Impactors or explosive missiles have the advantage of not having to match velocity with an Earth-bound asteroid, so they are cheaper and quicker to launch (or so I understand), and detection need not be as early. But they run the risk of fragmenting the asteroid and making the problem harder to deal with.
So why not send a missile that releases a cloud of dense gas, such as xenon or even one of those hexaflouride gases, into the path of the incoming asteroid? The cloud would not be very dense, but it would be huge, and the relative velocities could be enormous. The cloud would not slow the asteroid with kinetic impact, but with aerodynamic drag. I suppose the tricky part would be timing the release so that the gas does not dissipate too much. Could it deflect the asteroid appreciably?
The scenario I envision is this: we spot an incoming asteroid. We launch a rocket on a trajectory to pass closely in front of the asteroid. When the rocket reaches that point, it releases a cloud of gas so that the asteroid then has to pass through it. This could be done several times in sequence.
I do not envision a cloud surrounding the Earth (we already have one) or hanging in space for days or weeks. I envision a high-speed probe aimed to intersect an incoming asteroid's path, then exploding perhaps a millisecond before it crosses the asteroid's path. In that millisecond, the cloud of gas would expand to some tremendous size, perhaps kilometers wide, that the asteroid must pass through. Several probes could create a series of expanding clouds, one after the other. While the gas cloud will expand rapidly, reducing its density, a proper calculation would take that into account, by integrating over time. Concepts from fluid dynamics such as drag or shock obviously apply, because they apply to mediums as rarified as the solar wind.
So the question really is, how much aerodynamic drag could a cloud of gas expanding into vacuum exert on a largish asteroid? Could it significantly alter the asteroid's course or speed?
(At least two other people, one the famous astronomer Eugene Shoemaker, have suggested similar ideas, according to Wikipedia.)
(Another question asks whether a cloud of gravel could deflect an asteroid with kinetic impact without breaking it apart. This is a different suggestion. Other questions about the behavior of liquids or gas are also different.)