Not one, but five asteroids are whizzing past Earth today, all making their closest approach.
The biggest was discovered just weeks ago; the potentially hazardous 2024 BR4. Measuring 140-300m in length, it’s a sizeable chunk of space rock. At its maximum estimate, that’s roughly the same size as an oil tanker, six end-to-end Olympic-size swimming pools, or as tall as the Chrysler Building in New York City.
And like that box of chocolates you might be tucking into, the day has a few more surprises in store. Dr Darren Baskill, an astronomer from the University of Sussex tells us more:
“On Wednesday 14 February 2024, we have a grand total of five asteroids flying past the Earth – all within five million kilometres!”
That may seem like a long way out, but in astronomical terms, it’s a mere stone’s throw.
“The closest of those five is asteroid 2024 CU1, which will pass by some 52 per cent further away from us than the Moon, and is about 20 metres across.”
2024 CU1 makes its closest approach at 1:31pm GMT (+/- 1 minute). But at only 20 metres in diameter, it’s much smaller than the “city-killer” called 2024 BR4 that’s been making the headlines.
How close will asteroid 2024 BR4 get?
One of a collection of asteroids whizzing past Earth today, asteroid 2024 BR4 will make its closest approach at a distance of 4.6 million km (2.86 million miles). In terms of AU, that’s around 0.03AU – well within the specified criteria for being a potentially hazardous object (see What does ‘potentially hazardous’ asteroid mean? below).
(To convert kilometres/miles to AU, divide the distance in km/miles by the distance of 1AU in the same unit.)
Close approach will occur today at 11:03 GMT (+/- 1 minute).
Will Asteroid 2024 BR4 hit Earth?
Although the Valentine’s Day asteroid is hurtling through space at around 44,880km/hour (27,887 miles per hour) – just a little faster than the average speed Galileo clocked up on its six-year mission to reach Jupiter – we don’t need to worry about it impacting our celestial oasis.
It will safely pass us and poses no immediate threat to the Earth:
“Asteroid 2024 BR4 will pass by the Earth 12 times further away from us than the Moon,” explains Baskill.
What do we know about Asteroid 2024 BR4?
Unfortunately, very little. In general, the bigger an object the brighter it is (the brighter an object, the lower the magnitude), and NASA have assigned Asteroid 2024 BR4 a magnitude of 21.457. That’s not very bright.
But you also need to know the albedo (surface reflectivity) of an object, which we currently don’t know. Current best estimates put the asteroid’s size somewhere between 140 and 300 metres in diameter. We do however know, with relative certainty, its orbit. And today's visit will be the closest that 2024 BR4 will come in 120 years.
“While we don’t need to worry about 2024 BR4 due to its distance, it is interesting due to it being some 200 meters in size,” says Baskill. “That is larger than the 50-metre asteroid that hit Tunguska in Siberia back in 1918. Almost 100 million trees were felled within 30 km of the asteroid burning up in our atmosphere,” he explains.
As the name suggests, 2024 BR4 was only discovered recently. It was first spotted on 30 January 2024 by the Catalina Sky Survey in Arizona and has been classified as an Apollo asteroid. These are a group of asteroids that have the potential to intersect Earth's orbit, making them of particular interest to astronomers (and space agencies) who monitor for potential impact risks.
There are currently over 18,000 known Apollo asteroids, most of which originate from the asteroid belt between Mars and Jupiter.
What does ‘potentially hazardous’ asteroid mean?
A ‘potentially hazardous’ object (PHO) can be either an asteroid (PHA) or a comet (PHC) with the potential to make a threatening close approach to Earth. If an asteroid is large enough and its orbit brings it close enough to Earth, it’s considered potentially hazardous. There is a chance, however small, that it could collide with our planet.
Officially, a space rock must be 0.05 astronomical unit (AU) or less away from Earth to be classified as ‘potentially hazardous’ – that’s around 7.48 million kilometres (or 4.65 million miles).
To put that into context, 1 AU is the average distance between the Earth and the Sun, which is approximately 93 million miles (150 million kilometres). Mercury is around 0.39 AU from the Sun, Venus is around 0.72 AU from the Sun, and Mars is approximately 1.52 AU from the Sun.
So, when you compare the distances of our celestial neighbours, 0.05 AU is a relatively short distance.
What is a ‘city killer’ asteroid?
A ‘city killer’ asteroid refers to one that, if it were to hit Earth, has the potential to cause significant damage to a city (or other densely populated area). These asteroids are usually smaller than the ones that could cause global catastrophes, but large enough to pose a serious threat.
There are currently around 7,000 known asteroids that fall into the ‘city-killer’ category.
How many asteroids do we have close encounters with?
More than you might think. Dr Baskill explains:
“Over the last week alone, we have had three asteroids pass us by closer to us than the Moon! 2024 CY1 passed a mere 120,000km away on 12 February 2024, 2024 CJ1 was at 370,000 km (a similar distance to the Moon) on 9 February 2024, and 2024 CK1 passed us by at 228,000 km on 8 February 2024.”
2024 CY1 came so close to us on Monday, that it actually passed between the Earth and the Moon. It skimmed past at just 121,148 km (75,278 miles) away from Earth. But don't worry - even though we're only six weeks into the year, we've already had 10 (known) asteroids passing us within 1 lunar distance without bothering us (1 lunar distance is the distance between the Earth and the Moon).
“Fortunately, these three objects were all a mere 5-10 meters across, so while they would be a very impressive sight to watch burning up in our atmosphere, we don’t need to worry about objects of this size,” Baskill adds.
Here are the five asteroids making a close approach to Earth today:
2024 CD1
Closest approach: 3:03am (+/- 14 minutes)
Distance at closest approach: 0.025 AU
Diameter: 14m – 31m
Magnitude: 26.4
2024 CK3
Closest approach: 5:29 am (+/- 14 minutes)
Distance at closest approach: 0.045 AU
Diameter: 31m – 69m
Magnitude: 24.7
2024 CL1
Closest approach: 7:11am (+/- 1 minute)
Distance at closest approach: 0.00865 AU
Diameter: 18m – 41m
Magnitude: 25.8
2024 BR4
Closest approach: 11:03am (+/- 1 minute)
Distance at closest approach: 0.03 AU
Diameter: 140m – 300m
Magnitude: 21.5
2024 CU1
Closest approach: 1:31pm (+/- 1 minute)
Distance at closest approach: 0.004 AU
Diameter: 12m – 26m
Magnitude: 26.8
Why are some asteroids only detected at the 11th hour?
It all comes down to size and reflectiveness:
“The main problem with finding these smaller asteroids is that they are incredibly difficult to spot,” says Baskill.
“For every 10-fold decrease in size, there are 100 times more objects. But they are also 100 times fainter. While we know of ten thousand objects in our solar system that are 10km in size, there are a million 1-metre-sized objects.”
“But finding these smaller and fainter objects is a real challenge, even if they are more numerous, as they are so faint that they are usually only discovered within days of any close approach.”
About our expert, Dr Darren Baskill:
Dr Darren Baskill is an outreach officer and lecturer in the department of physics and astronomy at the University of Sussex. He previously lectured at the Royal Observatory Greenwich, where he also initiated the annual Astronomy Photographer of the Year competition.
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