Earth's closest brush with an asteroid in 2019

An asteroid set to skim the Earth in March will come closer than any other large space rocks over the rest of this year - that we know of.

2015 EG is expected to pass on March 4 at only 442,000km away - barely more than the moon.

Little is known about the asteroid, except that it's travelling at 9.6km/s and could be up to twice as large as the meteor that blew up over the Russian city of Chelyabinsk in 2013, injuring 1500 people.

According to Imperial College London's Impact Earth calculator, an asteroid this size hits the Earth about every 1400 years on average. If it did hit, it has the potential to create a crater 1.3km wide and 285m deep, and an explosion with the force of 3.5 megatons, making it three times more devastating than the US military's current most powerful nuclear weapon, the B83.

In comparison, the asteroid that killed the dinosaurs 66 million years ago has been estimated at being between 11 and 81 kilometres across.

There are three other little-known asteroids on NASA's radar which could potentially come closer than 2015 EG - 2012 KT12, 2016 NO56 and 2016 GE1 - but they're more likely to zip by about 1 or 2 million kilometres away.

While that sounds far, considering the Earth is 150 million kilometres from the sun, it's close by galactic standards.

The uncertainty stems from the fact tiny asteroids flying through distant space are hard to track, and astronomers often have little data to work with.

Already this year there have been two close calls with previously unknown asteroids - 2019 AS5 and 2019 AE9 - the former missing by just 15,000km, about the same distance from Christchurch to Boston.

The latter passed at a distance of about 100,000km, and measured between eight and 25 metres wide.

Tens of thousands of asteroids hit the Earth every year, but the vast majority are small and harmlessly burn up in the atmosphere.

NASA measures an asteroid's risk to civilisation on the Torino scale - zero means no risk at all, while 10 equals "global climatic catastrophe that may threaten the future of civilization as we know it". It's a combination of an asteroid's size, speed, composition and its likelihood of hitting the planet.

Presently, there are no objects rated above a zero. The highest rating ever given was to a 350m-wide asteroid named Apophis, which was downgraded to zero in 2006 when astronomers calculated a possible 2029 impact wasn't going to happen.

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