Water-laden exhaust from a space shuttle can drift over the North Pole and create elusive high-altitude clouds visible only at night, according to a surprising new study.

The discovery was made serendipitously with data collected by a German satellite launched and retrieved eight days later during a Space Shuttle Discovery mission. Surprised scientists watched the clouds develop from water that had been shuttled into the upper atmosphere by the very craft that lofted the satellite into orbit. Ground-based observations helped confirm the exhaust plume's travels.

The findings, from mission STS-85 in 1997, were reported May 31 in the journal Geophysical Research Letters.

At first thought, it might seem like a shuttle's exhaust would be a drop in the atmospheric bucket of moisture.

"Indeed, this was a surprise to us," said study leader Michael Stevens, a physicist at the Naval Research Laboratory in Washington.

But the outer atmosphere is thin and tenuous. A little goes a long way.

"It's a little like pouring a bucket of water onto your driveway -- spatially, the repercussions can be quite widespread," Stevens told SPACE.com. "Lower in the atmosphere, there is much more water, so it would be like pouring a bucket into a swimming pool."

He noted that mysterious noctilucent clouds have been spotted since at least the late 19^th Century, so rockets are not their only cause.

Much of the study was based on the Naval Research Laboratorys Middle Atmosphere High Resolution Spectrograph Investigation (MAHRSI) instrument, which was aboard the satellite. The research was funded in part by NASA.

The shuttle-inspired clouds are not very similar to condensation trails left by commercial airliners, which are created just as the exhaust is emitted, and at much lower altitudes.

Jet contrails, as they're often called, involve particles in exhaust that give water vapor a place to condense. While the exhaust can be humid, the process relies on water vapor already in the atmosphere.

A study performed in the days after the Sept. 11, 2001 terrorist attacks, when U.S. skies were clear of airplanes, found that contrails have a small but measurable effect on daily temperatures on Earth. The temperature range was more than one degree Celsius (about 1.8 degrees Fahrenheit) larger than when contrails were present, scientists reported in the journal Nature.

Meteorologists sometimes glean information from jet contrails. How readily one forms, and how long it persists, suggests how much moisture is in the air. Contrails also contribute about 1 percent of manmade greenhouse effects, a 1999 report showed.

Stevens and his colleagues never intended to track Discovery's emissions.

But over the mission's first two days, they noticed signatures of the water vapor plume high above the Arctic that "got our attention, and one thing led to another. This was not the plan; the shuttle plume was never considered in any of our operations. Sometimes, some of the most interesting science comes to you."

More work is needed to understand how the plume moved northward so quickly.

Also, with only one study of rocket-fueled clouds to go on, it's not clear if the Space Age alters the atmosphere much beyond creating a few high, mostly invisible clouds. Stevens said the study found no evidence for any other environmental effects.

Other rocket launches might also contribute to the formation of noctilucent clouds, he said, but further study is needed to find out.

"The shuttle injects by far the most water per launch into the upper atmosphere of any launch vehicle currently operating," Stevens said.

With dozens of other rockets going up in a typical year, his team would like to study their possible cumulative effect in generating the mysterious polar clouds.