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SIM Lite and Gaia
SIM Lite and Gaia are both astrometric missions. Gaia is an all-sky survey mission currently under development by the European Space Agency. Are both needed? The answer is emphatically yes, for two reasons. First, the advent of microarcsecond-level astrometric precision opens up a wide array of topics in astrophysics for which astrometry can now play a major role. Far from being a specialist technique, astrometry is once again becoming a fundamental tool for astronomy. Second, these missions are complementary in a way that every astronomer appreciates: Gaia is a broad-survey instrument and will fly first. SIM Lite is a powerful, sensitive, pointed instrument that will build on the results from Gaia.
Is there science overlap between SIM Lite and Gaia? The simple answer is — surprisingly little (see figure). This is because the SIM Lite science program is designed to complement, not duplicate, Gaia science. In general, Gaia will pursue those programs for which the science is derived from measurements of an ensemble of a very large number of targets. SIM Lite will focus on science that requires the highest precision on individually selected targets. Many examples can be found in this book. Two of these are the search for Earth-like planets orbiting the closest Sun-like stars and probing the Galactic potential by measuring the trajectories of individual hypervelocity stars.
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