  Both the radio International Celestial Reference Frame (ICRF) and the optical
Gaia Celestial Reference Frame (Gaia-CRF2) are derived from observations of
jets produced by the Super Massive Black Holes (SMBH) powering active galactic
nuclei and quasars. These jets are inherently subject to change and will appear
different at different observing frequencies, leading to instabilities and
systematic errors in the resulting Celestial Reference Frames (CRFs). Recently,
the Event Horizon Telescope (EHT), a mm-wave Very Long Baseline Interferometry
(VLBI) array, has observed the 40 micro-as diameter shadow of the SMBH in M87
at 1.3 mm, showing that the emitting region is smaller than the black-hole
shadow. Use of these SMBH "emission rings" (and the associated photon rings) as
astrometric references will enable the resulting CRF to be anchored directly in
SMBH shadows; the ultimate reference points for any CRF for the forseeable
future. A properly equipped space VLBI mission devoted to the observation of
SMBH event horizons could lead to a two-orders-of-magnitude improvement in the
accuracy and stabilty of the ICRF in the relatively near future.
