EXPLORATORY SUBMM SPACE RADIO-INTERFEROMETRIC TELESCOPE (ESPRIT) W. Wild 1,4 , Th. de Graauw 1,4,5 , F. Helmich 1,4 , J. Cernicharo 2 , A. Baryshev 4,1 , A. Bos 3 , J.-W. den Herder 1 , A. Gunst 3 , B. Jackson 1 , H-J Langevelde 6 , P. Maat 3 , J. Martin-Pintado 2 , J, Noordam 3 , A. Quirrenbach 5 , P. Roelfsema 1,4 , L. Venema 3 , P. Wesselius 1,4 , N. Whyborn 1 , P. Yagoubov 1 1 SRON, P.O.Box 800, Landleven 12, 9747 AD Groningen, The Netherlands, Email: W.Wild@sron.rug.nl 2 CSIC, Madrid, C/Serrano 113-121, 28006 Madrid, Spain 3 ASTRON, Dwingeloo, P.O.Box 2, 7990 AA, Dwingeloo, The Netherlands 4 Kapteyn Astronomical Institute, P.O.Box 800, 9700 AV Groningen, The Netherlands 5 Leiden Observatory, P.O.Box 9513, 2300 RA Leiden, The Netherlands 6 Joint Institute for VLBI in Europe, P.O.Box 2, 7990 AA, Dwingeloo, The Netherlands Abstract Angular resolution in the far-infrared (FIR) wavelength regime is still very limited although the FIR range has become of prime importance for astrophysics. Investigations of the star- and planet formation process occurring in interstellar clouds, and on the lifecycle of gas and dust in general require, besides high angular resolution, also high spectral resolution. To combine both characteristics, we propose a heterodyne aperture-synthesis mission concept, ESPRIT, to operate in a wavelength regime that is neither accessible from the ground by ALMA (Atacama Large Millimeter Array), nor with JWST (James Webb Space Telescope). Keywords: FarInfrared, Submillimeter, Interferometry, Space Instrumentation, Space mission, Star Formation, Spectroscopy. 1. INTRODUCTION The study of star and planet formation is one of the prime topics in modern astrophysics. Important questions include the physical conditions for star-formation to occur, the evolution of circum-stellar disks, the decoupling of dusty proto-planetary regions from the gas, and the chemistry that leads to the pre- biotic conditions of early Earth-like planets. In addition, we also would like to know what role star- formation, and in particular starbursts, play in external galaxies and how this interacts with the general interstellar medium. As the FIR wavelength range holds the most important spectral signatures of the material (atoms, ions, molecules) involved, a FIR space mission is required. The low extinction at these long wavelengths allows unique observations of details of the star formation process, in particular during its early phases, when these regions are completely obscured by the surrounding dust. The Earth atmosphere severely limits the possibility to observe at THz frequencies from ground-based observatories. ALMA will cover the atmospheric windows at very high angular resolution up to about 1 THz (the limit for observations even from high-altitude sites). Astronomical observations above 1 THz need to be done from space. However, all past, current and planned missions have limited angular resolution. The relatively small ratios of aperture diameter to wavelength, like for example in ISO, Spitzer Space Telescope and Herschel, provide only angular resolutions of the order of 5 arcsec in the 100μm region. This does not match the 0.1 arcsec resolution which is required for these studies. In order to achieve the required angular resolution, to investigate for example the distribution of key molecules in a circum-stellar disk, application of interferometer techniques in space is the only way forward. At the same time, high spectral resolution is required to measure the chemical composition, the dynamics and other physical conditions. In particular, studies of water and other hydrides, together with the isotopic/deuterated versions, are of prime interest for the star formation process. We will present a mission concept that combines all these capabilities and is therefore uniquely suited to address these questions: a free-flying, 6 element, far-infrared imaging interferometer using heterodyne detection. The results from ESPRIT will be highly complementary to TPF/Darwin as it will fill the gap and establish the link between formation of a planet and detection of a planet. It is also complementary to JWST-MIRI since ESPRIT will observe the evolution of these regions in the epoch prior to becoming observable in the mid IR.