A European satellite is set to provide major new insights into how water is cycled around the Earth.
The Smos spacecraft will make the first global maps of the amount of moisture held in soils and of the quantity of salts dissolved in the oceans.
The data will have wide uses but should improve weather forecasts and warnings of extreme events, such as floods.
A Rockot launcher carrying Smos lifted off from the Plesetsk Cosmodrome in Russia at 0450 (0150 GMT) on Monday.
Some 70 minutes later, the upper-stage of the Rockot released the spacecraft, and telemetry confirming all was well with the mission was acquired by the Hartebeesthoek ground station, near Johannesburg, South Africa.
The Soil Moisture and Ocean Salinity (Smos) satellite is part of an armada of European spacecraft being sent into orbit over the next few years to study the planet.
"We had a very beautiful launch," said Jean-Jacques Dordain, the director-general of the European Space Agency (Esa).
"This is not just a satellite; this is a very important event. This is the second of our Earth Explorers and with that we confirm that Esa is the space agency of the world making the best efforts for Earth science and a new understanding of climate change."
Smos carries a single instrument - an interferometric radiometer called Miras. Some eight metres across, it has the look of helicopter rotor blades.
Miras will measure changes in the wetness of the land and in the salinity of seawater by observing variations in the natural microwave emission coming up off the surface of the planet.
It carries 69 antennas positioned on a central structure and along the lengths of its three arms.
The whole system is folded for launch to fit inside the rocket, and its correct unfurling on day two of the mission will be a critical procedure.
"We do it in two steps," said Francois Bermudo, the Smos project manager with the French space agency (CNES).
"The arms are attached by 12 nuts, four on each arm. First, the pyrotechnic sequence will cut nine nuts so that each arm is held by one nut. Then we will fire the final nuts to have a synchronous opening of the arms. It will take about three minutes," he told BBC News.
Smos data will result in a better understanding of the hydrological cycle - the description of how water is constantly exchanged between the Earth's land and ocean surfaces and the atmosphere.
Information from Smos is expected to help improve short and medium-term weather forecasts, and also have practical applications in areas such as agriculture and water resource management.
In addition, climate models should benefit from having a more precise picture of the scale and speed of movement of water in the different components of the hydrological cycle.
The satellite is part of Esa's Earth Explorer programme - eight spacecraft that will acquire data on issues of pressing environmental concern.
The first is already in orbit - a mission called Goce, which is mapping variations in the pull of gravity across the Earth's surface.
Smos is the second Explorer to launch; and a third spacecraft, known as Cryosat, is due to go into space early next year. Cryosat will assess the state of the world's ice cover.
"We've been waiting a long time to get the Earth Explorers up," explained Dr Volker Liebig, the director of Earth observation at Esa.
"That is partly due to the waiting for launchers but also due to some technical problems, because all these missions are at the edge of technology and very innovative," he told BBC News.
"Now, we are very happy because if Cryosat goes up in February, within 12 months we will have launched three of our Earth Explorers."
The Smos programme cost is about 315m euros ($465m; £280m). It is led by Esa but with significant input from French and Spanish interests. The satellite is expected to operate for at least three years.
"If the instrument is still in good shape and we get the funding from our delegations, we would extend the mission for another two years and then review it again," said Dr Susanne Mecklenburg, the Esa Smos mission manager.
"From experience, Esa missions have lived for a lot longer than they were designed for."
Smos shared its ride into space with a small Esa demonstration satellite called Proba-2 (PRoject for OnBoard Autonomy). The spacecraft will test hardware and software that might be incorporated into future missions. These include new types of computer, battery, thruster, and solar panel systems.
Confirmation of Proba-2's successful separation from the Rockot upper-stage came three hours after launch.