Over the next several weeks, astronomers will be looking closely at an asteroid that could be as big as a football field as they try to determine just how likely it is to strike Earth in 2032.
The chances currently stand at 2.1% or about 1 in 47, astronomers stress that as they refine orbital calculations for 2024 YR4, as the asteroid is called, the odds are likely to fall to zero. (There's even a slimmer chance that the asteroid could impact the moon.)
Here's a look at how these objects are found, classified and tracked — and what, if anything, can be done to prevent an Earth impact.
What is a "Near-Earth Object"?
Near-Earth Objects (NEOs) are asteroids or comets nudged into our neighborhood of space by the gravitational attraction of nearby planets. They are chunks of rock, metal or ice left over from the formation of the solar system some 4.6 billion years ago.
Most are too small to worry about, but an impact from larger NEOs could cause considerable localized damage, such as the meteor that struck the Chelyabinsk region of Russia, in 2013. It shattered windows and caused hundreds of injuries over a wide area. Destruction from the largest of these objects, such as the asteroid that likely caused the extinction of the dinosaurs about 65 million years ago, could wipe out humans.
How are NEOs located?
Each night, teams of astronomers in the U.S., Europe and Asia photograph the night sky using a worldwide network of large ground-based telescopes.
The instruments used for these sky surveys are typically "wide field" telescopes designed to take in a large chunk of the sky at a time, according to Larry Denneau, an astronomer at the University of Hawaii's Institute for Astronomy, which runs the NASA-funded Asteroid Terrestrial-impact Last Alert System (ATLAS).
"We try to cover as much sky as possible," he says. "We don't need to see as far as possible out in the universe."
What Denneau and others are looking for is an unexpected, telltale movement against the background of stars that indicates the presence of a previously unidentified NEO.
Objects that are closer to Earth appear to move faster than those farther away. "Imagine you pull up to a railroad track and there's a train whipping by," explains Carson Fuls, director of the Catalina Sky Survey at the University of Arizona. "Now imagine you see a railroad track in the distance you. That doesn't appear to be moving that fast."
In the same way, an asteroid that is close to the Earth will appear to be moving faster than one that is farther away, even if they are traveling at the same speed.
ATLAS operates four telescopes. When it's dark, they are constantly taking pictures of the sky, says Denneau, whose team was the first to spot 2024 YR4 from observations made on a remotely operated telescope in Chile.
"These [are] big 100-megapixel images that come back to Honolulu and our computers reduce them and look for objects that are star-like, but moving in the exposure," he explains.
With the help of computer algorithms, the astronomers can then winnow down the data. "You can kind of put the good ones in one bin and the bad ones in the other bin and have, you know, a few that are in between" to screen out false detections, Denneau says.
"We do that for hundreds of footprints at each telescope every single night," he says.
Catalina Sky Survey operates three telescopes. The telescope with the widest field "can get a little over half of the visible sky" at a time, Fuls says.
"The real trick … when we find and track these objects is [that] we're kind of looking at a 2D image," Fuls says. That makes it difficult to determine the exact distance, he says. "We can start to guess how far away it is once we observe its orbit."
How do astronomers know how far and how big it is?
Optical telescopes are the best bet for spotting NEOs, but radio telescopes are the surest way to determine distance, according to Anne Virkki, a research fellow at the University of Helsinki who has tracked asteroids using radar. "And that's kind of the third dimension that's really important," she says.
Radio telescopes, which use a giant dish to collect radio waves emitted from celestial objects, can also be used to bounce a radar off NEOs to determine their size, Fuls says. Unfortunately, 2024 YR4 is already too far away for that. But there are other ways, he says.
Observing how bright an asteroid is and how that brightness changes over time can help in determining its shape, how fast it's spinning and what it's made of. But without knowing how reflective it is, that method is still a lot of guesswork, Fuls acknowledges. Putting together a coherent picture is a bit like solving a puzzle. "You'll first of all start to piece together the orbit and then we'll say, 'Aha, it's this, right?' and 'We think it's at this location or at this distance,' so we can get an estimate of the size from that," he says.
How do scientists know if an asteroid could impact the Earth?
Once an object is photographed, it gets run through a database to determine if it's already known to astronomers. That only takes about 10 minutes, Fuls says. If it appears to be new, follow-up observers report it up the chain and put it on a list to re-observe in an hour or two when it's moved a bit, as a second check, he says.
