The Herschel Space Observatory

The ESA Space Observatory Herschel will be launched from the Guiana Space Centre, Kourou, French Guiana, at April 16th, 2009 by an Ariane-5 ECA, together with ESA's Planck spacecraft, a mission to study the cosmic microwave background radiation.
Both spacecrafts will separate after launch and will be directly injected into a Lissajous orbit around L2, the second Lagrange point of the Sun-Earth system, at a distance of around 1.5 million km from Earth. L2 is chosen since the Sun, Earth and Moon are intense sources of both straylight and thermal radiation, and at this location these sources are all easily shielded from the payload.

Mission

Herschel represents a landmark mission in many regards. It is the only space facility to cover the far infrared to sub-millimetre parts of the spectrum (from 60 to 670 µm), which cannot be observed well from the ground. Many interesting astronomical phenomena become observably within this spectral range like dust obscured and cold objects, but also the forming of galaxies in the early universe and their evolution. Furthermore, the prime mirror of the Herschel telescope with 3.5 m in diameter is the largest mirror ever built for a space telescope and finally, three extraordinary spectrometric instruments comprise the Herschel science payload, provided from scientific institutes in ESA member states, Canada and the USA.
Herschel has a nominal routine operational lifetime of three years, with a possible extension of one year. About 7000 hours of science time will be available per year. Herschel is a multi-user observatory accessible to astronomers from all over the world. We describe in the following some scientific and technical aspects.

A) Astronomy

Aurora Sicilia-Aguilar, Max-Planck-Institute for Astronomy Heidelberg, Germany

Among many others, one important field of study for Herschel will be the formation of stars and planetary systems. Herschel will trace not only the disks where planetesimals are forming, but also reveal the initial conditions in a collapsing molecular cloud that will produce a cluster of stars with their protoplanetary disks.

The Complexity of Proto-Planetary Systems

The formation of planetesimals and planets takes place inside protoplanetary disks, composed of gas and dust, which are usually too far to be imaged in detail or even resolved. Multiwavelength observations, covering the range from ultraviolet to millimetre wavelengths, help to construct a picture of the whole system since the wavelength at which the emission peaks depends on the temperature (Fig. 1). Optical images trace the young star (with a temperature around 4000-6000 K) providing its luminosity, age, and radius. Near-IR observations show the innermost, warm (T~1000 K) part of the disk. Mid-IR data reveal the colder (~300 K) planet-forming regions. Finally, millimetre data trace the bulk of the cold (~20 K) disk material, which comprises most of the mass available for planet formation. Planet formation is thought to open inner holes and gaps in the disks, which can be observed as a lack of near-IR emission from the disk.

The Origin of Planetary Systems

The NASA Spitzer Space Telescope showed in singular detail the structure of the innermost disk. While most of the mass in the disk, optically thick, produces a continuum emission (similar to a collection of black bodies integrated over the range of disk temperatures), the material in the optically thin upper layers of the disk (disk atmosphere), overheated by the external radiation, shows an emission spectrum with the lines of some gaseous components and the solid state features of small (micron-sized) silicate particles. The analysis of the dust emission reveals grains with different silicate composition (olivine, forsterite, enstatite, silica), variable sizes, and in crystalline or amorphous state, which indicates strong dust processing and grain growth in protoplanetary disks. Spitzer, together with ground-based facilities operating in the millimetre range, confirmed that a large fraction of the dust accumulates into large grains, which remain “hidden” from the view of current instrumentation. Moreover, the disk structure varies from the inner to the outer parts, and may determine the type of planetary system that can arise from the disk.

