3I/ATLAS

• 1. Introduction

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  3I/ATLAS first showed up on the 1st of July 2025 as a faint speck of apparent magnitude 18. A NASA-funded survey telescope at Río Hurtado, Chile, caught it near the border of the constellations Serpens Cauda and Sagittarius. Its first name was only a placeholder, A11pl3Z. Within a day it had a proper designation, because astronomers realized the object came from outside the Solar System. It was the third interstellar object ever confirmed, after 1I/Oumuamua and 2I/Borisov. This documentary follows the questions it raised. How do astronomers tell a comet from a rock at such a distance? How fast was it traveling, and did it ever threaten Earth? What is it made of, and could it really be older than the Solar System itself? And why did one well-known astrophysicist suggest it might be a spacecraft?

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• 2. Sorting Rock From Comet

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  The Minor Planet Center first placed A11pl3Z on its Near-Earth Object Confirmation Page. The earliest observations hinted at a highly eccentric path that might approach Earth's orbit. The listing was meant to be temporary, lasting only until the orbit could be confirmed. Follow-up work by professional and amateur astronomers soon showed the path would never come near Earth. Instead it was hyperbolic, the shape of an object that passes through once and leaves.

  Records from before the discovery settled the question. The Zwicky Transient Facility had imaged the object on the 28th and the 29th of June 2025, and those frames turned up within a few hours of the first report. More Zwicky frames from the 14th to the 21st of June followed, along with ATLAS frames from the 25th to the 29th of June. Amateur astronomer Sam Deen found further ATLAS images from the 5th to the 25th of June. He suspected the object had gone unnoticed because it sat in front of the dense star fields of the Galactic Center. Zwicky frames from as early as the 22nd of May turned up too, and a Weizmann Astrophysical Observatory image from the 21st of May was published on the 18th of July.

  The newly commissioned Vera C. Rubin Observatory had also caught the object by chance, between the 21st of June and the 3rd of July 2025, during its science validation tests. Had those tests started two weeks earlier, Rubin would have beaten ATLAS to the discovery. Its 3200-megapixel LSST Camera took nearly 100 exposures before the comet could no longer be seen from Chile on the 20th of July.

  Was it even a comet? Alan Hale and other astronomers reported no cometary features at first. Then, on the 2nd of July 2025, the Deep Random Survey in Chile, the Lowell Discovery Telescope in Arizona and the Canada-France-Hawaii Telescope on Mauna Kea all saw a marginal coma. They also saw a possible tail-like stretch 3 arcseconds long. David Jewitt and Jane Luu, using the Nordic Optical Telescope that same day, found it "clearly active". Miguel Alarcón and a team from the Instituto de Astrofísica de Canarias reached the same verdict from Teide Observatory.

  The Minor Planet Center announced the discovery that day. It added the prefix 3I and the comet designation C/2025 N1 (ATLAS). By then it held 122 observations from 31 different observatories, and those were enough to pin down an orbit that turned out to be extreme.

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• 3. An Extreme Orbit

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  When 3I/ATLAS entered the Solar System, it was moving at 58 km/s relative to the Sun, far faster than 1I/Oumuamua or 2I/Borisov. At that speed the Sun's gravity cannot hold it. The comet sped up while falling toward the Sun and slowed as it climbed away. It will still escape for good.

  Orbital eccentricity measures how curved a path is. Elliptical orbits score below 1, and hyperbolic orbits score above 1. 3I/ATLAS scores 6.141, higher than 1I/Oumuamua at 1.2 and 2I/Borisov at 3.4. A value that high makes its path look almost straight.

  At 11:45 UT on the 29th of October 2025, the comet reached perihelion, its closest point to the Sun. The distance was 1.36 AU, where one AU is the average distance from Earth to the Sun. That put the comet between the orbits of Earth and Mars. Its speed peaked at 68 km/s.

  The tilt of its path was a coincidence. The trajectory lies within 5 degrees of the plane of the planets, although it runs backward at 175 degrees to the ecliptic. That alignment carried the comet near several worlds. It passed Mars at 0.194 AU on the 3rd of October 2025 and Venus at 0.65 AU on the 3rd of November. Earth came next, on the 19th of December, at 1.798 AU, so there was never any threat. Jupiter followed at 0.358 AU on the 16th of March 2026.

