Meteor guide: science and safety of Earth-bound rocks

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The meteor that fell on Russia today took the world by surprise ? not least because it hit the very same day that an asteroid is due to fly by Earth. These rocks have the potential to be highly destructive, but they can also reveal mysteries about our solar system. Our guide takes you through the science, and the safety, of Earth-bound space rocks.

What is the difference between a meteor, a meteorite, a meteoroid and an asteroid?
It's all about location and size. Roughly speaking, an asteroid is a relatively small body, usually rocky or metallic that isn't a comet ? these are composed of dirt and ice. Small asteroids are also called meteoroids. When an asteroid or meteoroid enters the atmosphere and streaks through the sky, as seen in Russia this morning, it then becomes known as a meteor. Anything that survives impact is a meteorite.

Whatever you call them, where do these things come from?
Most are from the asteroid belt, a jumble of rocks between the orbits of Mars and Jupiter that range in size from nearly 1000-kilometres across to microscopic dust particles. Collisions within the belt can send objects hurtling towards Earth, and heat from the sun can dislodge smaller rocks by warming them more on one side, resulting in a gradual push called the Yarkovsky effect. This can build up over millions of years to give asteroids significant speed.

Larger asteroids hitting the moon or Mars can also send shrapnel our way. There have been no confirmed meteorites originating from Mercury or Venus in this way, because they would have to fight against the Sun's gravitational pull to reach us.

How often do they actually hit Earth?
Tiny rocks enter Earth's atmosphere nearly every day, but burn up unnoticed. Larger impacts are rarer: NASA says that an object the size of a car should hit Earth every year. The exact size of today's Russian meteor isn't known, but it is likely to have been bigger than a metre and smaller than asteroid 2012 DA14, which will swoop close by us today and is 45 metres across. Asteroids on this scale are expected to hit once every 2000 years. The most damaging meteorite strike in recent times was the Tunguska event, a megatonne-scale explosion that destroyed a swathe of Siberian forest in 1908.

Has anyone ever been killed by a meteoroid impact?
There are no confirmed reports of human death being caused by rocks from space. However, in 1911, a 40-kilogram boulder from Mars struck and killed a dog in Egypt, and a boy in Uganda was hit, but not seriously injured, by a small meteorite in 1992. The vast majority of Earth's surface is unpopulated, so the odds of something landing on your head are small.

Why didn't we see today's meteor coming?
It was too small to reliably spot, although not impossible. In 2008, astronomers detected an asteroid just a few metres across, probably smaller than the Russian one, on a collision course with Earth , before it eventually landed in Sudan. "That was just fortuitous, there was a telescope scanning the right bit of sky just as it was coming in," says Simon Green of the Open University in Milton Keynes, UK. There are millions of similarly sized asteroids out there, but we don't really know anything about them.

What can we do to protect ourselves?
Asteroid monitors believe they have identified 90 per cent of the really huge rocks with the potential to hit Earth, but the smaller asteroids are, the harder it is to spot them. That is not too worrying, because these are likely to be less devastating if they do hit.

"Once you get below a few hundred metres in size, you're in the region of diminishing returns," says Mark Bailey of Northern Ireland's Armagh Observatory in College Hill, UK. As today's hit shows though, they can still cause injury and damage and Bailey believes we should devote more resources to the search. "In the immediate aftermath of such an event there is a tendency to close stable doors, but ideally they should have been closed in advance."

What can scientists learn from the Russian meteor?
Asteroids and meteoroids are the leftover building blocks of planets, so studying fragments like the ones that fell in Chelyabinsk can tell us more about the early days of the solar system.

"It's like looking at an ancestor of Earth," says William Bottke of the Southwest Research Institute in Boulder, Colorado. Some types of meteorites are very rich in carbon and can answer questions about how organics ? carbon-based compounds ? which are key to life on Earth, arrived on our planet.

Others are solid iron chunks that are probably the cores of big asteroids broken apart by collisions in space, giving us a glimpse at the deep interior of what was once a much larger space rock.

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