Comet likely culprit in Tunguska blast

Night-shining clouds created after space shuttle launches may offer clues into the cause of the Tunguska event, a mysterious blast which rocked southern Siberia more than a century ago.

Thin clouds have appeared at abnormally high altitudes over polar regions following space shuttle launches on several occasions in the past decade. These noctilucent, or night-shining, clouds typically occur in summer and lie at altitudes of about 85 kilometers, in a layer of the atmosphere called the thermosphere, says Michael C. Kelley, an atmospheric physicist at Cornell University. Kelley and his colleagues suggest in the July 28 Geophysical Research Letters that data gleaned from analyses of these high-flying clouds, as well as knowledge about the speed at which shuttle exhaust wafted to polar regions, now hint that the Tunguska blast of June 1908 (SN: 6/21/08, p. 5) resulted from a comet slamming into Earth’s atmosphere.

Each launch of a space shuttle, which burns a combination of liquid hydrogen and liquid oxygen as fuel, pumps about 300 metric tons of water vapor into the atmosphere at altitudes between 100 and 115 kilometers. Soon after the January 16, 2003, launch of the shuttle Columbia, a liftoff that took place just after the height of summer in the Southern Hemisphere, noctilucent clouds appeared over Antarctica. Similarly, a widespread display of the night-shining clouds showed up over Alaska two days after the shuttle Endeavour blasted off on August 8, 2007. Previous studies show that in both instances those clouds included material from the shuttle plumes.

 “Conventional wisdom says that the plumes shouldn’t reach the poles that quickly, but they do,” Kelley notes. So sustained high-altitude winds must be carrying the plumes to the poles.

Michael S. Stevens, a research physicist at the Naval Research Laboratory in Washington, D.C., agrees: Experiments show that “winds can be strong at these altitudes, but they’re not well understood.”

Using data based on shuttle plume movement, the researchers suggest that the Tunguska blast could have been responsible for unusually bright noctilucent clouds over Europe soon thereafter. The likely composition of those clouds further suggests that a comet caused the blast.

Some of the thickest and brightest noctilucent clouds ever observed — ones that cast enough light to read a newspaper in the middle of the night — appeared over Europe on July 1, 1908, Kelley says. Not coincidentally, he and his colleagues argue, the Tunguska blast occurred over southern Siberia about 22 hours earlier. The team’s models suggest that winds and diffusion could have transported material from the site of the blast to the skies over London, a distance of about 5,000 kilometers, in little more than a day.

Scientists at the time suggested that the night-shining clouds over London were made of meteoritic dust. But those aerosols are typically too small to reflect sunlight efficiently, Kelley argues, suggesting the clouds above Europe were made of ice crystals. This assumption, along with the new analysis of shuttle plume movement, strongly suggests that the object that blazed into the atmosphere and disintegrated above Siberia was a moisture-rich comet rather than a relatively dry asteroid.

“That’s an interesting idea, and worth considering,” Stevens says. In an alternate scenario, he notes, rather than the moisture being transported to Europe and then coalescing into clouds, the clouds may have formed over Siberia and then been transported to the west.