With the launch of Europe's Gaia star-mapping mission, scientists are one step closer to deciphering the history of the Milky Way, and to predicting how it might evolve in the future.

Equipped with twin silicon-carbide telescopes built around a one-billion-pixel focal array—the largest ever built—the European Space Agency's (ESA) Gaia mission is designed to survey a billion stars in the galaxy, providing scientists with the most precise 3-D map to date for understanding its composition and evolution.

“For the first time we will have a fair sample of what is out there, where it is, how it is moving, how [dark] matter is distributed, where and when stars formed, and where and when the chemical elements of which we are made were created,” says Gaia's U.K. Principal Investigator Gerry Gilmore.

ESA says the spacecraft and its sophisticated instruments are designed to measure the angular position of stars between 7-300 microseconds of arc—100 times the accuracy of ESA's 1989 Hipparcos mission, and equivalent to a terrestrial measurement of an astronaut's thumbnail on the lunar surface.

The resulting census will help astronomers determine the position, motion and properties of each star—attributes that can provide clues about their history—to create a “family tree” of the galaxy. ESA says the motions of these stars can then be viewed rapidly in forward or reverse, to determine more about how the galaxy was formed and to learn more about its ultimate fate.

Gaia blasted off at 6:12 a.m. local time on Dec. 19 atop a Soyuz ST-B rocket from ESA's spaceport in Kourou, French Guiana. About 10 min. later, after separation of the first three stages, the Fregat upper stage ignited, delivering Gaia into a temporary parking orbit at an altitude of 175 km (109 mi.). A second firing of the Fregat 11 min. later took Gaia into its transfer orbit, followed by spacecraft separation at 42 min. after liftoff. With the 2,034-kg Gaia on its way to the L2 Lagrange point 1.5 million km from Earth, ESA said it has established ground telemetry and attitude control from its operations center in Darmstadt, Germany, and that the spacecraft is now activating its systems.

Engineers commanded Gaia to perform the first of two critical thruster firings Dec. 20 to ensure it is on the right trajectory toward L2. About 20 days after launch, a second critical burn is expected to insert the spacecraft into its operational orbit, beginning a four-month commissioning phase in which all of the systems and instruments will be turned on, checked and calibrated before beginning its five-year mission.

Led by France, Germany and the U.K., the €940 million ($1.3 billion) Gaia mission was equipped by EADS-Astrium, starting with a €317 million contract awarded by ESA in May 2006.

In the course of its five-plus year mission, Gaia will observe more than 40 million objects per day, collecting 100 terabytes of raw data and yielding 1 petabyte of processed and archived data. Even after being compressed by software, the data produced is expected to fill over 30,000 CD ROMs.

Over a decade in the making, Gaia is two years behind schedule and 16% over budget, owing to technical issues with the satellite's instruments and delays due to a crowded manifest in Kourou. Much of the cost growth resulted from technical issues that extended manufacturing and testing of the spacecraft subsystems, notably the polishing of Gaia's 10 mirrors—including two large primary mirrors—and assembly and test of the focal plane, a 0.38-square-meter (4-sq.-ft.) camera comprising 106 charge-coupled devices (CCD), each of which is effectively a miniature camera.

More recently, a technical glitch that slipped the launch one month to Dec. 19 involved faulty transponder components (built by Thales Alenia Space of Italy) that generate timing signals for downlinking science data. ESA did not identify the in-orbit satellite with the faulty transponder component that prompted the agency to delay the mission one month, but said it would replace the parts prior to Gaia's launch as a precautionary measure.

Gaia is the second consecutive Soyuz launch from French Guiana to be delayed in 2013. O3b Networks postponed a planned late-September Soyuz launch of four communications satellites built by Thales Alenia Space due to a technical problem discovered on four similar O3b satellites launched earlier in 2013. The four spacecraft are now slated to launch in 2014.