Rosetta, which runs on solar power, went into sleep mode over 2 years ago, when its orbit pushed it well outside proximity to the sun. With people from all over the world watching, ESA planned to re-awaken it today, when it got a little closer to the sun (well, relatively at least — it's still over 673 million km away).
Finally, at 18:18 GMT, Rosetta pinged us with this:
That spike that you see is the signal that ESA received, and the evidence that, after its long hibernation, Rosetta has woken up and is ready to start exploring comets.
Of course, it still has another 9 million kilometers to travel and several rounds of health checks (to ensure that everything remains operational) before it can do so. But, researchers expect it to start delivering long-range photos by May, and by August, it should have reached comet 67P/Churyumov-Gerasimenko, where scientists eventually plan to attempt to land it.
Top Image: ESA-C.Carreau; Second Image: ESA