Good questions.  I think a great deal depends on the distance and quantity of the asteroids, as well as the math, which isn't available in the Galactopedia (you have to check in game to see how many sheilds they are producing).  As asteroids get further away, their value diminishes greatly.  But if you have a bunch near your planet, then when you have some breathing space, kick out a miner or two. 

At the colony rush stage, I will usually have my miner build 2-3 mines before I upgrade it to a colony ship.  Other than that I am with you, one rarely has time to spit out spaceminers and when you do, you are usually already winning.  If there are no galactic resources left to mine, and I am not ready to go to war, then I will usually produce a couple space miners.  That being said, think about the math (and please correct where I am wrong):

A manufacturing center produces 10 shields and takes, later in the game, oh, 10 turns to build.  A level II mine spits out, err, 10 shields and also takes 10. But, the number of space mines are  limited only by the number of asteroids, which, depending on your settings, can be very numerous.  I may only be able to build 5 manufacturing centers on the planet but could be surrounded by a dozen asteroids.  In this case, I may get more production from the asteriods than the manufacturing centers.  So from the math, as you can see, they can be very competitive with production planetary improvements.  This is assuming that the mines are close by.  If they are two squares away you usually get 4/5 shields for Level I, which is very helpful. Why is this worse/different than a Xeno Factory, which produces only 6 mp?  You could make the reverse argument, why bother with xeno factories when I could just build space mines. They take 5 turns to get 4/5 MPs? Great deal.

There is another aspect to the math:  space mining is expensive to research and other civs will not trade it to you under any circumstances.  I think it is actually a bug.  I remember trying to trade my home planet for space mining and the AI civ told me that "Space Mining is too important to our strategic goals to trade at this time."  So, I'll usually only have time to research the other levels when I'm producing so much research that I can knock it out in 1-2 turns.  This usually means I'm winning anyway.  The argument could be made, though, that instead of researching manufacturing centers, you could research SM and then trade for MC's when the AI researches it (they will trade MC's, for a lot). 

Therefore, one thing to try is to research space mining and offset this by trading for the production improvements, and have space miners continuously a-building.  Once again, provided that the asteroids are close by.

Here are my questions: 

1) Do the MP's from space mines contribute to your base production or are they added to your production after bonuses are applied? If the latter is the case, then much of the reasoning here is weakened.

2) Do space mines have maintenance like PI's?

3) I have noticed in the campaigns, against the Dread Lords, that they tend to ignore your ships if they are hiding in asteroid fields.  I have observed this time and again, although it is not 100% of the time.  There is simply a drastic reduction in the number of times the Dread Lords go after your ships if they are hidden.  Is this an illusion or is it part of the AI?

 I don't use the all-factory strategy?  So-so planet I have 3 MC's producing 10 MP each = 30 MP, Capital producing 14 MP, total 44 MP.   150/44 = 3.4 (or 4) turns, and that's the case if I fund social 100%.    That's for a new MC, with no advance factories needing to be upgraded, etc.  33/33/33 and we have 3.4x3 = 10.2 = 11 turns for new MC. 

I'm not sure how you build MC's every 3 turns on average without funding social 100%.  I don't get that very often.  I might get that on a planet with the manufacuring capital and quite a few factories.  We must be playing different games or one of us needs some sleep.  Probably me.

Thanks for the answers.  Space mines have no maintenance and they contribute to pre-bonus production.  Check.  Both good things.  I stated clearly in the post that one's decision to utilize space mining depends on abundance and distance.  No disagreement there.  One also must consider that asteroid production doesn't take up a tile on the planet as an MC does, freeing that tile for other uses, so there is additional benefit there.  Finally, you are calculating the cost of the space mine forgetting that in practice, most level II mines are upgrades from basic, which gets 4/5 MPs the whole time that the upgrade is occuring (right?).  So it is normally not the case that I am getting nothing from my investment in miners until the 15 turns are up.  From what you are saying, it seems you are actually agreeing that space mining can be cost-competitive with MC's over the long haul. 

