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Scientific American
17-07-2025
- Science
- Scientific American
7 Big Mysteries about Interstellar Object 3I/ATLAS
Earlier this month astronomers were thrilled to discover only the third known interstellar object ever seen in our solar system. Now dubbed 3I/ATLAS, the suspected comet has just zoomed past the orbit of Jupiter, traveling so fast that it's bound to slip through our sun's gravitational grip. The high speed and hyperbolic trajectory of 3I/ATLAS means it must have come from another star and was cast adrift in the Milky Way by some unknown process before it eventually, by chance, briefly swooped by our sun. It will reach about the orbit of Mars before it boomerangs back toward interstellar space, never to be seen again, at the end of this year. That's why astronomers have been racing to study 3I/ATLAS since July 1, when Larry Denneau of the University of Hawaii first spied it using a telescope in Chile that's part of the globe-spanning Asteroid Terrestrial-Impact Last Alert System (ATLAS). Soon more powerful observatories, including the James Webb Space Telescope (JWST) and Hubble Space Telescope, will scrutinize the object—which, thanks to its alien, interstellar provenance may be the oldest comet anyone has ever seen. 'I didn't get any sleep for like 35 hours,' says Bryce Bolin of Eureka Scientific in California, who rushed to release a preprint paper and arrange additional observations following 3I/ATLAS's discovery. 'It ruined my weekend.' On supporting science journalism If you're enjoying this article, consider supporting our award-winning journalism by subscribing. By purchasing a subscription you are helping to ensure the future of impactful stories about the discoveries and ideas shaping our world today. Stefanie Milam of NASA's Goddard Space Flight Center is part of a group that had reserved time on JWST to observe an interstellar object—if the researchers were fortunate enough for one to be discovered. But the group's luck was tested when it couldn't reach the lead of its program—Martin Cordiner, also at Goddard—to kick the observations into action. 'He was hiking in Maine when the object was discovered, and we could not reach him—he was completely off the grid,' Milam says. 'When he finally got back, his phone just blew up. I said, 'You're never allowed to go on vacation again!'' So why exactly are astronomers so eager to observe this object, and what do they hope to learn? Where did 3I/ATLAS come from? The first major question to answer about 3I/ATLAS is its origin. Tracing it back to an individual star is likely impossible, given the mixing of myriad stars in their orbits around our galaxy across billions of years. But we might be able to work out roughly the region it came from. One team of astronomers has already begun doing just that, using the high velocity of the object with respect to our sun—60 kilometers (37 miles) a second—to argue that it might have come from the vicinity of our galaxy's thick disk. This is a puffy torus of older stars moving at high velocities above and below the main flat plane of the Milky Way—which is where our sun serenely orbits. A thick-disk origin might mean that 3I/ATLAS is extremely ancient, more than eight billion years old. 'It's from a star that's potentially not even there anymore,' says Michele Bannister of the University of Canterbury in New Zealand, a co-author on the work. Aster Taylor of the University of Michigan performed a different age analysis based on the trajectory of 3I/ATLAS and suggests the object is 11 billion to three billion years old. 'We get similar answers,' Taylor says. Such estimates might soon be revised if subsequent observations can show just how much space weathering the object has endured during its interstellar sojourn. How big is it? Currently, 3I/ATLAS is inside the orbit of Jupiter and approaching the orbit of Mars, which it will cross in October, passing about 0.2 astronomical unit (one fifth the Earth-sun distance) from the Red Planet. Although early observations have led astronomers to categorize 3I/ATLAS as a comet, at the moment, it's not behaving exactly like one. The object doesn't display a large tail or enveloping coma of cast-off gas, only a hint of dust—but that is expected to change soon. As it traverses the asteroid belt between Mars and Jupiter and basks in the sun's radiance, its surface should warm enough to sublimate ice, venting sufficient material to form a large coma and perhaps a prominent tail. A substantial coma would be like a curtain drawn over astronomers' eyes, obscuring their view of the object and complicating efforts to gauge its dimensions. Before that happens, a team led by David Jewitt at the University of California, Los Angeles, is hoping to pin down its size with Hubble in August. (Other telescopes might be able to determine the size of 3I/ATLAS, too.) Initial estimates suggested 3I/ATLAS might be up 20 kilometers (12 miles) across—very big for a comet—but most astronomers now think it is much smaller. 