A new computer model shows that hours after an asteroid has been struck, gravity at the space rock's core can pull together the object's fragmented body.

When a science fiction plot portrays Earth in peril from a potentially devastating asteroid impact, a collection of heroes usually swoops in to save the day by detonating the enormous space rock into fragments.

But in reality, exploding a city-size asteroid may require more power than once thought, according to a new study.

Scientists had previously used computer models to estimate the impact needed to successfully shatter a big asteroid. However, a new model by another team of researchers recently came to a different conclusion by adding a variable that an older model omitted: how quickly cracks would spread through an asteroid after it was struck.

By looking more closely at small-scale changes in the asteroid's structure, the researchers developed a clearer snapshot of what would happen after an impact. Their new model suggests that gravity could help the asteroid hold itself together even after a powerful explosion and that more energy would be needed to smash the object to smithereens. [Top 10 Ways to Destroy Earth]

"We used to believe that the larger the object, the more easily it would break, because bigger objects are more likely to have flaws," lead study author Charles El Mir, a researcher with the Whiting School of Engineering at Johns Hopkins University in Baltimore, said in a statement.

"Our findings, however, show that asteroids are stronger than we used to think," El Mir said.

For their computer model, El Mir and his colleagues used the same scenario as in prior models created by other researchers: a target asteroid measuring about 16 miles (25 kilometers) in diameter gets struck by an object with a diameter of about 0.6 miles (1 km) traveling at 11,185 mph (18,000 km/h).

Calculations from earlier studies stated that such a high-speed impact would pulverize the target. But when researchers tested the new model, they saw a different outcome. Though the target asteroid was badly damaged, its core held together, the scientists reported in the study.

Their simulation separated what happened post-impact into two stages: seconds after the impact and then hours later. Immediately after the asteroid was struck, millions of cracks radiated inward, with the model predicting where and how they would spread through the asteroid's body.

In the new simulation, an impact ripples through an asteroid's body in moments. (Image credit: Courtesy of Charles El Mir)

But the asteroid didn't break apart. Instead, over the hours that followed, the gravitational pull of its damaged core gathered together the rocky fragments around the core, resulting in an asteroid that was fragmented but not completely blown to pieces, the study authors reported.

While big asteroid impacts on Earth are exceptionally rare, computer models such as these can help scientists to strategize how we might defend ourselves against potentially devastating projectiles in the future, Kaliat Ramesh, a professor of mechanical engineering at Johns Hopkins' Whiting School of Engineering, said in the statement.

"We need to have a good idea of what we should do when that time comes," Ramesh said. "Scientific efforts like this one are critical to help us make those decisions."

The findings will be published in the March 15 issue of the journal Icarus.

Originally published on Live Science.