A satellite that will track the changes in the major ice sheets covering the polar regions was launched on Sunday.

ICESat (Ice Cloud and Land Elevation Satellite) was blasted into orbit on a Delta rocket from Vandenberg US Air Force Base on the central California coast.

The spacecraft will give scientists the clearest picture yet of what is really happening in the Arctic and the Antarctic - whether the ice there is shrinking or growing and by how much, and the impact these changes might have on global sea levels.

"The geophysical community has been waiting for ICESat for 30 years," US space agency researcher Eric Rignot, who studies Antarctic ice, has told the BBC.

ICESat should have gone up last month. Its launch has been delayed by technical problems.

The double payload was carried on a Delta II rocket

It went up as part of a dual payload - a common occurrence these days as launch companies attempt to drive down costs in a competitive but depressed satellite market - at 1645 PST (0045 GMT).

The companion satellite riding on the Delta II rocket was the Cosmic Hot Interstellar Plasma Spectrometer, or CHIPSat. It will help astronomers study the hot gas that lies between the stars.

Space fleet

Growth or shrinkage in ice sheets strongly influence sea levels and are therefore critical in assessing climate change. But glaciologists do not have the precise data they need to determine in which direction the massive ice sheets in Greenland and Antarctica are really moving - or how they might change as the Earth warms.

In Antarctica for example, scientists know the West Antarctic Ice Sheet is smaller now than it was at the end of the last Ice Age but recently collected data suggest this retreat may have slowed or even reversed in more recent times.

ICESat is designed to provide some definitive answers.

It carries just the one instrument: the Geoscience Laser Altimeter System (Glas), which is the first space laser altimeter used to detect ice sheet mass balance.

ICESat is one of a fleet of new Earth observation satellites that aim to unravel some of the uncertainties associated with climate change.

The twin Gravity Recovery and Climate Experiment (Grace) spacecraft are already in orbit and just coming online. They will measure gravity variations across the Earth's surface - brought about primarily by shifts in the distribution of ocean waters.

	:	In Pictures
The first image from the Grace spacecraft

And Aqua, also launched last year, is collecting data on rainfall, snow, sea-ice, temperature, humidity, vegetation, soil moisture and clouds.