Earth had a near-miss with a 'city-killer' asteroid

Thursday, 25 July 2019

An asteroid about 100 metres in diameter and racing at 24 kilometres a second has just missed the Earth.

The rock, called Asteroid 2019 OK, sped by our planet on Thursday, passing within about 70,000 kilometres – which is a long way away but closer to us than the moon's orbit.

Due to the position of the asteroid – flying towards us from the direction of sun – astronomers had no warning it was headed our way.

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NASA

The asteroid passed close by on Thursday morning.

It is the largest rock to fly at such close quarters to the Earth this year, and possibly for many years.

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Astronomers believe it is between 57 and 130 metres in diameter.

Several dozen asteroids ranging from 6-12m fly close to Earth each year, but it's unusual for a rock the size of Asteroid 2019 OK to whizz past (file photo)

Telescopes only began to pick it up a couple of days ago, with a confirmation it was an asteroid only coming in the past 24 hours.

'It's impressively close. I don't think it's quite sunk in yet. It's a pretty big deal,' says Associate Professor Michael Brown, from Monash University's school of astronomy.

'[If it hit Earth] It makes the bang of a very large nuclear weapon – a very large one.'

How big?

'It would have hit with over 30 times the energy of the atomic blast at Hiroshima,' says Swinburne University astronomer Associate Professor Alan Duffy.

'It's a city-killer asteroid. But because it's so small, it's incredibly hard to see until right at the last minute.'

Asteroids this size tend to pass by once every decade.

Three other asteroids also raced past the Earth on Thursday, but none were as close or as large as 2019 OK.

The asteroid was picked up by separate astronomy teams based in Brazil and the US over the past few days.

NASA's Jet Propulsion Laboratory confirmed the discovery. The asteroid passed Earth by just 73,000 kilometres away, and was sized between 57 and 130 metres in diameter, according to the Lab's data.

'This is one of the closest approaches to Earth by an asteroid that we know of. And it's a pretty large one,' says Professor Brown.

By comparison, the rock that killed the dinosaurs was about 16 kilometres across, said Professor Gretchen Benedix, a planetary science researcher at Curtin University.

'It's not totally out to lunch these things happen. It's more rare they happen within a lunar distance,' she said.

'If that were to hit the Earth, that would be bad. Something 100 metres across would leave a noticeable hole on the planet.'

The Chelyabinsk meteor, which exploded over Russia in 2013, was only about 20 metres in diameter.

Astronomers typically try to pick up asteroids long before they pass by Earth.

But this one was particularly difficult to see because it was coming toward the Earth from the direction of the sun, making it very hard to see, Professor Brown said.

'It was faint, it was close to the sun. It's been getting closer to us, getting brighter and brighter, and finally some smaller telescopes have picked it up. Literally, right about now, it's about 70,000 kilometres from Earth,' he said.

A person armed with a pair of binoculars and looking at the right spot in the night sky may even have been able to spot it, he said.

Several dozen smaller asteroids in the six-to-12 metre range fly past Earth at a distance closer than the moon every year, according to NASA.

But such a large rock being so close is unusual.

'These events are rare. But we know, sooner or later, there's going to be one with our name on it,' says Professor Duffy.