University of Hawaii researchers have been investigating how asteroids can evade detection, discovering that some approaching objects can appear stationary in the sky because of a quirk of the planet's daily rotation.
A rugby-pitch-wide asteroid passed within 70,000km of Earth in 2019, with surprised astronomers only spotting it 24 hours before its closest approach.
Photo: 123rf
The study was undertaken by astronomer Dr Richard Wainscoat who said it is extremely rare for Earth to be hit by large space rocks.
"Nobody in recorded history has been killed by one of these," he said.
He said most near Earth objects have been catalogued.
"We've catalogued about 90 percent of the objects that are 1 kilometre or larger and we almost reached 40 percent of the objects that are 140 metres or larger."
But Dr Wainscoat said a 140m object could do catastrophic damage over a city or a state and even a 20m object could do damage.
He said the meteor that exploded over Chelyabinsk in Russia on 15 February 2013 was 20m and although it did not kill anybody, a lot of people had to be hospitalised.
An asteroid explodes above the Russian city of Chelyabinsk, on 15 February, 2013. Photo: AFP
Dr Wainscoat explained why in some cases telescopes will be able to detect an asteroid but the viewer will not perceive that the object is nearby or moving.
"If an object is approaching, sort of very roughly from the area of the sky that might be high up around 1am or 2am in the morning, that object is going to have a natural eastward motion towards the Earth where it's going to impact.
"But because we're on planet Earth and we're rotating around the Earth's axis we're also moving eastward - so the combination of those two motions tends to cancel out the motion of the object and it appears to be stationary."
Dr Wainscoat said unusual motion is what is used to detect objects that are close to Earth.
In this case because the object is perceived as stationary it might be thought to be a supernova, a nova or a star that has become brighter, he said.
Dr Wainscoat said if an object is coming straight at you it is difficult to detect its movement.
Every night the Pan-STARRS (Panoramic Survey Telescope and Rapid Response System) telescope at the Haleakalā Observatory in Hawaii looks for moving objects.
"We find maybe 10 or 20,000 objects a night and the vast, vast majority of those objects kind of have an ordinary motion and they're just objects in the main asteroid belt," he said.
"We are trying to look for these needle in a haystack things which have unusual motion which is a characteristic of being really close to us or possibly hitting the Earth at some point in the future."
There are people who operate telescopes the entire night and then an astronomer in a different time zone tries to review the objects during the day so any information can be quickly reported, Dr Wainscoat said.
He said if the asteroid that passed within 70,000km of Earth in 2019 had hit it would have likely been worse than the recent undersea volcanic eruption in Tonga in terms of explosive power.
NASA recently launched the Double Asteroid Redirection Test to try and determine if an asteroid can be deflected, Dr Wainscoat said.
NASA announced last year that it would send a spacecraft to crash into Dimorphos, the moon of the asteroid Didymos.
"So what NASA is doing right now is sending a spacecraft that they intend to impact upon an asteroid to see how much they can deflect it, just by kinetic impact, just by the energy of the motion of the spacecraft."
Dr Wainscoat said trying to deflect an asteroid is a tough international problem with several potential issues.
This artist's illustration shows the DART spacecraft from behind prior to impact at the Didymos binary system. Photo: AFP / NASA / Johns Hopkins APL
One issue is that if you succeed in breaking up an asteroid it is possible that many pieces could hit the Earth rather than just one piece, he said.
He said if an asteroid is deflected, it must be deflected far enough that it completely misses hitting the Earth and does not just hit a different country.
Dr Wainscoat said having information about a large object likely to hit the Earth well in advance might mean action could be taken.
"The idea is to find them 10 years or 20 years before they hit and you have a lot of time to do something about it.
"If you find it early enough then only a little push is needed to sort of push it aside, so a little push added up over 20 years of orbits might be enough to push it aside and make it miss the Earth."
He said until recently the ability to survey the skies in the southern hemisphere has not been as good, however two new telescopes in South Africa and Chile have just started to survey the southern sky.
"As of now we're basically surveying the whole sky just about every night."
Dr Wainscoat said that meant people would get a lot of advance warning if a comet was to hit the Earth.
"For an asteroid there are still some difficult orbits and we still haven't found all of big asteroids but pretty soon we'll have found basically all of the big asteroids and we'll know that nothing big is going to hit, or at least none of the dinosaur killer sized asteroids."
He said in Hawaii they receive help from New Zealand astronomers Alan Gilmore and Pamela Kilmartin in the Mount John observatory in the South Island.
"The reason why they're so valuable is they're just a little west of Hawaii, so if we discover an object with the Pan-STARRS telescope and it looks interesting, we can actually reach out to them when they're observing on the 1 metre telescope and ask them to get it and they can get it maybe a few hours after we see it."
Dr Wainscoat said this type of close collaboration would be invaluable were something ever likely to hit the Earth.
"The aim of what we're doing is to find out if anything is going to hit and potentially stop it from hitting and if something really is found to be dangerous then the research is actively being done to find out how to deflect it and stop it from hitting."