  The journey is far from over. The comet will reach the inner Oort cloud, the Solar System's distant outskirts at 2000 AU from the Sun, around the year 2189. It will cross the outer edge, beyond 100000 AU, in about 8000 years.

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• 4. Watching The Visitor

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  At its brightest, 3I/ATLAS was not expected to pass about apparent magnitude 11.5 as seen from Earth. It never showed to the naked eye or through small binoculars. On the 14th of September 2025 it measured magnitude 14.2. Seeing even a point-like star of that brightness takes a telescope with an aperture of at least 200 mm, and a diffuse comet may need 300 mm. By mid-November the comet reached the borderline of magnitudes 10 and 9, bright enough for tripod-mounted 20x80 binoculars.

  Comparison with other comets of that autumn shows how modest it was. At magnitude 12 during the Mars approach, it was about 120 times fainter than comet C/2025 A6 (Lemmon). It was about 190 times fainter than comet C/2025 R2 (SWAN).

  On the 21st of October 2025 the comet slid behind the Sun as seen from Earth, eight days before perihelion. Ground observers lost it. The GOES-19 weather satellite, which can see objects down to magnitude 12, followed it from the 18th to the 24th of October. The Solar and Heliospheric Observatory and the Polarimeter to Unify the Corona and Heliosphere satellite watched as well.

  In November the comet reappeared just before sunrise. Scientists from the Physical Research Laboratory took spectra with the 1.2 m infrared telescope on Mount Abu in mid-November. On the 11th of December it passed less than two degrees from the half Moon. Opposition, when it stood opposite the Sun and was up all night, came on the 22nd of January 2026, when it was much fainter.

  Sending a probe was out of the question. A July 2025 study led by Atsuhiro Yaginuma found that any launch from Earth after the 1st of July would need more velocity change than any available propulsion could supply. A probe leaving Mars would have needed far less.

  Between the 1st and the 7th of October 2025, two European Space Agency Mars orbiters, Trace Gas Orbiter and Mars Express, looked for the comet. The CaSSIS camera on Trace Gas Orbiter caught its coma using 5-second exposures, even though the comet was about 50,000 times fainter than what the orbiter normally views. Mars Express did not see it, probably because of a much lower exposure time. China's Tianwen-1 orbiter also imaged it. NASA's Mars Reconnaissance Orbiter observed it too, but the data was not released because of the NASA shutdown.

  Some spacecraft had chance close passes. Astrophysicist Marshall Eubanks calculated that Psyche passed about 0.302 AU from the comet on the 4th of September 2025, and that Juice passed within 0.428 AU on the 4th of November. Redirecting them risked their main missions, yet Juice still observed the comet in November with cameras, spectrometers and a particle sensor. Juno, low on fuel and with engine problems, could not be redirected. Europa Clipper was predicted to sit inside the comet's ion tail from the 30th of October to the 6th of November, and Hera from the 25th of October to the 1st of November. Scientists also hoped to spot a magnetic draping structure in the solar wind around the comet.

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• 5. Anatomy Of A Visitor

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  Hubble took its first images of the comet on the 21st of July 2025, revealing the coma in fine detail. The coma is the cloud of gas and dust around the nucleus, formed when the Sun's heat turns ice into gas that lifts dust away. Images from the 10.4 m Gran Telescopio Canarias on the 2nd of July showed it spanning about twice the diameter of Earth. Rubin data traced its growth from 13040 km on the 21st of June to 18760 km on the 2nd of July.

  The coma was reddish, much like 2I/Borisov, Solar System comets and D-type asteroids. The likely cause is irradiated organic compounds called tholins. Most grains were relatively large, several micrometers in radius. Small grains left the nucleus quickly, while large ones moved slowly. Hubble images from July suggested the comet shed 6 kg of small dust per second and 60 kg of large dust per second. That resembles 2I/Borisov on its approach, but it is lower than typical distant Jupiter-family comets.

