Herschel was launched on 14 May 2009, and was designed to provide the astronomical community with a facility to carry out routine astronomical observations for a period of three years. The current best estimate of the total mission lifetime (thus including the early mission phases up to and including the science demonstration phase) counting from the launch is in the range of 3.5-4 years.

The available observing time is divided into Guaranteed Time (GT) and Open Time (OT). In the nominal mission, providing 3 years of routine science phase operations, 19,776 hours of schedulable observing time is available, 32% is GT and the remainder is OT. The GT is owned by contributors to the mission, primarily by the instrument Principal Investigator (PI) consortia. The OT is made available to the worldwide astronomical community, including the GT holders, through a standard competitive proposal procedure.

The current AO is the second (and final) in-flight call for proposals. It has been preceeded by the Key Programmes AO in 2007, and the first in-flight call, AO-1, in 2010.