The possible trajectories are quite specific: https://twitter.com/Marco_Langbroek/status/97903409781211136... because it will re-enter basically over its own ground track. So although we don't know exactly where this will happen, we have a very good picture of where it won't happen (and that is most of the planet).
The alternatives occur because it is losing speed only very gradually with each orbit through the upper atmosphere. Tiangong-1 goes around the planet every 88 minutes, and one of those orbits will lose enough speed and altitude that the vehicle disintegrates due to heat or drag. It's just that due to variations in the upper atmosphere, we don't know which orbit exactly will finally tip the balance.
This is in contrast to a controlled re-entry vehicle e.g. Soyuz that washes off something like 150m/sec in a single engine burn to place it on an known and specific sub-orbital trajectory to get home.
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The possible trajectories are quite specific: https://twitter.com/Marco_Langbroek/status/97903409781211136... because it will re-enter basically over its own ground track. So although we don't know exactly where this will happen, we have a very good picture of where it won't happen (and that is most of the planet).
The alternatives occur because it is losing speed only very gradually with each orbit through the upper atmosphere. Tiangong-1 goes around the planet every 88 minutes, and one of those orbits will lose enough speed and altitude that the vehicle disintegrates due to heat or drag. It's just that due to variations in the upper atmosphere, we don't know which orbit exactly will finally tip the balance.
This is in contrast to a controlled re-entry vehicle e.g. Soyuz that washes off something like 150m/sec in a single engine burn to place it on an known and specific sub-orbital trajectory to get home.