WEST LAFAYETTE — Ever think about a meteoroid racing through Earth's atmosphere and striking our planet?

A Purdue University professor certainly has, but his recent research shows that the atmosphere is a better shield from such potentially destructive objects than previously thought.

“In the past, we’ve been missing an important piece of our understanding of what happens during the meteoroid entry,” said Jay Melosh, professor of Earth, Atmospheric and Planetary Sciences at Purdue.

His team studied the meteoroid explosion over Chelyabinsk, Russia, in 2013. As the 20-meter, 10,000-ton asteroid entered Earth’s atmosphere, it created a bright fireball seen by people on the ground. It exploded about 29 kilometers over land and emitted a shock wave equivalent to 10 Hiroshima bombs.

They found that air pressure builds up in front of a meteoroid. A vacuum is created behind it. Depending on the meteor’s surface and porousness, air will rush into those passages and blow off pieces.

“It’s like putting an air pressure hose into a bucket of sand. It blows the sand out of the bucket,” Melosh said.

"Across the meteor there's a big difference between the high-pressure pad of air in front and the vacuum behind it. If the meteor is at all porous, that high pressure of air will get to the inside and essentially blow it apart."

Previous studies of meteoroid break-ups have generally been based on the re-entry of space vehicles. His research team found a better explanation of why meteors break up in the atmosphere.

Most non-scientists simply believe that meteoroids and asteroids “burn up” in the atmosphere.

“The big incoming asteroids do not burn up and that’s what impact craters like Meteor Crater in Arizona were all about and that’s why the dinosaurs died,” Melosh said. “But for an intermediate-sized one, the atmosphere is pretty good at stopping these things.”

Although Melosh, who has done extensive studies on the effects of meteoroid impacts, has handled fragments of the Russian meteoroid, his team used computer models to decode the explosion.

The composition of such extraterrestrial bodies vary so there's no sure safeguard against them striking Earth.

The Russian object started breaking up 70 kilometers above Earth.

“In this size range I think our best safeguard is actually vigilance, finding these things before the impact and then doing good old civil defense, warning people about what’s coming and, if it’s something bigger, possibly evacuating the area if you have time.

“Foreknowledge is the best defense for objects of this size,” he added.

The team's research paper was published recently in "Meteoritics and Planetary Science." The research was supported by a grant through NASA’s Office of Planetary Defense.

According to NASA, meteoroids are rocky bodies hurtling through space. A meteor is the “shooting star,” which is seen as it enters the atmosphere. When they break up, the pieces hitting the planet become meteorites.

And don’t assume that Hollywood disaster movies accurately reflect the true nature of meteors.

"I don't think yet I've seen a Hollywood movie in which the meteor was realistic," Melosh said. "Realistic stuff doesn't make a good story. Hollywood usually manages to get it wrong."

Entry points

Purdue University researchers have studies predicting the impact of extraterrestrial bodies if they reach Earth. The results depend on the velocity of the rock and the angle of entry into the atmosphere.

In one example from the Impact: Earth! calculator, a porous 381-meter sedimentary rock, about the size of the Empire State Building, would leave the likely following impact if it traveled at 11 kilometers a second and entered the atmosphere at a 45-degree angle:

• The body would begin breaking up at 215,000 feet over Earth. The broken projectile fragments might leave a crater about two miles wide. But the pieces striking Earth could leave little damage in buildings of good design and construction. There would be slight to moderate damage in well-built ordinary structures and considerable damage in poorly built or badly designed structures. Some chimneys would be broken and heavy furniture could be overturned.

Source: Purdue University Impact: Earth!

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