Most likely Gene and Badoer alternating, though a few are rather optimistically calling for MS to return.
It was the damper spring from Barrichello's car, 4 seconds up the road (you can just see it going around turn 4). It wasn't bouncing back to Massa but going in the same direction, but due to the closing speed it appears as if it hurtles out from the trees towards him. So long as it stayed on a hard surface it could quite easily bounce around for a lot longer than those 4 seconds - keep in mind it would have been doing 130mph+ itself when it came off the car.
It's believed to be the spring marked 1 in this picture and weighs around 800g (or nearly the weight of a bag of sugar)... which is quite a lot when you hit it at around 150mph.
For the record, the nasty cut above his eye appears to have been made by the conveyor belt on the tyre wall, not as a result of the initial, more serious impact.