Scientists at the European Space Agency are expecting an important call.
Their comet-chasing probe Rosetta is due to wake from an almost three-year hibernation at 11 a.m. Monday (1000 GMT; 5 a.m. EST) and phone home to say all is well.
But because the spacecraft's systems will take hours to power up and the signal has to travel more than 800 million kilometres (500 million miles) back to Earth, the first sign of life isn't expected before early evening.
The agency is turning the tense wait into a social media event by encouraging space enthusiasts to "Wake up Rosetta" in case its internal alarm clock fails.
The probe will rendezvous with comet 67P/Churyumov-Gerasimenko in the coming months and drop a space lander onto its icy surface in November.