The European Space Agency (ESA) Rosetta spacecraft is now closing in on its destination, Comet 67P/Churyumov–Gerasimenko, as it moves farther into the inner solar system.
The wakeup call was programmed for midday but the procedure should take a number of hours followed by a tiny "all is well" signal, which will take 45 minutes to cross 800 million kilometres.
Once awake, Rosetta will carry out braking and steering manoeuvres until August 2014, when it will go into orbit above the comet 67P/Churyumov–Gerasimenko.
The one-billion-euro spacecraft carries 11 cameras, radar, infrared sensors and a fridge-sized robot laboratory called Philae which will harpoon the surface of a small planet known as the "dirty snowball" to carry out experiments.
Because comets are witnesses of the birth of our solar system, Rosetta should reveal secrets going back 4,6 billion of years ago, including, perhaps, how life on Earth started.