Asteroid mining: on purposely moving a near-Earth asteroid into Earth orbit

There's a pretty interesting discussion going on right now on Reddit about mining asteroids, and specifically about moving an asteroid into Earth's orbit in order to do so. The idea of giving the Earth a second moon (however small) has always been an appealing idea, and even though the asteroid proposed here is extremely tiny, it would still be a pretty impressive feat.

The idea is quite simple: there is a certain size of asteroid that is still fairly large in terms of resources, but small enough that Earth's atmosphere would cause it to break up before hitting the ground. A near-Earth asteroid like this would be the ideal candidate, since even in a worst-case scenario it would simply burn up in the atmosphere.

So how much material would be inside an asteroid of this type? Luckily the poster there (Lucretius, a poster with some pretty interesting ideas and one that first got me thinking about colonizing Cruithne 3753) has done some of the calculations for us.

First, asteroid 2008 TC3 is given as an example, since it broke up on its way down when it hit Earth last year. It was 2-5 metres in diameter. A solid asteroid with a diameter of 5 metres gives a mass of 73,000 kg, which is the equivalent of 4.9 launches of SpaceX's Falcon-9 Heavy, which is the most massive rocket of theirs they are planning to make, and at best (since SpaceX is much cheaper than other methods of launching payload to space and they still haven't developed that rocket either) that would still cost a minimum of $382 million.

But let's also remember that asteroid 2004 FH (diameter 30 metres) would likely have burnt up in the atmosphere too if it had hit, so there should be no problem in going with an asteroid some half its diameter. 2004 FH is some 28,000 tons, so let's be really safe and say an asteroid with some 10,000 tons of mass should be doable, which works out to the amount of mass that could be carried up with some 660 launches or so.

So just how big would an asteroid like this be? Here's what a 15-metre asteroid would look like next to the ISS.

Looks quite small. Don't forget, however, that the ISS is very thin and composed mostly of space, whereas the asteroid next to it is completely solid. Also, the ISS has taken over 11 years to reach its present state. The ISS has a mass of 300 tons, so no comparison to the mass of the asteroid next to it there.

One other potential use for an asteroid like this mentioned in the thread is a possible steady surface for telescopes that would otherwise have to use gyroscopes to remain steady.

Also note that a 15-metre asteroid really is quite large when you compare it to the size of a human. Here's a stick man standing on the surface to show you what it looks like.