Can Anyone Provide Some Astronomical Help Please?

[MattyK]

I think I'm going to have this problem a bit but hopefully if someone can help me with the current occurance of it, I'll be able to work out the answers myself when I get to it again. Basically, in my story, a spaceship is about to land on Saturn's moon, Titan (where there is a moon base of some kind). I've been reading up on Titan facts but I don't know how to translate some of it into every day English. For example:

The surface gravity is 1.352 m/s2 (compared to Earth which is 1 m/s2) and the atmosphere is more dense (98% nitrogen as opposed to Earth's 72%), apparently the equivilent to being five meters under water on Earth. I could do with knowing how this would effect a shuttle leaving orbit and landing on the surface. I'm guessing the entry speed would be slower than an Earth entry but I don't know whether the external heat would be greater or less.

I'm also assuming that the weaker gravity would mean that anyone born and raised on Titan would be thinner and taller than people from Earth (I think!). I've read that the body adjusts to different gravity in about 72 hours at the most so I'm guessing that a person from Earth would have to exercise for the three days to fend off the symptoms and after that they'd be more or less okay.

I think that's about it. I'm not dwelling on the science much in my story as it's not relevant but I want it to be as believable as possible so any help would be very much appreciated.

 

[Sapheron]

Being the resident geek when it comes to chemistry and geology and such like, I'll give it a shot. I must admit, shuttle re-entry isn't a strong point, but hey, I'll adapt. Cut to the last paragraph if you just want the answer instead of the gabble.

On Earth, a falling object accelerates at 0.98m/s2 until it reaches terminal velocity; the point at which air resistance equals the force of gravity. Then it falls at a steady speed until it reached the ground, at which point it splatters messily. Luckily, the effects of gravity go out pretty far; 500km above the surface of Earth will still see a gravity of about 0.85m/s2 or so (that's an approximate, don't quote it).

Now, if you increase surface gravity to 1.352m/s2 as on Titan things will fall more quickly because there's a greater downward force. The stronger force also means that with a similar atmosphere it will reach a higher terminal velocity. Obviously, the faster it falls the hotter it gets, because air resistance is increased. Basically, there's more of a mess when it reaches the bottom... and it's on fire.

If we change the atmosphere as well though, things are getting more complex. Nitrogen is less dense than oxygen, meaning that under the same gravity Earth should have the denser atmosphere. As Titan has a higher gravity though, it effectively pulls the atmosphere in, pressurising it against the surface in a way. This gives Titan gains a denser atmosphere.

Now, during re-entry a denser atmosphere will provide more air resistance, which in reality is just a particular type of friction. As we all know, friction causes heat and slows things down, so the denser atmosphere reduces the terminal velocity. To move at the same speed as a re-entry to Earth though (you'd need another source of energy... like a jet engine or something) would result in things getting much hotter than on Earth.

Luckily, for this the thickness (physical distance from surface to space) of the atmosphere isn't too big a problem. Both are thick enough for things to reliably reach terminal velocity or, if they are so inclined, disintergrate completely on the way.

Anywho, so in the end we have the denser atmosphere and higher gravity, leading to a big mess. Basically, anything re-entering will be falling a third again as fast, but being resisted against more. The object doing the re-entering would therefore have to be able to take much higher temperatures than those it would face on Earth. As I haven't worked it all out I'm not sure, but my educated guess says the final terminal velocity would still be more than on Earth.

I'll leave the biology questions to those more qualified on them. Hope that helped and, as far as I know, I haven't rememebered anything wrong.

 

[MattyK]

I think my brain needs a higher heat resistance, after reading all that it's starting to melt! Just one thing I want to clear up as I think I've confused myself, when I originally read 1.352m/s2 gravity on Titan, I thought "ooh, higher number, stronger gravity" but then I read somewhere that theoretically on Titan, a man with some sort of wing attached to his arms would be able to fly so it sounds like weaker gravity and you've just said higher gravity in your description...am I confusing the gravity with the dense atmosphere?

Apart from that, thank you very much. That's exactly the sort of answer I was looking for!

 

[TheEndIsNigh]

No I think you're wrong there Matty.

If your facts are right the denser atmosphere would be more buoyant.

