The US space agency on Friday launched a satellite to observe levels of salt on the surface of the world's oceans and measure how changes in salinity may be linked to future climate.

The $400 million Aquarius/SAC-D spacecraft, a partnership with Argentina, launched from Vandenberg Air Force Base in California at 7:20 am Pacific time (1420 GMT).

The orbiting science instrument will aim to map the entire open ocean every seven days from its position 408 miles (657 kilometers) above Earth, producing monthly estimates that show how salt levels change over time and location.

"Data from this mission will advance our understanding of the ocean," said Michael Freilich, director of NASA's Earth Science Division in Washington.

NASA said the mission will survey salinity at the ocean's surface in "the most detailed summary of conditions ever undertaken."

Previously, such measurements were taken largely by ships moving along their trade routes.

The mission, whose name refers to US-Argentine Aquarius Satelite de Aplicaciones Cientificas (SAC)-D observatory, is set to last for three years.

A European satellite was launched in 2009 to measure soil moisture and ocean salinity.

The European Space Agency's Soil Moisture and Ocean Salinity (SMOS) mission's main focus is soil moisture, while Aquarius is aimed primarily at measuring ocean salinity, which plays a key role in exchanges of water and heat in the atmosphere.

The Aquarius/SAC-D is a global collaboration with partner Argentina as well as France, Brazil, Canada and Italy, NASA said.

"This mission is the most outstanding project in the history of scientific and technological cooperation between Argentina and the United States," said the Argentine space agency's director Conrado Varotto.

"Information from the mission will have significant benefits for humankind."

Earlier this year, NASA lost Glory, a $424 million Earth-observing satellite that failed to separate properly from its rocket launcher and plunged into the ocean.

But Aquarius/SAC-D steered clear of that problem, and the payload fairing protecting the spacecraft separated and fell away as planned, allowing the craft to enter orbit.

The satellite observatory is carrying seven additional instruments to collect a range of environmental data for studies of natural hazards, air quality, land processes and epidemiology, NASA said.