In the OT2 process all remaining Herschel observing time will be awarded. At the present time there is some uncertainty as to exactly how much this is. By now Herschel has a known track record with respect to observing efficiency, thus the remaining main contribution to this uncertainty is the overall mission lifetime uncertainty.

At all times during the mission it is necessary to have a 'buffer' of at least 6 months of observations available for scheduling in the mission planning process. Initially this was an estimate, but it has been validated as a good number by actual in-flight experience. What 'at all times' means is that also at the end of the mission there will be no less than 6 months-worth of observations remaining. These observations will thus never be executed, but they do need to be allocated observing time.

Both of these aspects, the uncertainty in the amount of available observing time and the fact that an 'over-allocation' of at least 6 months of observations is required, must be taken into account in the OT2 process.