Bringing iron asteroids down to Earth?

[Antiphon]

Is there an official space elevator thread on this forum? You know, one where it's shown that the space elevator or tether can't possibly work even if you had the materials because you can't supply angular momentum to the thing you're trying to hoist?

 

[rrw4rusty]

But hey, it's your book so if you want to pilot house sized rocks, go for it. It is sci fi after all.

The reason I'm out on the forums and researching everything is because I want my sci-fi stories to be plausible.

Did you read all the posts? The hobby scientists (like myself) at my writer's meet-up group said house size iron asteroids would mostly burn up in the atmosphere and their repeated impacts would kill the planet with sun blockage. The first posts here were the same.

However, then someone who sounded more informed and who came with links to studies on this very thing posted saying that they would not burn away that much and the craters they would cause would be relatively safe and manageable. The real problem I face according to them (also an astronomer friend chimed in with pretty much the same) was guiding my house sized rocks to the safe and possibly preprepared landing area.

In any event, rewriting this part would have me rewriting perhaps 25% of the 1st book (completed) and even some of the 2nd book (80%). Why didn't I do my homework while I was in the process of writing this part. I was doing research but on the 'really' fantastic stuff in my book (like black holes traveling close to the speed of light) and just didn't think to check on this.

I do know that mining the rocks in space makes much more sense but there are some differences here. They need it not only for domestic construction but also for the huge superconducting rail gun to get them into space. There are A LOT of asteroids coming in from around their solar system (I say 100,000 in the book which I now think is a ridiculous number, what do you think?). They need most of the existing resources on the surface to process the ore. Building enough resources in orbit to handle the large number of asteroids would take too long and cost too much. They can barely afford trips to space now let alone building orbital factories.

Anyway... you sound like an informed person if you have any ideas to help with what I'm stuck with they would be greatly appriciated. Overall it looks doable based on studies already done.

Thanks so very much for your thoughts and help -- I do agree with you completely on a general level.

Rusty

 

[JaredJames]

Is there an official space elevator thread on this forum? You know, one where it's shown that the space elevator or tether can't possibly work even if you had the materials because you can't supply angular momentum to the thing you're trying to hoist?

I refer you to this post: https://www.physicsforums.com/showpost.php?p=2895250&postcount=129

"The core problem with your space hose is connected to the conservation of angular momentum. You cannot simply send an object into a state of higher angular momentum without robbing that angular momentum from something else. That's what the big rock at the top of the space elevator does. We have to capture an asteroid with a lot of angular momentum so that we can slowly impart its momentum to the satellites we send up there. The elevator cable doesn't work like a normal elevator cable, it must strain SIDEWAYS. The upward force is TRIVIAL compared to the sideways force that must be imparted by the cable on the satellite. This is why the vast majority of a rocket's flight is on its side and not upright. "

It isn't an official thread but it explains the concept of using the counterweight to impart the angular momentum on the cargo being raised.

 

[rrw4rusty]

My thread's been hijacked! Just kidding. I don't mind at all as I'm learning stuff I need to know possibly for future projects.

r

 

[jambaugh]

Dear Jambaugh,
... I also do not know how many g a human being can take or if there are other considerations.

That seems to make rail-guns for launching people rather expensive. I figure 5g's max for healthy general population and 10g's max for trained professionals. The record is almost 180g's in a rocket sled test.

I

You don't mention anything about height. The stratosphere extends from about 7 to 31 miles. Well, somewhere in this area for sure... now its construction limitations (at least for this know-nothing LOL). For now my new dimensions for the superconductive rail gun will be 1,150 miles long and 10 miles high (nice round numbers).

I figure height isn't an issue, the drag due to atmo. simply means your escape velocity will be reduced to a decent orbital velocity. Getting above the atmosphere is prohibitive with a rigid structure so once that's decided one needn't even angle the exit ramp up too much. (And one can use a lifting body to get an extra bit of lift and course adjustment on the way out.) Simply dig a "subway" tunnel install the mag-lev track (say 3 tracks at 120deg angles) and seal the tunnel with an iris opening on the end (or thin aluminum endcap which is pierced by the exiting missile.

My thought is that on an Earth sized planet with atmosphere we'd use linear accelerator launch to lift g-force hardened equipment and supplies (especially fuel and reaction mass). That coupled with tether slingshots would provide a good upward supply route until we can mine resources in space.

However on the moon it's a different matter as one may recall from reading Heinlein's "The Moon is a Harsh Mistress". Using my rule of thumb, the Moon's radius is 1737km so that's how long the track needs to be to escape the Moon's grav. at 1 Lunar surface acceleration. But that's only 1/6 of Earth's. At 1 Earth g, it need only be 1/6th as long. About 290km.
Allow 6g's and it need only be about 50km long. Very very do-able once we have mining and industry up there. (If we ever do. ).

For all the clever ideas; tethers and slingshots and rail guns and such, I think the most practical human launch system will be a NERVA (nuclear rocket) shuttle launched from high altitude jets. I don't foresee any real safety issues if it's launched over the ocean and if the engine isn't activated until high up in the atmosphere.

 

[D H]

I'm writing a science fiction story and in the future we are having to bring iron asteroids down to Earth.

I read science fiction on occasion, but I toss the story if the basis is so ludicrous that I cannot suspend my disbelief. This falls into that category.

