(The Hill) – An asteroid the size of a football stadium zipped between Earth and the Moon Saturday morning – the second of two astronomical close-ups in three days.
In this case, it’s a relative term: Saturday’s asteroid, 2024 MK, came within 180,000 miles of Earth. Meanwhile, on Thursday, asteroid 2011 UL21 flew within 4 million miles.
But Saturday’s passage of 2024 MK — which scientists discovered just two weeks ago — coincides with a sobering reminder of threats from space.
Sunday is Asteroid Day, the anniversary of the 1908 blast of a rock from space above a Russian city — the kind of danger that, astronomers warn, is always lurking as Earth moves through space.
Here’s what you need to know about asteroids, space danger and Saturday’s flyby.
What is an asteroid?
Asteroids are rocks in space that orbit the sun, rather like the planets they pass by from time to time.
Also 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 – making them in effect time capsules of a time far before the formation of the Earth or the sun.
Scientists have identified about 1.3 million of them, mostly orbiting in the vast space between Mars and Jupiter. Both individually and in aggregate, they tend to be small—the total weight of all asteroids in the solar system is believed to be less than that of the moon.
Throughout long history, asteroid impacts may also have been crucial to life on Earth.
In other asteroid news last week, scientists announced Wednesday the results of a 2023 mission to the asteroid Bennu that returned samples, suggesting the possibility that it was full of the ingredients for water.
These findings suggested a positive impact on asteroids. “Asteroids like this may have played a key role in delivering water and the building blocks of life to 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 do damage. In 2013, for example, an asteroid about 62 meters in size that broke up nearly 20 miles above Siberia released 30 times more energy than the atomic bomb that hit Hiroshima.
While most of the impact’s energy was absorbed by the atmosphere, the explosion sent 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 occurred over Siberia.
In that event, the Russian newspaper Sibir (Siberia) reported that villagers looking up saw a “strangely bright (impossible to see) bluish-white celestial body, which for 10 minutes moved downward.”
The body appeared to be a cylinder “tube” which began to “smudge” as it hit the denser atmosphere above the forest and disintegrated into billowing black smoke,” the article said.
“There was a loud crash (not thunder) as if large rocks were falling, or artillery 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 2024 MK, 500 to 800 feet in diameter, were to strike instead of pass by Earth on Saturday, it wouldn’t be the end of the world — at least, not completely. Such an impact “would have an equivalent impact energy in the hundreds of megatons approaching a gigaton,” Peter Brown of Canada Western University told the Canadian Broadcasting Service.
That’s a huge potential impact – for context, the explosion would be 10-20 times larger than that of most hydrogen bombs that have been tested, which are in the 50 megaton range.
“It’s the kind of thing that if it hit the east coast of the US, you’d 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 2011 UL21, the asteroid that flew by on Thursday, would be far more catastrophic. While it was very far out in space and had no chance of hitting Earth, it was also very large: the approximate size of Mount Everest.
At 1.5 miles in diameter, that asteroid was about a quarter the size of the asteroid that hit Earth 65 million years ago, wiping out all the walking dinosaurs as well as most life on Earth.
How high is the risk of a collision?
Research suggests it is very, very low. NASA has estimated that a civilization-ending event (like a Thursday-sized asteroid colliding with Earth) should only happen 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 is no impact in the next 1,000 years.”
NASA’s catalog of large, dangerous objects like 2011 UL21 is now 95 percent complete, Technology Review reported.
But as the 1908 and 2013 explosions suggested, a relatively small asteroid could “do a lot of damage,” warned Áine O’Brien of the University of Glasgow in Technology Review.
The map of asteroids the size of the one that passed between Earth and the moon on Saturday — which could destroy a city if it 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, looking for relatively small, fast-moving objects. The Asteroid Last Earth Impact Alert System that detected 2024 MK is one of many surveys looking for hazards.
These surveys provide early warnings that can help prevent asteroid impacts, Alan Fitzsimmons of Queen’s University Northern Ireland told the CBC.
“It is the only natural disaster that 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 strike, at least in theory.”
NASA managed to knock an asteroid off course in 2022, when its Dual Asteroid Redirect Test (DART) crashed a satellite about the weight of a small car into Dimorphos, a rock the same size as 2024 MK – slightly changing its orbit.
The DART mission, which required NASA to execute a precision collision 7 million miles away, showed “that NASA is trying to be ready for whatever the universe throws at us,” agency Administrator Bill Nelson said during a conference in that time.
But there is an old saying in science that while in theory there is no difference between theory and practice, in practice there is. Pulling off a success like the DART mission to stop an asteroid from hitting Earth “is certainly possible, but it would be a difficult and expensive task,” University of Manchester astronomer Alistair Gunn wrote for the British Broadcasting Corporation.
“The key would be to divert the asteroid from its collision course with Earth, rather than break it up into equally dangerous debris,” Gunn added.
He also noted that pulling this off would take a certain amount of time of at least five years – which is why early warning is “vitally important”.
This need for early warning is one reason why the 2024 MK crossing is so worrying: scientists only discovered it this month.
Earlier this week, NASA announced plans to deflect an asteroid that still had “high-level voids,” USA Today reported.
“We’re using the capabilities that we have to really try to hopefully pull away from that danger, figure out what’s out there and know if anything poses a threat,” Kelly Fast, the planetary protection officer of the NASA.
Could Americans See Saturday’s Asteroid?
Yes – if they were in the right region and were very prepared and lucky.
Americans in the southwestern US – or Hawaii – who were light pollution-free and willing to get up in the pre-dawn hours may have a chance to see 2024 MK as a fast-moving point that will approaches Earth around 9:46 a.m. ET.
That’s 90 minutes before dawn in Hawaii and about an hour after dawn on the West Coast — though the asteroid will be visible before it makes the pass.
For everyone outside those areas, the Virtual Telescope Project is streaming the passage live.
Even those in the right region may find viewing the passage challenging, Fitzsimmons of Queen’s University told CBC. Skywatchers will need a telescope and be prepared to spot a faint, fast-moving object. “You have to know exactly where to look,” he said. “It’s the engine.”