If it's confirmed as a new discovery, the object is reported to the Minor Planet Center run by the International Astronomical Union.
"From the short motion we have over a span of about 30 minutes, we can tell whether the object is likely to be close to the Earth," Denneau says.
But astronomers have to deal with a serious blind spot: If an object approaches the Earth with the sun behind it, it's difficult or impossible for optical telescopes to separate the light reflected off a tiny asteroid from the intense glare of the sun. That's how the meteor in Russia snuck up on the Earth without being noticed in 2013. "The Chelyabinsk meteor … came from the sun side," Denneau says. "So no telescope saw it before it impacted."
What happens if it looks like a possible hit?
It depends on how large it is. If the object is big enough to be a real threat and has a 1% or greater chance of hitting Earth based on initial calculations, it is reported to the International Asteroid Warning Network (IAWN), a global collaboration started in 2013 to monitor and track space objects that could impact Earth. For 2024 YR4, IAWN issued its first-ever Potential Asteroid Impact Notification to put the astronomical community on alert to collect as much information as possible on it to try to determine if it will impact.
Astronomers then get busy making follow-up observations, hoping to find out more about the object — how big it is, what it's made of and whether it's a threat to the Earth.
One group of amateurs, known as the International Occultation Timing Association, or IOTA, often use their own telescopes and sensitive cameras to look for the sudden dimming of light from a distant star as an asteroid or comet passes in front of it — something known as an occultation. With multiple observers pooling their measurements of that dip in the light from slightly different locations, they can help determine the size of the asteroid.
"There are some very, very professional amateurs ... who've got some very advanced capabilities," says Kelly Fast, the acting planetary defense officer for NASA's Planetary Defense Coordination Office. "They can provide additional observations once a new object is discovered."
Pulling together all the data, astronomers are able to do something similar to what the National Hurricane Center does to show the forecast probability of a tropical storm track, Catalina Sky Survey's Fuls says.
"Think about a cone or a circle of probability where this thing can be. And as we get more observations, it should start to shrink down," he says.
Another way of visualizing it is to imagine "a cloud that's ... maybe hundreds or thousands of miles in diameter," Denneau of ATLAS says. "Some of that intersects the Earth in 2032."
But astronomers are playing for time. Soon, the asteroid will be too dim even for the largest ground-based telescopes to see and it's already too far away for checking its distance with radar. If they don't get a good fix on 2024 YR4 before then, they won't get another crack at observing it until it comes around again in 2028 — just four years before a potential impact.
"What we really need is the object to move further along its orbit," to get more data to help tell astronomers where it's heading, he says. "That will give us a chance to constrain the possibilities of its orbit better."
Can anything be done to prevent an impact?
In 2022, NASA's DART mission ("Double Asteroid Redirection Test") successfully crashed into the asteroid Dimorphos about 7 million miles away from Earth, shifting its orbit a tiny bit. Although Dimorphos was no threat to Earth, the project was designed to prove that redirecting an asteroid on a collision course with Earth is possible. The mission was considered a success.
So, it is possible, but there are a number of caveats.
First, the object needs to be solid to have something to push against. So determining what it's made of is paramount. "Is it rocky? Does it have a lot of metallic content? Is it a rubble pile? Is it a solid, monolithic object?" NASA's Fast wonders aloud. "All of that is useful information."
Even so, space missions take years to get going and such a mission would require considerable lead time. In the case of 2024 YR4, there might be years to plan something like that and to get a spacecraft to the target object. But that might not always be the case, Fuls says.
A 2023 NASA report says the very biggest asteroids like the one that killed the dinosaurs are few and far between and the space agency thinks all of them have been identified and tracked. Another 95% of those a size smaller (which NASA describes as able to "cause global devastation [and] the possible collapse of civilization") have been discovered.
After that, the objects become less lethal, but fewer of them are known. Asteroid 2024 YR4 is thought to be between 40 meters and 90 meters in diameter (130 feet to about 300 feet), which puts it between two of NASA's lower classifications: 50 meters (165 feet), which could cause "local devastation" — of which only an estimated 7% are known and 140 meters (460 feet), with 40% unknown and the potential to be deadly over metro areas and states with likely mass casualties.
Is there any good news?
Yes! At this point, it's much more likely that 2024 YR4 won't hit the Earth. And while it could do considerable if it hit over a populated area, it's not a planet killer!
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