  During July and August 2025 the coma seemed stretched westward, toward the Sun and the direction of travel. Early reports called this a tail, but it is really a plume of dust blown off the sunlit surface, where ice sublimates faster. Images from the 26th of August still showed the inner coma fan-shaped and slightly brighter on the Sun-facing side.

  The true tail pointed the other way. Hubble images from July showed a very faint, broad tail directed away from the Sun, which suggested small dust grains leaving at high speeds. By the end of August the 8.2-meter Gemini South telescope saw it growing to 30 arcseconds, roughly 56000 km. By the 15th of September it measured 50 arcseconds, about 100,000 km. Michael Jäger recovered the tail after perihelion, on the 8th of November. In April 2026 the European Space Agency reported that the gas tail stretched 5 million km from the nucleus. Dust tails on ordinary comets can reach about 10 million km.

  The nucleus is harder to measure, because glare from the dust makes it look larger and brighter. Hubble limited its diameter to below 5.6 km. Early estimates from July ran higher, though astronomers knew the true size should be far smaller. The dust loss rate suggests a nucleus under a kilometer across, like 2I/Borisov.

  Rotation has proved just as elusive. Raul de la Fuente Marcos and colleagues, using the Gran Telescopio Canarias from the 2nd to the 5th of July, reported a rotation period of 16.79 hours. Toni Santana-Ros and colleagues found 16.16 hours. They also saw brightness swings shrink from 0.3 to 0.2 magnitudes through July, as dust hid the nucleus. In December 2025 Miquel Serra-Ricart and colleagues measured 15.48 hours from the wobble of a gas and dust jet. Virginio Oldani and other Italian astronomers proposed that the north pole points toward either RA 154 degrees, Dec +25 degrees, or RA 334 degrees, Dec -25 degrees.

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• 6. Chemical Fingerprints

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  NASA's Transiting Exoplanet Survey Satellite had already recorded the comet from the 7th of May to the 3rd of June 2025, two months before discovery. It was bright and active even at about 6.4 AU from the Sun. At that distance water ice cannot sublimate fast, so something more volatile must have been driving the activity.

  Water ice was first reported on the 20th of July 2025, from near-infrared observations by Gemini South and the NASA Infrared Telescope Facility on the 5th and the 14th of July. Ultraviolet data from the Swift Observatory around the 30th of July and the 1st of August suggested water vapor and hydroxide ions. On the 21st of August, astronomers from NASA's SPHEREx mission and Caltech reported water ice and bright carbon dioxide emission.

  The James Webb Space Telescope made its first observations on the 6th of August 2025 with its NIRSpec instrument, and NASA announced results on the 25th of August. The coma was unusually rich in carbon dioxide. It also held small amounts of water ice, water vapor, carbon monoxide and carbonyl sulfide, plus a trace of carbon-13. The water ice seemed to be fine grains smaller than 1 micrometer, probably amorphous. A second Webb visit in December 2025 added methane to the list.

  The Very Large Telescope found atomic nickel vapor on the 20th of July and cyanide gas on the 14th of August. Both rose "rapid and steady" through that period as the comet neared the Sun. Nickel appearing before cyanide was odd. No iron vapor was seen, although iron and nickel usually come out of comets in roughly equal amounts. Rohan Rahatgaonkar and colleagues proposed that space weathering breaks down nickel-bearing organic compounds, such as nickel tetracarbonyl. The concentrations of nickel and cyanide still matched those of Solar System comets at similar distances.

  A different gap stood out. The VLT saw no dicarbon, tricarbon or amino radicals, and the MDM Observatory found no sign of carbon chain compounds. That makes 3I/ATLAS one of the most carbon chain-depleted comets ever known.

  The carbon dioxide itself is the real puzzle. Its ratio to water was among the highest ever seen in a comet at that distance, and only the Oort cloud comet C/2016 R2 (PanSTARRS) breaks the usual trend similarly. Martin Cordiner and a team of NASA researchers offered explanations. The nucleus may be rich in carbon dioxide because of heavy radiation or because it formed near the frost line. Or an unusually thick, insulating crust may hide water ice below. In April 2026 the European Space Agency reported emissions equal to "70 Olympic Swimming Pools" of water vapor per day, mostly directed toward the Sun.

