'Comet of the century' nears Earth

I've always found the concept of capturing a comet interesting and potentially very easy. Here is the idea, a comet is coming in for a close pass of the Sun, so we send a ship to the comet ahead of time with essentially a large kevlar bag with a hole on one end. As everyone knows, as comets come in closer to the Sun their volatiles come off in the form of the tail. Instead of wasting that material from the tail, encase the comet in a kevlar bag or some other material and 'point' the hole in the opposite direction you want to go. Essentially turning the comets volatile derived tail into a rocket. Most likely into a more circular orbit, closer to Earth, though obviously not too close, for use by space settlements. (If you are ambitious you could even crash it (or land it) into a lunar pole or if you are terraforming, into the Mars surface.)
Brooks