Scientists Trace 85-Year Trail to Asteroid : Astronomy: Researchers say a mysterious 1908 blast in Siberia was the result of a midair breakup of a speeding chunk of space matter.

An explosion with the punch of a small atomic bomb pounded Siberia near the Tunguska River in 1908, flattening forests, lightening the night sky in Europe and posing one of the century’s most intriguing scientific riddles: What could cause such a catastrophic event?

Exotic and entertaining theories from black hole phenomena to flying saucer crashes have been offered for the blast, but scientists have focused primarily on comets and asteroids. However, vital evidence of a collision--notably a crater--has never been found.

In Thursday’s journal Nature, three scientists from the National Aeronautics and Space Administration and the University of Wisconsin propose a solution: the explosive, aerodynamically induced midair breakup of a 100-foot-wide asteroid. The scientific team, led by physicist Christopher F. Chyba of NASA’s Ames Research Center in Mountain View, used discoveries about craters on Venus to estimate the size, composition and trajectory of the possible asteroid, as well as the altitude at which it may have exploded.

Key to their explanation is an understanding of how aerodynamic forces break open a speeding asteroid in midair. As the rock crumbles, the spreading fragments create a huge shock wave.

Chyba and his colleagues said their work shows that the Tunguska event was not just a scientific curiosity, but “the typical fate for stony asteroids . . . entering the atmosphere at common hypersonic velocities.”

The event, they said, can help scientists and government officials better assess the hazards posed by impacts of small comets and asteroids.

Although the Tunguska blast was felt over a large area and reportedly brightened the sky thousands of miles away in Europe for several nights, scientists did not visit the suspected explosion site for nearly two decades.

H.J. Melosh of the University of Arizona, who has studied Tunguska, said the first scientist on the scene found thousands of acres of downed trees with “boggy depressions” at the center. He concluded that a meteorite caused the damage, but no one could find a crater or telltale meteorite fragments.

Scientists later proposed that the area had been hit by an icy comet. But all known types of comets are so dense that they would have almost certainly left a crater on impact.

Later events, including the midair explosion of a small asteroid over Revelstoke, Canada, in 1965, persuaded a later generation of scientists that a meteor was the most likely cause of the Tunguska explosion.

But how it occurred remained unclear until one of Chyba’s colleagues, Kevin J. Zahnle, studied how asteroids break up while passing near Jupiter and the sun and applied the data to those heading toward Earth.

The researchers, who included Paul J. Thomas of the University of Wisconsin in Eau Claire, concluded that the Tunguska asteroid was probably made of stone because an iron-based asteroid would have hit the ground intact and a more porous asteroid should have exploded high in the atmosphere.

The team’s calculations show that the Tunguska asteroid probably descended about 32,000 m.p.h. at a 45-degree angle and exploded about 5 1/2 miles above the ground. The altitude correlates with earlier estimates that were based on the pattern of fallen trees.

The resulting fireball of superheated air probably evaporated a large amount of water, creating high altitude ice clouds, the scientists said. Winds would have pushed these noctilucent clouds over Europe, where they lighted the night sky by reflecting distant sunlight.