  We show that a 10 year Gaia mission could astrometrically detect the orbital
motion of ~1 sub-parsec separation supermassive black hole binary in the heart
of nearby, bright active galactic nuclei (AGN). Candidate AGN lie out to a
redshift of z=0.02 and in the V-band magnitude range $10 \lesssim m_V \lesssim
13$. The distribution of detectable binary masses peaks at a few times $10^7$
solar masses and is truncated above a few times $10^8$ solar masses.
