(The Hill) – An asteroid the size of a football stadium threaded the needle between Earth and the moon Saturday morning – the second of two astronomical near misses in three days.
Near minutes is a relative term in this case: Saturday’s asteroid 2024 MK came within 180,000 miles of Earth. Meanwhile, on Thursday, asteroid 2011 UL21 flew by at a distance of 4 million miles.
But Saturday’s passage of 2024 MK – which scientists only discovered two weeks ago – coincides with a sobering reminder of threats from space.
Sunday is Asteroid Day, the anniversary of the explosion of a rock from space over a Russian city in 1908 — the kind of danger that astronomers warn is always lurking as Earth hurtles through space.
Here’s what you need to know about asteroids, space hazards and Saturday’s close flyby.
What is an asteroid?
Asteroids are rocks in space that orbit the Sun, much like the planets they occasionally cross paths with.
Like the planets, asteroids formed more than 4.6 billion years ago from the condensing cloud of dust and gas that formed the Solar System — effectively making them time capsules from a time far before the Earth or the Sun formed.
Scientists have identified about 1.3 million of them, mostly orbiting in the vast space between Mars and Jupiter. Both individually and in the aggregate, they tend to be small—the total mass of all asteroids in the solar system is thought to be less than the mass of the Moon.
Throughout long history, asteroid impacts may have also been essential for life on Earth.
In other asteroid news from last week, scientists on Wednesday announced the results of a 2023 mission to the asteroid Bennu that returned with samples suggesting the possibility that it was full of ingredients for water.
These findings suggest that asteroid impacts are upside down. “Asteroids like this could have played a key role in supplying water and the building blocks of life on Earth,” said co-author Nick Timms of Curtin University.
What happens if one hits Earth now?
An asteroid doesn’t have to be particularly large to cause damage. For example, in 2013 an asteroid about 62 feet in diameter that disintegrated nearly 20 miles over Siberia released 30 times more energy than the atomic bomb that hit Hiroshima.
While most of the energy of the impact was absorbed by the atmosphere, the detonation created a shock wave that blew out windows and injured more than a thousand people.
Asteroid Day on Sunday commemorates an even bigger impact, the Tunguska event of 1908, which also took place over Siberia.
In this case, the Russian newspaper Sibir (Siberia) reported that peasants looking up saw a “strangely bright (impossible to look at) blue-white celestial body moving downward for 10 minutes.”
The body looked like a “tubular” cylinder, which began to “blur” upon impact with the denser atmosphere above the forest and disintegrated into billowing black smoke,” the article reads.
“There was a loud knocking sound (not thunder) as if large rocks were falling or artillery being fired. All the buildings shook. At the same time, the cloud began to emit flames of uncertain shapes. All the villagers panicked and took to the streets, the women were crying, thinking it was the end of the world.
If the 500- to 800-foot-diameter 2024 MK were to hit Earth on Saturday rather than fly by, it wouldn’t be the end of the world — at least not quite. Such an impact “would have an equivalent impact energy in the hundreds of megatons approaching a gigatonne,” Peter Brown of Canada’s Western University told the Canadian Broadcasting Service.
That’s a huge potential impact – for context, the explosion would be 10-20 times larger than most hydrogen bombs tested, which have a range of 50 megatons.
“It’s the kind of thing that if it hit the East Coast of the US, it would have catastrophic effects on most of the East Coast.” But it’s not big enough to affect the whole world,” Brown said.
The impact of a hypothetical collision with asteroid 2011 UL21, which flew by on Thursday, would be far more catastrophic. While it was comfortably far out in space and had no chance of hitting Earth, it was also very large: about the size of Mount Everest.
With a diameter of 1.5 miles, this asteroid was about a quarter the size of the asteroid that hit Earth 65 million years ago and wiped out all the dinosaurs that walked, as well as most life on Earth.
How high is the risk of a crash?
Research suggests it is very, very low. NASA estimates that a civilization-ending event (such as a Thursday-sized asteroid colliding with Earth) should only occur every few million years.
And such an impact from an asteroid half a mile in diameter or larger will be nearly impossible for a very long time, according to findings published last year in The Astronomical Journal.
“It’s good news,” study leader Oscar Fuentes-Muñoz of the University of Colorado Boulder told MIT Technology Review. “As far as we know, there will be no impact for the next 1,000 years.”
The catalog of large and hazardous objects, such as the 2011 UL21, is now 95 percent complete, Technology Review reported.
But as the 1908 and 2013 explosions suggested, a relatively small asteroid can still “do a lot of damage,” warned Áine O’Brien of the University of Glasgow Technology Review.
A map of asteroids the size of the one that will fly between Earth and the moon on Saturday — which could destroy a city if they hit the planet — is still only 40 percent complete, the magazine reported, according to Big Think.
How do scientists detect and track asteroids?
They do this by constantly scanning the sky for relatively small, fast-moving objects. The asteroid impact warning system that detected 2024 MK is one of many surveys looking for hazards.
These surveys offer early warnings that could help prevent asteroid impacts, Alan Fitzsimmons of Queen’s University in Northern Ireland told the CBC.
“It’s the only natural disaster we can stop. You can’t stop a tsunami, you can’t stop an earthquake, you can’t stop a volcano,” he said. “You can actually stop or prevent an asteroid impact, at least in theory.”
NASA managed to knock an asteroid off course in 2022 when its Double Asteroid Redirection Test (DART) rammed a satellite about the weight of a small car into Dimorphos, a rock about the same size as 2024 MK — slightly altering its orbit.
The DART mission, which required NASA to perform a precise collision at a distance of 7 million miles, showed “that NASA is trying to be ready for whatever the universe throws at us,” agency administrator Bill Nelson said during a briefing at the time.
But the old adage in science says that while in theory there is no difference between theory and practice, in practice there is. Achieving a feat like the DART mission to prevent an asteroid from hitting Earth “is certainly possible, but it would be a difficult and expensive task,” University of Manchester astronomer Alistair Gunn wrote to the British Broadcasting Corporation.
“The key would be to divert the asteroid from its collision course with Earth rather than shattering it into equally dangerous debris,” Gunn added.
He also noted that it would take at least five years to remove it – making early warning “vital”.
This need for early warning is one of the reasons why the passage of 2024 MK is so worrisome: Scientists discovered it just this month.
Earlier this week, NASA announced plans to deflect the asteroid, which still had “large gaps,” USA Today reported.
“We’re using the capabilities that we have to really try to eliminate that danger, to understand what’s out there and to know if something is a threat,” Kelly Fast, NASA’s acting planetary defense officer, told the paper.
Were Americans able to see Saturday’s asteroid?
Yes – if they were in the right region and were both very prepared and lucky.
Americans in the US Southwest — or Hawaii — who were out of light pollution and willing to get up before dawn may have a chance to see 2024 MK as a fast-moving point that will make its closest approach to Earth at about 9:46 a.m. ET. .
That’s 90 minutes before dawn in Hawaii and about an hour after dawn on the West Coast — although the asteroid will be faintly visible before the flyby.
For anyone outside of these areas, the virtual telescope project is live streamed through the walkthrough.
Queen’s University’s Fitzsimmons told CBC even those in the right field. Sky watchers will need binoculars and be prepared to spot a faint, fast-moving object. “You have to know exactly where to look,” he said. “It’s motoring.