The Milky Way has been mapped in greater detail than ever before. And a first quick look indicates that our home galaxy is larger in extent than scientists had thought before, says Gisella Clementini, an astronomer at the Astronomical Observatory of Bologna in Italy.

Today, at the European Space Astronomy Centre in Madrid, the European Space Agency (ESA) released the first data from its €750 million Gaia star-mapping mission. The new catalog contains sky positions for 1.1 billion stars, 400 million of which have never been seen before. For many stars, the positional accuracy is 300 microarcseconds—the width of a human hair, seen from a distance of 30 kilometers—positions that will help astronomers better determine the 3D layout of the galaxy. “This is far better than anything we’ve ever had before,” says project scientist Timo Prusti of ESA’s science and technology center ESTEC in Noordwijk, the Netherlands. “It’s a milestone.”

Gaia was launched in December 2013 and has been scanning the skies ever since with its gigapixel camera, despite minor problems with stray light getting in and some recurring contamination of ice on the two main telescope mirrors. So far, it has carried out many hundreds of billions of individual measurements, yielding 40 gigabytes of data every single day. By combining the new measurements with older ones from ESA’s Hipparcos satellite, data analysts were able to derive accurate distances and motions for a subset of 2 million stars, providing more precise information on their physical properties and on the Milky Way’s gravity field. "Knowing stellar distances is extremely important for many fields of astrophysics," Prusti says. "To me, this is the most exciting part of this first data release."

A second data release, planned for late 2017, will include even more accurate positions—in some cases up to 10 microarcseconds, or a human hair at a distance of 1000 kilometers. The second release will also contain distances and motions for all 1.1 billion stars, says astronomer Anthony Brown of Leiden University in the Netherlands, who chairs a 450-member consortium of Gaia data analysts. In addition, Gaia will discover tens of thousands of new star clusters in the Milky Way, and yield accurate positional data for about a million remote galaxies. “Future facilities like the Large Synoptic Survey Telescope, the James Webb Space Telescope, and ESA’s Euclid satellite, will gratefully exploit the Gaia catalog,” Brown says.

As of today, scientists worldwide will have unlimited access to the Gaia data. “Please enjoy with us,” Prusti said at the Madrid press conference. The new information is of particular interest to astronomers studying stellar evolution and the formation history of the Milky Way, including the role and distribution of dark matter. In the future, Gaia is also expected to discover new asteroids in our solar system and thousands of Jupiter-like planets orbiting other stars. An alert system to notify astronomers about Gaia discoveries of transient events like supernovae (exploding stars) is already in operation. According to Brown, many scientists are eager to start working on the data. “I expect the first scientific papers based on this data release within a couple of weeks,” he says.

Gaia’s primary mission will be over in mid-2019. ESA still has to decide on a possible mission extension to 2024, which would even further increase the accuracy of the final catalog. As for the distant future, astronomers dream of an infrared counterpart to Gaia, which would be able to peer through the Milky Way’s dust cloud into its very center, and also would excel at detecting and measuring faint red and brown dwarf stars in the solar neighborhood.