Why go after iron in space for the purpose of sending it down to Earth? Iron ore is cheap, cheap, cheap. Current prices of high grade iron ore are on the order of 10 cents per kilogram. Current cost of putting something into low Earth orbit is well in excess of $10,000 per kilogram, but with some promise that that might fall by a factor of 2 or 3. You are going to need to send fuel and equipment into space (lots of it) to go and retrieve those asteroids, mine them, etc. A five or six order of magnitude increase in the cost of iron or decrease in the cost of entry into space is needed to make asteroid mining for iron viable.

Sans a space elevator, sending stuff into space will remain very expensive for a long, long time. Use the iron in those asteroids up in space. Don't send it up from the Earth, but don't send it down to the Earth, either. You can collect your volatiles in space, too. You need to make the stuff that is sent up into space worth the cost of doing so. People, and maybe some equipment that just can't be manufactured in space. Similarly, you need to make the stuff that is sent back down to Earth worth the cost of doing so. Iron just won't cut it.

There are things that do cut it. Iridium costs over $10,000 per kilogram, for example. We know that there are asteroids out there with a much greater relative abundance of iridium than occurs on the Earth: One such asteroid hit the Earth 65 million years ago. There have to be more of those out there.

 

[JaredJames]

DH, he isn't using Earth in his book. He explained that the planet he is using has limited resources and so has to get resources off-world.

 

[D H]

Still, yech. I would once again throw the story down on the basis of "come off it". Iron is one of the most common elements in the universe precisely because it is the most stable of all elements. Iron is more abundant in the universe than are nitrogen, silicon, sulfur, or calcium (some other elements that are essential for life as we know it).

As iron is essential to life as we know it, how did life even arise on such a planet in the first place? Assuming this came to pass, how do this alien civilization advance to the stage where they could venture into space? For that matter, how did this alien civilization advance beyond the ability to throw rocks and shoot arrows at one another? Assuming this civilization did get past these hurdles, what exactly is the need for iron if they learned how to get by without it?

 

[JaredJames]

I'm not sure if my recall is correct, but there was something about people going to this planet (not evolving there) and they need the resources to develop it. So far as a plot goes not the worst I've ever heard.

Out of curiosity, how do they calculate what is most abundant in the universe?

 

[Office_Shredder]

Out of curiosity, how do they calculate what is most abundant in the universe?

Counting

 

[JaredJames]

But then how do they know how much of a substance there is in another solar system / galaxy? You can't count that. (Can you?)

Just because Earth has a lot of it, doesn't mean everywhere does.

 

[DaveC426913]

Still, yech. I would once again throw the story down on the basis of "come off it". Iron is one of the most common elements in the universe precisely because it is the most stable of all elements. Iron is more abundant in the universe than are nitrogen, silicon, sulfur, or calcium (some other elements that are essential for life as we know it).

You are being unfair and judgmental. You do not know the premise of the story. You cannot judge it as unrealistic without understanding the elements in place that make the story in the first place. The are countless reasons why the peoples might be motivated to have their iron mining on-planet. The 'why' is the premise of the story.

 

[JaredJames]

You are being unfair and judgmental. You do not know the premise of the story. You cannot judge it as unrealistic without understanding the elements in place that make the story in the first place. The are countless reasons why the peoples might be motivated to have their iron mining on-planet. The 'why' is the premise of the story.

Plus, the moment you realize it's about another planet and mining asteroids it should be blatant that it's fiction. So the writer can do what he likes and you shouldn't expect it to be completely factual (or even slightly for that matter).

Reminds me of the debate in the 'faux pas' thread regarding sci-fi.

 

[DaveC426913]

Plus, the moment you realize it's about another planet and mining asteroids it should be blatant that it's fiction. So the writer can do what he likes and you shouldn't expect it to be completely factual (or even slightly for that matter).

What? No!

There is no reason why science-fiction can't be far-future and still adhere to a plausible and possible storyline. In fact, arguably, that is a requirement of sci-fi. If it blatanly violates plausibility, it enters the realm of fantasy.

 

[JaredJames]

What? No!

There is no reason why science-fiction can't be far-future and still adhere to a plausible and possible storyline. In fact, arguably, that is a requirement of sci-fi. If it blatanly violates plausibility, it enters the realm of fantasy.

I used fiction here not science fiction. I used it because I find the idea of landing house sized asteroids with any accuracy to be rather implausible, even in the far future (at least without some form of space elevator). I'm not saying it's impossible, just not very realistic as I see it.

 

[D H]

You are being unfair and judgmental.

Well, yeah, but I do value my rather limited spare time. I am particularly unfair and judgmental when it comes to hard sci-fi. I am a bit more forgiving with soft sci-fi, where the science fiction is really just a plot device.

I used fiction here not science fiction. I used it because I find the idea of landing house sized asteroids with any accuracy to be rather implausible

The Moon is a Harsh Mistress

 

[JaredJames]

The Moon is a Harsh Mistress

And that swooshing sound is this one going right over my head...

EDIT: Thanks to Google, I now know where that is from and have learned a new word 'polyandry'.

 

[D H]

Google that phrase then.

 

[DaveC426913]

I used fiction here not science fiction.

OK, hm. Well, fiction simply means it isn't an actual recounting of real events - which, being in the future - sort of goes without saying.

Being fiction says nothing about how fantastical the story might be.

 

[JaredJames]

OK, hm. Well, fiction simply means it isn't an actual recounting of real events - which, being in the future - sort of goes without saying.

Being fiction says nothing about how fantastical the story might be.

I agree, which is why I believe the sci-fi channel has become sci-fy.