Since its 1989 launch, Europe's pioneering Hipparcos Space Astrometry Mission has provided unprecedented detail of the Milky Way galaxy, charting stars and other celestial bodies with a precision akin to measuring the height of an astronaut standing on the Moon when seen from Earth.

The European Space Agency (ESA) ended the Hipparcos campaign in 1993, but the data it cataloged on positions, distances, motions, brightness and colors of more than 100,000 stars has paved the way for Europe's next-generation star-mapper, a €940 million ($1.25 billion) mission known as Gaia.

Equipped with twin silicon-carbide telescopes built around a single 1-billion-pixel focal array, the largest ever built, Gaia will measure the angular position of stars between 7-300 microseconds of arc—100 times the accuracy of Hipparcos and equivalent to a terrestrial measurement of an astronaut's thumbnail on the lunar surface.

From its intended orbit around the L2 Lagrange point 1.5 million km (930,000 mi.) from Earth, the 2,030-kg (4,475-lb.) spacecraft will survey the brightness of 1 billion, or 1%, of the stars and other celestial bodies in the galaxy, 10,000 times as many as Hipparcos.

The resulting census will allow astronomers to determine the origin and evolution of the galaxy while uncovering tens of thousands of previously unseen objects, including extrasolar planets, brown dwarfs, quasars, asteroids and comets, and other galaxies, monitoring each target object up to 100 times to precisely chart its characteristics. Over 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.

Built by EADS Astrium under a €317 million contract awarded in May 2006, Gaia was shipped Aug. 23 to Europe's equatorial spaceport in Kourou, French Guiana, in preparation for launch. Slated to lift off Nov. 20 atop a European variant of Russia's Soyuz rocket, the mission is running nearly two years behind and roughly 16% over budget.

ESA officials say 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.

Given the size of the spacecraft's focal plane, and the mass and volume limits of its Soyuz launcher, Gaia's design affords only minimal radiation shielding of its focal arrays, the performance of which will degrade near the end of the mission. “This effect was well-recognized at the beginning of the program and is normal for all CCDs flying in space,” ESA explains. “However, in Gaia, the effect is very much visible due to the high measurement accuracies required.”

ESA says it will attempt to operate the CCDs at an optimum temperature and rely on specific operational modes. More important, the agency adds, are complex ground tests aimed at characterizing Gaia's CCD behavior under the influence of radiation. To this end, Astrium started a dedicated test program in 2005 that was completed earlier this year with the support of E2v Technologies and a handful of academic institutions. ESA spent €3 million on the test program but says it is confident the problem is resolved and Gaia's performance requirements will be met.

In addition to manufacturing and tests, ESA says 4% of Gaia's cost growth stems from issues associated with the spacecraft's launch, including an increase in the cost of the Soyuz.

When ESA approved Gaia in 2000, it was planned to launch on a European Ariane 5. A subsequent funding reduction de-scoped the mission to fit on Russia's mid-sized Soyuz Fregat 2-1a, operated by the Starsem affiliate of European launch provider Arianespace from Baikonur Cosmodrome, Kazakhstan. When the launch contract was later awarded in 2009, it called for lofting Gaia on a new European variant of the Soyuz STB/Fregat-MT, which began operations in Kourou in 2011.

This year, Arianespace's crowded Soyuz manifest has postponed the mission twice. Between Gaia's development and launch delays, ESA says it is paying an additional €3 million per month, with another €2 million being racked up by Gaia's data processing teams at European research institutions and computing centers, which account for €200 million of mission costs.