The `Herschel Space Observatory' (or simply `Herschel' - formerly FIRST) is the fourth Cornerstone mission in the European Space Agency (ESA) science programme. It will perform imaging photometry and spectroscopy in the far infrared and submillimetre part of the spectrum, covering approximately the 57 - 670 μm range.
The key science objectives emphasize current questions connected to the formation of galaxies and stars, however, having unique capabilities in several ways, Herschel will be a facility open for observing time proposals from the entire astronomical community. Because Herschel to some extent will be its own pathfinder, the issue of instrument calibration and data processing timescales has special importance.
Herschel will carry a 3.5 metre diameter radiatively cooled passive monolithic telescope. The science payload complement - two cameras/medium resolution spectrometers (PACS and SPIRE) and a very high resolution heterodyne spectrometer (HIFI) - will be housed in a superfluid helium cryostat. Herschel will be placed in a transfer trajectory towards its operational orbit around the Earth-Sun L2 point by an Ariane 5 (shared with the ESA cosmic background mapping mission Planck) in 2007.
Once operational Herschel will offer a minimum of 3 years of routine observations; roughly 2/3 of the available observing time is open to the general astronomical community through a competitive proposal procedure.
This paper intends to provide a selfstanding overview of the Herschel mission, and to serve as an introduction to the more specialised Herschel papers that follow in this volume.