Herschel: Probing Colder Regions, Larger Grains, and Disk Chemistry

The instruments available so far have shown the importance of studying the larger dust grains as well as colder disk regions. Observations in the 100-600 µm range are crucial to obtain the size distribution of dust grains in disks, which is necessary to estimate the total dust mass available to form planets. It is also required to determine the structure of the planet forming region in solar-like planetary systems during and after planet formation. Colder systems, similar to our Kuiper Belt, will be also traced by observations in this wavelength range. Herschel fills in the gap between near- and mid-IR instruments like Spitzer, and millimetre and submillimetre wavelength detectors like APEX, the precursor of ALMA (Fig. 2). It will also allow to study the disk mineralogy of non-silicate components, as well as the chemistry in the planet-forming regions, in particular, the water-related chemistry. In addition, Herschel will probe the early stages of star formation, which occur in heavily obscured molecular clouds and condensations, opaque at IR wavelengths. Most of the energy of a collapsing protostar is emitted at Herschel wavelengths, and Herschel is powerful enough to detect the initial phases of formation of objects down to brown dwarf masses, as well as details of the inner structure of protostellar condensations. These initial stages of star formation determine the initial disk characteristics and probably the fate of the planet-forming disk and the kind of system (multiple star, different types of planetary systems) that will be formed.

(See also the Engelberg Lectures 2007 -> "Stars, Disks and Newborn Planets: Imaging Proto-Solar Systems".)

B) Technology

Bernhard Braunecker, SPS-Secretary

The Spacecraft

The Herschel spacecraft is approximately 7.5 m high and 4 x 4 m in overall cross section, with a launch mass of around 3.3 tonnes. The spacecraft comprises three sections, the telescope, protected by a sunshade, the scientific instruments in the focal plane of the telescope and third, the service modules with the electronics for the instruments and the satellite.

Telescope

The Herschel telescope is a Cassegrain design, i.e. a concave parabolic prime and a convex hyperbolic secondary mirror, the latter acting as entrance pupil (Fig. 3). The overall focal length is 28.5 m with f-Number 8.7. The diameter of the secondary is kept rather small with 34 cm, not to obscure too much the collecting area of the prime. The distance between both mirrors was specified to only about 1.6 m to achieve a compact packaging size of the telescope.
Both requirements, however, lead to an extremely ‘fast’ prime mirror with f-Number 0.5 at a diameter of 3.5 m! Consequently, the optical performance of the telescope is enormously sensitive to any mechanical deformations of the mirror surfaces or to changes of the spacer lengths. Since the telescope will be operated at 70 K, and since no active refocusing is foreseen, its construction must be athermal. Thus EADS Astrium in Toulouse, who fabricated the unit, used sintered SiC-100, a silicon carbide technology, for both mirror structures and the spacers, together with a special cryogenic Invar M93 for the spacer fittings inside the optical cavity.
But cryogenic tests performed at the Focal 6.5 facility in Centre Spatial de Liège, at the University of Liège, Belgium emphasized that large and ‘fast’ telescopes always surprise with unexpected problems. Some serious discrepancy was found during testing between the prediction and measurement of the telescope back focal length at its cryogenic operating temperature of 70 K.
A team of engineers and scientists including the author were collocated at ESTEC / NL, the technology centre of ESA, to review and independently rework the test results in the context of the mission requirements and predictions for the behaviour of the telescope in its operational environment. An insufficient CTE measurement accuracy was identified as the root cause of the unexpected discrepancy (see Engelberg Lectures 2007 -> "Technology Review of Large Space Telescopes").

Instruments

Herschel carries three scientific instruments:

• HIFI (Heterodyne Instrument for the Far Infrared), a very high resolution heterodyne spectrometer to cover two bands, 480–1250 GHz and 1410–1910 GHz, using superconducting mixers as detectors (see Engelberg Lectures 2007 -> "Terahertz Astronomy on the Herschel Space Observatory").
• PACS (Photoconductor Array, Camera and Spectrometer), an imaging photometer and medium resolution grating spectrometer, operating simultaneously in two wavelength bands, 60–130 µm and 130–210 µm, with bolometer and photoconductor array detectors.
• SPIRE (Spectral and Photometric Imaging REceiver), an imaging photometer and an imaging Fourier transform spectrometer, providing broadband photometry simultaneously in three bands centred on 250, 350 and 500 µm.

All three Herschel instruments will be cooled by the cryostat filled at launch with more than 2000 litres of superfluid helium, kept colder than –271 °C. Further cooling down to 0.3 K is required for the SPIRE and PACS ‘bolometer’ detectors. When the helium has been evaporated, Herschel will no longer be able to perform observations. Enough data will hopefully have been collected to satisfy astronomers’ needs for the next decade.