'It's probably somewhere in the range of one or two kilometers,' says John Noonan at Auburn University in Alabama. That would be somewhat comparable in size to our first two interstellar visitors: 1I/ʻOumuamua, which was discovered in 2017 and was up to about 400 meters (0.25 mile) long, and 2I/Borisov, which was found in 2019 and was about one kilometer (0.6 mile) wide. If 3I/ATLAS turns out to be much bigger, 10 kilometers (six miles) or more, this would pose problems for preexisting estimates of many big interstellar objects reside in the galaxy. 'It's statistically extremely unlikely we should ever see something that size,' Noonan says. 'Theorists don't like that. But as an observer, I would love to see a really weird, big object.' How fast is it spinning? As well as its size, one of the key properties astronomers want to know about 3I/ATLAS is its rotation rate—something they might discern by watching the object's changing brightness as it spins. The spin of 3I/ATLAS could carry clues as to how the object was ejected from its home star in the first place. 'Certain ways of kicking these objects out tend to make them spin up,' Taylor says. A close pass of a gas giant planet, for instance, could easily set the object twirling while hurling it away from its home star. Conversely, a slow rotation period would suggest the object experienced a more gentle ejection. 'You could do this when stars die,' Taylor says. 'They lose a lot of mass, and so the gravitational force on objects at the outer edge of their system goes away. Those objects become unbound and just flow out into the galaxy.' The rotation period can also tell us more about the shape of 3I/ATLAS—a steady rotation suggests a fairly spherical form, whereas a fluctuating rotation speed might suggest a 'wonky shape,' Taylor says, like that of 'Oumuamua, which was estimated to be cigar- or pancake-shaped. What is 3I/ATLAS made of? If 3I/ATLAS really is an ancient cometary castaway that has been drifting through the galaxy for eons, it might be full of ice that has never been heated by a star. If so, then as it gets closer, the object might suddenly erupt into activity. While that could be bad news for measuring its size, it would aid efforts to determine 3I/ATLAS's chemical composition. JWST and Hubble would be best suited for the task of picking apart the different species of molecules that might erupt from 3I/ATLAS. Unfortunately, however, in October, when the object will be at its warmest, closest point to our star (called perihelion), Earth will be on the other side of the sun. This will make observations from our planet almost impossible. In November, post-perihelion, Noonan will use Hubble to study 3I/ATLAS and its emissions, looking for signs of substances such as hydroxide and hydrogen that can help clarify its composition. If the object is several billion years old, as predicted, then it might be rich in water because of the suspected formation environment around older stars. 'You would expect a lot of hydrogen coming from these water-rich irradiated objects, if this is really as old as [thought],' Noonan says. Milam and her colleagues, meanwhile, will use JWST in August and December to observe 3I/ATLAS before and after perihelion. Thanks to its keen infrared vision, JWST is better suited for teasing out the presence of molecules such as water, carbon monoxide, carbon dioxide and ammonia. 'We can really home in and see what this thing looks like,' she says. 'Borisov had a pretty boring chemistry, but it wasn't like any object in our solar system—there was hardly any water at all but a lot of carbon monoxide and hydrogen cyanide. With JWST, we're hoping to see a lot of carbon dioxide [on 3I/ATLAS], maybe even water, if it's as pristine as people are projecting.' Although the overall view from Earth degrades as the object approaches perihelion, some telescopes will be less visually impaired. Those operated by the Lowell Observatory in Arizona, for instance, are primed to observe 3I/ATLAS at dawn and dusk, when the sun is below the horizon. This will allow for studies even when the object will be close to our star from our planet-bound perspective. 