32nd URSI GASS, Montreal, 19-26 August 2017

The Herschel Space Mission

F. Herpin(1), E. Caux(2), and L. Pagani(3) (1)Laboratoire d’Astrophysique de Bordeaux, (2)IRAP, Toulouse, France (3)LERMA, Observatoire de Paris & CNRS, PSL, UPMC, France

1. Extended Abstract

Herschel[1] was a spatial submillimetre observatory with spectroscopic and imaging capabilities covering the range from 55 to 671 μm (0.44 to 5.5 THz) partly explored for the first time here. With a primary mirror of 3.28 m (effective diameter) and a surface accuracy better than 6 μm, it will remain the largest telescope launched until the arrival of the James Webb end of 2018. The observatory was equipped with a dewar that contained 2367 liters of liquid Helium at launch in May 2009. Positioned at the 2nd Lagrangian point, it functioned till April 2013. The telescope was passively cooled to 85 K. The total weight of the spacecraft was 3.4 tons at launch, including 315 kg for the mirror, 426 kg for the instruments and 335 kg of He. The payload was made of 3 instruments, 2 spectro-photometric cameras PACS [2] and SPIRE[3] equipped with bolometer arrays, and 1 heterodyne instrument, HIFI[4].

With Herschel, the last window of the electromagnetic spectrum opened up to our eyes. Herschel project started in 1982 when no IR astronomical had been launched yet. The handful of IR spatial observatories which appeared in between had in common a small primary mirror (60 to 80 cm) actively cooled to 4 K to push their sensitivity to its best. They also covered mostly the mid and far IR domains, between 3 and 200 μm. They were sensitive to the emission of objects warmer than 15 K thereby excluding of the dust, too cold (∼10 K) to be detected with these observatories. These instruments had also limited spectral resolution due to the compulsory use of optical devices (CVFs, Fourier transform, grating and Fabry-Pérot spectrometers). Herschel was conceived from the start as a totally different observatory: a much larger but passively cooled primary mirror with far IR and submm cameras with limited spectral resolution and a heterodyne spectrometer with a single pixel but very high spectral resolution. As a spatial heterodyne spectrometer, Herschel was preceded by two small spacecrafts, SWAS [5] and Odin [6] with limited capabilities. We will present the three Herschel instruments, highlighting HIFI which was a major step forward because of: - higher spatial resolution (3-5 w.r.t. SWAS/ODIN, 8 with ISO-LWS) - higher sensitivity (10 w.r.t. SWAS/ODIN) - higher spectral resolution than other missions (ISO, SPITZER, Astro-F,…) - larger colder aperture, better ‘site’, and more observing time than balloon- and airborne instruments.

Its primary targets were the cold dust, the light hydrids, with a special focus on H2O, and a few species of high interest like + C and O2 in both our Galaxy and other galaxies. Its main focus was on in all its possible aspects including cosmological metal enrichment evolution, prestellar cores, chemistry, outflow, cooling etc.

We will present a small selection of interesting scientific results covering the formation and evolution of galaxies, star formation and evolution, the molecular universe, and the planet formation and system. For instance, the revision of the + H2 content in our galaxy using C as new tracer to map the invisible hydrogen gas, the importance of filaments in star formation, clues to creation of Earth's oceans, cosmic dust origins, or the origin of water on giant planets.

Figure 1. Herschel and Vela C. Copyright ESA/PACS & SPIRE Consortia, T. Hill, F. Motte, Laboratoire AIM Paris-Saclay, CEA/IRFU – CNRS/INSU – Uni. Paris Diderot, HOBYS Key Programme Consortium.

2. References

1. G. L. Pilbratt, et al., “Herschel Space Observatory. An ESA facility for far-infrared and submillimetre astronomy”, 2010, A&A, 518, L1, doi :10.1051/0004-6361/201014759.

2. A. Poglitsch, et al., “The Photodetector Array Camera and Spectrometer (PACS) on the Herschel Space Observatory”, 2010, A&A, 518, L2, doi :10.1051/0004-6361/201014535.

3. M. J. Griffin, et al., “The Herschel-SPIRE instrument and its in-flight performance”, 2010, A&A, 518, L3, doi :10.1051/0004-6361/201014519.

4. T. de Graauw, F. P. Helmich, T. G. Phillips, et al., “The Herschel-Heterodyne Instrument for the Far-Infrared (HIFI)”, 2010, A&A, 518, L6, doi :10.1051/0004-6361/201014698.

5. G. J. Melnick, et al., “The Submillimeter Wave Astronomy Satellite: Science Objectives and Instrument Description”, 2000, ApJL, 539, L77, doi :10.1086/312856.

6. A. Hjalmarson, et al., “Highlights from the first year of Odin observations”, 2003, A&A, 402, L39, doi :10.1051/0004- 6361:20030337.