On the long-term ICRF stability

The International Celestial Reference Frame (ICRF) based on the VLBI-derived positions of extragalactic radio sources was adopted by the International Astronomical Union (IAU) in 1998. Later, in 2009 and 2020, two extended ICRF versions, ICRF2 and ICRF3, were released, respectively. The latter is adopted by the IAU as the current implementation of the ICRF. Due to large distance to ICRF objects, mainly active Galactic nuclei (AGN), the ICRF is more systematically stable than the replaced FK5 optical frame. However, the ICRF3 system is also evolving with time primarily due to physical processes in AGN. Several studies were recently devoted to investigation of this problem. In this work, new strategies will be discussed to assess the long-term stability of the ICRF.