Asteroid flies within 2,950km of Earth. We didn’t see it coming

Published by SCMP.COM

Instead, the Palomar Observatory in California first detected the space rock about six hours after it flew by Earth.

Chodas confirmed the record-breaking nature of the event: "Yesterday's close approach is closest on record, if you discount a few known asteroids that have actually impacted our planet," he said.

Nasa knows about only a fraction of near-Earth objects like this one.

Many do not cross any telescope's line of sight, and several potentially dangerous asteroids have snuck up on scientists in recent years.

This recent near-Earth asteroid was initially called ZTF0DxQ but is now formally known to astronomers as 2020 QG. Business Insider first learned about it from Tony Dunn, the creator of the website orbitsimulator.com.

"Newly-discovered asteroid ZTF0DxQ passed less than 1/4 Earth diameter yesterday, making it the closest-known fly-by that didn't hit our planet," Dunn tweeted on Monday.

Newly-discovered asteroid ZTF0DxQ passed less than 1/4 Earth diameter yesterday, making it the closest-known flyby that didn't hit our planet.

Tony Dunn Early observations suggest the space rock flew over the Southern Hemisphere just before 4am Universal Time on Sunday.

The animation above shows 2020 QG flying over the Southern Ocean near Antarctica.

The International Astronomical Union's Minor Planet Centre calculated a slightly different trajectory.

The group's rendering, suggests the asteroid flew over the Pacific Ocean hundreds of kilometres east of Australia.

As far as space rocks go, 2020 QG wasn't too dangerous.

Telescope observations suggest the object is between 2 metres and 5.5 metres wide - somewhere between the size of a small car and an extended pickup truck.

Even if it was on the largest end of that spectrum and made of dense iron, only small pieces of such an asteroid may have reached the ground, according to the "Impact Earth" simulator from Purdue University and Imperial College London.

Such an asteroid would have exploded in the atmosphere, creating a brilliant fireball and unleashing an airburst equivalent to detonating a couple dozen kilotons of TNT. That's about the same as one of the atomic bombs the US dropped on Japan in 1945.

This doesn't make the asteroid's discovery much less unnerving, though - it does not take a huge space rock to create a big problem.

A simulation of a 20-metre-wide asteroid burning up in Earth's atmosphere.

Take, for example, the roughly 20-metre-wide asteroid that exploded without warning over Chelyabinsk, Russia, in February 2013.

That space rock created a superbolide event, unleashing an airburst equivalent to 500 kilotons of TNT - about 30 Hiroshima nuclear bombs' worth of energy.

The explosion, which began some 20km above Earth, resulted in a blast wave that shattered windows in six Russian cities and injured about 1,500 people.

In July 2019, a 130-metre asteroid called 2019 OK , or less than 20 per cent of the distance between Earth and the moon.

Astronomers detected that rock less than a week before its closest approach, leading one scientist to tell The Washington Post that the asteroid essentially appeared "Out of nowhere".

In an unlikely direct hit to a city, such an asteroid might kill tens of thousands of people.

Nasa is actively looking for dangerous space rocks, as Congress has required it to do since 2005.

The agency is only mandated to detect 90 per cent of "City killer" space rocks larger than about 140 metres in diameter.

In May 2019, Nasa said it had found less than half of the estimated 25,000 objects of that size or larger.

Of course, that doesn't count smaller rocks such as the Chelyabinsk and 2019 OK asteroids.

"There's not much we can do about detecting inbound asteroids coming from the sunward direction, as asteroids are detected using optical telescopes only, and we can only search for them in the night sky," Chodas said.

The agency is in the early stages of developing a space telescope that could detect asteroids and comets coming from the sun's direction.

Nasa's 2020 budget allotted nearly US$36 million for that telescope, called the Near-Earth Object Surveillance Mission.