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.
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.
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.