A Herschel target is considered a 'Target of Opportunity' (ToO) if the observations are linked to the occurrence of an event whose exact timing and/or location in the sky are unknown at the time of the proposal submission deadline. ToO targets include objects which can be identified in advance but which undergo unpredictable changes (e.g. some recurrent novae) as well as objects that can only be identified in advance as a class (comets, novae, supernovae, gamma ray bursts, etc). Herschel proposals consisting of ToO targets in full or in part must present a detailed observing plan for the observations to be performed if the triggering event occurs. A generic name and dummy coordinates (if necessary) will be entered through HSpot for this purpose. Integration times will be estimated as a function of the brightness assumed for the event. Backup strategies can also be proposed (e.g. for not-so-bright comets). The triggering conditions and the reaction times necessary to accomplish the scientific goals proposed must be clearly described and justified in the proposal.

The HOTAC may recommend the tentative allocation of observing time provided these conditions are met and the reaction times required are compatible with Herschel operational constraints (typically a minimum of 5-7 days, although under exceptionally favourable circumstances reaction times of 3-4 days can be achieved, and this will be tried only for critical ToOs). It will be the responsibility of the proposer to urgently submit a ToO activation request to Helpdesk in case the conditions approved by HOTAC are satisfied. He/she will provide as well all the necessary information to initiate the preparation of the observations updating the details of the observations originally included in HSpot, if needed, so that the observations can be programmed at the earliest possible time, considering the reaction times solicited in the proposal.

In the event of a sudden phenomenon of a nature that could not have been anticipated, Herschel observations can also be requested through Director's Discretionary Time (DDT).