After all on Earth even you can fly through the atmosphere in the oceans cos it's denser than the air

 

[Sapheron]

Quite so. The higher number does indeed mean stronger gravity. It is the density of Titan's atmosphere (I didn't realise it was that much denser) which would allow this.

 

[MattyK]

Cool, gotcha, thanks!

 

[chrispenycate]

Um, could you check your figures for Titan's surface gravity? I find one tenth of that.

 

[Sapheron]

Although I can't provide the precise figure for Titan's surface gravity from my textbooks, I can say with certainty that Titan is nearly five times as massive as Earth. As such I feel it's unlikely that Titan would have a tenth of the gravity.

 

[chrispenycate]

Sorry, in which case I mean Earth's gravity at 981cm /sec/sec, ie. close to 10M/sec, not 1. That is Titan has about 15% of Earth's gravity, not 150%.

Also, percentage of nitrogen does not give absolute atmospheric pressures-

In many ways Titan is similar to the Earth. It's about the size of our moon, so its gravity is less than Earth's, one-seventh. But it's the only world in the solar system that's got an atmosphere that has anywhere near the atmospheric pressure on Earth. Mars has 100 times less than Earth's pressure, and Venus has 100 times more. Those are our so-called "sister" planets - and they're 100 times different in either direction. Titan's atmosphere is only one and a half times as thick as Earth's.

So, the lower gravity will make the drop off in pressure slower, better for atmospheric braking, but leading to a slower approach. But do you really need atmospheric braking with your 'capable of turning corners like an X-wing fighter' freighter? If it can manage manoeuvres as described in the critiques section, it has something well in advance of a simple reaction drive, and the only reason to use atmospheric braking would be to save fuel; and one thing Titan is not short of is potential fuel.

 

[MattyK]

Okay, sorry folks, I've crapped up big time! I read on a comparison table that Titan had the gravity quoted in my OP (1.352 m/s2) and that was right. What I got wrong, however, is that Earth's gravity is actually 9.780327 m/s2 which does indeed mean that Titan has considerably lower gravity.

Right?

 

[chrispenycate]

Correct. And my first comparison chart gave me the result in g's - Earth gravities – rather than Metres per second per second.

It didn't sound reasonable that a body so much smaller than Earth, and out in the 'hydrogen hangs around' zone for temperatures, would have a surface gravity higher than one g, and it didn't.

Which makes the atmospheric density even more amazing (and the place even more interesting as a base. Volatiles for fuelling incoming vessels, carbon, oxygen, nitrogen and hydrogen available for recharging a life support system, and a thick enough atmosphere to protect you from charged particles caught in Saturn's magnetic field). Not terraforming, of course; but in many ways easier than Mars or an asteroid, except for the long walk home.

 

[ktabic]

That's right, a much lower gravity. Slightly less than the Moon in fact.

Quite an active little place is Titan, there is really geological activity happening there, which is presumably why it still has an atmosphere.

 

[Sapheron]

Heheh. I assumed the figures for Earth were correct and only checked the Titan ones. Turns out that I had the figures for Earth wrong in my head instead (which is particularly silly, as it's the one I most often have to deal with). Well pointed out, Chris. Obviously, the opposite to what I said should now happen.

 

[Nik]

Phew !!

You had me worried there...

 

[Glitch]

One question I would ask is how much does this affect the story.

You say you want it to be as believable as possible but if it doesn't affect the plot much I wouldn't about it. Of course it's your story so write it how you want to, but don't get caught up in facts that the average reader wouldn't know anyway.

 

[ktabic]

And in a serendipitous moment, or possibly he just reads chrons, and who can blame him, XKCD does Titan

 

[chrispenycate]

One question I would ask is how much does this affect the story.

You say you want it to be as believable as possible but if it doesn't affect the plot much I wouldn't about it. Of course it's your story so write it how you want to, but don't get caught up in facts that the average reader wouldn't know anyway.

And what about your occasional non average reader? I have been known to get very miffed, and even write to authors about imprecision in SF novels. A factor of ten out (definition of 'right to within an order of magnitude' is 'wrong') in a distance, or time to accelerate to a particular speed can take all the joy out of a book. Certainly, we're a vocal minority, bur SF readers are already a minority – actually, those who read anything for pleasure are a minority – so why not avoid the hassle and get the details right?