WASHINGTON (AP) -- It wasn't visible, even to a pilot flying through it, but a cloud of sooty burned kerosene drifted all the way from a Russian rocket launch to California, where instruments on a high-altitude research plane detected it.

"When we first started looking at this data we referred to it as the mystery plume," said Martin Ross of The Aerospace Corporation in Los Angeles. "It was a surprise it held together for so long."

It was the first time such emissions have been detected high in the stratosphere.

Near the Earth's surface, rocket emissions normally disperse into the air in a matter of hours, said Paul A. Newman of NASA's Goddard Space Flight Center in Greenbelt, Md. Seeing this cloud still together in the upper atmosphere after 12 days "was a surprise to us," he said.

The cloud encounter on April 18, 1997, is described in a paper scheduled to appear in the March 15 edition of the journal Geophysical Research Letters, published by the American Geophysical Union. The cloud was investigated by a team of scientists led by Newman.

"Typically, when you release a plume in the atmosphere it gets sheared out ... like stretching a wad of taffy," said Newman, an atmospheric physicist. The material will get smaller and smaller until it "will smear out to nothingness," he said.

He said scientists don't think such rocket emissions currently pose any hazard to the atmosphere, but studying this one gives them a good basis for estimating any future threat if the number of launches should be increased significantly.

Ross said the any effect from rocket combustion emissions was insignificant compared to other industrial activity.

The chance encounter 12 miles above the California coast near San Francisco disclosed a cloud more than 100 miles across but only about 100 yards thick. It was not visible, but instruments detected high concentrations of soot and sulfates of the type produced by burning kerosene rocket fuel.

After ruling out aircraft as the source of the soot, the scientists studied wind patterns to work its possible route. Researchers concluded the cloud most likely came from one of two Russian rockets: a launch from the Baikonur Cosmodrome in Kazakhstan on April 6 or one from Plesetsk, Russia, on April 9.

They said the air trajectories and trace gas concentrations suggest the Baikonur launch, a trip to resupply the Mir space station, was the more likely source.

This would mean that the plume traveled more than 6,000 miles over 12 days while remaining fairly intact, they reported.

The researchers noted that this was the first time emissions have been observed in the stratosphere from liquid-fueled rockets, although alumina particles from large solid-fuel rocket motors have been found in the past.

Besides Newman and Ross, the team included researchers from the University of Denver; National Oceanic and Atmospheric Administration; Raytheon Co. and Science Systems and Applications Inc.