Duration: 9 months
Very long baseline interferometry (VLBI) at radio wavelengths has achieved record angular resolutions and provided the first images of black holes by combining a global array of telescopes. Further resolution improvement, which is essential for precision studies of black hole spacetimes, requires submm space-based VLBI. To perform such observations, key challenges need to be solved. To alleviate the need to launch large (10+ meters) dishes, long integration times are required to build up signal-to-noise. Such long integrations require a phase-stable interferometer, imposing challenging system requirements: (i) clocks that have a relative stability far beyond current H-maser capabilities, and (ii) sub-wavelength level knowledge of the relative orbits. Furthermore, (iii) the wide bandwidths required would imply challenging data downlink rates of tens of Gbit/s. We propose a small system study into SHARPEx, the first space-space VLBI experiment. SHARPEx will consist of two twin satellites, possibly Cubesats, imaging Active Galactic Nuclei at cm wavelengths. It will address the three challenges outlined above using novel approaches: (i) clock signals will be shared over an intersatellite link (ISL) and mixed at each satellite, creating a syntonized clock, and (ii) GNSS receivers will be used to determine the orbits in 3D at sub-wavelength level. (iii) The observed data will also be shared over the ISL, and will be correlated on board, strongly reducing the required downlink data rate. Previous ESA activities have demonstrated clock syntonization in the lab, and performed relative navigation and correlation studies. Hence, a small in-orbit demonstration is timely. The OSIP study outcome will form the basis for a SHARPEx mission proposal. Space-to-space VLBI with small dishes, enabled by clock syntonization, relative navigation, and on-board correlation, will open up a new regime in image resolution and quality for astronomy, and lead to razor-sharp images of black holes.
