Five Years at the Movies
  GAIA is an astrometric satellite which has been approved by the European Space Agency for launch in about 2010. It will measure the angles between objects in fields that are separated on the sky by about a radian. Data will stream continuously at 1 Mbps from GAIA's three telescopes, providing information on the positions and velocities of the billion or more astrophysical objects brighter than 20th magnitude. The motion of objects across the sky caused by their space motion and their parallactic motion, and the variability of objects in 15 wavebands, will be measured because each object is observed at least 150 times during the 5 year mission lifetime. From the raw time series, a three-dimensional movie of the motions of stars in the Galaxy will be synthesized. The alert despatcher in the GAIA mission will provide forewarning of many kinds of bursting and variable phenomena. We discuss two applications of the GAIA movie show in detail here. First, the astrometric microlensing signal will allow us to take a complete inventory of all objects -- no matter how dark -- in the solar neighbourhood. Second, the catalogue of supernovae detections will be the largest ever taken. It will provide opportunities both for follow-ups in other wavebands and numerous examples of scarce phenomena (like subluminous SNe). 
N.W. Evans (Cambridge), V. Belokurov (Oxford)