'The Lowell Discovery Telescope is really well suited to observations close to the horizon,' says Nick Moskowitz, an astronomer at Lowell Observatory. 'We will be able to track it closer in to perihelion than other facilities.' An unlikely additional capability will be at Mars, where spacecraft such as NASA's Mars Atmosphere and Volatile Evolution (MAVEN) orbiter may be able to see 3I/ATLAS as it passes about 30 million kilometers (19 million miles) from the planet. 'It'll be pretty large and apparent in the sky,' Noonan says, providing the object kicks into activity as hoped. 'They'll be able to see the coma,' giving us an insight into 3I/ATLAS's activity near the sun that would otherwise be impossible to see from Earth. Will it survive? A big unknown about 3I/ATLAS is whether it will actually survive its close encounter with our sun. While 'Oumuamua did so, Comet Borisov was not so fortunate, with the object appearing to split and break apart on its way out of our solar system. The same fate could befall 3I/ATLAS. 'Borisov fragmented, which is pretty usual for comets,' Bannister says. All eyes will be on our latest visitor to see if the same thing happens again. An additional quirk of 3I/ATLAS's survivability is the impact of solar wind, which may snip away any cometary tail as it is ejected. By chance, the object is entering our solar system at quite a shallow angle, much flatter than that of most comets, which means it will experience stronger solar headwinds. Sarah Watson of the University of Reading in England and her colleagues are using this quirk to study how the solar wind traverses into the outer solar system. 'We can potentially calculate the speed of the solar wind,' she says, by noticing the impact of the solar wind on the purported comet's tail, if one materializes. Could we reach it? No spacecraft will be able to reach 3I/ATLAS. It is moving too fast and is too far from Earth for us to consider launching something in time. Yet an upcoming European Space Agency (ESA) mission called Comet Interceptor, set to launch in 2029, might attempt to visit another interstellar object, if we find one within its reach. The spacecraft will be positioned past the moon's orbit away from the sun and, if a suitable target is found, will be commanded to fire its engines and try and intercept the incoming alien object. If no suitable interstellar object is found, Comet Interceptor will instead be sent to one of several intriguing comets of our solar system. 'It is possible we could get an interstellar object, but we have to be really lucky,' says Colin Snodgrass, an astronomer at the University of Edinburgh, who is deputy lead on the mission. How many are there? One of our biggest outstanding questions about interstellar objects concerns their unknown abundance. The object 3I/ATLAS is our third interstellar visitor in eight years—a real but weak hint of how many are out there, waiting to be found. Predictions estimate there are trillions upon trillions of interstellar objects drifting around our galaxy, and perhaps one in our solar system at any given time—but they're typically just so faint that they're unlikely to be found by most telescopes. This is expected to change when a new telescope called the Vera C. Rubin Observatory begins a 10-year survey of the sky later this year. Rubin is expected to see somewhere between six and 51 interstellar objects in its 10-year survey. Seeing such a population will tell us 'how unique, or varied, planetesimal formation is across different parts of the galaxy,' Bannister says, referring to kilometer-scale objects thought to coalesce around newborn stars that become the feedstock for planets—and, when kicked to a system's hinterlands, become a reservoir of comets. One puzzling question is why we haven't seen much smaller interstellar objects, Moskowitz says. If smaller objects are more plentiful than larger objects, as scientists expect, then we should have seen some small interstellar objects entering our atmosphere, appearing as meteors streaking across Earth's skies at speeds and trajectories that clearly convey their interstellar origins. Detections of such objects have been claimed, but the evidence behind them has failed to convince most experts. The apparent absence of small interstellar interlopers 'is telling us something, but we don't know what that is yet,' Moskowitz says. 'I think that's going to be one of the major questions: Why are we seeing these big cometlike things coming through the solar system, but we're not seeing things that are smaller? It may have to do with the survivability of stuff out there in the galaxy, but we need more data.'
