Rosetta comet-hunter narrows the gap
Europe's Rosetta comet-chasing spacecraft has started the process of zeroing in on its quarry.
Controllers lit the thrusters on the satellite on Wednesday, enabling the mission to start to match its pace to the huge ball of ice and dust.
The 45-minute burn was the first of 10 planned manoeuvres up to 6 August, when Rosetta will drop into orbit around Comet 67P/Churyumov-Gerasimenko.
The probe will then map the object to find a site to put down a lander.
This contact event is scheduled for mid-November.
Rosetta and 67P are currently some 538 million km from Earth, with the spacecraft running slightly ahead of the comet.
The separation distance between the pair is narrowing daily. At the moment, it is about 1.8 million km.
But Wednesday's burn will have refined the rendezvous trajectory and rate of closure.
Together with the nine subsequent manoeuvres, Rosetta's velocity relative to the comet should eventually be reduced to a mere 1m/s and bring it to within just 100km.
Without all these burns, the spacecraft would simply fly past 67P with the closest approach getting no nearer than 40,000km.
The mission is one of the most complex and ambitious ventures ever undertaken by the European Space Agency (Esa).
Launched back in 2004, Rosetta has taken a rather circuitous route out to its icy target.
This has involved making a number of flybys of the inner planets, using their gravity to pick up sufficient speed for the eventual encounter.
It has already delivered some fascinating science, particularly from the close passes made to two asteroids - the rocks Steins, in 2008, and Lutetia, in 2010.
The plan is for Rosetta to escort the comet as it moves closer towards the Sun, monitoring the changes that take place on the 4km-wide body.
The lander - called Philae and currently riding piggyback on the Esa probe - will report changes that occur at the surface of 67P.
They should get a grandstand view of gas and dust lifting away from the comet as its ices are heated by our star.
Bigger to come
Rosetta was put into hibernation in 2011 because its trajectory through the Solar System was about to take it so far from the Sun that its solar panels would harvest minimal energy.
Power constraints have since eased, and the probe was awoken on 20 January this year.
Engineers, subsequently, have been checking over Rosetta's systems and instruments.
And, in that sense, Wednesday's burn was also something of a test, given that the thrusters were last fired in anger several years ago.
Esa's mission control in Darmstadt, Germany, will pore over the telemetry coming back from Rosetta following the burn.
If the data is satisfactory, the go-ahead will be given for manoeuvre number two to be conducted on 21 May. This will be much bigger.
Whereas the first thruster firing was intended to slows Rosetta's speed relative to the comet from 775m/s to 755m/s, burn two will remove almost 300m/s. It will also take about eight hours to complete.
Jonathan.Amos-INTERNET@bbc.co.uk and follow me on Twitter: @BBCAmos