, produce water vapor which is an extremely potent greenhouse gas, get yourself a nice positive feedback loop going on.

If you establish any kind of water cycle, the ice in the polar caps will eventually "fall" into the low-lying areas on Mars. (We see something similar happen on Mercury and Titan, for example.) There are questions about how much water is tucked away on Mars. May need to import more.

And Mars' lower gravity means you need a lot more atmosphere than that present on Earth to get the same surface pressure. And Mars' distance from the sun means it's just plain colder. May want to setup giant space mirrors to direct more light at the surface.

And yes, there's the problem that Mars' low gravity and lack of a magnetosphere means its more susceptible to atmospheric escape. Timescale on this is pretty long, but still something to worry about.

It's all pie in the sky right now. Transporting that much material through the solar system doesn't seem to be physically impossible, but we currently have no experience with any of that. How do you build a planet-wide magnetic field? How do you create a biosphere from scratch? How do you make sure you don't accidentally create Venus? No solid answers to those questions right now.

Thank you. So the practical approach at this time would still be the Total Recall method - do whatever you do underground?

Okay, that raises another stupid series of questions. I presumed atmosphere retention was strictly a function of gravity, but if it isn't does that mean that the less massive Mars requires a stronger magnetic field to retain an atmosphere? If all planets don't have magnetic fields, why do some have them and others not? The dynamo effect of a molten core? The  massive presence of nickel and iron ? A radiation belt ? Or is that a side effect of a magnetic field?

[Lorizael]

Okay, that raises another stupid series of questions. I presumed atmosphere retention was strictly a function of gravity, but if it isn't does that mean that the less massive Mars requires a stronger magnetic field to retain an atmosphere?

In the absence of solar wind, particles in the atmosphere escape when they are moving too fast (high temperature) and not being held on to tightly (low gravity). The solar wind is a stream of high-energy particles that can knock bits of atmosphere away. Weaker gravity means particles are more easily knocked loose, but the solar wind is weaker out at Mars than at Earth. Not sure how this all works out.

If all planets don't have magnetic fields, why do some have them and others not? The dynamo effect of a molten core? The  massive presence of nickel and iron ? A radiation belt ? Or is that a side effect of a magnetic field?

Yep. In general, bigger planet = hotter core = dynamo effect. Some gas giant moons gets protection from their planets. Jupiter itself has a very strong magnetic field which probably comes from the super weird and poorly understood "metallic hydrogen" in the core. So classical metals not strictly necessary. Smaller planets (Mars, Mercury) have cooled down and don't have magnetic fields anymore.

But neither does Venus, even though it's the same size as us. Why? Prooobably because it spins slowly. Jury's still out on this. We can say "dynamo" and that conjures an image of some simple 19th century device that we must surely understand completely by now, but planets are big and complicated.

[ColdWizard]

Thank you. So the practical approach at this time would still be the Total Recall method - do whatever you do underground?

But boring. I like the giant magnetic balloon(s).

[Lorizael]

So conservation of angular momentum is a variation of voodoo magic. Got it.

Well yeah.

https://www.youtube.com/watch?v=r__nGqGpTD8

[Rusty Edge]

Thank you. So the practical approach at this time would still be the Total Recall method - do whatever you do underground?

But boring. I like the giant magnetic balloon(s).

That's a cool idea, but how much energy does it take to make a planetary scale magnetic field?

[Lorizael]

Alright, so here's a really rough estimate. If the magnetic field is roughly the size of Mars + decent Martian atmosphere, and the strength is the same as Earth's (and uniform all the way through, which is probably a very bad assumption), and Mars is actually empty space... then you've got ~200 million gigajoules of energy stored in the field. Mars is really iron+nickel+rock, all of which are better at holding magnetic fields, so the answer may be lower by 2 or 3 orders of magnitude.

The largest nuclear plant in the US operates at ~4 Gigawatts, so you need to commandeer its output for a year and a half to produce your magnetic field. But if you do need less because you're dealing with the actual composition of Mars and not a vacuum, we're talking at most just a few days of that plant's output. It's honestly not an astronomical sum because Earth's magnetic field is in truth very weak (~50 microteslas, compared to the 5 milliteslas of a refrigerator magnet). It's only a lot because it's a huge volume of space.

