Some asteroids can 'sneak' into us thanks to a peculiarity of the Earth's rotation that makes them appear to be barely moving - making them difficult to detect.
This is the warning from NASA-funded experts who studied how telescopes nearly missed a 328-foot-wide asteroid that came within 43,500 miles of Earth back in 2019.
The space rock, called '2019 OK', was the first object of its size to come so close to our planet since 1908 - but it was only discovered 24 hours before its nearest approaching.
The reason, the team determined, is that it moved toward us in such a way that its motion across the night sky was counteracted by the Earth's spin.
Thus - for early warning systems like Pan-STARRS1 at Hawaii's Haleakala Observatory - 2019 looked OK stationary, so the automatic detection software was not activated.
In fact, experts said that up to half of asteroids approaching Earth from a danger zone east of the 'opposition' are likely to undergo periods of such apparent slow motion.
(An asteroid is said to be in opposition when its position in the night sky places it along a line that intersects both the Earth and the Sun.)
This means that half of these asteroids at the moment can also be difficult to detect - and computerized telescopes need to be updated to take into account the effect.
Some asteroids can 'sneak up' on us thanks to a characteristic of the Earth's rotation that makes them appear to be barely moving - making them difficult to detect (stock image)
Pictured: Some asteroids approaching Earth from the east of the opposition (the yellow line) appear at almost the same point in the sky as they get closer. This is because as the asteroid appears to be moving east across the night sky, such a movement is counteracted by the Earth's rotation - meaning it is seen from exactly the same angle from Earth, even as it gets closer (represented by the row of Parallel, dotted orange lines)
WHAT IF ... 2019 OK HAD BEEN BEATING THE EARTH?
Although not big enough to cause a global catastrophe, 2019 OK would have wreaked significant havoc if it had affected the Earth - especially in densely populated environments.
"It's a city killer asteroid," Swinburne University astronomer Alan Duffy told the Sydney Morning Herald.
'It would have hit with over 30 times the energy from the nuclear explosion at Hiroshima.'
The study was conducted by astronomer Richard Wainscoat of the University of Hawaii at Manoa and his colleagues.
'Near-earth objects approaching from a direction east of opposition - especially 0-2 hours [0–30°] east of opposition - are prone to periods of slow motion during their approach, 'the researchers explained in their paper.
'The induced topocentric motion coming from the Earth's rotation cancels the natural eastern motion in the sky, making the object appear almost stationary. This makes discovery difficult.
'Studies should be extra careful when examining the sky in this direction, and aggressively follow up on new slow-moving objects.'
Had the apparent slow-motion phenomena not been in play with the asteroid 2019 OK, the researchers said, the object near Earth would probably have been discovered up to four weeks before it approached our planet.
As NASA defines it, a terrestrial object - or 'NEO' - is any body that comes within 28 million miles (45 million kilometers) of Earth's orbit around the Sun.
Any NEO whose orbit crosses our planet and is larger than 460 feet (140 meters) in diameter is further classified as a 'potentially dangerous object' (PHO).
In 1994, the US Congress mandated that NASA should catalog at least 90 percent of NEOs larger than 0.6 miles (1 kilometer) across - that is, large enough that they would cause a global catastrophe if ever should hit the Earth.
That goal was reached in 2011. In 2005, however, the directive was updated to include the cataloging of 90 percent of all PHOs by the year 2020 - a goal that has not yet been reached to date, with the number currently about 40 per cent.
"We have a way to go," Professor Wainscoat told the Telegraph.
But, he added, 'when we have cataloged more than 90 percent, the number that can sneak up on us from [the danger zone] will be small. '
For early warning systems like Pan-STARRS1 at Hawaii's Haleakala Observatory, 2019 OK - seen here at four different times on July 7, 2019, before it was marked - it looked stationary, so it did not start the automatic detection software
The risk of devastating impactors was recently highlighted in the Netflix movie 'Don't Look Up', starring Leonardo DiCaprio and Jennifer Lawrence as scientists trying to warn an uninterested public about a comet about to wipe out humanity.
However, in case you are worried about an Armageddon-themed death, Professor Wainscoat said people 'should not lose sleep' over the possibility.
But he added: 'If we find something that will hit the Earth, we want to do something about it!
'It's not a matter of finding them and sitting there and letting it hit.'
In fact, NASA is embarking on a mission to explore the possibility of redirecting an asteroid's course by crashing a space probe into it.
The DART 'Double Asteroid Redirection Test' mission was launched from Vandenberg Space Force Base in California last November and is expected to reach its target - the small planet-moon Dimorphos - around the end of September this year.
The full results of the study were published in the journal Icarus.
Explained: The difference between an asteroid, meteorite and other space rocks
An asteroid is a large part of rocks left over from collisions or the early solar system. Most are located between Mars and Jupiter in the Main Belt.
ONE comet is a rock covered with ice, methane and other compounds. Their orbits lead them much further out of the solar system.
ONE meteor is what astronomers call a flash of light in the atmosphere when waste burns up.
This waste itself is known as one meteoroid. Most are so small that they evaporate into the atmosphere.
If any of this meteoroid comes to Earth, it's called one meteorite.
Meteors, meteorids and meteorites usually originate from asteroids and comets.
For example, if the Earth passes through the tail of a comet, much of the debris burns up into the atmosphere and forms a meteor shower.