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A powerful green light lances toward the stars above Utah State University in Logan, stretching miles into the night sky - is it a signal from ET trying to phone home?

Actually, the beam is a fancy sort of laser shining from the top of USU's science building. It's used to detect fluctuations in the atmosphere.This week, scientists are flashing it into the heavens while space shuttle Discovery passes overhead, shining its own laser light downward. Eventually, researchers hope to correlate the results of the ground-based studies with those carried out from Discovery.

The system, called "lidar" for "light detection and ranging," illuminates the atmosphere between 20 and 60 miles above the ground, said Professor Thomas D. Wil-ker-son of the University of Maryland and USU. The light that bounces back can be detected in all sorts of conditions, and it works best during clear weather.

Although the reflections are captured by using an 18-inch-diameter telescope, the light can be seen from the ground.

"It seems like a long, bright green beam that extends from the top of our building here," said Professor Robert Schunk, director of USU's Center for Atmospheric and Space Sciences.

"You can see it at great distances. And particularly you can see it as it bounces off a cloud."

The beam is so bright that the university had to get a special clearance from the Federal Aviation Administration to make certain that it shines where planes aren't flying over. Otherwise, pilots looking down might have their vision impaired.

How bright is the laser? "It's bright enough that if you put a piece of paper up there (immediately in front of the device) it burns the paper," Wilkerson said.

Pulses of light flash upward at 30 times a second, so frequently that they fool the eye and look like a steady beam. At other times, a smaller laser is used that flashes at 10 times a second, which makes the beam appear to blink.

Each pulse lasts only 10 one-billionths of a second. For that infinitesimal moment, the laser puts out 70 megawatts of power.

By studying the way light bounces back to the telescope, scientists can calculate what kind of air pressure changes, water vapor or other conditions it is encountering at different heights.

The region of most interest to them is generally called the middle atmosphere. Wilkerson joked that sometimes, because it's so hard to get balloons or planes to that level to gather data, "it's called the ignorosphere."

Wilkerson said the USU lidar station and other laser observatories have been given schedules of the passages of Discovery, so they can make measurements at the same time and in the same general area as the shuttle's laser experiments.

"We are particularly focusing on those times when they are going to be flying over," he said.

Lidar experiments at USU have been going on since August 1993. They are run by Consortium Lidar, which includes Professor Vincent Wickwar at USU; Wilkerson; Professor John Meriwether at Clemson University, Clemson, S.C.; and Professor David Rees, University College, London.

The project is funded partly by the institutions and partly by the National Science Foundation.