European Space Agency engineers have identified a potential fault with the Gaia spacecraft, and are delaying its 20 November launch to make repairs before kicking off the mission to map the Milky Way.

The spacecraft, being readied for integration with its Soyuz launcher at ESA’s spaceport in Kourou, French Guiana, is carrying time signal transponders of a type that have been linked to a problem with at least one satellite currently in orbit. The transponders will be removed from Gaia at Kourou and returned to Europe, where the potentially faulty components will be replaced and verified.

After replacements have been made, the transponders will be refitted to the spacecraft and a final verification test carried out. ESA has set Gaia revised launch window as 17 December to 5 January.

The delaying action is understood to have followed an alert by transponder manufacturer Thales Alenia Space. The company apparently supplied similar units for telecommunications satellites owned by O3b Networks – four of which were launched in June, also by Soyuz. O3b is understood to have delayed a subsequent launch.

Gaia will create a catalogue of 1 billion stars in the Milky Way. This will include trajectory data, which will give astronomers a picture of our galaxy’s past and future configuration. The five-year, €740 million ($1 billion) mission will fill what is expected to become astronomers’ standard star catalogue for decades to come.

In order to achieve its scan of the skies, Gaia will be placed 1.5 million km beyond Earth’s orbit at the so-called Lagrange L2 point, where the balance between the Earth and Sun's gravity will keep it orbiting the Sun in line with Earth. The location is ideal for a mission whose instruments must be kept extremely cold. This will ensure accuracy and stability – minimal station-keeping thrust is needed, and the instrument-bearing side of the spacecraft will always face deep space, protected by a 10m sunshield.

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