[Rusty Edge]

Alright, so here's a really rough estimate. If the magnetic field is roughly the size of Mars + decent Martian atmosphere, and the strength is the same as Earth's (and uniform all the way through, which is probably a very bad assumption), and Mars is actually empty space... then you've got ~200 million gigajoules of energy stored in the field. Mars is really iron+nickel+rock, all of which are better at holding magnetic fields, so the answer may be lower by 2 or 3 orders of magnitude.

The largest nuclear plant in the US operates at ~4 Gigawatts, so you need to commander its output for a year and a half to produce your magnetic field. But if you do need less because you're dealing with the actual composition of Mars and not a vacuum, we're talking at most just a few days of that plant's output. It's honestly not an astronomical sum because Earth's magnetic field is in truth very weak (~50 microteslas, compared to the 5 milliteslas of a refrigerator magnet). It's only a lot because it's a huge volume of space.

Thanks, I didn't mean to make you do so much math.

[Lorizael]

I like math.

Plus this was mostly just back of the envelope algebra. If I wanted to be semi-rigorous I would have integrated the magnetic field strength as a function of radius and maybe not treated the composition issue as just dropping a few zeros.

[Buster's Uncle]

Lori:  a bit of Star Trek universe fanwank I came up with and posted five years ago.  I'm interested in your opinion of the likelihood of the physics, proceeding from the assumptions...

Well, it starts with the plainly observable fact that everyone's artificial gravity is insanely reliable.

I conclude this:  in the ST universe, there's stuff about how gravity works that was discovered by the mid 90's.  Khan's ship had artificial gravity, and it wasn't spinning or accelerating.  So there's a way to make, dunno, a gravity deck plating cheaply that works for a very long time with little or no power input.  Every race discovers this application of the law of gravity pretty soon after they go into space.  The same, or similar, techniques make for a nifty non-reaction gravity drive, which Starfleet calls "impulse".  A slightly more sophisticated application involving the interaction of fields from two gravity generators distorts warps space-time and makes for a nifty FTL drive.  Thus, everyone and his mother has a FTL starship with two drive pylons of some sort.  Both types of drive take a lot more juice then the deck plates because the gravity fields, by the nature of the thing, are not static, but have to expand and contract and vary in intensity.  That Warp is probably by an order of magnitude more power-hungry than Impulse naturally follows.

[Lorizael]

Hey BU!

So what we know about gravity is that it propagates outward radially and that it's very weak. The first part is tricky, because each deck on a starship is at 1 gee and so are the Jefferies tubes. So you need to make sure the artificial gravity on deck 1 doesn't bleed over onto deck 2, which means you need to be able to sharply reconfigure the field across short distances. Hard to imagine that not requiring a lot of energy.

Gravity being weak means, traditionally, you need an entire planet to get a reasonable amount of it. You need less than a planet if you're much closer to the source of gravity, but still on the order of, like, a small moon. But if you know some extra cool facts about gravity, like, say, how to generate gravitons, maybe you don't need all that mass.

[Buster's Uncle]

Oh, the way the artificial gravity works is bull, and nothing but -though it has to be low/no power, or they'd lose gravity in emergencies roughly every episode- but is the drive speculation worth anything?

[Lorizael]

Well, depends on whether or not you're obeying Newton's third law/conserving momentum. If you can just randomly generate a gravitational field in space, then sure, maybe you won't run into any problems with Newton. But if impulse relies on a gravity generator producing thrust somehow, then your momentum needs to come from somewhere.

[Geo]

But if impulse relies on a gravity generator producing thrust somehow, then your momentum needs to come from somewhere.

Quantum Space Foam!

[Unorthodox]

-though it has to be low/no power, or they'd lose gravity in emergencies roughly every episode

You sure?  I was under the impression (not sure where not really a trek tech guy) the deflector dish was constantly channeling tremendous power just to keep dust from punching holes in the ship.  I haven't really seen it FAIL either.  In fact, you generally see life support fail before the deflector. 

Which...how DOES the deflector...uh, deflect?  Is IT directed gravity?  Fits with the tractor/repel beam it shoots out too...