Forbes
12-04-2025
- Science
- Forbes
‘Earth Crosser' Asteroid May Now Strike The Moon, Scientists Say
An artist's illustration of 2024 YR4 Asteroid 2024 YR4, until recently thought to have a chance of striking Earth on December 22, 2032, could now be headed for the moon instead, according to new data. After new observations, 2024 YR4 is thought to have a diameter of about 98–213 feet (30–65 meters). The James Webb Space Telescope studied it in March and found it to be about 197 feet (60 meters). That's about the same width as a football field. The stony asteroid — first discovered on December 27, 2024, when it was passing just 1.5 million miles (2.5 million kilometers) — is one of the largest objects in recent history that could impact the moon, according to a paper accepted for publication in The Astrophysical Journal Letters. A lunar impact would be a book for scientists, who know little about the relationship between the size of an asteroid and the size of its resulting impact crater — despite the moon being covered in craters. 'If it does, it will give scientists a rare chance to study how the size of an asteroid relates to the size of the crater it creates — something we haven't been able to measure directly before," said Bryce Bolin, the lead author. For now, there's a roughly 2% chance YR4 could collide with the moon. An artist's illustration of 2024 YR4 in relation to Earth. Data from the Gemini South Observatory in Chile and the Keck Observatory in Arizona revealed that the asteroid rapidly rotates backward and may be shaped like a hockey puck. 'This find was rather unexpected since most asteroids are thought to be shaped like potatoes or toy tops rather than flat disks,' said Bolin. '2024 YR4 is a solid rock, likely chipped off from a larger rubble-pile asteroid in the central Main Belt between Mars and Jupiter." That 2024 YR4 appears to come from the main asteroid belt is a surprise. 'This region was not previously known to produce asteroids on Earth-crossing paths,' said Bolin. It's thought that gravitational interactions with Jupiter have forced it into the vicinity of Earth. This composite image of asteroid 2024 YR4 was captured with the Gemini South telescope in Chile. 2024 YR4 came to prominence in late January 2025 when the International Asteroid Warning Network placed the asteroid on its watch list after calculations of its orbit suggested it had over a 1% probability of hitting Earth in 2032. However, more calculations showed that it had dropped below 1% by late February. At the end of February, NASA announced that the chances of 2024 YR4 striking Earth during a close pass in 2032 were near zero. 'Studying this asteroid was vitally important in understanding the population of Earth crossers that have the potential to be Earth impactors and are poorly understood," said Bolin. This is a still from an animation showing asteroid 2024 YR4 as it passes by Earth and heads toward ... More its potential impact with the Moon. The news about 2024 YR4 comes as astronomers prepare for a very close pass of a much larger asteroid, 99942 Apophis, in exactly four years. The 1,100-foot (340-meter) wide asteroid will get to within 20,000 miles (32,000 kilometers) of Earth on Friday, April 13, 2029, creating a once-in-a-thousands-year opportunity for science. It will be so close that it will be seen eyed by observers across Western Europe and Western Africa. When Apophis was discovered in 2004, scientists calculated it might strike Earth in 2029, 2036 or 2068 — hence its "God of Chaos" nickname. NASA and the European Space Agency will send spacecraft to orbit Apophis before, during and after its close pass, just in case its trajectory changes and it becomes Earth-bound. Wishing you clear skies and wide eyes.
Yahoo
10-04-2025
- Science
- Yahoo
That 'city-killer' asteroid has surprised scientists yet again with its 'unusual' shape, and it could smash straight into the moon
When you buy through links on our articles, Future and its syndication partners may earn a commission. Scientists have revealed the unusual shape of a large asteroid with the potential to smack into the moon in the year 2032, according to new research. Asteroid 2024 YR4 sparked a flurry of interest earlier this year after NASA determined there was a chance it could hit Earth. The risk of an Earth impact has since dropped to zero, but there's still a slim chance 2024 YR4 could hit the moon in 2032 — 3.8% likelihood as of early April. A team of astronomers recently studied the asteroid in more detail using the Gemini South telescope in Chile. The researchers found that 2024 YR4 likely came from the asteroid belt between Mars and Jupiter and has an unusually flat, disk-like shape, similar to a hockey puck, according to a statement released by the National Science Foundation's National Optical-Infrared Astronomy Research Laboratory (NOIRLab). "This find was rather unexpected since most asteroids are thought to be shaped like potatoes or toy tops rather than flat disks," study lead author Bryce Bolin, a research Scientist at Eureka Scientific, a