Atmospheric scientist Gary Thomas of the University of Colorado has seen thousands of noctilucent cloud (NLC) photos, and he ranks this one among the best. "It's lovely," he says. "And it shows just how high these clouds really are--at the very edge of space."

He estimates the electric-blue band was 83 km above Earth's surface, higher than 99.999% of our planet's atmosphere. The sky at that altitude is space-black. It is the realm of meteors, high-energy auroras and decaying satellites.

What are clouds doing up there? "That's what we're trying to find out," says Thomas.

People first noticed NLCs at the end of the 19th century after the 1883 eruption of Krakatoa. The Indonesian supervolcano hurled plumes of ash more than 50 km high in Earth's atmosphere. This produced spectacular sunsets and, for a while, turned twilight sky watching into a worldwide pastime.

One evening in July 1885, Robert Leslie of Southampton, England, saw wispy blue filaments in the darkening sky. He published his observations in the journal Nature and is now credited with the discovery of noctilucent clouds.

Scientists of the 19th century figured the clouds were some curious manifestation of volcanic ash. Yet long after Krakatoa's ash settled, NLCs remained.

"It's a puzzle," says Thomas. "Noctilucent clouds have not only persisted, but also spread." In the beginning, the clouds were confined to latitudes above 50o; you had to go to places like Scandinavia, Siberia and Scotland to see them.

"This year's apparition over Iran was splendid," says Thomas. The Persian clouds appeared on July 19th, just a few days before the ISS display, and were photographed from latitude 38o N. "That's pretty far south," he says.

The genesis and spread of these clouds is an ongoing mystery. Could they be signs of climate change? "The first sightings do coincide with the Industrial Revolution," notes Thomas. "But the connection is controversial."

NASA is investigating. The AIM satellite, launched in April 2007, is now in polar orbit where it can monitor the size, shape and icy make-up of NLCs. The mission is still in its early stages, but already some things have been learned. Thomas, an AIM co-Investigator, offers these highlights:

1. Noctilucent clouds appear throughout the polar summer, are widespread, and are highly variable on hourly to daily time scales. A movie made from daily AIM snapshots shows the 2007 NLC season unfolding over the north pole: watch it.

2. There is a substantial population of invisible noctilucent clouds. Thomas explains: "NLCs are made of tiny ice crystals 40 to 100 nanometers wide-just the right size to scatter blue wavelengths of sunlight. This was known before AIM.

The spacecraft has detected another population of much smaller ice crystals (< 30 nm) that don't scatter much sunlight." Clouds made of these smaller crystals are stealthy and hard to see, but a key part of the overall picture.

3. Some of the shapes in noctilucent clouds, resolved for the first time by AIM's cameras, resemble shapes in tropospheric clouds near Earth's surface. AIM science team members have described the similarities as "startling." The dynamics of weather at the edge of space may not be as unEarthly as previously supposed.

These findings are new and important, but they don't yet unravel the central mysteries:

Why did NLCs first appear in the 19th century?

Why are they spreading?

What is ice doing in a rarefied layer of Earth's upper atmosphere that is one hundred million times dryer than air from the Sahara desert?

AIM has just received a 3-year extension (from 2009 to 2012) to continue its studies. "We believe that more time in orbit and more data are going to help us answer these questions," says Thomas.

Meanwhile, it's a beautiful mystery. Just ask anyone at the edge of space.

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