Sniff, our most successful scientific Spacecraft by publications. The new Voyage 2050 programme certainly provides science topics that could lead to a worthy successor.
Some more information in the Guardian:
There is currently no mission planned for her as Australia just paid the training cost when she did not get selected into the ESA astronaut chor.
Australia would also now have to pay for a ride to the ISS. The tourist ticket info I found was about 30 MUSD but that's including the training I think.
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So Gaia relies on maintaining very accurate pointing at the stars while measuring their position. For this purpose it uses special cold gas (Nitrogen) thrusters with very low flow noise. These cold gas tanks are now empty and Gaia simply cannot continue to operate.