California-based astrophysics and astronomy research corporation, said in the statement. The researchers posted their findings to the preprint database arXiv, and will soon publish them in The Astrophysical Journal Letters. Related: 'Just the tip of the iceberg': Why risky asteroids like 2024 YR4 will pester Earth for decades to come Astronomers discovered the asteroid 2024 YR4 in December 2024, hence the "2024" in its name. In February, the likelihood of 2024 YRY hitting Earth in 2032 rose to 1 in 32, or 3.1%, which was the highest impact probability NASA had ever recorded for a space object of 2024 YR4's size or larger. However, astronomers always expected the threat level to drop once they learned more about 2024 YRY's trajectory. Within days, the likelihood plummeted to effectively zero, and has a 0.00078% chance of hitting Earth in 2032 at the time of writing. However, there remains a risk for the moon — not that the asteroid would significantly damage the moon even if it does hit. For the new study, researchers captured images of the space rock at different wavelengths of light. The team used 2024 YRY's pattern of light output over time, or lightcurve, to study the asteroid's shape, composition and orbital characteristics, according to the statement. From the rock's reflective pattern, the researchers concluded that 2024 YR4 is likely an S-type asteroid, which means it is rich in silicates — a class of minerals that includes quartz. The team also determined that the asteroid is rotating very fast, about once every 20 minutes, and estimated its size to be 98 to 213 feet (30 to 65 meters) wide, according to the study. RELATED STORIES —Watch potential 'city-killer' asteroid 2024 YR4 as it hurtles through space —Enormous, mountain-size asteroid will be visible from Earth this weekend in rare 'once in a decade' event —Potentially hazardous asteroids: How many dangerous space rocks lurk near Earth — and can we stop them? Researchers have been trying to accurately calculate the size of 2024 YR4 since its discovery, and this latest approximation is similar to previous estimates. Recent data from the James Webb Space Telescope suggested it could be slightly larger, 174 to 220 feet (53 to 67 meters), Live Science previously reported. 2024 YR4 is one of the largest objects in recent history with the potential to strike the moon, according to the statement. Back when there was a rising likelihood of 2024 YR4 hitting Earth, it was dubbed a potential "city killer" because it was large enough to wipe out a major city. While unlikely, a lunar impact would provide researchers with an unprecedented opportunity to study how the size of an asteroid relates to the size of the crater it creates, according to the statement.
Yahoo
10-04-2025
- Science
- Yahoo
Asteroid That Threatened Earth Shaped Like a Saucer, Scientists Find With Large Telescope
After taking another peek at the "city-killer" asteroid that once had a small but uncomfortable chance of smashing into our planet, astronomers have figured out what it actually looks like: a flat, spinning disk. The preferred analogy has been to compare the shape to a hockey puck. To us, this sounds like a huge missed opportunity to say that the Earth was — however briefly — under threat by a literal flying saucer, just like in that one movie. The findings come from a preprint study, accepted for publication in The Astrophysical Journal Letters. Using the Gemini South Observatory in Chile, the astronomers imaged the roughly 200-foot asteroid, 2024 YR4, in multiple wavelengths, revealing that it was rapidly rotating once every 20 minutes, in addition to its unusual UFO-like appearance. "This find was rather unexpected since most asteroids are thought to be shaped like potatoes or toy tops rather than flat disks," lead author Bryce Bolin from Eureka Scientific said in a statement about the work. The chunky space rock was first spotted in late December of last year. Soon, early observations predicted that there was a small but disquieting chance of one percent that it could hit our planet. By February, the odds surpassed 3 percent, before the space rock was finally determined determined to be on a harmless trajectory. At the time, from what scientists could tell from visible light observations, 2024 YR4 appeared to be around 300 feet long, a size large enough to wipe out a metropolis should it strike one. Recent research using the James Webb Space Telescope's infrared capabilities, however, determined it was actually no more than 220 feet long, or about the size of a 10-story building, and as small as 174 feet. This latest study backs up that estimate, independently concluding that YR4 is between 98 to 213 feet in diameter. Digging deeper, the astronomers studied the asteroid's lightcurves, finding that 2024 YR4 is what's known as an S-type asteroid rich in silicates. That's not surprising, but its origins are: the asteroid appears to have been nudged our way by Jupiter out of the solar system's main asteroid belt, a ring of rocky objects surrounding the Sun that stretches between the orbits of