asteroids Incoming asteroids could crumble harmlessly before they hit us

[Reefa]

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The skies may be safer than we assumed. Many asteroids are weak and brittle – which could be good news for us on Earth.

 

More than 90 per cent of asteroids and comets larger than a kilometre across in Earth’s neighbourhood have already been discovered, and scientists think the region is mostly clear of them.

 

Should one wander near to us, though, it could have devastating results. Scientists have ideas about how to push it away with thrusters or solar sails. However, the success of these plans depends on understanding what the rocks are made of and whether they might break apart.

 

Space rocks fall to Earth as meteorites all the time, but few are recovered, so scientists are reluctant to crush them to study their contents and behaviour. Earth rocks usually serve as models instead.

 

Recently, Desireé Cotto-Figueroa of Arizona State University in Tempe and her colleagues sacrificed some centimetre-sized cubes taken from well-studied meteorites, one that fell in Mexico in 1969 and another that landed in Morocco in 2008. They wanted to find out how terrestrial rocks really compare with meteorites, and whether we can trust Earth rocks in tests of asteroid-deflection schemes.

 

When the researchers crushed the cubes, they found that they were nearly as brittle as concrete. By extrapolating the strengths and weaknesses of the tiny samples to much larger scales, the team calculated the likely break-up rate for different meteorite sizes and types.

 

It turns out that overall, space rocks from the asteroid belt are less tough than Earth rocks, says Cotto-Figueroa. Crumblier asteroids are more likely to fall apart in the atmosphere, producing brilliant fireballs rather than wide craters.

Only the strong survive

This lines up well with what we see on the ground, says study co-author Erik Asphaug, also at Arizona State University.

 

“We do see a lot more fireballs than craters, and in fact only the iron meteorites – the strong, dense ones – survive to hit the ground and make craters smaller than a few kilometres,” he says. “That means we can explain why meteors that are the size of a car break apart so high up in the atmosphere. They are weak, about the strength of cruddy sandstone.”

 

That’s good news, says Dan Durda at the Southwest Research Institute in Boulder, Colorado – given that the biggest space rocks, which would do the most damage on colliding with Earth, are the likeliest to fall apart before reaching the ground. “It’s a saving grace for us,” says Durda.

 

But Daniel Britt at the University of Central Florida in Orlando isn’t sure we’re that much safer. “It is clear these are piles of rubble. But that doesn’t mean the boulders are strengthless,” he says. “These guys are just a pain to crush.”

 

The findings also complicate matters for protecting Earth, because weaker asteroids would be harder to deflect.

 

Asphaug agrees: some asteroids could still be dangerous. “Just because something is weak doesn’t mean it has time to come apart and explode in the atmosphere before it hits,” he says. “Once an object is about 100 metres in diameter or so, it hits Earth before it hits enough air that could stop it.”

Better models

The real message is that we need to be more careful about which materials we use to study how asteroids and other small bodies behave, he says. The meteorites that survive the trip through Earth’s atmosphere represent the strongest part of the original rock – with the 90 per cent lost during the journey probably even weaker.

 

“We have a strong selection bias in the meteorites we have in the lab,” says Asphaug. If we want to understand these bodies well enough to protect ourselves from them, we need to grapple with that, he says.

 

Recent history also shows that asteroids can still do plenty of damage in the atmosphere. The 20-metre-wide meteor that exploded over Chelyabinsk, Russia, in February 2013 contained between 20 and 30 times the energy of the atomic bomb dropped on Hiroshima, and injured more than 1500 people.

 

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