[ido_now]

NASA knew they were dead before re-entry, or suspected as much.

Is it impossible to have another shuttle always ready for a rescue effort if trouble is expected?

People are less expensive I quess.

[jeremiah]

How about, they knew nearly immediately upon launch, and dithered for days. It was too late to send a rescue, because they wasted days in debate and paperwork.

[GingisK]

Is it impossible to have another shuttle always ready for a rescue effort if trouble is expected?

Yes. Due to budget cuts there is not a set of main engines for each orbiter. The main engines must be moved from the bird that just landed to the one that is being prepared for flight.