The ESA's Rosetta spacecraft wakes up after three years in hibernation and sends message to Earth, ahead of its comet-chasing mission.
It returned a signal sent out by the European space agency on Monday.
After the successful waking up the ESA tweeted 'Hello World' mimicking the signal sent back from deep space.
The ESA 's webcast showed scientists cheering and hugging as the signal came in. It followed hours of anxious waiting.
Rosetta 's on-board alarm clock went off at 05:00 on Monday, but it took the space craft all of seven hours to warm up and turn on its transmitter to beam a message back to Earth.
The spacecraft has been hibernating for most of the last three years to save power. It's due to reach a comet called 67P in August. But unlike previous comet probes Rosetta won't just sail by.
The space craft is designed to put itself into orbit around 67P for more than a year of close up studies.
Comets are believed to be the pristine leftover remains from the formation of the solar system some six billion years ago.
Scientists hope the mission will help provide the clues about how the solar system came into existence.
The spacecraft gained Irish attention when 'it' began tweeting from the Rosetta Twitter account.
It tweeted a varying greeting in several languages, but got Irish people particularly excited when it tweeted a greeting 'as Gaeilge'.
Prior to the tweeting spree, users were tweeting with the hashtag #wakeuprosetta.
Dia dhaoibh uilig ar domhan!— ESA Rosetta Mission (@ESA_Rosetta) January 20, 2014