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Rare Interstellar Comet 3I/ATLAS Sparks Debate Amid Global Observation Campaign By BBC Science Correspondent The recent passage of the interstellar comet 3I/ATLAS through the inner solar system has become one of the most intense and scrutinized astronomical events of the decade, prompting a massive global observation effort and sparking a vigorous scientific debate about the nature of objects originating beyond our Sun. Detected in July 2025 by the Asteroid Terrestrial-impact Last Alert System (ATLAS) in Chile, 3I/ATLAS is only the third confirmed object of interstellar origin—after 1I/ʻOumuamua and 2I/Borisov—to be tracked by astronomers. Its high velocity and complex chemical signature are providing scientists with a unique, fleeting opportunity to sample material from another star system, fundamentally challenging existing models of planetary formation. The comet, currently travelling at approximately 210,000 km/h (130,000 mph) on a hyperbolic trajectory, recently completed a crucial flyby of Mars. The event on 3 October saw a fleet of robotic explorers, including the European Space Agency's (ESA) ExoMars Trace Gas Orbiter (TGO) and Mars Express, pivot to capture close-range observations. While the comet posed no threat, passing roughly 30 million kilometres from the Red Planet, the proximity offered an unparalleled vantage point for instruments usually dedicated to Martian atmospheric science. The sheer speed and faintness of the object made the task technically demanding for the orbiting spacecraft. "The observation was certainly very challenging," said Nick Thomas, principal investigator for the CaSSIS camera aboard the ExoMars TGO. "The comet is around ten thousand to one hundred thousand times fainter than our usual target. We are now in the laborious process of analysing the images and spectroscopic data, but every tiny bit of information is precious when dealing with an object from outside our own stellar neighbourhood. " Unlocking the Building Blocks of Distant Worlds The core scientific interest in 3I/ATLAS centres on its chemical composition. Early observations by both the NASA/ESA Hubble Space Telescope and the James Webb Space Telescope (JWST) confirmed the presence of typical cometary materials, including water ice, carbon monoxide (CO), and carbon dioxide (CO
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However, the object has demonstrated unusual activity, beginning to shed gas and dust—forming a glowing cloud known as a coma—while still much farther from the Sun than is typical for Solar System comets. This early outgassing suggests a unique thermal history or a specific abundance of highly volatile ices. In a recent study, researchers detected hydroxyl (OH) gas, a by-product of water, being released at a rate equivalent to a "fire hose running at full blast. " Auburn University postdoctoral researcher Zexi Xing, lead author of a paper on the water activity, noted the object’s surprising behaviour. “Every interstellar comet so far has been a surprise,” Xing stated. “'Oumuamua was dry, Borisov was rich in carbon monoxide, and now ATLAS is giving up water at a distance where we didn't expect it. Each one is rewriting what we thought we knew about how planets and comets form around stars. ” The analysis of these exotic elements could reveal whether the elemental building blocks of planetary systems elsewhere in the Milky Way are similar to our own, providing a direct link to conditions around distant stars. Scientists also suspect that 3I/ATLAS, based on its trajectory and speed, could be the oldest comet ever observed, potentially billions of years older than our own 4. 6-billion-year-old Solar System. The Debate: Comet or Classified Artifact? Adding a layer of public controversy to the scientific discovery is the ongoing suggestion from some quarters that 3I/ATLAS might not be a natural phenomenon. Harvard astrophysicist Avi Loeb, known for his unconventional theories regarding previous interstellar visitor 'Oumuamua, has suggested in papers and public forums that the object’s characteristics could be consistent with an artificial probe or remnant of alien technology.
Dr. Loeb points to a combination of its estimated size (potentially larger than previous interstellar objects) and a potentially rare orbital alignment near the ecliptic plane as points of interest that warrant further investigation beyond traditional cometary models. However, the consensus among the wider astronomical community strongly leans toward a natural origin. Scientists studying the extensive data gathered from multiple telescopes point to the object's visible coma and tail, which are clear signs of cometary activity driven by solar heating. Tom Statler, NASA’s lead scientist for solar system small bodies, countered the technological theory. “It looks like a comet. It does comet things. It very, very strongly resembles, in just about every way, the comets that we know,” he explained. Statler acknowledged that while 3I/ATLAS exhibits some properties that differ from Solar System comets, the evidence overwhelmingly supports its classification as a natural body. What’s Next for 3I/ATLAS The next phase of the comet’s journey will be critical for observation. Having passed Mars, 3I/ATLAS is now accelerating toward its closest approach to the Sun, known as perihelion, which is expected around late October 2025. During this period, the increased solar radiation will cause maximum activity, likely increasing the size and brightness of its coma and tail.
Though the comet will be difficult to observe from Earth due to its close proximity to the Sun, deep-space missions are poised for action. Following perihelion, the comet will pass near Jupiter, allowing the ESA’s Jupiter Icy Moons Explorer (JUICE) to monitor the object between November and December 2025. This observation window offers a chance to study 3I/ATLAS in its most active state. Data from JUICE will be particularly valuable as the comet begins its swift, one-way journey back out of the Solar System and into the blackness of interstellar space, a transit expected to be complete by early 2026. The rarity of these interstellar visitors has also fueled strategic planning for future space exploration. ESA's Comet Interceptor mission, slated for launch in 2029, is designed to wait in a parking orbit for the possibility of intercepting either a long-period comet or, ideally, a future interstellar interloper like 3I/ATLAS. As the scientific community continues to sift through the vast dataset captured during this flyby, the consensus remains that 3I/ATLAS, whether natural or not, represents an invaluable messenger. It carries primordial ice and dust from a star system light-years away, offering astronomers a direct connection to the complex processes that govern planet formation across the galaxy. The data gathered now will define models of interstellar chemistry for years to come.
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