The Swarm mission to map the Earth's global magnetic field in unprecedented detail has launched from Russia.
The trio of European Space Agency (Esa) satellites left the Plesetsk Cosmodrome at 12:02 GMT, riding a Rockot vehicle.
They are expected to be deployed at an altitude of 490km, in a polar orbit, shortly after 13:30 GMT.
Swarm's data should help scientists understand better how the field is generated, and why it appears to be weakening.
The strength has fallen by some 15% in the past two centuries. The movement of the north geomagnetic pole has also accelerated.
Researchers have speculated that Earth may be on the cusp of a polarity reversal, which would see the direction of the field flip end to end. North would become south, and vice versa.
This has not happened for 780,000 years, but the phenomenon has nonetheless been a regular occurrence through geological time.
The major part of Earth's global magnetic field is generated by convection of molten iron within the planet's outer liquid core, but there are other components that contribute to the overall signal.
These include the magnetism retained in rocks, and there is even an effect derived from the movement of salt water ocean currents.
Swarm will attempt to tease apart these various factors, to get a clearer picture of the field's true origins and its changing behaviour.
Other uses of the Swarm data will include investigations of the electrical environment of the high atmosphere and the way this interacts with the solar wind - the continuous stream of charged particles billowing away from the Sun.
The wind carries its own magnetic field which clashes with Earth's, producing "storms" that can on occasion disrupt satellites, radio communications and even electricity grids at the planet's surface.
A successful launch will only be confirmed when a signal from all three satellites is picked up just after their separation from the Rockot's Breeze upper-stage.
This signal should be acquired by ground stations in Kiruna, Sweden, and the Arctic Svalbard archipelago at about 13:37 GMT. This information will be relayed to Esa's mission control centre in Darmstadt in Germany, where the operation of the satellites will be managed.
All three satellites in the Swarm constellation are identical. Their super-sensitive instrumentation acts rather like a 3D compass, enabling the precise strength and direction of the magnetic field to be determined all around the globe.
The trio's construction was led by the Astrium company, predominantly in Germany and the UK.
Engineers have had to ensure the magnetism generated by the satellites' own internal electronics does not obscure the subtle scientific measurements the mission will be attempting to make.
This has meant putting the instruments on the end of a long boom to keep them separate from the main spacecraft body. It has given Swarm a very distinctive look - like giant mechanical rats with long tails.
A key moment following launch will be the deployment of the booms. Stowed for launch, the instrument tails will have be opened for there to be an operational mission.
"The first boom will open a few hours after launch," explained Andy Jones, the Astrium UK Swarm project manager.
"Each one is held in place by a nut, which will release the boom and allow it to swing open. Springs will initially pull the boom well past the hinge line, and it will waggle back and forth until eventually settling down. The waggling lasts about a minute and a half."
The Darmstadt space operations team hopes to have all three booms open by early Saturday.
Swarm is the latest satellite in Esa's Earth Explorer series. Missions have already flown to measure Earth's gravity field, its ice cover, the salinity of its oceans and the moisture retained in its soils.
Jonathan.Amos-INTERNET@bbc.co.uk and follow me on Twitter: @BBCAmos
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