When asteroids fall through Earth's atmosphere, a variety of things can happen. Large iron-heavy space rocks are almost sure to slam into the planet. Their stony cousins, however, can't take the pressure and are more likely to explode above the surface.

Either outcome can be dismal. But the consequences vary.

So scientists who study the potential threat of asteroids would like to know more about which types and sizes of asteroids break apart and which hold together. A new computer model helps to quantify whether an asteroid composed mostly of stone will survive to create a crater or not.

A stony space rock must be about the size of two football fields, or 720 feet (220 meters) in diameter, to endure the thickening atmosphere and slam into the planet, according to the study, led by Philip Bland of the Department of Earth Science and Engineering at Imperial College London.

"Stones of that size are just at the border where they're going to reach the surface -- a bit lower density and strength and it'll be a low-level air burst, a bit higher and it'll hit as a load of fragments and you'll get a crater," said Bland, who is also a Royal Society Research Fellow.

The distinction would mean little to a person on the ground.

Two ways to destroy a city

"An airburst would be a blast somewhere in the region of 500-600 megatons," Bland said in an e-mail interview. "As a comparison, the biggest-ever nuclear test was about 50 megatons."

A presumed airburst in 1908, over a remote region of Siberia called Tunguska, flattened some 800 square miles (2,000 square kilometers) of forest. The object is estimated to have been just 260 feet wide (80 meters). Bland said the event was probably equal to about 10 megatons.

"If most of it made it to the ground you might actually be a bit better off, because the damage would be a little more localized," he said. "A lot of energy would still get dumped in the atmosphere, but you'd probably also have a ragged crater, or crater field, extending over several kilometers, with the surrounding region flattened by the blast."

Smaller stony asteroids, say those the size of the car, enter the atmosphere more frequently but typically disintegrate higher up and cause no damage. In fact, as many as two or three dozen objects ranging from the size of a television to a studio apartment explode in the atmosphere every year, according to data from U.S. military satellites.

Separate research in recent years has shown that stony asteroids are often mere rubble piles, somewhat loose agglomerations of material that may have been shattered in previous collisions but remain gravitationally bound.

Pieces and parts

The new computer model is detailed in the July 17 issue of the journal Nature. It was created with the help of Natalia Artemieva at the Russian Academy of Sciences.

Previous models treated the cascade of fragments from a disintegrating asteroid as a continuous liquid "pancake." The new model tracks individual forces acting on each fragment as the bunch descends.

The researchers can plug in asteroid size, density, strength, speed and entry angle at the top of the atmosphere. With "reasonable confidence" a computer program then details how that rock should behave in the air and what will happen at the surface.

The model has implication not just for land-based impacts, but also splashdowns in the ocean that can trigger devastating tsunamis. An airburst is not likely to generate much of a tsunami, possibly lowering that risk compared to what scientists had figured.

The results suggest rocks about 720 feet across (220 meters) are likely to actually hit the surface every 170,000 years or so. Some previous research has suggested a frequency of every 4,000 years or less.

Looking back

The model can also "hindcast" what sort of rock might have generated a certain known crater.

"You see a crater field on Mars, we can tell you what sort of object caused it," Bland said.

In fact, he and Artemieva have done just that. In their most recent tests, which are not discussed in the Nature paper, they plugged in the atmospheric details of Mars, as well as Venus, and hurled some hypothetical space rocks at those planets.

"The simulated crater fields that the model produces look almost exactly like the real thing," Bland said.

For now, the model does not handle very large asteroids, those that could cause widespread regional or even global damage, though Bland said the flaw may be fixable. He is careful to point out that computer models do not provide solid proof for what might happen.

"There are still a lot of unknowns in this," he said.