By Jonathan Amos
BBC News science reporter, in Baltimore
A Nasa satellite mission will be launched this year to study the highest and most mysterious clouds on Earth.
"Night-shining" clouds form brilliant silver-blue structures
Noctilucent, or "night-shining", clouds appear as thin bands in twilight skies, some 80km (50miles) above the surface.
Recent records suggest they have become brighter, more frequent and are being seen at lower latitudes than usual.
Scientists cannot say for sure but they suspect human activity may be altering the conditions in the mesosphere that drive the clouds' formation.
"Noctilucent clouds were first seen in 1885 by a British amateur astronomer, Robert Leslie," explains James Russell from Hampton University, Virginia, US.
"They're very beautiful. They have distinctive features - bands, and ripples we call billows - and form right on the edge of space."
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
Russell is the principal investigator on the AIM (Aeronomy of Ice in the Mesosphere) spacecraft, which will be lofted to 600km (370 miles) to make a detailed study of the clouds.
The 195kg (430lb) satellite will be put in space by a Pegasus rocket launched from beneath the wing of an aircraft.
AIM's three instruments will investigate the recipe needed to make the clouds - 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 think most of the dust comes not from below but from above - from space. It is extremely hard for dust in the lower atmosphere to be pulled so high, while meteoritic dust is known to be settling onto the planet all the time as rocky space debris falls to Earth.
The AIM spacecraft will look through and over the clouds
If this has stayed reasonably stable over time, then the explanation for the observed changes in the occurrence and properties of the clouds will have to be sought in the satellite's temperature and water/ice data.
The mesosphere is already very cold, down to about -125C and more, but researchers say it is getting even colder.
Although the extra carbon dioxide (CO2) put into the atmosphere by human activities has warmed the air near the Earth's surface, it is thought to have had the opposite effect in the middle and upper atmosphere by radiating heat more efficiently into space.
"In addition to CO2, methane has been increasing in the atmosphere," added Dr Russell. "Once methane makes it into the high atmosphere, the sun breaks down the molecule and forms water - so, that's another source for water vapour in addition to the water vapour coming from below.
They form more than 70km above most clouds
"These are all likely causes for the changes we are seeing. Our mission will collect the data that can be put into the models to help us get to sound conclusions about what is really going on."
AIM is a US space agency Small Explorer mission. It has a number of partner organisations, including the British Antarctic Survey.
James Russell gave details of the mission here at the American Geophysical Union Joint Assembly.