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• 7. Older Than The Sun

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  Running the comet's path backward leads to interstellar space in the direction of Sagittarius, near the Galactic Center. Unlike the previous two interstellar visitors, it came from the southern celestial hemisphere, opposite the solar apex in the north. The apex is the direction of the Sun's motion relative to local stars. Astronomers had predicted more objects from the apex and expected southern ones to be harder to spot. Either 3I/ATLAS is a rare find, or southern interstellar objects are more common than thought.

  Its motion through the galaxy offers a clue to its age. The comet rose through the galactic plane at a speed quite high compared with nearby stars and other interstellar objects. That means it follows a tilted orbit around the Milky Way, which places it in either the thin disk or the thick disk. The thick disk mainly holds older stars with lower levels of heavy elements than the Sun.

  A July 2025 study led by Matthew Hopkins estimated, with 68 percent confidence, an age between 7.6 and 14 billion years. That estimate assumes it came from the thick disk. The Solar System is only 4.6 billion years old, so 3I/ATLAS may be the oldest comet yet seen. An independent analysis by Aster Taylor and Darryl Seligman, also in July 2025, gave 3 to 11 billion years.

  Its home star cannot be identified. After billions of years in the Milky Way, the comet has been mixed in with countless other stars. Its speed may have shifted through gravity assists from close encounters with stars and nebulae. Yiyang Guo and collaborators found in September 2025 that it may have passed within 1 parsec of 25 known stars in the past 10 million years.

  The composition still tells a story. If the comet comes from the thick disk, its parent star might have had at least 40 percent of the Sun's heavy element abundance. The carbon dioxide suggests the comet formed far from that star, beyond the frost line, in a protoplanetary disk of gas and dust. Later, a close encounter with a giant planet or a star flung it into the dark.

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• 8. The Spacecraft Debate

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  On the 16th of July 2025, astrophysicist Avi Loeb and other researchers of the Initiative for Interstellar Studies posted a paper on arXiv. They speculated that 3I/ATLAS could be an extraterrestrial spacecraft. They cited "anomalous" characteristics, including an apparently large size, a lack of identifiable chemicals and a path aligned with the plane of the planets.

  Other astronomers pushed back at once. Chris Lintott criticized the idea immediately. Live Science reported that "the overwhelming consensus is that it is a comet", and that many researchers were disappointed because the paper distracted from other scientists' work. Scott Manley and David Kipping pointed out flaws in Loeb's statistical analysis of the trajectory. Darryl Seligman, who led the first published study of the object, said that numerous telescopic observations showed "classical signatures of cometary activity". He added that the chemicals might not yet be detectable because the object was still far from the Sun.

  Later measurements answered that point. Webb confirmed outgassing of carbon dioxide, water and carbon monoxide. Swift found tentative water vapor and hydroxyl in late July and early August, and the Very Large Telescope found nickel and cyanide, which are commonly found in comets. In 2026 the European Space Agency's JANUS camera on Juice still picked out a distinct coma from 60 million km away, with both a dust tail and an ion tail. That backed the view that this is an ordinary comet.

  Loeb had earlier suggested that 1I/Oumuamua and a possible interstellar meteor, CNEOS 2014-01-08, might be spacecraft, and many researchers criticized him for it. On his blog he wrote that "the most likely outcome will be that 3I/ATLAS is a completely natural interstellar object, probably a comet". He defended his hypothesis as an "interesting exercise in its own right, and is fun to explore, irrespective of its likely validity". Astronomer Samantha Lawler said that remaining open-minded is important, but that the paper pushed this sentiment to the limit. She added that extraordinary claims need extraordinary evidence, and that "the evidence presented by Loeb et al. is absolutely not extraordinary."

  The watching has not ended. In November 2025, Hubble performed ultraviolet spectroscopy to measure the comet's gas emissions and its sulfur-to-oxygen ratio, and it will keep monitoring 3I/ATLAS on the way out of the Solar System.

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