Mars and the gas giant — specifically, a region called the central main belt. Previously, astronomers suspected 2024 YR4 came from the inner main belt, based on its composition — but its retrograde spin revealed in these latest observations indicate otherwise. "We are a bit surprised about its origin in the central main asteroid belt, which is a location in the asteroid belt that we did not think many Earth-crossing asteroids could originate from," Bolin said. While it no longer poses a threat to Earth, there's still a 3.8 percent chance YR4 could veer into the Moon, according to the latest Webb observations. If it does, the impact will leave a nasty scar — but shouldn't alter the lunar world's orbit. More on space: Scientists Intrigued by Stars Singing Ancient Songs
Forbes
09-04-2025
- Science
- Forbes
Moon-Threatening Asteroid Has Weird Shape, Scientists Say
Asteroid 2024 YR4's star has faded when it comes to the general public, but scientists are pumped to be learning more about it. The space rock reached celebrity status earlier this year when initial data showed it had a small chance of striking Earth in 2032. Further observations ruled out the scary scenario—but the moon is still in play. Now we know more about the famous asteroid. Scientists teamed up to study the asteroid and discovered some surprising information about its possible origin and shape. 'Studying this asteroid was vitally important in understanding the population of Earth crossers that have the potential to be Earth impactors and are poorly understood,' said astronomer Bryce Bolin of Eureka Scientific in a statement on April 8. Bolin is the lead author of a paper on the asteroid set to be published in 'The Astrophysical Journal Letters.' Bolin and his colleagues used the Gemini South telescope in Chile to image the asteroid. 'Detailed analysis of the asteroid's lightcurve (pattern of light output in time) allowed the team to determine its composition, orbital characteristics and 3D shape,' the United States National Science Foundation National Optical-Infrared Astronomy Research Laboratory said. The asteroid likely came from the solar system's main asteroid belt, a region between Mars and Jupiter. It's quite the hot hangout for asteroids, with NASA estimating it contains between 1.1 and 1.9 million asteroids larger than 0.6 miles in diameter—and many more smaller ones. 'We are a bit surprised about its origin in the central main asteroid belt, which is a location in the asteroid belt that we did not think many Earth-crossing asteroids could originate from,' said Bolin. The asteroid has a fast rotation of only about 20 minutes. The data shows YR4 has a shape reminiscent of a hockey puck. That qualifies as another surprise. 'This find was rather unexpected since most asteroids are thought to be shaped like potatoes or toy tops rather than flat disks,' Bolin said. The W. M. Keck Observatory in Hawaii also got in on the asteroid fun and contributed data to the team's paper. Keck's ability to see in infrared helped the researchers study the asteroid's composition. It's likely made of solid rock. Larger asteroids are often called 'rubble piles' since they're conglomerates of rock fragments. Asteroid YR4 may have once been a boulder on one of those larger asteroids. Whether an asteroid is solid or a rubble pile makes a difference when it comes to planetary defense. That's crucial information that could help with planning a mission like NASA's DART test. DART involved crashing a spacecraft into an asteroid to alter its orbit. Asteroid 2024 YR4 was first spotted in December. There had been some uncertainty about the asteroid's size, but NASA's James Webb Space Telescope turned its powerful instruments on the rock and found it measured in at about 200 feet wide, roughly the size of a 15-story building. That's within the initial estimates of between 130 and 300 feet wide. At one point, early observations suggested 2024 YR4 had a 3.1% chance of impacting Earth. As more data came in, researchers revised that down to near zero. However, the asteroid now has a 3.8% chance of impacting the moon on Dec. 22, 2032. Don't worry about the moon, though. 'In the small chance that the asteroid were to impact, it would not alter the moon's orbit,' NASA said in a statement on April 2. If YR4 does smack into the moon, it will be a thrilling time for astronomers. It would 'provide an unprecedented opportunity to study the relationship between the size of an asteroid and the size of its resulting impact crater—a previously unknown quantity,' NOIRLab said. Researchers aren't done with YR4 yet. Webb is set to check it out again in late April or early May, even as the asteroid becomes too far away and too faint for ground-based telescopes to make observations. Asteroid 2024 YR4 is no longer a big mystery. Its composition, size, shape and origin are better understood. We can also breathe a sigh of relief that it won't be barreling into our planet in 2032. The moon, however, may need to brace for impact. We'll know even more as scientists continue to study the space rock.
