Workers prepare to enclose NASA's Orbiting Carbon Observatory, or OCO, inside a protective structure that fits flush with the outside surface of the rocket. ((NASA/Robert Hargreaves Jr., VAFB))

A NASA satellite designed to track carbon dioxide emissions has landed in the ocean near Antarctica after failing to reach orbit early Tuesday.

The Orbiting Carbon Observatory lifted off from Earth aboard a Taurus XL rocket at 1:55 PT from Vandenberg Air Force Base on California's central coast.

Several minutes into the flight, however, the launch managers had to shift into a contingency plan after the moulded structure surrounding the satellite failed to separate properly from the spacecraft.

Taurus program manager John Brunschwyler said the rocket splashed into the ocean. The rocket carried hydrazine fuel but NASA officials said they had no indication any part of the rocket or satellite posed a threat to anyone.

The OCO was NASA's first satellite dedicated to monitoring carbon dioxide on a global scale, and took eight years of planning and design and $280 million US to bring to the launch stage.

It was expected to help determine where carbon dioxide originates and what role forests and oceans play in sequestering the greenhouse gas.

Japan's Ibuki satellite, launched in January 2008, is already in orbit about 670 kilometres and monitors the levels of carbon dioxide and methane from 56,000 locations.

NASA officials said at a press conference Tuesday morning the loss of the satellite is a setback to CO2 observation, but that other satellites capable of monitoring CO2 exist and were meant to complement each other.

There are also more than 280 land-based sites to monitor carbon dioxide, but using satellite observations allows scientists to get a picture of emissions from developing nations, where monitoring is difficult, and also track the movement of the gas over oceans.

With files from the Associated Press