I Know This Isn't Realistic

POSTED BY: Haken
UPDATED: Saturday, September 28, 2002 05:47
VIEWED: 4955
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DELVO

Friday, September 27, 2002 9:48 AM

Thinking there's a problem with this is SERIOUSLY off-track from real-world physics and geometry. Where did such an idea even COME from?

For one thing, that gas giant (the big planet) looks almost exactly like a NASA simulation I've seen of how Saturn would look from one of its moons (I don't remember which).

Second, where did this idea of gravity ripping planets apart if they get too close come from? Stars do that, but they have much stronger gravity and are GAS. Planets can have very close orbits and not be damaged; a collision, not an orbit, is where there'd be trouble.

Third, multiple moons of big planets ROUTINELY pass close to each other. So all we're looking at here is a coincidence on the level of a "blue moon" (something that happens once every few months or years).

Fourth, haven't you ever seen the photographs of something with the moon behind it with the moon looking much too large? All you need to do is back off a bit with the camera, and presto, the foreground objects shrink in your viewfield and the distant ones like moons don't.

Also, the moon is currently drifting away from the Earth at a rate that puts it less than half as far away a few billion years ago. So it would have taken up four times as much of the sky. And the only nasty horrible consequence was that there'd be a bit more vertical difference between high tide and low tide. Closer yet would have been possible; there's no reason to think not.

ZICSOFT

Friday, September 27, 2002 4:31 PM

Quote:
Originally posted by LivingImpaired:
Yeah, yeah, yeah... these are all good points. But we're getting away from the important point: It looks pretty.

Which is always a priority with Mister Whedon. And is actually the main thing I like about the show, despite all my nitpicking. (Hacken, sorry if I got too lectury.) Still, I think that Whedon is trying to give Firefly a semi-solid scientific basis. Those of you who don't care about that stuff can just ignore it. But some of us enjoy SF specifically for that kind of detail, and enjoy arguing about it.

JOSS, WHERE'S MY CHECK???!!!

JASONZZZ

Friday, September 27, 2002 5:19 PM

I think the problem with the scenario is that there is a bluesky and people walking around on there without the aid of environmental protection. I wouldn't have a problem the whole deal if the environment is violently chaotic and people are basically sealed in barametric chambers. I can deal with fantasy, but just a tiny bit of realism would do.

Quote:
Originally posted by Delvo:
Thinking there's a problem with this is SERIOUSLY off-track from real-world physics and geometry. Where did such an idea even COME from?

For one thing, that gas giant (the big planet) looks almost exactly like a NASA simulation I've seen of how Saturn would look from one of its moons (I don't remember which).

Second, where did this idea of gravity ripping planets apart if they get too close come from? Stars do that, but they have much stronger gravity and are GAS. Planets can have very close orbits and not be damaged; a collision, not an orbit, is where there'd be trouble.

Third, multiple moons of big planets ROUTINELY pass close to each other. So all we're looking at here is a coincidence on the level of a "blue moon" (something that happens once every few months or years).

Fourth, haven't you ever seen the photographs of something with the moon behind it with the moon looking much too large? All you need to do is back off a bit with the camera, and presto, the foreground objects shrink in your viewfield and the distant ones like moons don't.

Also, the moon is currently drifting away from the Earth at a rate that puts it less than half as far away a few billion years ago. So it would have taken up four times as much of the sky. And the only nasty horrible consequence was that there'd be a bit more vertical difference between high tide and low tide. Closer yet would have been possible; there's no reason to think not.

HAKEN

Friday, September 27, 2002 7:45 PM

Quote:
Originally posted by Delvo:
Thinking there's a problem with this is SERIOUSLY off-track from real-world physics and geometry. Where did such an idea even COME from?

I wasn't going to say anything, but, this is high school physics.

http://www.astronomynotes.com/gravappl/s8.htm

And please read up on on the Roche Limit.

http://scienceworld.wolfram.com/physics/RocheLimit.html

But, in the end, like LivingImpaired said, all that matters is that it looked pretty.

DELVO

Friday, September 27, 2002 9:03 PM

Quote:
And please read up on on the Roche Limit.

OK, you got me for badly imprecise writing. I didn't mean to imply that there's no Roche Limit... just that there's PRACTICALLY not one for planets/moons because the limit would be so close that they practically never get that close without colliding anyway... and that the picture doesn't tell us that the Roche Limit has been violated.

