By Jonathan Amos
Science correspondent, BBC News, Darmstadt
19 January 2014
Last updated at 14:26 GMT
Mission controllers are waiting for a very simple signal that will tell them the great endeavour remains on course
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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-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.
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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.
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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.
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Before it can send the signal, Rosetta must work through a sequence of activities that will last several hours.
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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.
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"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.
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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.
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Europe's Rosetta spacecraft was launched from Earth in 2004
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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.
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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.
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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.
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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."
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Philae robot's task in November: No mission has ever attempted to make a soft landing on a comet before
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