Tiangong-1 crash--why it's so hard to predict where space debris will land and what can be done about it

The now defunct Chinese space station Tiangong-1 is en route to crash into Earth - completing its "atmospheric reentry phase". While experts have been aware that this would happen for more than a year, there has been huge uncertainty around the exact timing. As the station's orbital altitude has decreased, however, this uncertainty has gradually reduced and it is now possible to determine that it will deorbit within a few days.