NASA launched a spacecraft in April, 2007 on a two-year mission to study mysterious “noctilucent” clouds. The AIM spacecraft will monitor the clouds from Earth orbit to learn what they may be telling us about our planet.

Noctilucent clouds (NLC), also known as Polar mesospheric clouds, are very high, bright “cloudlike” phenomena visible in a deep twilight, mostly in higher latitudes. The name means “night shining” in Latin. They are most commonly observed in the summer months at latitudes between 50° and 60° north and south of the equator.
NLC are higher than any other visible clouds, located in the “mesosphere” layer at altitudes of around 60 miles. They are visible only when illuminated by sunlight from below the horizon while the ground and lower layers of the atmosphere are in the Earth's shadow; at other times they are too faint to be seen.
Scientists have wondered about the composition and origin of NLC since they were first reported, following the eruption of Krakatoa in 1885. It was once proposed that they were composed of volcanic or meteoric dust, but they are now known to be primarily composed of water ice, like cirrus clouds (which are at much lower altitudes). Dr. Michael Stevens of the United States Naval Research Laboratory believes space shuttle exhaust may contribute to the formation of noctilucent clouds, but so far that remains speculative.

Noctilucent clouds can be studied from the ground, from space, and in situ by sounding rockets, but they are too high to be reached by weather balloons. It is hoped that the AIM satellite mission will provide the best information yet.

NLC are visible only against a twilit sky background before sunrise or after sunset; they are never seen in daylight skies. The best time to look for NLC is during the deep twilight's of summer, particularly just prior to and for a few weeks after the summer solstice – the last half of June in the Northern Hemisphere.
Optimal geographic locations are those between 50 and 60 degrees latitude (for example, southern Canada), though they are occasionally reported beyond this latitude band. Through most of June and July (in the northern hemisphere) the skies are never completely dark, and twilight persists all night.
Here’s a fine description of NLC, from the Noctilucent observers’ home page (http://www.kersland.plus.com/): “They appear as complex interwoven streaks or knots of "cloud". Colour is generally white or a distinctive pearly-blue tone, sometimes with a golden lower edge. Structure is reminiscent of daytime cirrostratus formation.”

On August 28, 2006, scientists with the Mars Express mission announced that they found clouds of carbon dioxide similar to noctilucent clouds over Mars that extended up to 100 km above the surface of the planet. So maybe Earthlings are not the only ones who see these things?
Jim Parr was kind enough to share some photographs of NLCs he took in June, 2007.

Thanks, Jim!

AIM spacecraft image from June 11:
NASA's AIM spacecraft took this composite UV-wavelength picture
on June 11th from a vantage point 600 km over Earth's north pole: