![]() Gaia’s mission statement is to “map a billion stars,” but even this bold promise undersells the space observatory’s true abilities. The results are already changing astronomers’ perspective of our galaxy. But in September 2015, the Gaia science team released a first round of data from 14 months of observing, and welcomed scientists around the globe to jump in. Gaia won’t complete its mission until 2019, and the final data analysis won’t be available until years afterward. The ESA mission originally stood for “Global Astrometric Interferometer for Astrophysics.” Though many of the parameters changed, ESA kept the name for mission continuity. Gaia was a Greek goddess who was regarded as a sort of Mother Earth. In 2013, ESA launched Gaia, which will return even higher-precision data on over a billion stars. In 1989, the European Space Agency (ESA) launched the Hipparcos satellite to measure the positions of 2.5 million stars, a catalog that wasn’t released in full until 2000. And even tracking a star’s motion across the sky in two dimensions is difficult without years of data. We’ve come a long way from thinking of the stars as a two-dimensional projection on the sky, but measuring a star’s distance remains quite tricky. Over millennia, huge advances have been made in astronomy, but some surprisingly basic questions remain: Where exactly are the stars? Where, in the grand scheme of things, is Earth? What is the shape and structure of our home galaxy? Since ancient times, humans have stared at the sky, cataloged its residents, marked new arrivals, and charted the constant stars and wandering planets. When combined with newly-developed technology, the same measurement principle employed by Hipparcos can be used to gain, simultaneously, a factor of more than 100 improvement in accuracy, a factor 1000 improvement in limiting magnitude, and a factor of 10000 in the numbers of stars observed.Astronomy is often called the oldest branch of science. Gaia builds on the proven techniques established by Hipparcos to bring astrometry into the 21st century. ![]() Another important feature of global astrometric data is the capability of determining the astrometric parameters of double and multiple systems, including extra-solar planetary systems and brown dwarfs. Global astrometry has many intrinsic advantages over pointed observations: a global instrument calibration can be performed in parallel with the observations and the interconnection of observations over the celestial sphere provides the rigidity and reference system needed for the kinematical interpretation of the observations themselves. (In ground-based parallax measurements the transformation of relative parallaxes to absolute distances is a non-trivial problem.) In addition, the global nature of the measurements, the fact that the positions and changes in positions caused by proper motion and parallax are determined in a reference system consistently defined over the whole sky, leads to the determination of absolute parallax measurements. The first space-based astrometry mission, ESA's Hipparcos satellite, demonstrated that milliarcsecond accuracy was achievable by means of a continuously scanning satellite which observed two directions simultaneously. Despite the huge progress made since then, ground-based measurements are limited by several factors (for example, fluctuating atmosphere, instrument flexure, limited sky coverage per observing site) which ultimately limit the measurement accuracy which can be achieved. Some of the earliest recorded astrometry measurements date back more than 2000 years. Measuring the trigonometric parallax for a star yields the only fully reliable way of measuring distances in the local Universe.
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