The Smart 1 spacecraft has fired up its innovative solar-electric engine. Mission controllers sent instructions to the lunar probe to start its ion drive as they checked out the health of all Smart's onboard systems. The European spacecraft, which blasted off from French Guiana on Saturday, will take 15 months to reach the Moon.
When it arrives, Smart will use its miniaturised instruments to map the rocky body's chemical make-up to confirm theories about its creation.
The 110 million euro (Ј78m) spacecraft is Europe's first solo mission to the Moon.
The unmanned probe was put into its initial Earth orbit after a flawless launch on an Ariane 5 vehicle from the Kourou spaceport.
Now engineers at the European Space Agency's operations centre in Darmstadt, Germany, are running tests on the probe's electronics and scientific instruments to make sure they survived the shaking they received as the Ariane hurtled skywards.
The ion drive was switched on at 1225 GMT on Tuesday.
The commissioning of the probe will continue through the week before a longer firing on the drive pushes the probe out in earnest towards the Moon. If the engine can be shown to work well, it is likely to become a standard form of propulsion on many of the inter-planetary explorers built by Esa in the coming years.