'Potentially hazardous' asteroids to fly by Earth

Two asteroids are set to zip by the Earth in the next week, followed by an enormous skull-shaped rock in November.

But don't worry - when NASA calls an object "potentially hazardous", they're talking on a galactic timescale. There's no need to panic right away.

The first, 2016 NF3, will pass by the Earth on Thursday (NZ time). It's been described in the British tabloids as bigger than the Great Pyramid of Giza and flying at 32,400km/h - which while true, doesn't mean anything to the average person if it's not going to hit the Earth.

And it won't even be close - 2016 NF3 will fly by 5 million kilometres away, 13 times further away than the moon.

Pyramids are big, and if one hit the Earth, it would be bad for anyone nearby. Photo credit: Getty

"There is absolutely nothing for concern by this pass of 2016 NF23," Lindley Johnson, NASA planetary defence officer, told space.com.

"This object is merely designated a potentially hazardous asteroid (PHA) because its orbit over time brings it within 8 million kilometres of Earth's orbit, but there is nothing hazardous to Earth or even unique about this pass of the asteroid."

The second asteroid this week putting the fear of god into headline writers is 1998 SD9. It'll be much closer than 2016 NF3 - in fact the closest pass by any "potentially hazardous" asteroid in the next two months - but still four times further away than the moon.

Which brings us to 2015 TB145 - the suspected ex-comet that looks remarkably like a human skull. It last passed us on October 31, 2015, and was appropriately dubbed the 'Halloween asteroid'. Its next appearance in our part of the solar system will be on November 11, when it will be about 39 million kilometres away - a quarter the distance from here to the sun, and despite its fearsome appearance, not a hazard.

In 2015 when it was first spotted, it was only 486,000km away - slightly further than the moon.

What would happen if they did hit us?

In an alternate universe when all three asteroids hit the Earth, the effects would be potentially more than just hazardous.

1998 SD9 is between 38 and 86m wide and travelling at 10.7km per second. No one's sure what it's made of, but if it was dense rock and hit the Earth at a 90-degree angle, according to Purdue University's Impact Earth tool it would make a crater a kilometre across and 300m deep. The resulting air blast would topple nearby buildings, collapse highways and blow down trees. The explosion would be roughly equivalent to the legendary Tunguska blast of 1908.

2016 NF3 is up to 160m across and travelling at 9km/s. It would have nearly twice the energy as 1989 SD9, and the explosion would register 5.5 on the Richter scale a kilometre away. The crater would be 2km across and 460m deep. 

JA Penas/SINC Halloween
Artist's impression of the 'Halloween asteroid'. Photo credit: JA Penas/SINC

2015 TB145 - the Halloween asteroid - is about 600m in diameter and going a blistering 35km/s. The resulting impact would be far, far stronger than any hydrogen bomb ever made, and the crater would be 14km across and 650 m deep.

The surrounding area would be buried in rubble 10m deep, and a fireball 12km wide would burn hot enough to set clothing on fire. People standing 20km away would avoid being burned alive or inside the crater, but would feel a quake 32 times stronger than the one that rocked Christchurch in 2011. They might also be deafened by a 130db bang.

But none of that's going to happen - at least for now.