  The Star Watch extreme-precision astrometry mission (0.1 - 1.0 uas) builds on
technology developed, and validated, during the SIM (Space Interferometry
Mission) project. The sole science instrument is an optical interferometer with
50-cm collecting apertures, separated by a 6-meter baseline.
  The heart of this detector is the Astrometric Beam Combiner (ABC). This is
flight-quality hardware that underwent testing at high levels of integration,
retiring most technical risk (achieving TRL-6) after 10 years and $600 million
of investment. The ABC is in storage at JPL, ready to complete testing,
followed by integration with the mission support structure.
  Star Watch incorporates advances in technology since the end of the SIM
project. These include smaller, lighter beam launchers and corner cubes for
laser metrology; attitude-control micro-thrusters, allowing deletion of
reaction wheels; and advanced fringe detectors. The technology pioneered by
Star Watch, the first long-baseline Michelson interferometer in space,
represents an important investment in the future of space astronomy. NASA's
proposed Vision Missions (Exo-Earth, Black Hole and Cosmic Dawn Mappers)
require the use of precision interferometers.
  No other Astrophysics Probe concept comes close to this level of technical
readiness. There are no analogs to Star Watch. It will provide access, for the
first time, to the realm of temperate Terrestrial worlds circling nearby
sun-like stars, measuring true masses and orbits. This can be achieved by the
close of the 2020s.
