Submillimeter interferometry has the potential to image supermassive black holes on event horizon scales, providing tests of the theory of general relativity and increasing our understanding of black hole accretion processes. The Event Horizon Telescope (EHT) performs these observations from the ground, and its main imaging targets are Sagittarius A* in the Galactic Center and the black hole at the center of the M87 galaxy. However, the EHT is fundamentally limited in its performance by atmospheric effects and sparse terrestrial (u, v)-coverage (Fourier sampling of the image). The scientific interest in quantitative studies of the horizon size and shape of these black holes has motivated studies into using space interferometry which is free of these limitations. Angular resolution considerations and interstellar scattering effects push the desired observing frequency to bands above 500 GHz.This paper presents the requirements for meeting these science goals, describes the concept of interferometry from Polar or Equatorial Medium Earth Orbits (PECMEO) which we dub the Event Horizon Imager (EHI), and utilizes suitable space technology heritage. In this concept, two or three satellites orbit at slightly different orbital radii, resulting in a dense and uniform spiral-shaped (u, v)-coverage over time. The local oscillator signals are shared via an inter-satellite link, and the data streams are correlated on-board before final processing on the ground. Inter-satellite metrology and satellite positioning are extensively employed to facilitate the knowledge of the instrument position vector, and its time derivative. The European space heritage usable for both the front ends and the antenna technology of such an instrument is investigated. Current and future sensors for the required inter-satellite metrology are listed. Intended performance estimates and * The research work reported in the paper was partly supported by the Project NPI-552 "Space-to-space Interferometer System to Image the Event Horizon of the Super Massive Black Hole in the Center of Our Galaxy" co-funded by the European Space Agency (ESA) and the Radboud University of Nijmegen (ESA contract 4000122812), and by the NWO project PIPP "Breakthrough Technologies for Interferometry in Space".

The International X-ray Observatory (IXO) is an L class mission candidate within the science programme Cosmic Vision 2015-2025 of the European Space Agency, with a planned launch by 2020. IXO is an international cooperative project, pursued by ESA, JAXA and NASA. By allowing astrophysical observations between 100 eV and 40 keV, IXO would represent the new generation X-ray observatory, following the XMM-Newton, Astro-H and Chandra heritage. The IXO mission concept is based on a single aperture telescope with an external diameter of about 3.5 m, a focal length of 20 m and a number of focal plane instruments, positioned at the focal point via a movable platform. A grating spectrometer, enabling parallel measurements, is also included in the model payload. Two parallel competitive industrial assessment studies are being carried out by ESA on the overall IXO mission, while the instruments are being studied by dedicated instrument consortia. The main results achieved during this study are summarised.

No abstract

ESA's Soil Moisture and Ocean Salinity (SMOS) mission, together with NASA's Soil Moisture Active Passive (SMAP) mission, are both providing a wealth of information to the user community for a wide range of applications. Although both missions are still operational, they have significantly exceeded their design life time. For this reason, ESA is looking at future mission concepts which would adequately address the requirements of the passive L-band community beyond SMOS and SMAP. This paper proposes one mission concept, TriHex, that has been found capable of achieving high spatial resolution, radiometric resolution and accuracy, approaching the user needs. This is possible by the combination of aperture synthesis, formation flying, the use of general circular orbits and alias free imaging.

No abstract