Wall Street Journal op-ed on private space development and asteroid mining

The Wall Street Journal has an op-ed today here on private companies in space and asteroid mining in particular, with an estimate as to the average value of a 500-metre asteroid at $20 trillion. The problem with an asteroid of that size though is that it would be impossible to move into Earth orbit and mining would have to be done remotely, either by using humans living on the asteroid (which means an extremely long mission) or by mining it with robots. Far better would be to use this idea, which would involve capturing a smaller asteroid and moving it into Earth orbit. The upper limit for such an asteroid should be about 20 metres, since anything larger than that wouldn't break up in Earth's atmosphere were something to go wrong. Even placing an asteroid into an orbit much farther than the Moon would be acceptable, since at least this would mean that the asteroid was orbiting us and not the Sun, so there would be no waiting for launch windows to occur, which always take at least a year and sometimes two or more.