On February 15, astronomers and enthusiasts watched the skies as a small near-Earth asteroid passed uncomfortably close to our planet (only 27,500km away). Just as asteroid 2012 DA14 was approaching, however, an unexpected meteorite stole its thunder by exploding in the skies above the Russian city of Chelyabinsk, shattering windows and injuring more than 1500 people. The disintegration of the 10,000-tonne chunk of rock – the remnants of which hit the Earth north of the city – released energy equivalent to more than 500 kilotonnes of TNT, says Nasa.
We didn’t know the meteorite was coming, but it wasn’t a total surprise. “We would expect an event of this magnitude to occur once every 100 years,” said Paul Chodas of Nasa’s Near-Earth Object (NEO) Program Office, whose main role is keeping an eye on space rocks that might hit us.
Evidence of past collisions is widespread, with impact craters of up to 300km in diameter identified all over the planet. Meteorites have helped to shape the geology and biology of our planet, bringing different elements to Earth including, perhaps, some scientists suggest, even water.
They’ve also contributed to extinctions. Geologists have linked the mass extinction that happened at the end of the Cretaceous period – 75% of all terrestrial creatures disappeared from the fossil record – to an impact event 66 million years ago.
Sedimentary layers all over the world, including some found in New Zealand, show a thin layer of clay rich in iridium deposited 66 million years ago. This layer of iridium – which is rare on Earth but often found in meteorites – led scientists to suggest the extinction was caused by a massive meteorite, the impact of which could have caused firestorms, a mega-tsunami, dust clouds and climate change.
A 180km-wide impact crater discovered in the 1970s on the Yucatan peninsula in Mexico was dated at 66 million years and is now acknowledged as the event that contributed to the dinosaurs’ extinction.
The NEO Program is keeping tabs on 1382 potentially hazardous asteroids (PHAs). Bodies are considered PHAs if their diameter is greater than 150m and they have an orbit that could come close to the Earth’s.
But Nasa’s interest isn’t confined to watching out for imminent threats. These asteroids are also considered potential stores of valuable resources in the future, with hopes that raw materials sourced from astral bodies could be used for developing space structures or for providing fuel for future exploration.
As for asteroid 2012 DA14, it won’t come our way again until 2046 – at which time it’s likely to pass within 1.5 million kilometres of Earth.
Rocks of the solar system
A comet is made of rock and ice. Some comets orbit on the far edges of the solar system, taking thousands of years to circle the Sun. Others have highly eccentric orbits: as they near the Sun the ice melts, forming a nucleus and a long tail of dust and gas.
An asteroid is a small rocky object that – mostly – orbits the inner solar system between Mars and Jupiter. Some asteroids have orbits that intersect with the orbits of Earth and other planets.
A space rock that’s bigger than a grain of dust but smaller than an asteroid.
When a meteoroid enters the Earth’s atmosphere it’s called a meteor. As it heats the surrounding air molecules it produces a bright streak that looks like a “shooting star”.
When a piece of meteor survives its passage through the atmosphere to land up upon the Earth we call it a meteorite.
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