The Herschel Space Observatory (Herschel) is an ESA (European Space Agency) project with instruments funded by ESA member states. It was operated from May 2009 till April 2013, offering unprecedented observational capabilities in the far-infrared and sub-millimeter spectral range (55-671 microns [um]). Herschel carried a 3.5m diameter passively cooled Cassegrain telescope, which was the largest of its kind and utilizes a novel silicon carbide technology. The science payload comprised three instruments: two direct detection cameras/medium resolution spectrometers, the Photodetector Array Camera and Spectrometer (PACS) and the Spectral and Photometric Imaging Receiver (SPIRE), and a very high-resolution heterodyne spectrometer, the Heterodyne Instrument for the Far Infrared or HIFI, whose focal plane units were housed inside a superfluid helium cryostat. PACS comprised two mutually exclusive sub-instruments: a bolometric camera designed to perform photometry in three spectral bands (70, 100 and 160 um) and an integral field unit grating spectrometer operating over the spectral range from 57 to 210 um with a spectral resolution ranging from 1000 to 5000. SPIRE comprised a three-band photometer, operating in spectral bands centered on 250, 350 and 500 um, and an imaging Fourier-Transform Spectrometer (FTS), which provided low resolution spectra over the 195-670 um band. Both instruments used germanium bolometers operating at 0.3 K and coupled to the telescope with hexagonally conical feedhorns. The photometer and the spectrometer were not designed to operate simultaneously. HIFI was designed to obtain spectra with very high resolution (up to 10⁷) in the far-infrared and sub-millimeter wavelengths not directly observable by ground-based telescopes. The HIFI instrument was an heterodyne receiver which provided spectroscopy in the continuous frequency range 480-1250 GHz (240-625 microns) and in the frequency range 1410-1910 GHz (157-213 microns). Herschel had two Announcement of Opportunities (AOs) for Open Time (OT) observations. The first in-flight AO for Open Time (OT1) was opened on 20 May 2010, with a deadline of 22 July 2010. For OT1, 241 observing programs were accepted and the total allocated observing time amounts to 6576.9 hours. The second in-flight AO for Open Time (OT2) was opened on 9 June 2011, with a deadline of 15 September 2011. There were parallel AOs for Guaranteed Time observations, GT1 and GT2, with separate deadlines. The Announcement of Opportunity (AO) for Herschel Key Programs (KP) was issued on 1 February 2007, with separate deadlines for guaranteed time (GT) and open time (OT) proposals. The whole Key Program AO process has now been completed, and by coincidence there were exactly the same number of KP GT and OT programs, in both cases 21 programs were awarded observing time. Taken together, these 42 observing programs contained 11,650 astronomical observation requests or AORs (AORs are the primary units of Herschel observing time and are effectively the Herschel 'observation units'). The total allocated observing time for these programs was 11,257.7 hours, corresponding to approximately 57% of the nominally available Herschel routine mission science time. Herschel successfully made over 37,000 scientific observations before its helium cryogen was exhausted. The HSA is available at the Herschel Science Centre at <a href="http://herschel.esac.esa.int/Science_Archive.shtml">http://herschel.esac.esa.int/Science_Archive.shtml</a>;, the Herschel help desk is at <a href="http://herschel.esac.esa.int/esupport/">http://herschel.esac.esa.int/esupport/</a>;, the Herschel User Provided Data Products are available at <a href="http://herschel.esac.esa.int/UserProvidedDataProducts.shtml">http://herschel.esac.esa.int/UserProvidedDataProducts.shtml</a>;, the Herschel Postcard Server is at <a href="http://archives.esac.esa.int/hsa/aio/doc/postcardGallery.html">http://archives.esac.esa.int/hsa/aio/doc/postcardGallery.html</a>;, and the Herschel Observation Log is at<a href="http://herschel.esac.esa.int/logrepgen/observationlist.do">http://herschel.esac.esa.int/logrepgen/observationlist.do</a>; This table was created by the HEASARC in October 2013 based on <a href="https://cdsarc.cds.unistra.fr/ftp/cats/VI/139">CDS Catalog VI/139</a> file herschel.dat. This is a service provided by NASA HEASARC .