An Event Horizon Imager(EHI) Mission Concept Utilizing Medium Earth Orbit Sub-mm Interferometry

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 simulation results are given.

Space-to-space Radio Interferometry System from Medium Earth Orbits

This paper presents a new concept to perform space-to-space Very Long Baseline Interferometry which enables the imaging of cosmic sources at high-resolution and high-sensitivity with small antennas.Several individual apertures are embarked on separate identical satellites staggered in height into Polar or Equatorial Circular Medium Earth Orbits(PECMEO orbits).These orbits are stable and allow GNSS-based on-the-fly centimeter-level relative positioning.Coherent operation is possible by exchanging local oscillator components and measured signals through Inter-Satellite Links(ISL).On-board cross correlation is performed at each satellite over a delay window compatible with the accuracy of the on-the-fly relative positioning and the result sent to the ground.Image reconstruction is completed on the ground thanks to sub-millimeter baseline retrieval from accurate GNSS orbits,ISL ranging and spacecraft attitude information.The application of this concept to image the Super Massive Black Hole Sgr A*is hinted.