Continued hold for Aura satellite

Aura completes the land, water and air series

The launch of Aura, a US space agency satellite designed to check the health of the Earth's atmosphere, has been delayed for a third time.

A problem with the rocket's battery system stopped the countdown three minutes before lift-off on Wednesday.

Launch attempts on Sunday and Tuesday were also scrubbed because of technical issues with the rocket and its payload.

Aura will peer through the stratosphere and troposphere, to study in detail the thin layer of gas in which we live.

Among its many tasks, it will test whether international atmospheric treaties, such as the Montreal Protocol to repair the ozone layer, are working.

Launch officials said the flight would be put off for at least 24 hours, with further delays possible because possible bad weather might prevent support aircraft from tracking the rocket.

The spacecraft promises significant returns for climate studies

Aura is going into orbit from the Vandenberg Air Force Base, California.

Nasa describes the spacecraft as one of the most sophisticated environmental monitoring satellites ever built.

It is the third in the agency's series of satellites aimed at providing definitive data on the global environment.

The first two, Terra and Aqua, are studying the ground and the oceans. Aura will concentrate on the atmosphere, looking at gases, pollutants, and chemical reactions.

The three-tonne spacecraft will help scientists understand how atmospheric composition affects and responds to Earth's changing climate.

"Aura's going to be very helpful in tracking whether the ozone layer is recovering, in establishing the relationship between particulates and atmospheric gases and climate change; and Aura will hopefully also be a very useful tool in developing better predictions of air quality," Rick Pickering, Aura Project Manager at the Goddard Space Flight Center, told BBC News Online.

Last year, scientists produced the first concrete evidence that the ozone layer is in the process of recovering (even if the hole is presently getting bigger) after decades of damage caused by substances like CFCs.

The ozone "hole" is still growing - but not as fast as it once was

But a return to full health will take about 50 years by current estimates - and global warming could change that timescale.

On climate change itself, one outstanding issue is the role of tiny particles in the atmosphere.

These aerosols, typically containing sulphur or carbon, come from natural sources, such as volcanoes, and from human sources, such as the soot from fossil fuel burning.

Aerosols are an important but uncertain agent of climate change. By absorbing or scattering radiation, they can either warm or cool the troposphere. They can also modify clouds and affect precipitation.

If Aura can help scientists understand precisely what these particles are doing, how they behave and what they mean for the future of the global climate, that would be a significant return on the satellite's billion-dollar cost.

Aura has four instruments, the High Resolution Dynamics Limb Sounder (HIRDLS); the Microwave Limb Sounder (MLS); the Ozone Monitoring Instrument (OMI); and the Tropospheric Emission Spectrometer (TES).

HIRDLS was built with a substantial UK contribution.