WASHINGTON — A high-altitude research plane checking the skies over California discovered a cloud of sooty burned kerosene, probably produced by a Russian rocket launch nearly two weeks earlier.
It was the first time such emissions have been detected high in the stratosphere. Researchers had expected such rocket plumes to disperse in the air.
Paul A. Newman of NASA's Goddard Space Flight Center in Greenbelt, Md., said that at the Earth's surface mixing of emissions into the air occurs within a few hours. Seeing this cloud still together in the upper atmosphere after 12 days "was a surprise to us."
"It was a surprise it held together for so long," added Martin Ross, an atmospheric chemist at the Aerospace Corp. in Los Angeles. "When we first started looking at this data we referred to it as the mystery plume."
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 has since dissipated.
"Typically, when you release a plume in the atmosphere it gets sheared out . . . like stretching a wad of taffy," explained 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 threat if the number of launches should be increased significantly.
"It's a nice neat little piece of information," Newman said.
Ross added that "emissions from rocket combustion do affect stratospheric chemistry, but it's an insignificant effect compared to other industrial activity."
The cloud, more than 100 miles across, was only about 100 yards thick.
After ruling out aircraft as the source of the soot, the scientists studied wind patterns to work its possible trajectory. While the data weren't perfect, researchers concluded the cloud most likely originated in 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 means that the rocket's plume (traveled) more than (6,000 miles) over a 12 day period while remaining fairly intact and well-defined horizontally and vertically," they reported.
The researchers noted that this is 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 detected 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.