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Rebecca Morelle explains how the mission will unfold
One of the most daring space missions ever undertaken reaches a key milestone on Monday.
Europe's Rosetta probe was launched a decade ago on a long quest to chase down and land on a comet, and has spent the past two-and-a-half-years in hibernation to try to conserve power.
But at 10:00 GMT, an onboard "alarm clock" is expected to rouse the spacecraft from its slumber.
Rosetta will then warm its systems before sending a signal to Earth.
Receipt of this "I'm awake" message will confirm the great endeavour is still on course.
Rosetta is due to rendezvous with Comet 67P/Churyumov-Gerasimenko in August.
And after spending a couple of months studying and mapping this 4km-wide ball of ice and dust, it will drop a small robot on to the comet's surface to gather samples and panoramic pictures.
Controllers at the European Space Agency's (Esa) operations centre here in Darmstadt, Germany, do not know precisely when Monday's all-important message will arrive, but they anticipate receiving it sometime between 17:30 and 18:30 GMT.
Before it can send the signal, Rosetta must work through a sequence of activities that will last several hours.
First, it must raise the temperature of the sensor chips in its navigation instruments, then it must stop the stabilising spin into which it was placed for hibernation, and finally it must find Earth on the sky and point its main antenna in that direction.
"It will be transmitting just the 'carrier signal', so at that point there's no data coming down from the spacecraft," explained Andrea Accomazzo, Rosetta's operations manager.
"We just receive a firm frequency. In theory, it would be like a continuous beep if you were to convert it into sound. We will see it on a screen that is basically a spectrum analyser.
"Although, we will have no information from the spacecraft, we will know just from that transmission that it must have done everything it had to do automatically and is in a safe status; and that everything that happens next is in our hands," he told BBC News.
If the onboard computer encounters an anomaly during any phase in the pre-programmed sequence, it will go back to the start of the last command step and begin again.
This means the "I'm awake" message might not actually arrive until late on Monday night.
Controllers will certainly not intervene until Tuesday morning. If required, they can send commands to Rosetta that will be picked up by its low-gain antenna. Everyone here, however, is confident the probe will come back to life in the normal way.
Rosetta was put into hibernation in June 2011 because its trajectory through the Solar System was about to take it so far from the Sun that its solar panels would produce minimal power. The decision was therefore taken to put the spacecraft in a deep sleep.
Launched back in 2004, the probe has taken a rather circuitous route out to its comet target.
This has involved making a number of flybys of the inner planets, using their gravity to pick up sufficient speed for the eventual comet encounter.
It has already delivered some fascinating science, particularly from the close passes it made to two asteroids - the rocks Steins, in 2008, and Lutetia, in 2010.
Once controllers have a full assessment of the health of Rosetta, they will initiate a series of burns on its thrusters to close the gap to 67P. Currently at a separation of nine million km, this will be reduced to a mere 10km by mid-September.
The landing of the three-legged Philae robot is scheduled at the moment for 11 November.
The intention is for Rosetta to follow the comet as it moves closer towards the Sun, monitoring the changes that take place on the body. Philae will report changes that occur at the surface.
Comets - giant "dirty snowballs", as some have called them - are believed to contain materials that have remained largely unchanged since the formation of the Solar System 4.6bn years ago, and Rosetta data should therefore help researchers understand better how our local space environment has evolved over time.
Esa director general Jean-Jacques Dordain told BBC News: "Rosetta is a unique mission - unique technologically, unique scientifically, and unique philosophically because comets may be at the origin of who we are."
Jonathan.Amos-INTERNET@bbc.co.uk and follow me on Twitter: @BBCAmos
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