There are improvements in the game that almost universally pointless (fusion power plants, e.g.).  I don't think space mining is one of them.

I love asteroids, so I play with the 'dense asteroid' settings on. Asteroids everywhere. The difference in productivity is so huge that I get used to planets having the extra production boost. Planets that receive no asteroid mining bonus, even though they have 4 Industrial Sectors and a Quantum Power Planet, seem to be absolutely lame in production. In fact if you have a Galactic Project to complete, by running all asteroid mines production to that 1 planet, you can usually cut the social prodcution time in half.

I don't see why people hate asteroids so much, and why they don't use them... I usually have 1 planet keep pumping out the cheap ole Space Miners and just let them do their thing. Once a turn or so you need to tell it where to mine next when one is done, but it's not even a micro-manage issue. And over time, you don't notice the increase in production since it so slowly becomes avaiable, but if you destroy all your asteroid mines, you would notice an enormous difference. warships would probably take double the time to be built on an average planet.

I'm liking the direction of this post. Willy & GD, I think you are on to something with the "set one planet to spam tiny miners". What I think would make this work particularly well, which Willy obliquely mentions, is the fact you can set the miner to "auto" and just let them do their thing. I had forgotten about this and trying to manage miners and trying to determine how far an asteroid is up the development cycle was alienating me. I'd love to have all the production points of those asteroids without all the BC cost, I just didn't want to micromange the process. Voila, turn on the auto feature, you dummy!

Another thing I have found is that the AI will trade his space miner to you as well, so if you want some in the early game you can get them through the diplomacy screen. For my two cents I'll probably let the AI keep them and just buff my influence ability so as soon as the AI miner completes a mine it flips to me anyway.

This principle applies to any of the modules (constructors, transports, spore, survey, etc) as miniturization improves.  A good tactic in general. Remember that you get civ-wide speed bonuses for ion, impulse, warp and some race specific techs,  in addition to starting speed bonus if you have selected it.  You can have a constructor move at 4-5 with no engines on a tiny hull.

FYI, Two of the tournament boards ("Arcea's Last Stand" and "Rock and a Hard Place") are really good places to test your space mining theories, in addition to being really fun.

I read that as from a time perspective, personally.

Thats 9 Industrial Sectors worth, and that's just the base MP. Now granted, asteroid mines are way too easy to destroy, but I have yet to ever see an enemy destroy one. They usually fly right past towards a planet or starbase. Usually you don't even need to protect them.

Because the last thing I would want to do is rebuild a mining asteroid for the next 75 turns because of a Korath Super Duper Corvette popped up right next to it with nothing better to do. (This did happen, but the Corvette just sat right next to my mines while I was at war. In fact it sat there for almost a year).

See, if they could do that, I'd use them outside of the first 6 months.

But until then...fat chance.

I used to enjoy blowing the AI's asteroid mines to smithereens....until I realized it was better to have them switch allegiance to my glorious empire instead. I haven't yet seen the AI attack an asteroid mine and I've been playing on suicidal for a few weeks, so it's probably a rare event in general.

The problem with letting the space miners be automated, is that it seems they often will not keep improving the same asteroid field until it is fully built up... instead they often leave them at the basic level and will fly halfway across the galaxy to some remote asteroid in the very corner of an immense map. I tend to keep a close eye on my space miners to make sure they don't do dumb things like this.

Since the AI probably uses this same method as the human 'automate' option, that is the reason you will rarely ever see an AI opponent ever have an advanced asteroid mine.

They love to research SM I.  In my games they rarely research SM II and never III.  To be honest, I rarely have the luxury of researching SM II and III unless the game is already in the bag.  On games where I have pursued SM more diligently I try to time the research to correspond exactly to when the miners are finishing available asteroids. I might take a break from whatever I'm researching to give the miners the tools they need.

You're totally right though, they don't build enough miners.  If they're going to go through all the trouble of researching the tech, they need to prosecute that advantage.

Asteroids are a waste of time.