ZICSOFT

Saturday, September 28, 2002 12:13 AM

Quote:
And please read up on on the Roche Limit.

Aha! I had forgotten about that. Nice simple model for planet break-upage. Here's a page with a slightly simpler description, accesible to anybody with a little basic math:

http://curriculum.calstatela.edu/courses/builders/lessons/less/les1/mo
ons/roche.html

The Roche limit of two bodies of the same size and density is 2.43 radiuses. Earth's radius is a little over 6,000 km, so two earth-like bodies start destroying each other when the get within about 15,000 km of each other. (If you're metrically challenged, that about 9300 miles.) The terraformed moon in the picture is obviously not that close -- it'd fill the sky if it was.

Quote:
But, in the end, like LivingImpaired said, all that matters is that it looked pretty.

Math can be pretty too!

JOSS, WHERE'S MY CHECK???!!!

LIVINGIMPAIRED

Saturday, September 28, 2002 1:02 AM

Quote:
Originally posted by Zicsoft:
Math can be pretty too!

Not the way my physics professor teaches it.

________________

"You still don't get it. It's not about right, not about wrong... It's about Power." —Morph-O-Monster, "Lessons"

JASONZZZ

Saturday, September 28, 2002 5:24 AM

Check this out: I remember this from a couple of different SF novels I read. Planet bodies in the same solar orbit but very close to each other's Lagrangian points...

http://burtleburtle.net/bob/physics/kempler.html

Very interesting...
Check out his other weird and stable orbits:

Figure 8 orbits:
http://burtleburtle.net/bob/physics/eight.html

Dyson's Swarms:
http://burtleburtle.net/bob/scifi/dyson.html

Quote:
Originally posted by Haken:
Take a look at this from the 'The Train Job'

I'm guessing the largest one in the background is a planet. The other two are moons. One seems to be terraformed. The one Malcolm is on must be a moon as well because there's just no way for two planets to be that close together.

But how the heck do you explain all these terrestial objects being so close together in distance without gravity ripping everything apart? There's just no way.

A bit overzealous with the cut and paste I think.

JASONZZZ

Saturday, September 28, 2002 5:47 AM

Quote:
Originally posted by Zicsoft:
Quote:
And please read up on on the Roche Limit.
Aha! I had forgotten about that. Nice simple model for planet break-upage. Here's a page with a slightly simpler description, accesible to anybody with a little basic math:

http://curriculum.calstatela.edu/courses/builders/lessons/less/les1/mo
ons/roche.html

Actually, we are talking about being just close enough to pass thru the (tremendously strong)magnet field of the gas giant and just enough gravitational influence to rip away the atmostphere and create 100 metre tidal waves - the same gravitational influence would create havoc with the moon's crust. Certainly enough stress to not just create tidal waves with fluids, but with the crust itself. In any case, still not close enough to actually rip the moon apart. Check out a similar comparison between Jupiter and Io. Io and Jupiter's Roche limit is 60223km, but its actual orbit is around 422000km. It has exactly the type of conditions that I described above. and no blue sky.

Quote:
Originally posted by Zicsoft:

The Roche limit of two bodies of the same size and density is 2.43 radiuses. Earth's radius is a little over 6,000 km, so two earth-like bodies start destroying each other when the get within about 15,000 km of each other. (If you're metrically challenged, that about 9300 miles.) The terraformed moon in the picture is obviously not that close -- it'd fill the sky if it was.

slight correction needed there. That result of 15447km is described from center of mass to center of mass. Earth has a radius of ~6350+km. So those two Earth like objects will actually be within staring distance of 2300km from each other surface to surface. With the top of the thermosphere at about 122km. We are talking about really maybe 2000km of separation.

In comparison, driving from Buffalo, NY to San Francisco would cover about 4300km.

Quote:
Originally posted by Zicsoft:

Quote:
But, in the end, like LivingImpaired said, all that matters is that it looked pretty.
Math can be pretty too!

JOSS, WHERE'S MY CHECK???!!!

In either case, you can be about 10x further away and still feel the influence described in my earlier case above. It's likely that all life and life sustaining type environment would get boiled and ripped away wayway before the planetary object actually catastrophically breaks apart.

I don't disagree, it is pretty and thought provoking.

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