Gaia space mission and quasars
  Quasars are often considered to be point-like objects. This is largely true
and allows for an excellent alignment of the optical positional reference frame
of the ongoing ESA mission Gaia with the International Celestial Reference
Frame. But presence of optical jets in quasars can cause shifts of the optical
photo-centers at levels detectable by Gaia. Similarly, motion of emitting blobs
in the jet can be detected as proper motion shifts. Gaia's measurements of
spectral energy distribution for around a million distant quasars is useful to
determine their redshifts and to assess their variability on timescales from
hours to years. Spatial resolution of Gaia allows to build a complete magnitude
limited sample of strongly lensed quasars. The mission had its first public
data release in September 2016 and is scheduled to have the next and much more
comprehensive one in April 2018. Here we briefly review the capabilities and
current results of the mission. Gaia's unique contributions to the studies of
quasars are already being published, a highlight being a discovery of a number
of quasars with optical jets.
