Tunguska: the explosion of a stony asteroid.

On the morning of June 30, 1908, an enormous fireball streaked across the sky over the Siberian wilderness. The tremendous atmospheric explosion that followed released enough energy to flatten and char trees over an area 60 kilometers wide near the Tunguska River. But the blast left no crater.

Explanations of this cataclysm have ranged from an encounter with a lump of antimatter or a miniature black hole to the entry into Earth's atmosphere of a "puffball" comet or an asteroid. Now Christopher F. Chyba of NASA's Ames Research Center in Mountain View, Calif., and his collaborators suggest a stony asteroid as the likely perpetrator.

To explain the peculiar pattern in which energy was deposited at the Tunguska site, many investigators have favored the explosion of a comet with such a low density that it would rapidly decelerate and come to a stop in the atmosphere. In a computer simulation, however, Chyba and his colleagues show that the nuclei of comets explode too high in the atmosphere to account for the Tunguska phenomena.

The researchers calculate that taking aerodynamic forces into account, an asteroid would fracture when mounting air pressure exceeds the object's strength. Thus, a stony asteroid about 30 meters in diameter and moving at 15 kilometers per second would disintegrate at roughly the same height at which the Tunguska object apparently exploded, whereas an iron-rich asteroid would explode too low - if at all - and a carbonaceous asteroid would explode too high.

The researchers report their results in the Jan. 7 Nature.

"Given that the Tunguska object was kind of typical, it was probably a stony asteroid," Chyba concludes. But the simulation doesn't entirely rule out an unusually fast, iron-rich asteroid or a very strong carbonaceous asteroid.

Nonetheless, comments H. Jay Melosh of the University of Arizona in Tucson, "instead of an implausibly low-density comet, the Tunguska projectile [is] more likely a representative of the most common class of meteorites."

To improve estimates of the probability of atmospheric explosions or direct impacts caused by cosmic projectiles striking Earth, Chyba and his colleagues intend to use their computer model to check systematically the fates of different types of objects over a wide range of sizes as they enter the atmosphere. Chyba has already found that of about eight small objects known to pass close to Earth, half would cause Tunguska-like explosions if they hit the atmosphere.

"A lot of these objects wind up being bad news," Chyba says.

Heightened interest in the likelihood of devastating explosions and impacts caused by such objects has also prompted researchers to take a closer look at Earth-approaching asteroids. Last month, astronomers at NASA's Jet Propulsion Laboratory (JPL) in Pasadena, Calif., used a large radar antenna to obtain the sharpest images yet of one of these asteroids - Toutatis - when it passed within 2.2 million kilometers of Earth.

The radar images reveal that Toutatis consists of two irregularly shaped, cratered chunks, about 4 and 2.5 kilometers in diameter, which may be held in contact by gravity "Three years ago, we were startled by the initial evidence for contact-binary asteroids," says JPL's Steven Ostro. "Now it seems that double bodies might be very common in the Earth-approaching asteroid population."