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3i/Atlas - Is it an asteroid, is it a comet, or other?
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19-08-2025 05:39AM#1
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Detection of an Anti-Solar Tail for 3I/ATLAS
Deep images of 3I/ATLAS, taken by the Gemini South telescope on August 27, 2025. The different panels show images from short to long wavelengths in the u (upper left), g (upper right), r (lower left) and i (lower right) spectral bands, centered on wavelengths of 0.365, 0.467, 0.616 and 0.747micrometers, respectively. The images show the first evidence for a teardrop shaped tail in the anti-Sun direction behind 3I/ATLAS. (Credit: B. Bolin et al. 2025)
On August 27, 2025, deep imaging of the interstellar object 3I/ATLAS by the Gemini South 8.2-meter telescope — aided by the Gemini Multi-Object Spectrograph (GMOS), revealed a weak tail with a teardrop shape in the anti-Sun direction (reported here). At that time, 3I/ATLAS was at a distance from Earth of 2.59 times the Earth-Sun separation. The Gemini South Observatory is located on a mountain called Cerro Pachón in the Chilean Andes.
The images were taken in the u (upper left), g (upper right), r (lower left) and i (lower right) spectral bands, centered on wavelengths of 0.365, 0.467, 0.616 and 0.747 micrometers, respectively. They show clear evidence for a teardrop shaped tail in the anti-Sun direction behind 3I/ATLAS. The observed tail is about 30 arcseconds, or equivalently 56,000 kilometers long, pointing towards the South East. The coma is about 10 arcseconds or equivalently 18,800 kilometers wide, significantly more extended than its compact appearance in images of 3I/ATLAS from early July 2025.
Data collected on August 6, 2025 by the Webb Telescope (accessible here) confirmed the existence of a carbon dioxide (CO2) gas plume around 3I/ATLAS with an order of magnitude lower levels of water (H2O) and carbon monoxide (CO), as reported earlier by the SPHEREx space observatory team (here). SPHEREx mapped the CO2 plume out to 348,000 kilometers around 3I/ATLAS. The inferred mass loss rates from 3I/ATLAS are 130 kilograms per second for CO2, 6.6 kilograms per second for H2O and 14 kilograms per second of CO. The H2O mass loss rate is only 5% of the CO2 output, unlike expectations from a water-rich comet.
It therefore comes as no surprise that the plume of gas around 3I/ATLAS is shaped by the solar wind and the solar radiation pressure to a teardrop configuration.
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I'm deeply fascinated by the metals in the halo.
The Mysterious Object Cruising Through Solar System Is Releasing Strange Metals, Paper FindsAstronomers just made an "extremely puzzling" discovery.
![object-producing-metals-nickel-iron[1].jpg](https://us.v-cdn.net/6034073/uploads/UDOJSTKEJ5CT/object-producing-metals-nickel-iron-5b1-5d.jpg)
Astronomers are still poring over data to get a better sense of 3I/ATLAS, the third interstellar object to ever have been observed careening through our solar system.
The unusual visitor is set to scream past Mars this week as it continues on its extremely rapid trajectory through our cosmic neighborhood.
Now, in a new yet-to-be-peer-reviewed paper, an international team of astronomers analyzed data collected by the European Very Large Telescope’s Ultraviolet and Visual Echelle Spectrograph (UVES) — and finding that 3I/ATLAS shows an “extreme abundance ratio” of nickel and iron in its gas plume.
That makes it stand out when compared to both familiar solar system comets and 2I/Borisov, the second interstellar object to have been observed in 2019, which was also a comet.
The team was met with a conundrum, further highlighting how 3I/ATLAS’ highly unusual nature keeps throwing astronomers for a loop.
“At the distances at which comets are observed, the temperature is far too low to vaporize silicate, sulfide, and metallic grains that contain nickel and iron atom,” they write. “Therefore, the presence of nickel and iron atoms in cometary coma is extremely puzzling.”
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