Understanding of the origin and evolution of galaxies, stars, planets, our Earth and of life itself are fundamental objectives of Science in general and Astronomy in particular. Although impressive advances have been made in the last twenty years, our knowledge of how the Universe has come to look as it does today is far from complete. A full insight of the processes involved is only possible with observations in the long wavelength infrared waveband of the electromagnetic spectrum. It is in this range that astronomical objects emit most of their radiation as they form and evolve in regions where obscuration by dust prevents observations in the visible and near infrared.
Over the past quarter of a century successive space infrared observatories (IRAS, ISO, Spitzer and AKARI) have revolutionised our understanding of the evolution of stars and galaxies. Mid to far infrared observations have led to stunning discoveries such as the Ultra Luminous Infrared Galaxies (ULIRGS), the basic processes of star formation from "class 0" pre-stellar cores through to the clearing of the gaseous proto-planetary discs and the presence of dust excesses around main sequence stars. The Herschel Space Observatory launched in 2009 will continue this work in the far infrared and sub-mm and JWST, due for launch in 2018, will provide a major boost in observing capability in the 2 – 28 µm range.
Previous infrared missions have been hampered by the requirement to cool the telescope and instruments to < 5 K using liquid cryogens. This has limited the size of the apertures to < 1 m and our view of the infrared Universe has been one of poor spatial resolution and limited sensitivity. The Herschel mission addresses the first of these by employing a 3 m mirror to dramatically increase the available spatial resolution but, because it is only cooled to 80 K, only offers a modest increase in sensitivity in the 55 – 210 µm range compared to previous facilities. JWST will provide a major increase in both spatial resolution and sensitivity but only up to 28 µm.
It is in this context that the JAXA led mission Spica (SPace Infrared telescope for Cosmology and Astrophysics) is proposed. Spica is an observatory that will provide imaging and spectroscopic capabilities in the 5 to 210 µm wavelength range with a 3 m telescope like Herschel, but now cooled to a temperature less than 6 K. In combination with a new generation of highly sensitive detectors, the low telescope temperature will allow us to achieve sky-limited sensitivity over the full 5 to 210 µm band for the first time.
With its powerful scientific capabilities, Spica will provide unique and ground-breaking answers to these key questions, and it is with this goal that the Spica science objectives have been defined:
- Formation and evolution of planetary systems:
- Gas and dust in proto-planetary discs, including water, and their link to planetary formation;
- mineralogy of debris discs;
- gas exoplanets atmospheres;
- composition of Kuiper Belt objects.
- Life cycle of dust:
- Physics and chemistry of gas and dust in the Milky Way and in nearby galaxies;
- dust mineralogy;
- dust processing in supernova remnants and the origin of interstellar dust in the early Universe.
- Formation and evolution of galaxies:
- AGN/starburst connection over cosmic time and as a function of the environment;
- co-evolution of star formation and super-massive black holes;
- star-formation and mass assembly history of galaxies in relation with large scale structures;
- the nature of the Cosmic Infrared Background.
Spica will be launched with JAXA's H2A-202 from the Tanegashima Space Centre in 2020 and is planned as a nominal three year mission (goal 5 years) orbiting at L2.
A large amplitude halo orbit is baselined, with a period of around 180 days and a semi-major axis amplitude of about 750 000 km.
The LEOP (Low Earth Orbit Phase) duration will be of about 3 days, the in-flight commissioning phase will last 2 months and the cool-down will take about 168 days.