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Last Updated: Thursday, 26 April 2007, 18:53 GMT 19:53 UK
Mission aims at highest clouds
Noctilucent clouds (Pekka Parviainen)
They form more than 70km above most clouds
The US space agency (Nasa) has launched a mission to study the highest clouds on Earth - noctilucent clouds.

These silvery-blue structures appear as thin bands in twilight skies, forming some 80km (50miles) above the surface.

Recent records suggest they have become brighter, more frequent and are being seen at lower latitudes than usual.

The Aeronomy of Ice in the Mesosphere (AIM) spacecraft was launched at 1326 PDT (2026 GMT) on Wednesday on a Pegasus XL rocket.

The booster was first carried to an altitude of 11.9km (39,000ft) by a Stargazer L-1011 carrier aircraft before being released to make its way into space.

Altered factors

To form, noctilucent clouds need cold temperatures, the presence of water vapour, and small dust particles around which the water can condense and freeze out to create ice crystals.

Scientists cannot say for sure but they suspect human activity may be altering the balance of these conditions in the mesosphere.

Noctilucent clouds are seen when the Sun is below the horizon (BBC)
The changes in frequency and brightness have been observed over the past 20 years
They are normally apparent in summer at about 50-65 degrees north and south
They have been seen recently as low as 40 degrees North
One study has implicated water in space shuttle exhaust plumes as a contributor
It is known, for example, that increased concentrations of carbon dioxide, from fossil fuel burning, are cooling the highest reaches of the Earth's atmosphere.

But there may be other, more influential factors at play which could explain why these clouds have become brighter, increased in frequency and have started to appear at latitudes down to 40 degrees.

"The occurrence of these clouds at the edge of space and what causes them to vary is not understood," said AIM principal investigator James Russell.

"One theory is that the cloud particles grow on 'seeds' of meteoric dust or dust lofted up from below. AIM will provide the comprehensive data needed to test current theories for cloud formation or develop new ones, and allow researchers to build tools to predict how they will change in the future."

Dust count

The Aim spacecraft carries three instruments, and will look through and over the clouds.

One instrument is essentially a suite of four cameras that will provide multiple, panoramic views of the poles and clouds.

AIM spacecraft (Nasa/Hamilton)
The AIM spacecraft will look through and over the clouds
Another, called the Solar Occultation for Ice Experiment (Sofie), will measure cloud particles, temperature and atmospheric gases involved in forming the clouds.

The third instrument, called the Cosmic Dust Experiment, records the amount of space dust that drifts into the atmosphere from the cosmos.

Noctilucent clouds were first identified in 1885 by the British amateur astronomer Robert Leslie.

The Nasa mission is run out of Hampton University, Virginia; and has international partners, including the British Antarctic Survey.

Sky at night takes on a blue hue
05 Jun 06 |  Northern Ireland
Mission to target highest clouds
26 May 06 |  Science/Nature
Spacecraft seek climate clarity
28 Apr 06 |  Science/Nature

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