Space very long baseline interferometry in China

Abstract

Space very long baseline interferometry (VLBI) has unique applications in high-resolution imaging of fine structure of astronomical objects and high-precision astrometry due to the key long space-Earth or space-space baselines beyond the Earth's diameter. China has been actively involved in the development of space VLBI in recent years. This review briefly summarizes China's research progress in space VLBI and the future development plan. Keyword: Radio Astronomy; space science; Very long baseline interferometry BackgroundVery long baseline interferometry (VLBI) is a modern observational technique based on combining geographically distributed radio telescopes to form an observation network, thereby enabling the highest possible angular resolution. The imaging resolution provided by VLBI is inversely proportional to the maximum baseline (the separation between two telescopes in the network) length and proportional to the observation wavelength. VLBI allows astronomers to observe the fine structure of compact celestial objects and acquire high-precision astrometry with sub-mas resolutions. Routinely operational VLBI networks, such as the European VLBI Network (EVN), the Very Long Baseline Array (VLBA) in the USA and East Asia VLBI Network (EAVN), have maximum baselines of 5000-10,000 km, and they yield high resolutions of 0.08-0.5 mas ], which are 100-600 times higher than those of the Hubble space telescope. However this is still not sufficient for some extremely compact unresolved objects. For example, imaging the black hole shadow of Sgr A*, the supermassive black hole (SMBH) at the heart of the Milky Way, requires a resolution of about 0.02 mas, which is on a scale comparable to the event horizon of the SMBH [Goddi, et al.(2017)].Two conventional methods can be used to increase the imaging resolution: observing at highest possible frequency, and increasing the VLBI baseline length. The former method motivated the development of the Event Horizon Telescope [Doeleman(2017)] for directly observing the immediate environment of Sgr A* and M87 SMBHs at 230 GHz. The baseline lengths of ground-based VLBI networks are constrained by the size of the Earth. In order to acquire larger baseline length, astronomers have proposed sending radio telescopes into space to create a space-Earth VLBI network, i.e., space VLBI.Despite various technical difficulties and the huge associated costs, space VLBI can provide the highest resolution, as well as opening new observation windows in the electromagnetic spectrum by exploiting the lack of atmospheric disturbance in the space environment. Sub-millimeter (submm) wavelength and infrared observations from the Earth are highly constrained by the broad-band absorption of molecules in the atmosphere. Moreover, the turbulence in the atmosphere results in rapid fluctuations of the visibility phase to significantly reduce the coherence time for mm and submm wavelength interferometers, thereby decreasing the fringe detection sensitivity of VLBI. These factors motivated the development of spac...

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“…However, the size of the earth is a natural constraint. To acquire a larger baseline length, telescopes were sent into space to form the SVLBI [1] . But various technical bottleneck occurs simultaneously and inevitably.…”