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Both the radio International Celestial Reference Frame (ICRF) and the opticalGaia Celestial Reference Frame (Gaia-CRF2) are derived from observations ofjets produced by the Super Massive Black Holes (SMBH) powering active galacticnuclei and quasars. These jets are inherently subject to change and will appeardifferent at different observing frequencies, leading to instabilities andsystematic 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 M87at 1.3 mm, showing that the emitting region is smaller than the black-holeshadow. Use of these SMBH "emission rings" (and the associated photon rings) asastrometric references will enable the resulting CRF to be anchored directly inSMBH shadows; the ultimate reference points for any CRF for the forseeablefuture. A properly equipped space VLBI mission devoted to the observation ofSMBH event horizons could lead to a two-orders-of-magnitude improvement in theaccuracy and stabilty of the ICRF in the relatively near future.

Anchored in Shadows: Tying the Celestial Reference Frame Directly to Black Hole Event Horizons