When Otto Jesse pointed his telescope at the sky one June night in 1885, a cloud obstructed his view. Instead of being annoyed, however, the astronomer was delighted.
With good reason -- it was no ordinary cloud that was blocking his line of sight. Night had already fallen above Berlin, but the cloud continued glowing, its light almost as bright as the full moon. Jesse realized it must be located on the outer edge of the atmosphere, right on the border with outer space, in order to still be catching light from the sun.
Jesse, who was then in his mid-40s -- photographs show him with a stern expression and a well-groomed beard -- felt electrified that summer night. Apparently there were more things between heaven and earth than people had thought. Researching noctilucent ("night glowing") clouds became his life's work from that night on.
The clouds usually appear in the northern sky on summer nights. They're visible with the naked eye and are in some cases so bright that those standing in their light cast a shadow. Jesse used photographs to calculate their altitude for the first time: They were 82 kilometers (51 miles) above the ground -- more than 10 times higher than regular storm clouds.
Even today, more than 120 years later, the German scientist's observations are still considered groundbreaking. And even today, it remains unclear just how noctilucent clouds -- known to scientists as NLCs -- form. But this summer a breakthrough could finally be achieved: NLC researchers want to use robot cameras and a surveillance satellite to finally solve the mystery.
The conditions for observing NLCs were never more favorable than today. NLCs are glowing more brightly, they're being sighted more frequently, and they're appearing further south. They used to be sighted mainly north of the 50th parallel, in Sweden or Scotland. But a few years ago, they were sighted from Colorado for the first time -- on the 40th parallel, the same latitude as southern Italy.
NLCs have little in common with the Northern Lights, also known as the aurora borealis. They're produced not by particle streams but by ice, and they don't produce their own light but simply reflect that of the sun.
High and dry
It all sounds like a pretty simple meteorological phenomenon, but it's extraordinarily difficult to investigate, because it occurs in the barely accessible border region between the Earth and outer space. The mesosphere, as the region is called, is more than twice as high as the ozone layer. It lies far beyond the stratosphere and can be seen from the International Space Station (ISS).
The vast altitude also explains the luminosity of the clouds. The gaseous high-flyers are often still within reach of the sun when night has already fallen on the Earth below. It's only in the daytime that the mesospheric clouds disappear in the blue of the sky.
The zone in which NLCs are found is about 100 million times drier than the Sahara. That they can still form is due to the extreme cold there. Temperatures in the mesosphere can drop as low as minus 140 degrees Centigrade (minus 220 degrees Fahrenheit) -- colder than anywhere on the planets. The mesosphere shrouds the Earth like an ice-cooled crystal ball that clouds immediately when someone breathes on it. When a space shuttle travels to the ISS, for example, the hot exhaust fumes freeze and create man-made NLCs.
But these beauties of the night are capricious. They usually vanish as suddenly as they appear -- almost in the way that German playwright and poet Bertolt Brecht describes a cloud in one of his love poems: "There was a cloud my eyes dwelt long upon / It was quite white and very high above us / Then I looked up, and found that it had gone."
So whoever goes courting an NLC needs a lot of luck and plenty of patience. "Until now, the sightings were mainly left to chance," says Russian physicist Peter Dalin. He wants to change that. Dalin has been conducting research in Kiruna, Sweden ever since he got his doctorate at the Space Research Institute in Moscow.
Now he wants to ambush the heavenly fly-by-nights like a kind of stratospheric paparazzo. He's currently building a ring of six cameras around the North Pole -- from Siberia to Canada via Scotland. The plan is to use regular Canon cameras to take a picture of the sky every minute, each coordinated by a computer. The pictures will be taken for months -- throughout the entire NLC season, which lasts from May until August.
Four cameras have already been installed, and one of them is even connected to the Internet. "That way I get a kind of time lapse film," Dalin hopes. His hypothesis is that the formation of NLCs depends on lower air currents that travel the entire northern hemisphere and also transport warmth into the mesosphere. When the currents grow weaker, temperatures in the mesosphere drop even lower than usual. The little humidity which is found up there condenses, and ice clouds can form.
But even the hemispheric photo booth will probably miss many NLCs, since low clouds often obstruct the view from the ground. That's why astronomers are eager to see the completion of NASA's AIM (Aeronomy of Ice in the Mesosphere) satellite -- which will finally be able to analyze the clouds from the other side.
In late April, the satellite -- which weighs almost 200 kilograms (440 lbs) and costs almost $120 million -- will position itself at its observation post about 600 kilometers (373 miles) above the ground for two years. The first data could already be available by the end of the year.
AIM will use four cameras to survey the North Pole region. It will also be equipped with a second type of instrument, whose purpose is to determine whether NLCs are of heavenly or more earthly origin. Clouds normally form only where there are so-called "condensation nuclei." These nuclei usually consist of dust particles whose size is measured in nanometers, like that of the particles in cigarette smoke. Water vapor attaches itself to them. But where does this pollution of the border region between the Earth and outer space come from -- from above or below?
Otto Jesse already supposed back in 1890 that volcanic ash could be swept all the way up to the edge of the atmosphere. The Krakatoa volcano in Indonesia had erupted two years before he first sighted an NLC. Ash from the eruption was carried far up into the stratosphere, darkening the skies and causing bad harvests -- as well as spectacularly colorful sunsets all over the world.
A new hypothesis began attracting attention at the beginning of the space age: Are the luminous clouds perhaps caused by extraterrestrial rock clusters? About 100 tons worth of meteorites rain down on the Earth every day, with part of them burning up at about the altitude of the NLCs, where they appear as so-called shooting stars. A lot of extraterrestrial dust is left behind. A measuring instrument on board the AIM satellite that functions like a vacuum cleaner is now supposed to clarify the extent to which this extraterrestrial dust is related to cloud formation.
Scientists are pinning their hopes on SOFIE, as the instrument which is perhaps the most important measuring device on board AIM is known. SOFIE -- the acronym stands for Solar Occulation For Ice Experiment -- is a kind of thermometer, designed to answer the question of whether more NLCs are being sighted because the Earth is warming up.
That's exactly what Gary Thomas, a retired NLC specialist at the University of Colorado in Boulder believes. He's one of the advisors to the AIM mission. While it seems paradoxical at first, the phenomenon is not implausible: As more greenhouse gases isolate the Earth, thereby causing the lower strata of the atmosphere to heat up, less warmth radiates back to outer space and the memosphere gets colder -- ideal conditions for the formation of NLCs. Indeed, laser measurements at Germany's Leibniz Institute of Atmospheric Physics indicate that the temperature up there has dropped more than 15 degrees Celsius (59 degrees Fahrenheit) over the last 50 years.
"They are a beautiful phenomenon," Thomas says. "But these clouds could also be a sign from nature that we are upsetting the equilibrium of the atmosphere."