NASA launches spacecraft to test asteroid defense concept

[reACTIONary]

2. The target is the smaller of two asteroids...

... which is orbiting the larger one like a mini-moon. The orbit can be observed and precisely determined from earth. So any change in the orbit caused by the impact can be observed and measured using earth based observations.

This is what makes this experiment practical. Otherwise, you would have to build two spacecraft, an impactor and an observatory. It would be very costly and difficult to build and deploy a sufficiently sophisticated observatory. This way we can observe from the comfort of home, for as long as we need to.